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Optical processes in cadmium sulfide thin films

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Optical processes in cadmium sulfide thin films
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Clausen, Edward M., 1958-
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viii, 240 leaves : ill. ; 28 cm.

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Absorption spectra ( jstor )
Cadmium ( jstor )
Excitons ( jstor )
Heat treatment ( jstor )
Light refraction ( jstor )
Photoluminescence ( jstor )
Room temperature ( jstor )
Thin films ( jstor )
Wavelengths ( jstor )
X ray film ( jstor )
Cadmium sulfide ( lcsh )
Dissertations, Academic -- Materials Science and Engineering -- UF
Materials Science and Engineering thesis Ph. D
Optical data processing ( lcsh )
Semiconductors -- Optical properties ( lcsh )
Thin films ( lcsh )
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bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

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Thesis:
Thesis (Ph. D.)--University of Florida, 1987.
Bibliography:
Includes bibliographical references (leaves 235-239).
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Also available online.
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Typescript.
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Vita.
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by Edward M. Clausen.

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Full Text


















OPTICAL PROCESSES IN CADMIUM SULFIDE
THIN FILMS













By

EDWARD M. CLAUSEN, JR.


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY


UNIVERSITY OF FLORIDA


1987































Copyright 1987

by

Edward M. Clausen, Jr.















ACKNOWLEDGMENTS


Not enough can be said for the people who have contributed to the

start and particular the completion of this dissertation. It would

have been very difficult for me to get through this project without

their help.

I would like to express my deepest appreciation and gratitude to

Dr. Joseph H. Simmons, my academic adviser, who supplied much

encouragement, direction, assistance and the opportunity to work on

this project. His insight and ability to help me solve problems proved

to be an extremely valuable asset, although his confidence in my

abilities to "get the job done" was invaluable. I would also like to

express my appreciation to the members of my committee, Dr. Paul

Holloway, Dr. Robert Dehoff, Dr. Stan Bates, Dr. Tim Anderson and Dr.

Ramakant Srivastava for their suggestions, comments and guidance. This

is certainly one of the best combinations of abilities and expertise

for a committee and I thank the members for their help and enthusiasm.

My closest friends Guy Latorre, Richard Robinson and B.G. Potter

deserve special thanks for their personal support and reassurance. My

roommate and friend, Steve Wallace deserves the most thanks and credit

for having to endure the more trying times of this project. I would

especially like to thank my mother and father for their support

throughout all of my endeavors, and my closest and dearest friend Laura

Harmsen for her inspiration and emotional support.










Finally, I would like to express my graditude to the Air Force

Office of Scientific Research (AFOSR 84-0395) for funding this research

project.















TABLE OF CONTENTS


PAGE


ACKNOWLEDGMENTS . . . . . . . . . . . iii

ABSTRACT . . . . . . . . . . . . . vii

CHAPTERS

I INTRODUCTION . . . . . . . . .. 1

Optical Signal Processing . . . . . .. 1
Nonlinear Optical Materials . . . . . . 2
Thin Films . . . . . . . . . 5

II THEORY OF NONLINEAR OPTICS . . . . . . 8

Nonlinear Optical Susceptibility . . . . 8
Optical Bistability . . . . . . . .. 19

III BACKGROUND BIBLIOGRAPHICAL REVIEW . . . .. .28

Bulk Properties of Cadmium Sulfide . . . .. .28
Photoluminescence of CdS . . . . . .. .32
Resonant Raman Scattering in CdS . . . .. 37
Nonlinear Susceptibility . . . . . .. .38
CdS Thin Films . . . . . . . .. 39
Vacuum Deposition . . . . . . .. 39
RF Sputtering . . . . . . . .. 45
Semiconductor Doped Filter Glasses . . . .. .50

IV EXPERIMENTAL METHOD . . . . . . . .. .52

Vacuum Deposition System . . . . . .. .52
RF Magnetron Sputtering . . . . . . .. .60
Thin Film Deposition . . . . . . .. 63
Substrate materials . . . . . .. 64
Substrate temperatures . . . . .. .66
Deposition rates . . . . . . .. 68
Co-Sputter Alternating Deposition (COSAD) . 68
Post Deposition Treatments . . . . . .. .70
Thin Film Characterization . . . . . .. .71
Microstructure . . . . . . . .. 71
Chemical Analysis . . . . . . .. 73










Optical Measurements . . . . . . .. 76
Index of Refraction . . . . . . .. 77
UV-VIS Absorption . . . . . . .. 86
Photoluminescence and Raman . . . . .. .88

V RESULTS AND DISCUSSION . . . . . . .. .96

Thin Film Physical Properties . . . . .. .96
Microstructure Characterization . . . .. 96
Transmission electron microscopy . . .. .96
Scanning electron microscopy . . . .. .109
X-ray diffraction experiments . . . .. .123
Compositional Analysis . . . . . .. .132
Stoichiometry determination of target material 132
Analytical techniques . . . . . .. .134
Co-Sputter Alternating Deposition . . .. .149
Microstructure . . . . . . .. 149
Composition . . . . . . . .. 163
Thin Film Optical Properties . . . . .. .165
Absorption Spectra . . . . . . .. 167
Room temperature UV-VIS absorption ..... .168
Low temperature UV-VIS absorption . . .. .174
Index of refraction . . . . . .. 185
Photoluminescence and Raman Spectroscopy . 190
Photoluminescence spectra . . . . .. .193
Raman spectroscopy . . . . . .. .207
COSAD Optical Properties . . . . . .. .211

VI CONCLUSIONS . . . . . . . . . .. 218

VII FUTURE WORK . . . . . . . . .. 224

APPENDICES

A BASIC PROGRAM FOR LAMBDA 9 SPECTROPHOTOMETER . 227

B ELLIPSOMETRY . . . . . . . . .. 230

BIBLIOGRAPHY . . . . . . . . . . . .. 235

BIOGRAPHICAL SKETCH . . . . . . . . . .. 240
















Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy


OPTICAL PROCESSES IN CADMIUM SULFIDE
THIN FILMS

by

Edward M. Clausen, Jr.

December, 1987

Chairman: Joseph H. Simmons
Major Department: Materials Science and Engineering

The semiconductor, cadmium sulfide, has received much attention

for its optical, electronic and piezoelectric properties. Recently,

several investigators have demonstrated that its Wannier excitons hold

great promise for applications in nonlinear optics. However, the

results showed, as expected by the current theoretical thinking, that

the nonlinear optical behavior and the exciton and band gap energy

configurations are dependent upon the microstructure of the samples.

Since most applications in computer logic, or communications require

waveguide geometries, the object of this dissertation has been the

study of thin films.

The investigation presented here, therefore concerns the study of

the effect of microstructure and preparation conditions on the optical

properties of CdS thin films. High optical quality films were produced

by RF magnetron sputtering with a variety of microstructures and

crystallographic characteristics controlled by the deposition process

vii









and subsequent heat treatments. An in-depth study of the thin film

microstructure revealed the relationship between crystallographic

defects and band gap defects which lead to band tailing absorption.

Optical characterization related exciton photoluminescence, absorption,

resonant Raman scattering and band edge photoluminescence and

absorption to the microstructure of films. For the first time,

absorption bands and photoluminescence associated with exciton states

were observed in a polycrystalline thin film.

A supplementary study investigated composite films, consisting of

CdS crystals in a glass matrix, formed by a novel co-sputter deposition

process, in which alternating layers of CdS and a borosilicate glass

were deposited to form a thin film. A wide variation in the structure

of the deposited film was obtained by changing the amount of deposited

CdS and by post deposition heat treatments. Low concentrations produced

CdS microcrystallites as small as 70 A, small enough for quantum

confinement processes to affect the band energy. For all films

produced, a shift in the band absorption edge to higher energies was

observed; however, it was determined that this shift must be partially

associated with a chemical shift. Possible formation of CdO could

raise the band gap energy, although this alone could not produce the

shifts observed in the films with the smallest particles, and therefore

a confinement process might exist.


viii
















CHAPTER I
INTRODUCTION


Optical Signal Processing

With the very intense development of optical communications in

recent years, the need for integrated optical systems for the

processing of optical signals has greatly increased. To take full

advantage of the speed and information bandwidth available at optical

frequencies, an all optical system is desired. All of today's systems

operate by the high bandwidth transmission of optical signals in

optical fibers, followed by conversion to lower bandwidth electrical

signals before any processing, such as amplification and multiplexing,

can take place. The limiting drift velocity of charge carriers in a

semiconductor and the capacitive coupling between adjacent elements

present the fundamental limit for processing speed in these systems.

Also, the serial nature by which the electronic data must be

manipulated presents another speed barrier. An alternate approach

would utilize optical bistability and optical switching demonstrated in

certain materials for all-optical modulation, detection and

multiplexing. Optical switching is achievable with materials which

display a third order nonlinear susceptibility. A third order

susceptibility leads to an intensity dependent index of refraction or

absorption. This nonlinear refractive index can exhibit onset and

decay on a very fast time scale, which makes it an attractive









2

phenomenon for optical signal processing. With the proper material,

all optical operations could conceivably be carried out in a monolithic

thin film, which would act as a guiding medium with both active and

passive regions. An optical communications fiber could be coupled

directly to this thin film, thereby fully utilizing the speed and

bandwidth of the optical signal.

A complementary application of an integrated optics technology

which utilizes optical switching would be in the area of high speed

logic operations for the next generation of computers. Logic gate

operation has been demonstrated with several materials which exhibit a

nonlinear optical susceptibility and optical bistability. A few of

these gates have been shown to switch on a subnanosecond time frame,

which is competitive with present day high speed electronic systems.

The primary advantage of the optical gate is the possibility of

parallel processing on the fundamental logic cell level, which adds

tremendous speed advantages over electronic systems. Most optical

computer designs today are based on integrated optics, in which the

active regions consist of arrays of bistable devices arranged with a

high spatial density. A high density of gates is possible because

optical gates are not subject to capacitive coupling, thus making

possible massive parallel processing without the connection problems

encountered in today's electronic systems.



Nonlinear Optical Materials

A large number of architectures have been proposed for both

computer systems and multiplexing circuits based on optical switching.












While this field has greatly advanced, there is, however, a very great

need for suitable materials and systems. Many materials exhibit a

third order nonlinear susceptibility,1 but very few have the

characteristics necessary for an integrated optics system. A few of

the semiconductors which have been investigated include InSb, GaAs,

GaP, CdS, and CdTe. Most of these semiconductor materials which show

optical switching have been investigated in bulk form. Only the

multiple quantum well (MQW) structures made from gallium arsenide are

the notable exception and are made in thin film form.2 This material

is perhaps the most promising today for use in optical signal

processing systems, primarily because its nonlinearity occurs at the

same wavelength as the semiconductor lasers which are currently

available. For an integrated optical system this is an essential

consideration because most of the processes which produce the nonlinear

susceptibility require laser light of energy near the band gap of the

material.

Equally important considerations, however, include the value of

the nonlinear coefficient, the switching speed, and the absorption

coefficient, since these values determine how much power is required to

switch the device, and how fast recovery will be. Low power operation

is essential for any large scale integration, although the figure of

merit (FOM) most often quoted is given by


FOM = n2 (1.1)
t a



where n2 is the nonlinear index, T is the switch-off time, and a is the

absorption coefficient. Obviously the larger the FOM the more










4

attractive a system is. MQW gallium arsenide has an additional

advantage of room temperature operation, but suffers from a slow

switch-off time and very large absorption. There are other materials

which have been shown to exhibit a much larger nonlinear effect than

MQW gallium arsenide. A material which has been demonstrated to exhibit

one of the largest nonlinear coefficients is cadmium sulfide (CdS).3

The large coefficient was obtained by saturating a bound exciton level

in the band gap.4 The mechanism which leads to the large exciton

saturation effect in CdS is central to this dissertation and will be

described in a subsequent section. However, only a very few groups

have looked at CdS, and no one has either investigated the bound

exciton saturation mechanism in thin films or explored the possible

applications in integrated optics.

The most important consideration for any practical nonlinear

application is the temperature at which the nonlinear process

predominates. To the present day the largest nonlinear coefficients are

only measured at very low temperatures. For example, the large

coefficient obtained by saturating the bound exciton level in CdS

occurs at 2 K. Since the binding energy of the bound exciton only

corresponds to a few millielectron volts, the state is thermally

annihilated at higher temperatures, and the large nonlinear effect

disappears.

The electronic structure of the band gap, however, may be altered

to permit access of exciton levels at higher temperatures by

controlling the physical size of the material. The phenomenon by which

this occurs is known as quantum confinement. When the crystal size of










5

a material is on the order of the radius of the exciton state, new

boundary conditions can distort the translational motion of the

exciton, its binding energy and the individual orbits of the electron

and hole. At this size the electron and hole interactions with the

crystal surface begin to govern the electronic properties of the

semiconductor. Quantum confinement also occurs in the MQW structures

of GaAs; however the exciton is only confined in one direction. A much

stronger effect occurs when the confinement is in two or three

dimensions.



Thin Films

A study of CdS thin films was chosen for a number of reasons: 1)

very few semiconductors which display a nonlinear susceptibility have

been tested in a thin film form, despite the potentially dominating

role in applications. 2) Cadmium sulfide displays one of the largest

excitonic saturation effects, and thus the influence of film structure

due to deposition conditions or subsequent treatments could be

investigated. 3) Quantum confinement effects and their interaction with

the perturbation of the exciton absorption process are of great

interest to the future of nonlinear optical developments and

applications. Cadmium sulfide promises to offer a means of studying

both the effects and their interplay with the development of the thin

film structure. 4) Since the energies of the exciton states correspond

to wavelengths in the visible part of the spectrum, the experimental

optics for the measurement of these states is simplified, and many










6

investigative tools become available for following the underlying

processes.

The study of any thin film for this application should start with

examining the properties of the bulk material which contribute to the

specific origin for the nonlinear effect. In the case of CdS this

means looking at the electronic states of the material which lead to

the presence of excitons. These states have been thoroughly examined

for more than 30 years and they are probably the best understood in

this material. Exciton states have been shown to occur in thin

epitaxial films, but no one has investigated the presence of these

states in polycrystalline thin films, nor have the effects of

preparation conditions on the excitonic transitions of a thin films

been investigated. The objective of this study therefore is to

determine how exciton states would occur in polycrystalline thin films

of the material, how they are affected by structure and formation

conditions, and how size variations and grain boundary structures might

affect their energy level structure.

A supplementary part of this study will investigate the

possibility of producing thin film structures consisting of a glass

matrix with small isolated crystals whose sizes matched those needed to

develop quantum confinement effects. The investigation will use

spectroscopic techniques to determine if quantum confinement effects

can be induced on the exciton states in the material. The objective is

to produce a thin film of semiconductor doped filter glass.

The process of quantum confinement has been and still is under

investigation in bulk semiconductor doped filter glasses, which have










7

received considerable attention lately for nonlinear optics

application. These are crown base glasses which contain one or two

percent of CdS or mixtures of CdS and CdSe. It has been postulated

that the semiconductor crystals exist in the glass matrix as finely

dispersed microcrystallites, which are small enough to permit quantum

confinement effects to occur. This effect is still not well understood

and has not been clearly demonstrated. Many authors have observed

energy shifts that may be due to compositional effects rather than

microstructure size. However, it appears theoretically that with

sufficient confinement, the exciton state will be accessible at room

temperature.
















CHAPTER II
THEORY OF NONLINEAR OPTICS


Nonlinear Optical Susceptibility

The nonlinear refractive index which is observed in certain

semiconductor materials is a result of an electronic polarization which

is induced by the interaction with a monochromatic radiation field.

The susceptibilities of the polarization determine the values of the

experimentally measured optical properties. To understand the

relationships between the susceptibilities and the optical properties

we must first consider the electro-magnetic field, E, which is given by




E(t) = E(w)e-iwt + E*(w)eiwt (2.1)




The resulting polarization has frequency components at all multiples of

+/-w, but considering only those that occur at w




P(W) = X(1)E(w) + x(2)[E]E(w) + X(3)[E]2E(w) + . (2.2)




The first term in the series, X(1) is the linear susceptibility and by

using first order perturbation theory,5 the linear dispersion of the

refractive index below the band gap can be calculated. The higher










9

order terms in the series are the nonlinear susceptibilities, and

although their magnitudes are much smaller than the first term, under

very high field intensities a number of different effects can be

observed. For non-centrosymmetric crystals (i.e. crystals without an

inversion center), under appropriate conditions, the second order term

is manifested as two different effects. The first is a quadratic

variation of the refractive index with applied voltage, which is known

as the Kerr effect. The second is the generation of a second harmonic

radiation field. Second harmonic generation is a very useful effect

for doubling the frequency of a laser beam. Both of these effects have

many important applications; however, the primary interest of this

study is the effects which lead to the third order term X(3). One

consequence of the third order term is the generation of a third

harmonic radiation field. For the current topic of this study,

however, the manifestation of this term as a nonlinear refractive index

is of primary interest. The relationship between the third order

susceptibility and the nonlinear refractive index can be understood by

first considering basic dielectric theory for the displacement of a

charge in response to an applied electric field:




D(w) = E(w) + 4iP(w) = c E(w) (2.3)





where D(w) is the frequency dependence of the displacement and e is the

complex dielectric constant. The variable, e can be defined as











C (n + ica ) 2
2w


where ca/2w is the extinction coefficient. By combining these two

equations a relation between the polarization and the index of

refraction can be written.


(n + ica )2= 1 + 4m P(w)/E(w)
2w


(2.5)


The nonlinear susceptibility can be defined by expansion of the

refractive index in terms of the intensity I, of the radiation inside

the sample:5


n = nI + n21 + n312 +


(2.6)


Next we assume that the extinction coefficient is very small compared

to n. Then by using equations 2.5 and 2.6 and expanding the terms,



2 2 (21)2 (i) X(3) 2
n = n + 2n 1 n21 + (n21) = 1 + 4 X( + 47X [E]2. (2.7)





Finally we assume n2 is much smaller than nI and by comparing

coefficients of [E]2 and I


(2.4)













n2 1 + 4n X (2.8)

and


n 2 2Tr (2.9)
n1



There are essentially four electronic processes which can produce

a reactive nonlinear susceptibility in semiconductors. These are known

as 1) the induced free-carrier plasma, 2) the dynamic Burstein-Moss

effect, 3) the direct saturation of interband excitations, and 4) the

saturation of exciton absorption.5 Of these 1 and 2 are the most

commonly studied, 3 occurs in most direct band gap semiconductors, and

4 is the most promising for high speed operations. All four processes

can occur in some materials. The most dominating process which is

observed depends somewhat on the material, but mostly on the particular

experimental setup and measurement temperature. As shown in Table 1,

different processes result in widely different values of the reported

nonlinear index and saturation intensity.

The four processes listed above basically describe how the

transitions between different levels in a semiconductor and the

saturation of those levels result in the observed nonlinear

susceptibility. Depending on the band structure of a material and the

particular wavelength of light used for the analysis, one of the four

processes will dominate. The one process that can occur in nearly

every semiconductor, however, is the induced free-carrier plasma.

Assuming that photo induced transitions produce electron-hole pairs,

the number of these free carriers will be intensity dependent.




















TABLE 1


Listing of


Nonlinear Optical Values for Certain
Semiconductor Materials


Electronic
Proces


FES

DBM


DIS



BES

FCP


IS (W/cm2)


n2 (cm2/W)


toff (sec)


FOM


& a a a


150

500


2000



58,26

1.2X105


10-4





6X10-5



10-4,10-2


10-8




10-6



10-9

10-10


1




0.06



2


Key for Electronic Processes:


FES
DBM
DIS
BES
FCP


Free Exciton Saturation
Dynamic Burstein-Moss
Direct Interband Saturation
Bound Exciton Saturation
Free Carrier Plasma


Material


GaAs
(MQWS)

Bulk


InSb



CdS










13

Depending on the recombination time TR, and the absorption coefficient

a, the steady state density of free carriers will be given by


a I R

h w


(2.10)


Once these carriers are formed, they are allowed to diffuse and form an

electron-hole plasma. The plasma will respond to an applied electric

field and the resulting undamped oscillations will produce a

polarization which can be related to the index of refraction through

the dielectric constant6


2 = ( -
n = ( c -


2
4mNe )
2
m w


(2.11)


By use of equation 1.10, the nonlinear refractive index for a plasma

can be written as


2
n2(P) =e a R

3
n m W


(2.12)










14

where n is the refractive index of the material without the plasma and
*
m is the effective mass. The transient susceptibility is determined

by the time it takes for the free carriers to build up;7 however, this

can be a very short time. This process will also occur at room

temperature. The disadvantage of this process for nonlinear optical

applications is that the effect is very small because there is no

coupling between the states, and therefore no resonance effects take

place. The process also can suffer from a very slow recovery time

because the recombination time in certain semiconductors is on the

order of usec.

Another process that is based on an intensity dependent free

carrier concentration is the dynamic Burstien-Moss or blocking effect.

Again a steady state density of charge carriers is given by equation

2.10; however, the origin of the absorption is not considered

explicitly. At low temperatures the carriers are assumed to thermalize

by a phonon scattering process so that they fill the bottom of the

conduction band. The top of the valance band becomes empty and the

shift of the effective band gap to higher energy becomes intensity

dependent. In association with this shift there must be an intensity

dependent contribution to the refractive index.5 This is because the

filled conduction band effectively blocks absorptive transitions, and

the blocked transitions no longer contribute to polarization and

refraction. Also, some type of unspecified coupling takes place

between excited states, so that the effect is enhanced somewhat. The

nonlinear refractive index for this process is given by














n2(BM) = 2 r ( e P )2 N (2.13)
3 n h w h (w G W) I




where N is the free carrier density given by equation 2.10, P is a

momentum matrix element, and wG is the effective band gap. For

nonlinear optical applications the advantage of this process is that

the occupied states are closer to resonance so that a larger n2

results. There also is the possibility that this process can occur at

energies below the band gap, as saturation of excited carriers can be

produced not by direct optical absorption, but by scattering from other

excited states.6 The one disadvantage of the process that is similar

to the induced plasma process is that the interband relaxations

required for decay of the state can be very slow. In some materials a

faster decay process can occur by scattering to intraband transitions,

which effectively relaxes the system by transferring the population to

other states.6

Another mechanism for the nonlinearity observed in some materials

is by a direct interband saturation process. This process assumes that

the band structure in a direct gap semiconductor can be modeled as a

set of uncoupled two level systems which are homogeneously broadened by

a dephasing time T2. Homogeneous broadening means that the individual

transitions are indistinguishable. The T2-Lorentzian broadening

results in absorption below the band gap and excitation into the T2-

broadened "band-tail" is assumed to be responsible for the nonlinear

refraction.6 At some high level of intensity the two level system

should become saturated, and associated with this saturation is a










16

nonlinear contribution to the refractive index.5 The nonlinear index

of refraction for this process is expressed by


n2(S) = 1 ( eP )4 T 2 X
h3 h w T2 15 n2 c


( 2 m ) G W ) 3/2 (2.14)
h


where T is the relaxation time, and c is the speed of light. The

advantage of this process for nonlinear applications is that the

effective nonlinear refractive index is inversely proportional to the

band gap energy, so for small band gap materials this process leads to

a very large effect. As indicated above, the broadening results in an

effect which occurs below the band gap energy, so absorption losses are

reduced. The disadvantage of utilizing this process is the same as for

the other two processes; in some materials there is no fast mechanism

for decay of the excited state.

The final process that will lead to an electronic nonlinear

refractive index in certain semiconductors is the saturation of bound

exciton levels. These particular defect states are characterized by a

very narrow transition linewidth, which is comparable to atomic

resonances. A bound exciton is an associated electron-hole pair that

is bound to an impurity site. The oscillator strength of the bound

exciton, which is related to the polarizability, is extremely large in

comparison to oscillator strengths of molecules.' In addition, there

is a very high density of oscillators, which contributes to a very

large nonlinear effect. The absorption transition of the bound exciton









17

is modeled as a saturable two level system, inhomogeneously broadened

by a T2 dephasing time. The transition linewidth is inhomogeneously

broadened because excitons bound at different locations see different

environments.4 An inhomogeneously broadened system is described as a

distribution of groups or classes of transitions, and within each class

the transitions are assumed to be identical homogeneouslyy broadened).

The saturation of the inhomogeneously broadened system does not depend

on the homogeneous lineshape function, but rather on the linewidth of

these "homogeneous packets".8 This means that the saturation intensity

is inversely dependent upon the dephasing time T2 as shown by



2 2
I= 2 n hvAv (2.15)

s X2



where Av = (i T2)-~ (2.16)



and 0 is the ratio of the radiative lifetime to the spontaneous decay

time, which is usually taken as equal to one. In semiconductor systems

the dephasing time is on the order of 0.1 psec,4 which means that very

low saturation intensities are required to saturate bound exciton

transitions. The importance of the saturation intensity will be

described in a following section; however, a small value indicates that

the nonlinear refractive index is very large, as the two are inversely

proportional (see equation 2.25). As shown in Table 1, in CdS a

saturation intensity as small as 26 W/cm2 has been measured, which

corresponds to n2 value of 1.3 X 10-2 cm2/W.10 This is the largest n2










18

value ever reported. Additional advantages of bound exciton saturation

are that the state decays by a radiative transition and the lifetime is

on the order of 500 psec. This would make for a very fast, low power

switch. Also, since the excitons are bound to defect sites, there are

no carrier diffusion problems, which in the other three processes tend

to wash out the effect. The one primary disadvantage of utilizing this

process is that the strongest exciton resonance occurs at 2 K. As the

temperature is increased, the transition broadens, thereby requiring a

larger saturation intensity and hence a smaller n2 is observed. In

addition, since the exciton binding energy is only a few millielectron

volts, the state is thermally annihilated at higher temperatures.

As previously described the process of quantum confinement could

be used to access exciton levels at higher temperatures if the physical

size of the material could be made small enough. Multiple quantum well

structures produce quantum confinement in one direction because the

structure is made up of alternating layers in which the layers act as

infinite potential wells and the layer thickness is smaller than the

exciton radius. Although the exciton is not confined in the other two

directions, the effect is strong enough that nonlinearity can be

observed at room temperature. The effect would be larger if

confinement was made in the other two directions.










19

Optical Bistability

The primary means for measuring nonlinearity in materials is

through an internal feedback device known as a Fabry-Perot

interferometer. The saturation intensity of such a system is the

intensity at which the gain of the feedback saturates. This is the

point at which optical bistability occurs, and from the saturation

intensity and the interferometer parameters the nonlinear index of

refraction can be determined.

The Fabry-Perot consists of a cavity formed by two plane parallel,

highly reflecting mirrors. The transmission of monchromatic light

through the device is determined by the optical path length of the

cavity. If the cavity is not tuned to the wavelength of the light,

then a transmission of 1 % results. When the optical path length is

exactly equal to an integer number of wavelengths, then a resonance

effect occurs and the output intensity from the device reaches nearly

100 % of the input intensity. A diagram of this process is shown in

Figure 1. The optical path length is determined by the physical length

d, times the refractive index n of the material within the cavity. The

condition for resonance therefore is given by





2 n d = m X (2.17)


where m is the integer order number.

















Incident
Beam = 100





Reflected
Beam = 90


Transmitted
Beam =1


Figure 1 Schematic diagram of a Fabry-Perot interferometer showing
how interference of the forward and reverse beams changes
the output intensity.9


Forward
Beam = 10


Reverse
Beam = 9










21

When the cavity does not satisfy the above requirement, then a

linear relationship will exist between the incident and transmitted

intensity. If a material with a nonlinear refractive index is placed

in the cavity, then a positive feedback loop will occur where the

refractive index and the light intensity become mutually reinforcing.

As the incident intensity is increased, a change in refractive index

occurs which brings the device closer to resonance, which further

increases the intensity inside the cavity, which further changes the

index, etc. This continues until a saturation intensity is reached, at

which point a phase shift to resonance occurs within the cavity and the

transmitted intensity suddenly increases.

The ratio of the incident intensity to the transmitted intensity

as a function of the phase shift 6 is given by the Airy function A(8)





It
t = A(Q) (2.18)
I.
i


where

A(e) = 1 (2.19)
1 + F sin2(6/2)


and F is related to the reflectivities of the mirrors R by


F = (2.20)
(1-R)2
I -R )













\I \ I '--r2 = 0.04

I I
I I


I -0.8
-4- r .
0 I I--F=200
-21r 0 2 4x 6
PHASE SHIFT



a)




















b)
Figure 2 Descriptions of the operation of a Fabry-Perot. a) plot of
the Airy function as a function of the phase shift 6,
reflectivities r, and the finesse F; b) transmitted image
from a high finesse Fabry-Perot.53









23

Figure 2a displays how the relative transmitted intensity and the

sharpness of the transition is related to the reflectivities and

finesse of the cavity. As the reflectivity of the mirrors is

decreased, the finesse is decreased and the transition at the critical

phase shift broadens. When the reflectivities and the finesse of the

cavity are large, then the transitions are sharp. For these

conditions, the transmitted image from a diffuse source through the

Fabry-Perot will appear as a series of sharp concentric rings, as shown

in Figure 2b.

For a nonlinear Fabry-Perot, however, the Airy function must be

slightly modified to account for the nonlinear index by







A(e)NL = 1 (2.21)
1 + F sin2( t I eff 6)
eff





where i is a constant describing the nonlinear refraction,10 and Ieff

is the effective mean intensity within the cavity. The total Fabry-

Perot fractional transmission can be written asI0





22
T 1 R ) ( I -A) A(e) (2.22)
(1 -R ( 1 A ) 2










24

where the intensity absorption per pass A is given by





A = 1 ead (2.23)



and a is the absorption coefficient. A second equation can be written

which is parametric in Ieff for the Fabry-Perot transmission:I0






T = a d ( 1 R ) ( 1 A ) eff (2.24)
A (1 -R( 1 A ) I
o





The condition for optical bistability can be determined by

simultaneously solving equations 2.22 and 2.24. A graphical solution

of these equations which shows the criterion for optical bistability is

shown in Figure 3.

The critical intensity Ic for the onset of bistability is given by

the intensity Io which gives more than one intersection with the line

and curve. This is the intensity at which the saturation of the

feedback occurs. Once this saturation is achieved, it is found that if

the input intensity is reduced, the output intensity does not drop

until a finite decrease in the input has occurred. In other words, a

hysteresis effect is observed. The switching between the two intensity

levels can be considered as a change in logic state. The nonlinear

Fabry-Perot interferometer therefore can be used as an optical logic


















a) oc


SA'\


/


8c 8 >8
1 eff 8








Figure 3 Graphical solution to the Airy function showing the
critical phase shift required for the onset of
bistability.10










26

gate. The great interest for logic gate applications is that the

switching between the two levels can occur on a subnanosecond time

period in some materials.

The saturation or critical intensity Ic is found to be inversely

proportional to the nonlinear refractive index and is given by




I = 1 (2.25)
c



where


3 n
2 = (2.26)
X a





and p is a figure of merit value for the cavity, relating the

reflectivities of the two mirrors and the attenuation of light as

passes through the cavity. Equation 2.25 indicates that for fixed

cavity conditions, a small saturation intensity corresponds to a large

nonlinear refractive index. The total index of refraction is given by




nt = n1 + An (2.27)


An = n21c.


where


(2.28)










27

The value of nt is what actually determines the phase shift but as

indicated by equation 1.1, the figure of merit for nonlinear optical

applications also includes the switching times and the absorption

coefficient.















CHAPTER III
BACKGROUND BIBLIOGRAPHICAL REVIEW

Bulk Properties of Cadmium sulfide

Nearly all of the early work on cadmium sulfide was carried out on

single crystal platelets which were made by a chemical vapor phase

growth process. The natural crystal structure of cadmium sulfide is

hexagonal wurtzite, although single crystals of the cubic zincblende

structure have been fabricated. Within either of the two crystal

structures it is possible to have regions which are made up of the

alternate crystal structure. The transition from a hexagonal to a

cubic lattice or visa versa can occur through a well known twinning

mechanism,12 in which the twinned region is bound by stacking faults.

The twinned regions can be manifested during deformation of the crystal

or under particular growth conditions, although it is difficult to

differentiate these two sources when crystals are grown from the vapor

phase. In either case, the two crystal structures do not have a center

of symmetry or inversion, which leads to the unique properties of

noncentrosymmetric crystals such as piezoelectricity, pyroelectricity,

and third order optical susceptibility.

Another important physical property of CdS is the stoichiometry of

the crystal. Very little work has been done on the defect chemistry of

CdS; however, the work done on other II-VI semiconductors such as ZnS

and CdTe indicates that the range of nonstoichiometry at room

temperature is very small, e.g. 0.01 to 0.1 %.13 Early work by Collins










29

which involved sulfur atmosphere heat treatments and electron

bombardments, showed that sulfur vacancies were the predominant native

defect and that they acted as the recombination center responsible for

the green edge emission associated with CdS luminescence.14 Other

studies which investigated impurity doping effects are described in a

following section on photoluminescence.

A detailed knowledge of the band gap structure of CdS has come

from the extensive study of exciton states. Cadmium sulfide is found

to be a direct gap semiconductor with a band gap equal to 2.59 eV at 0

K. The wurtzite lattice of the material is described by a p-like

valance band consisting of two gamma-7 states and one gamma-9 state,

and a s-like conduction band made up of one gamma-9 state. A diagram

of the band extrema is shown in Figure 4. The three states in the

valance band are also known as the A, B, and C free exciton states.

These intrinsic exciton states are modeled as Wannier excitons; i.e.

the electron and hole behave like a hydrogen atom. The orbital

movements of the electron and hole are determined by their effective

masses within the band extreme. As shown in Figure 5, the solution for

the wave function of this model results in a series of discrete

parabolic bands below Eg which merge into a continuum at higher

energies.15 Because of the unique band structure of this material

there are a large number of possible exciton states. Any of the

exciton energy levels (i.e. n=1,2,3, etc.) can be associated with the

three primary states (A,B and C) in the valance band. In addition, any

one of these free excitons can be associated with an impurity center,
















E(k)







F7
r9




I~7



(0,0,0)
k

WURTZITE


Figure 4 Band gap structure for wurtzite crystals near k=0.14




















n=3
n-=2

n=l


E



J


U


Figure 5 Energy diagram for Wannier excitons as a function of
exciton momentum K, showing "hydrogenic" states which merge
into a continuum at energies greater than E .15












forming a bound exciton complex, which will have a lower energy than

the corresponding free exciton.

Of all the II-VI semiconductor materials, the exciton states in

bulk CdS have been studied the most and are perhaps the best

understood. Reflection, absorption, and luminescence studies dating

back to the mid-fifties have investigated the exciton states in this

material. Excitation of the states can be accomplished by either

electron bombardment or by photon absorption. When the emission is due

to the latter process it is known as photoluminescence and the results

reported for CdS are detailed below.



Photoluminescence of CdS

When CdS is excited by photons of energy greater than the band gap

the characteristic luminescence which results form the decay of excited

states is shown to consist of two primary emission bands. The first,

known as the "green-edge emission" is due to the edge emission of

various states in the band gap, i.e. shallow donor-acceptor

recombinations. Studies of the edge emission of CdS were made by

Kroger as early as 1940.16 At a slightly higher energy, a band known

as the "blue-edge emission" occurs and is due to emission from free and

bound exciton complexes.17 A typical low resolution spectrum

displaying these two bands is shown Figure 6, and a high resolution

spectrum of a portion of the blue band is shown in Figure 7. The

intensity of the bands is dependent upon the polarization of the

incident light with respect to the c-axis of the crystal.














1.00


0.90


0.80


0.70


0.60

0.50


0.40


0.30


0.20 -


0.10 -

0 _
4700


/ -. E11C -
05 4
II" I
00 5200 5300 5400 5500


WAVELENGTH IN ANGSTROMS







Figure 6 Low resolution photoluminescence spectrum of CdS single
crystal showing "green-edge" emission due to band edge
recombinations and "blue-edge" emission due to excitons.
Figure shows emissions are dependent upon the polarization
of the excitation source.18


























WAVELENGTH (nr)


484


PHOTON ENERGY (.V)


Figure 7 High resolution photoluminescence spectrum of CdS single
crystal at 4.2 K, showing bound exciton peaks for
excitation perpendicular to the c-axis. Peaks labeled P
are observed with parallel excitation.19










35

The exciton states were first extensively studied and

characterized by Thomas and Hopfield.19 They have shown that within

the higher energy band a number of sharp luminescence lines occur which

correspond to transitions of both free excitons (A, B, and C) and

excitons bound to neutral donors or acceptors. A designation of I1 was

given to excitons which are bound to neutral acceptors and 12 was given

to the excitons bound to neutral donors. These two peaks are labeled

in Figure 7. The distinctions between the various transitions were

made by using the Zeeman effect. When a strong magnetic field is

imposed on the sample, many of the luminescent lines split due to the

spin moments of the ground and excited states. From the group theory

of bound complexes,19 they were able to assign the transitions to the

different defect states.

A later study by Henry, Faulkner, and Nassau showed for the first

time donor-acceptor pair lines in the photoluminescence spectra of

CdS.20 These pair lines are narrowly spaced transitions which were

observed in the green-edge emission band and correspond to closely

spaced donor-acceptor pair-recombination bands. Again the confirmation

of these lines was made by Zeeman experiments. The significance of the

study is that for the first time direct spectroscopic evidence for the

existence of these states was made. This is important point because in

the study by Thomas and Hopfield it was assumed that these states must

exist based on the Zeeman experiments, but they had no direct evidence.

Henry, Nassua, and Shiever studied the impurity doping of CdS and

showed that Na and Li are the only shallow acceptors that can act as

substitutional impurities.21 These shallow acceptors give rise to the










36

I, bound exciton that was described by Hopfield and Thomas. Usually

two II lines are observed, and by varying the doping level these

authors proved the lines to only be due to Na and Li. High purity

crystals grown in clean reactor tubes were found to only exhibit the

I(Li) line. When Na was added a considerable broadening of the I,

line occurred, but as successive runs were made in the same tube, these

authors showed the Na line could be resolved. Doping with K, Rb, or

Cs, only resulted in a sharp I(Li) line, and P was found to give a

complex shallow acceptor. The identity of the shallow donor level

responsible for the 12 bound exciton could not be determined; however,

the authors showed by donor-acceptor pair line splitting that the donor

was not a native double donor such as a cadmium vacancy, or a sulfur

interstitial. The authors reasoned that the donor may be Na or Li

interstitials because of the small size of these atoms, and when

crystals are heavily doped, they become highly compensated.

Unfortunately they were unable to prove the identity of the donor.

Later work by Henry and Nassau involved measuring the

spectroscopic lifetimes of the two bound exciton complexes.22 They

were basing their work on another study by Thomas and Hopfield which

showed the measured oscillator strength of the bound excitons to be

very large, which meant that the radiative decay of the weakly bound

exciton would be very fast. Thomas and Hopfield determined an

oscillator strength of 9 +/-2 corresponding to a radiative lifetime of

0.4 +/-0.1 nsec.23 This oscillator strength is about 104 greater than

the strength of a free exciton.II Henry and Nassau with their

experimental setup were able to measure a radiative lifetime for the 12










37

exciton to be 0.5+/-0.1 nsec. The very fast decay time of this state

is what makes CdS so attractive for nonlinear optical application.



Resonant Raman Scattering in CdS

Further studies of exciton levels in CdS have involved the

measurement of multiple phonon scattering from exciton states. Leite,

Scott, and Damen found that the scattering of longitudinal optical (LO)

phonons was enhanced when the scattering-phonon frequency coincided

with that of excitons.24 They were able to show up to nine orders of

resonant Raman scattering occurring at frequencies shifted less than 1%

from multiples of the 305 cm-I line (the first LO line). These authors

were not able to determine the identity of the exciton state (i.e. free

or bound) which was acting as the intermediate state for the resonant

scattering in this study, butin a latter paper by Leite, Scott, and

Damen free excitons were proven to be the primary intermediate state.25

They also showed that bound excitons participated as intermediates by

observing phonon sideband features on the photoluminescence of the

bound excitons. The I1 exciton was found to be a stronger resonant

state compared to the 12 bound exciton.

In a more recent paper by Mashshenko,61 the temperature dependence

of the LO and 2LO lines associated with the A exciton were

investigated. At 77 K the A-LO phonon predominates the emission, but

as the temperature is increased to 110 K, this line decreases and the

A-2LO line increases. This increase in the 2LO line indicates that an

increase in the probability of a two-phonon process occurs at high

temperatures.












Nonlinear Susceptibility of CdS

Even with the obvious advantages of using CdS for nonlinear

optical applications, very few groups have investigated this material.

The primary amount of work in this area on CdS has been carried out by

Dagenais.3,4,11 As would be expected from the large oscillator

strength of the bound exciton, a very large nonlinear refractive index

results when this level is saturated, and the decay time is very short.

By use of a Fabry-Perot arrangement and a narrow bandpass tunable dye

laser, Dagenais reported a cw saturation intensity for this level of

only 58 W/cm2, which corresponds to a nonlinear index of refraction of

1 X 10-4O cm2/W.3 In a later publication, Dagenais and Sharfin1 report

that by using a high finesse Fabry-Perot, a saturation intensity of

only 26 W/cm2 is required, which for the experimental setup corresponds

to a nonlinear refractive index of 2 X 10-2 cm2/W. They also reported

a switch up and switch down time of one and two nanoseconds

respectively. These are the largest values of a nonlinear refractive

index ever to be reported.

Other work on CdS has been done by Bohnert, Kalt, and

Klingshirn.26 They did not, however, study nonlinearity by exciton

saturation, but rather by the formation of an electron-hole plasma.

High intensity laser pulses of energies just above the band gap energy

were used to study this effect. By measuring the temporal line shape

of the transmitted laser pulse they were able to determine the

renormalization of the band gap due to the formation of an electron-

hole plasma. Because this is a much smaller effect, intensities of 120

kW/cm2 were required to produce a change in the transmitted pulse.










39

CdS Thin Films

Vacuum Deposition

A great number of studies have investigated the thermal

evaporation of CdS for the deposition of thin films. Although all thin

films for this study were deposited by RF-magnetron sputtering, some of

the results of these investigations are relevant to the present work.

Many of the studies of thermal evaporation were undertaken to research

the electrical properties of CdS thin films, although some optical

properties have been studied. The problem is that most of these

studies present results which both show differences from single crystal

results and also differ from each other.27 The reasons for the

discrepancies are related to the difficulties in evaporating CdS, which

lead to problems with maintaining stoichiometry and crystal structure

in the deposited films. The difficulty in evaporating CdS, as well as

other chalcogenide compounds, is that complete dissociation of the

compound occurs during evaporation.28 If too high a temperature is

used, then the compound will dissociate incongruently and because Cd

has a higher volatility than S, non-stoichiometric thin films result.

Source temperatures between 650 and 700 must be accurately controlled

to avoid excess cadmium.28,'29 Due to differences in the sticking

coefficients of Cd and S, a non-stoichiometric thin film will also

result if the an improper substrate temperature is used. Cook and

Christy27 have found that if fused quartz substrates are used at

temperatures greater than 200 C, then non-uniform films result. In

comparison, Wohlgemuth et al.29 have found that if the substrate is










40

cooled to LN2 temperatures, then an amorphous film results which is Cd

rich.

The optical properties of CdS thin films are central to this

study; however, the reported results for vacuum vapor deposited thin

films are found to vary greatly. Most notably is the variation in the

reported optical band gap and absorption coefficient, and the

calculated index of refraction. Some of these differences can be

realized by examining Figure 8 and Figure 9 which show the refractive

index and absorption coefficient obtained by several authors.29

Referring to Figure 8 the results for single crystal CdS obtained by

Cardona and Harbeke30 are given by curve a. Curve b is for

polycrystalline film deposited onto fused silica at 180 C by Khawaja

and Tomlin. 31 Curve c corresponds to a polycrystalline film also

deposited on silica at 180 C by Wohlgemuth et al.29 and curve d is for

an amorphous film deposited at LN2 temperatures by Wohlgemuth et al.

The same authors correspond to the same curve letters of the reported

absorption coefficients shown in Figure 9. As shown in Figure 8 the

results by Khawaja and Tomlin show the closest resemblance to single

crystal results, while the Wohlgemuth et al. results show a marked

difference. In contrast, as shown in Figure 9, the absorption

coefficient of polycrystalline films deposited by Wohlgemuth et al.

model single crystal results best. The highest quality films in these

studies were deposited at high temperatures, although a more recent

study by Cook and Christy27 reports similar results for room

temperature depositions. It is obvious that subtle differences in the

deposition technique result in a wide variation of thin film




















3.0 3.0

2.8 -- 2.8

2.6 2.6
n/ n
2.4 2.4

2.2 -2.2

2.0 2.0

--I I I I I -- I 1 1
2000 4000 6000 8000 10000
X(A)








Figure 8 Variation of index of refraction reported by several
authors.29





















105


'E



10o


103- I I I
1.0 2.0 3.0 4.0

hfi w (eV)







Figure 9 Variation of the absorption coefficient reported by several
authors.29










43

properties. This is probably due to differences in thin film

microstructures, which in the above studies were only determined by X-

ray diffraction. No direct measurements (i.e. electron microscopy)

were used to examine thin films.

Although some of the optical properties of evaporated thin films

have been investigated, albeit there are differences in the results,

apparently very few studies have been made of the photoluminescence of

thin films produced by this technique. Christmann et al.32 describe

evaporation deposited epitaxial CdS films which displayed both green

and blue edge luminescence. This is one of the first studies to relate

both the morphology and composition of thin films to the observed

photoluminescence. Thin films were deposited on cleaved surfaces of

SrF2 at temperatures from 210 to 310 C. Variation of the

supersaturation by controlling either the source temperature or the

substrate temperature resulted in three basic morphologies. At low

supersaturations, smooth CdS thin films were produced which only

displayed broad band green photoluminescence. By decreasing the

substrate temperature or increasing the source temperature, films were

produced which displayed a structure with many hexagonal flat tops.

These films showed green edge emissions and very low intensity blue

edge emissions. Finally, at very high supersaturations, films which

displayed a morphology with many hexagonal pyramids were found to show

very intense blue edge, or bound exciton photoluminescence.

Through cathodoluminescence and microprobe studies it was

determined that the hexagonal pyramids were cadmium rich, and they did

not contribute to the luminescence. Only the areas adjacent to the












hexagonal pyramids were found to show bound exciton luminescence. The

pyramids were thought to have nucleated from Cd droplets, and as they

grew, the adjacent areas were depleted of cadmium. The required donor

states for 12 bound excitons were therefore provided by cadmium

vacancies. Other films were found to be uniformly slightly cadmium

rich, and it was thought that cadmium interstitials were responsible

for the broad band photoluminescence.

Humenberger et al.33 also reported thin films which displayed

bound exciton luminescence. These films, however, were made by a hot-

wall epitaxial technique, which is similar to the vapor phase technique

used to make bulk single crystal platelets. Films were deposited on

BaF2 substrates. The surface morphology was found to be smooth with a

low density of hexagonal flat tops. No compositional data were given

for thin films; however, acceptor states were provided by indium doping

thin films during the deposition process. Free carrier concentrations

as a result of the indium doping were measured to be on the order of

1017 to 1018 cm-3.33

The above two studies are the first to show by photoluminescence

the presence of exciton levels in thin films. Studies of several other

thin film deposition techniques such as chemical bath,34 or spray

pyrolisis35 have reported photoluminescence, but none have shown

exciton emissions. To see these transitions, it is apparent that very

high quality thin films are required, and it is obvious that standard

vacuum techniques or other techniques such as those described here are

not capable of producing thin films of the necessary degree of quality.

A deposition technique which permits greater control over the










45

deposition process is RF sputtering, and several investigations of the

deposition of CdS by this technique are presented below.



RF Sputtering

Only recently has the deposition of CdS thin films by RF

sputtering been investigated to any great extent. One of the first

investigations was reported by Lagnado and Lichtensteiger.36 They

described some of the properties of CdS thin films produced by RF-diode

sputtering. A very strong preferred orientation of the CdS

crystallites was found to occur with the c-axis parallel to the

substrate plane. Electrical resistivity measurements at different

temperatures revealed two activation energies for conduction,

interpreted to show the activation of an unspecified trap below the

conduction band, although no data on the chemical analysis for

impurities was presented.

Recently the technique of RF-diode sputtering has received

considerable attention for producing CdS thin films for solar cell

heterojunctions.37 The large photoconductivity of CdS makes it an

attractive material for both solar cell and sensitive photodetector

applications. A number of investigations on the RF diode-sputtering of

CdS for these applications have been reported by Martil et al.38-41

These workers have investigated the dependence of the physical,

electrical, and optical properties on both deposition parameters and

post deposition heat treatments.

The earliest publication by Martil et al.38 outlines the

dependence of deposition rate, thin film grain size and resistivity as












a function of sputtering power, pressure, substrate temperature and

substrate bias. Thin films were deposited onto fused silica

substrates. The first effect they describe is the dependence of the

deposition rate and resistivity on the pressure of gas used. A maximum

in the deposition rate and a minimum in resistivity occurred for a

sputtering pressure of 5 pm (5 X 10-3 torr). For higher pressures they

found the rate actually decreased, and the resistivity increased. The

decrease in deposition rate at higher pressures was explained by a

backscattering mechanism which increases the probability that more

sputtered atoms will return to the target at higher pressures. The

minimum in resistivity at 5 pm pressure was claimed to be a result of

the maximum deposition rate that occurred at this pressure. Although

no direct proof was given, the authors claim the high deposition rate

decreased the number or concentration of impurities that were trapped

in the film.

The second effect described in this early paper by Martil et al.38

was the dependence of thin film grain size on the substrate temperature

and substrate bias. The grain size was determined by SEM observations.

As would be expected, the grain size of films increased from 300 A to

3500 A as the substrate temperature was increased from 90 C to 300 C.

The deposition rate accordingly decreased as the temperature was

increased. When the substrate bias was increased to above -100 volts,

the resulting thin films were found to be amorphous. The temperature

effects can be explained by considering that with increasing

temperature 1) the critical size for a nucleus increases, 2) the

surface diffusion coefficient increases and 3) the sticking coefficient










47

for a material decreases.28 The appearance of an amorphous structure

with high substrate bias was explained by the authors as a result of

structural damage that occurred due to ion bombardment (which was

increased by the high negative bias).

The second report published by Martil et al.39 further explored

the effects of substrate temperature and bias on the electrical

properties of CdS thin films. They found that the resistivity

increased from 10 to 108 9 cm as the temperature was increased from 60

C to 250 C, which they claimed was due to a change in stoichiometry

(i.e. a loss of cadmium with increasing temperatures), although no

chemical analysis was presented to back up this claim. A minimum in

resistivity was also found with a substrate bias of -50 volts, or with

a floating substrate (which developed a self bias of -28 volts). This

effect was explained by the ion bombardment that results from these

bias voltages preferentially resputting oxygen and other impurities

from the growing film. Oxygen has been shown to be a acceptor-like

trapping center, located 0.9 eV below the conduction band.42 Again no

chemical analysis was reported to prove that a decrease in oxygen

occurs. Also the variation in activation energy for conduction that

occurs with temperature as seen by Lagnado and Lichtensteiger36 was not

explored by these authors.

The third publication by Martil et al.40 reports the influence of

the above sputtering parameters on the optical properties of thin

films. The structural changes which occur under different conditions

were also further explored. Increasing the substrate temperature

resulted in similar structural changes to those reported in the earlier









48

study; the grain size varied between 500 A and 3000-4000 A as the

substrate temperature was increased from 60 to 300 C. This report,

however described the sputtering pressure dependence of the

crytstallinity. The sputtering pressure determines how much structural

damage occurs due to ion bombardment. At low pressures, bombardment is

enhanced by a large self-bias that develops on the substrate, and an

amorphous structure results as previously described. At higher

pressures, the crystallinity was also found to decrease, which was

explained by the authors as due to a porous structure that develops

from trapped gases.

Optical properties were found to be a function of both substrate

bias and sputtering pressure. A maximum in the optical band gap (2.36

eV) was found to occur for a floating substrate bias, whereas the

minimum in the band gap (2.30 eV) occurred for a -110 volt bias. The

band gap was found to increase with an increase in sputtering pressure

and became nearly constant for pressures above 10 pm. The pressure

dependence of the refractive index however contradicts the pressure

dependence of the band gap. The index of refraction was found to be a

maximum with 5 pm pressure, and it decreased with increasing pressure.

This was explained in terms of the porous structure which occurred at

higher pressures; however, if the refractive index is reduced at higher

sputtering pressures due to a more porous, less crystalline structure,

then the band gap energy should also decrease. An increase in the

number of crystalline defects should cause tailing of the band gap,

which is seen as a decrease in the gap energy.29










49

The latest investigation to be reported by Martil et al.41

describes the effects of heat treatments on the electrical and optical

properties of sputtered films. Heat treatments were carried out under

H2 and N2 atmospheres at temperatures ranging from 100 C to 550 C and

for times ranging from 20 minutes to 5 hours. The primary effect on

the electrical properties was a two order of magnitude reduction in the

resistivity to 4 X 10-2 Q cm for heat treatments at 200 C. Carrier

mobilities accordingly increased to 50 70 cm2/Vs for this treatment.

Higher temperatures were found to increase the resistivity and decrease

the mobility. The decrease in resistivity was explained by claiming

that oxygen desorption from the grain boundaries occurs at 200 C. As

previously described, oxygen is a trapping center which in this case

decreased the carrier concentration and simultaneously increased the

scattering which reduces the mobility.41 No explanation was given for

the increase in resistivity with increasing temperature, although it

could be due to a loss of cadmium.

The optical absorption edge was shown to become sharper and occur

at a higher energy for heat treatments at 200 C. Temperatures higher

than this were not reported to significantly alter the absorption edge;

however, treatments above 550 C produced an overall decrease in the

transmission. This was explained by a dissociation or reevaporation

process that occurred as a consequence of the higher temperatures. The

optical band gap was found to increase from 2.36 eV to 2.39 eV for

treatments at 200 C, which was again explained in terms of oxygen

desorption from the grain boundaries, although no chemical analysis was

reported to prove this hypothesis.










50

Semiconductor Doped Filter Glasses

Sharp cutoff filter glasses are silica based glasses which contain

a fine dispersion of semiconductor microcrystallites. The variation of

the sharp absorption edge position is achieved by either varying the

composition and or heat treatment schedule. Great interest has been

generated recently because it is thought by many authors that the

crystal size developed in these materials by thermal treatments is on

the size order for quantum confinement effects to occur. For glasses

containing a mixture of CdS and CdSe many authors have reported a blue

shift in the absorption band edge. In addition, a blue shift of the

high energy exciton photoluminescence peak has been interpreted in

terms of quantum confinement.

Warnock and Awashalom43,'44 in two publications on mixed crystal

glass (those containing CdS.27/Se.73) report that the glasses which

displayed smallest size distribution (average size = 94 A) exhibited

the largest shift in the exciton photoluminescence, and this

photoluminescence was shown to decay on a time scale of only 18 psec,

which is nearly two orders of magnitude faster than the lifetime

displayed by excitons in bulk material.44 The shifts in peak position

and fast lifetime were interpreted in terms of a quantum confinement

effects.

A very recent paper by Borrelli et al.45 has shown that the blue

shift in the photoluminescence displayed by this particular mixed

crystal glass is not due to confinement effects, but rather to

compositional effects. A careful study of the crystal structure by X-

ray diffraction revealed that a change in the stoichiometry occurs










51

during the heat treatments which develop the microcrystallites. It was

postulated by these authors that more of the selenium remains in glass,

while sulfur is more easily incorporated in the crystallites.45

Therefore, at the lower temperatures which produce the smaller

crystallites, the crystallites end up containing more sulfur, so the

optical properties of glasses containing these crystallites are blue

shifted toward the optical properties of pure CdS. True quantum

confinement effects were, however, shown to occur in a series of

experimental glasses which contained either CdS or CdSe, but not both.
















CHAPTER IV
EXPERIMENTAL METHOD


Vacuum Deposition System

Thin films for this study were made with a specially designed

vacuum system. A schematic of the deposition chamber is shown in

Figure 10, and a picture of the complete system is shown in Figure 11.

The deposition chamber consists of a rectangularly shaped stainless

steel box which measures 12" X 18" X 18". This chamber was custom

built by MDC Vacuum Corp. (Haywood, CA.). Numerous ports and

feedthroughs on the chamber permit a wide variation of deposition

configurations. The vacuum pumping system utilizes a 330 1/sec

turbomolecular pump (Balzers, Hudson, NH), a molecular sieve trap (MDC

Vacuum Corp.), and a 300 1/min two-stage mechanical pump (Sargent

Welch, Skokie, IL). With the molecular sieve trap activated, pressures

in the high 10-5 torr range were possible using the mechanical pump

only. Pumpdown from atmosphere pressure to 1X10-6 torr could be

accomplished in 2 hours with use of the turbopump. Vacuum gauging was

performed with a Leybold Heraues (East Syracuse, NY) model CM 330

combined Penning discharge and thermocouple gauge controller.

Pressures for sputter deposition were controlled by using a

micrometer adjustable throttle valve (Sputtered Films Inc. Santa

Barbara, CA.) and a mass flow meter/controller (Matheson Gas Products

Norcross, GA). High purity argon gas (99.9995%) was used as the



































SHUTTER



RF MAGNETRON
SPUTTER
GUN


GAS INLET
SS FLOW METER


THROTTLE VALVE



I TURBOOLECULAR PUM1 P ,,,


Figure 10 Schematic diagram of sputter deposition chamber. Computer
figure shown to indicate computer control of system.





























































Photograph of complete deposition system showing
sputtering chamber and instrumentation.


Figure 11










55

sputtering gas. Initially the thermocouple vacuum gauge was calibrated

with a capacitance manometer (MKS Instruments Inc. Burlington, MA) for

different settings of the throttle valve and mass flow controller.

Based on the previous work on CdS sputtered films, a pressure of five

microns (5X10-3 torr) was chosen and the system was calibrated for this

pressure. To obtain this pressure an argon flow rate of 20 cc/m was

needed.

Film thickness during deposition was monitored with a Leybold-

Heraeus IC 6000 deposition controller. This instrument utilizes a

quartz crystal oscillator microbalance for determination of film

thickness. All parameters of calibration and deposition control are

software programmable with this unit, either via the front panel

controls, or through an RS-232 communications link to an external

computer. The IC 6000 permits monitoring of up to six different films,

and by using a sample and hold program, two different films can be

monitored and controlled simultaneously. Close loop control of

deposition rates was achieved by connecting the IC 6000 thickness

monitor to the RF power supplies.

For semi-automatic control and constant monitoring of the

deposition process a Zenith Z-158 personal computer with 512K RAM and a

20 megabyte hard disk was interfaced to the system. A Dascon 1 I/O

board (Metrabyte Corp.) which provides four channels of analog and 12

bits of digital input/output was installed in the computer. With this

interface board and with RS-232 communications to the IC 6000

deposition controller, it was possible to monitor nearly every aspect

of the deposition process. The RS-232 communications permitted direct









56

programming control over deposition parameters and constant monitoring

of deposition rate, thickness, power output and crystal oscillator

status. The analog portion of the I/O board was connected to the

pressure gauge controller and to the RF power supplies. Connection to

the power supplies permitted recording of the DC bias which develops on

the target as a result of the sputtering process. All of these process

parameters were stored on disk during a deposition run. This made it

possible to go back and review the deposition process, if a film was

later found to have anomalous properties.

A unique feature of the deposition system has to do with the

various substrate holders which could be used. For low temperature

depositions, a liquid nitrogen (LN2) cooled substrate holder was

employed. This holder is made up of two double concentric tubes in

which the center tube acts as a reservoir for LN2 and the outer tube,

which is open at the bottom to the vacuum system, acts as an insulating

vacuum jacket. An aluminum plate with a slightly recessed area to hold

the substrate is attached to the end of the inner reservoir tube with a

copper screw. To reduce the amount of contamination that would occur

on the cold substrate, an additional LN2 cooled coil was positioned

around the substrate holder assembly. This coil is referred to as the

cryoshield in Figure 10. For room temperature depositions, the same

holder was used, however, without filling the LN2 reservoir.

High temperature depositions were accomplished by replacing the

LN2 feedthrough assembly with a resistively heated substrate holder.

This holder was made by drilling holes lengthwise in a thin aluminum

block and inserting nichrome wound ceramic heaters. A Chromel-Alumel









57

thermocouple was embedded in the core of the holder, and control of the

temperature was made by controlling the input power to the holder with

a variable autotransformer. Substrates could be heated to 300 C with

this arrangement with a temperature control of +/- 5 C.

A single planar magnetron sputter gun supplied by US Guns

(Campbell,CA) was used for sputter depositing the CdS thin films. The

gun accepts two inch diameter targets, which can range from 1/16th to a

quarter of an inch in thickness. An Eratron RF power supply

(Campbell,CA) operating at 13.57 MHz with a power capability of 600

watts was used to supply RF power to the gun, through an auto load

match tuning network. The matching network is require to balance the

impedance of the gun and plasma to the 50 ohm output from the power

supply. If the impedance is not matched, then the RF power is

reflected, and the only thing that is accomplished is heating of the

heat sinks in the power supply. The utility of an auto load network is

that when conditions of the plasma change, the network will

automatically match the impedance, thereby eliminating any reflected RF

power.

High purity cadmium sulfide sputtering targets were obtained from

CVD Industries (Woburn, MA). These targets are made by a chemical

vapor deposition process so they are supplied with a bulk density of

nearly 98% of theoretical density. High bulk density reduces the

amount of outgassing during sputtering. All impurities were less than

1 ppm and the stoichiometry of the target material was slightly cadmium

rich (50.46%). Nearly all thin films for this study were made from CVD

targets; however, a different type of sputtering target was obtained












from EM Chemicals Inc. (Hawthorne, NY) for testing. This target was

made by a hot pressing technique, which results in a lower bulk density

and a higher impurity content. Impurity information was not given by

the company for these targets; however, thin films that were sputtered

from these targets were found to contain much higher levels of iron

than the levels found in films sputtered from CVD targets.

Targets that were used for COSAD were made from a piece of glass

obtained from Schott Glass Industries (Duryea, PA). The glass was a

special type of borosilicate crown glass (#8329) which is similar in

composition to a BK-7 glass, but it is processed in such a way so that

all the volatile impurities in the glass are removed. This glass is

sold by Schott as a special type of over coating glass used for thermal

evaporation. Targets were made by first sectioning the as supplied

cylinder of glass into 3/16 inch discs from which two inch blanks were

cut. Both sides of a blank were polished to 600 grit before being used

as a sputtering target. After it was found that a high power level was

required to sputter this glass, a 1/16th inch copper plate was bonded

to one blank to improve thermal contact to the sputter gun. The

bonding was accomplished with an indium tin solder.

For producing COSAD films, the deposition chamber was reconfigured

to accept a second RF planar magnetron sputter gun, for sputtering the

glass target. This gun was used with a second 600 watt Eratron supply

and the two power supplies were driven by a common oscillator to phase

match the two plasmas. In addition, a set of baffling plates were

mounted inside the chamber as shown in Figure 12. The center baffle

plate was used to isolate the two plasmas. A pair of three inch




















ROTATION
* FEEDTHROUGH
U


I CRYSTAL
THICKNESS
MONITOR i
F---lN
I-____
"ii l .. . .... .. ... ...


SUBSTRJ

SHUTTER


iAlt MU


ILULtK

SHUTTER


BAFFLE
PLATE


Figure 12 Schematic diagram of deposition chamber configured for
COSAD.


A Tr I IhI nrnm .?.'.'.- .%-z .%-.- -.-#










60

diameter apertures centered over the two sputter guns were cut in the

top plate to provide access to the two plasma sources. A dual

substrate holder was rotated in the plane above the two apertures, so

that the substrate was alternatingly exposed to one source and then the

other. The dual holder provided the capability of producing two films

during a single deposition run. Initially a DC motor with a reducing

gear drive was used to rotate the substrates with a linear speed. A

more sophisticated direct drive stepping motor was later added to the

system to permit variable exposure to the two plasma sources. The

stepping motor was interfaced with the Dascon I/O board in the Z-158

computer via an external digital control circuit.

One disadvantage of the COSAD configuration was that substrates

could not be heated or cooled and the temperature of the substrate

could not be monitored during a deposition. The utility of this

particular system configuration, however, is that single component

films could still be produced, simply by positioning the substrate

holders over the respective apertures, and exposing the substrate to

deposit a film.



RF Magnetron Sputtering

As described in the Bibliographic Review section, the only sputter

deposition of CdS was carried out by RF-diode sputtering. In this

configuration two parallel plates are used. The target is attached to

the cathode plate, and substrate is attached to the anode plate, which

can either be at ground or floating potential. A plasma is generated

between the two plates by applying RF power to the cathode plate.










61

Electrons in the plasma can be accelerated by the positive half of the

RF field, which results in both the ionization of argon atoms, and the

bombardment of the target. This bombardment results in the development

of a negative potential on the target surface, which is called the DC

bias. The argon ions in the plasma are too massive to be accelerated by

the RF field; however, they are accelerated towards the target by the

DC bias, which results in sputtering of the target material.

The configuration of diode sputtering results in exposing the

substrate and growing films to the full energy of the plasma. The

growing film is continuously bombarded by a number of energetic

particles, both charged and neutral. This bombardment can either be

enhanced or reduced by applying a potential to the substrate during

deposition. Substrate biasing can only reduce the amount of

bombardment to a certain extent, essentially because the substrate is

totally emersed in the plasma during deposition. Considerable heating

of the substrate and film occurs as a result of the bombardment, even

at moderate plasma powers.

RF planar magnetron sputtering reduces the amount of bombardment

by incorporating a strong permanent magnet behind the cathode assembly.

The configuration of a US sputter gun is shown schematically in Figure

13. Curved magnetic field lines force the electrons in the plasma into

circular orbits, which both enhances the plasma immediately above the

target, and reduces the amount of electron bombardment on the growing

film. The target is a circular disk that is secured to the magnet

housing, which comprises the cathode. The anode is comprised of a

circular cap (called the ground shield in the Figure) which fits over









GROUND
SHIELD

TARGET TARGET (CATHOOE)
CLAMP COOLING
TARGET SUPPORT \ "CAVITY
& MA N T--__
HOUSING
CENTER & R RO

MAGNETS---- GROUND
S-SHIELD
INSULATOR--\. M i SUPPORT




WATER COOLING
LINES


POWER
FEEDTHROUGH---






VACUUM
FEEDTHROUGH----



Figure 13 Cross-sectional view of RF magnetron sputter gun made by US
Guns Inc.46










63

the gun assembly. Most of the plasma is constrained to the area

immediately above the target, so a large reduction in substrate

bombardment and substrate heating is realized. The substrate and

growing film, however, can still be bombarded by energetic neutral

particles, which has been shown to be a major cause of substrate

heating.47 Still the heating is reduced and as will be pointed out in

the Results section (Chapter 5), a considerable difference in the as

deposited film properties occurs with RF planar magnetron sputtering,

compared to RF diode sputtering.



Thin Film Deposition

Prior to deposition of thin films, all substrates were subjected

to a three phase cleaning procedure. The first phase of the process

was an ultrasonic bath in DI water for 10 minutes. After this

treatment the substrates were removed from the water and rinsed with

isopropyl alcohol (IPA). A second ultrasonic cleaning was then carried

out for 10 minutes in IPA. The final phase of the cleaning procedure

was a 15 minute treatment in a vapor degreaser, which employed IPA as

the solvent. Substrates were slowly removed from the vapor, which

allowed the condensed vapor droplets to evaporate. To check for

cleanliness substrates were examined by edge illumination against a

black background. This permitted observation of any contaminates on

the surface of the substrate in addition to any small flaws in the

surface.

To deposit thin films, substrates were mounted on a substrate

holder and positioned in the deposition system immediately after












cleaning. The deposition system was pumped down to a pressure of < 1.0

X 10-6 torr before backfilling with the sputtering gas. The IC-6000

was usually programmed for a fast ramp up to the desired power level

and then a rate controlled presputter was initiated before the shutter

was opened. If a sputtering target was newly installed, a presputter

burn-in of 30 minutes was carried out; otherwise the target was

presputtered for 5 minutes before exposing the substrate to the plasma.

The IC-6000 automatically controlled the deposition rate and thickness.

Deposition rate was varied between 1 to 5 A/sec and films for optical

analysis were typically made 1 pm thick, although the film thickness

for other analysis was varied.



Substrate materials

Several different types of substrates were used to support thin

films, depending on how the films were to be analyzed. Standard

substrates for optical analysis were made of optical quality fused

silica, typically one inch square and 1 mm thick. These substrates

were found to have very few surface flaws and were essentially

transparent over the optical region in which films were analyzed. The

low index of refraction (n=1.46) of these substrates was also necessary

for measuring the refractive index of the film and for planar waveguide

measurements. Some films were also deposited on silica for TEM

analysis. Self-supporting TEM specimens were prepared by floating

these films off the silica with a 1% HF solution.

Other films for optical analysis were deposited onto single

crystal sapphire substrates. This was done to determine if the high










65

thermal conductivity of the sapphire would aid in reducing heating by

the laser beam during low temperature spectroscopic measurements.

These substrates were of very high quality with no detectable flaws and

a very smooth surface finish. Some COSAD films were deposited onto

sapphire substrates to aid in X-ray chemical analysis, since the

sapphire had no X-ray lines which overlapped the lines in these films.

For other TEM analysis, films were deposited on slabs of single

crystal NaCI and on TEM grids which held carbon support films. The TEM

grid-carbon film combination was used for quick analysis of deposited

films. Once the grid-carbon film assembly was prepared, thin films

could be deposited directly onto the carbon film, and no other sample

preparation was necessary. Films of thickness from 500 to 3000 A were

typically deposited for TEM analysis. The carbon support film was made

by thermally evaporating approximately 200 A of amorphous carbon onto

sheets of single crystal mica. After the carbon film was floated off

the mica, 300 mesh TEM grids were carefully placed on the floating

film. The film with grids was then picked up on a thin cardboard card

and allowed to dry.

Single crystal NaCl was used as a substrate because it was found

that after heat treatment or as a result of a high temperature

deposition, thin films of CdS could not be floated off silica

substrates. Normally the 1% solution of HF would readily float films

off silica, but after any type of heat treatment above 200 C not even

a high concentration of HF was successful for floating the films.

Apparently some type of chemical bond forms between CdS and the silica

even at relatively low temperatures. Single crystal NaCI substrates









66

were also required for TEM analysis of COSAD films, since the HF

solutions normally used to lift CdS off silica slides would strongly

attack these films.

Substrate temperatures

Several different substrate temperatures were used to investigate

the effect of deposition temperature on thin film grain size. With

the LN2 feedthrough assembly that was described above, substrates could

be cooled to LN2 temperatures in about 20 minutes. For some

depositions a low temperature thermal contact paste was use to mount

the substrate in the holder, although most frequently, standard hold

down clips were used. An Iron-Constantan thermocouple was initially

used to monitor the temperature of the substrate holder during a

deposition. To use a digital meter to measure the output from this

thermocouple during a plasma deposition, an RC circuit had to be used

to decouple the RF signal that was imposed on the DC thermocouple

signal. The RC circuit is essentially a low band pass filter. A

schematic diagram of the circuit is shown in Figure 14. If this filter

is not used, a very large RF signal can be induced in the lead wires,

which will destroy most D/A converters used in digital meters.

For depositing films at room temperature, the LN2 feedthrough

assembly (without LN2 in the reservoir) was used to hold substrates.

These depositions were very close to room temperature, since it was

found that even with the highest deposition rates, the holder would

only heat up to 35 C. When substrate temperatures greater than this

were required, the LN2 assembly was replaced with the resistively

heated holder. Temperatures as high as 300 C could be obtained in
































r O.1F------------
'' .OI^F


Thermocouple


'. 0.01 pF
O.... ..1


Schematic diagram of RC decoupling circuit used protect
digital equipment from RF transients.48


Figure 14












about 10 minutes with this holder, although most depositions were made

at 200 C. The unique feature of this holder is that it permitted

outgassing of the substrates prior to deposition. This was done by

heating to 200 C for 30 minutes in high vacuum.



Deposition rates

A variation in deposition rates was also studied to determine its

effect on thin film properties. For pure films of CdS, rates from 1

A/sec to 5 A/sec were used. These rates were monitored and controlled

by the IC 6000 deposition controller. Figure 15 shows a graph of the

sputtering rate versus power density for both CdS and BK-7 targets. As

shown, the glass target used for making COSAD films is much more

difficult to sputter, as an input of 275 watts (13.5 W/cm2) only

resulted in a deposition rate of 2 A/sec. Power levels greater than

300 watts could only be used with the BK-7 glass target which had a

copper backing plate bounded to the back of the target. Even at 500

watts, however, a deposition rate of only 4 A/sec could be obtained.

At this power level, considerable heating of the substrate was found to

occur, even when the substrate was rotated during COSAD runs.



Co-Sputter Alternating Deposition (COSAD)

By using the dual sputter gun configuration described above, a

very unique type of thin film could be produced. The acronym describes

the process by which these films were made; by rotating the substrate

above the two sputter guns, an alternating layer of glass and the CdS

was deposited, which was repeated continuously. Initially the two guns



























1 I

172
0 -T-
0











Figure 15


2 4 6 8 10 12 14
POWER DENSITY (W/cm2)










Graph of sputtering rate versus input power density for CdS
and BK-7 targets.










70

were set up to sputter deposit at a static rate of 3 A/sec. This is

the maximum rate that could be used with the glass target without

causing excessive heating. The circle over which the substrate was

rotated was eight inches in diameter, so with the three inch diameter

apertures used with the guns, the duty cycle was rather low, only 0.12

for each gun. With a rotation speed of 20 RPM, the resulting total

dynamic deposition rate is less than 1 A/sec. One possible way to

increase this rate would have been to reduce the rotation speed. This,

however, would also mean that the substrate would be spending more time

over areas which were not depositing film and were actually depositing

impurities.

The use of the computer controlled stepping motor to rotate the

substrate holder alleviated the problem with the linear drive system.

With this setup is was possible to increase the amount of time the

substrate spent over a given source. At the same time, it reduced the

amount of impurities introduced into the film because the stepping

motor could be driven full speed between the two sources, which was

about 120 RPM. By simply altering the computer program, this system

permitted easy variation of deposited structure.



Post Deposition Treatments

To improve many of the physical properties of thin films a post

deposition heat treatment had to be instituted. Generally, heat

treatments on CdS thin films were made in a fused silica muffle tube

furnace under a flowing argon atmosphere. The argon used for heat

treatments was of the same grade as the gas used for sputtering. For










71

annealing COSAD films high purity compressed air was used with the same

furnace setup. Flow rates of 100 cc/m were typically used.

Temperature control was accomplished with a single setpoint temperature

controller (Love Control Co.) which provided temperature control to +/-

1 C. Two thermocouples were used, one mounted outside the tube for

control and one positioned inside the tube adjacent to the sample.

Heating rates of up to 35 C/min could be realized, and cooling rates

of 100 C/min could be obtained by sliding the muffle tube out of the

furnace hot zone. Temperatures for heat treating thin films ranged

from 200 C to 700 C, with times ranging from 1 minute up to 5 hours.

To reduce the amount of grain growth that would occur as a result

of high temperature heat treatments, some films were heated by a

technique known as rapid thermal annealing. This process utilizes high

power quartz lamps to rapidly heat the sample. Because of the low

thermal mass that is achieved with this technique, heating rates in

excess of 500 C/sec are possible. The sample is sandwiched between

two thin sheets of graphite which act as black body absorbers and help

to transfer the heat generated by the lamps to the sample. In this

particular system lamps with a total power of 5000 watts were used.

Heat treatments up to 650 C with times ranging from 10 to 60 seconds

were done under a flowing nitrogen atmosphere.



Thin Film Characterization

Microstructure

Determination of thin film microstructure was achieved using

several analytical techniques. Direct examination of the









72

microstructure was made by electron microscopy. The various TEM

specimens that were described in a previous section were examined in a

JEOL 200 CX analytical microscope (Japanese Electron Optics Laboratory,

Boston, MA.). This instrument was used in both transmission (TEM) and

scanning transmission (STEM) modes to directly observe and measure the

ultra-fine structure of thin films. Selected area diffraction (SAD)

mode was used to conduct electron diffraction experiments for

determination of crystal structure and orientation. In the SAD mode it

was possible to obtain diffraction information from areas as small as 5

um. The line to line resolution observed in the TEM mode was 8 A.

Another technique used for determination of crystal orientation was

dark field imaging. By tilting the primary beam in the microscope, the

contrast that is observed in a dark field image is only due to those

crystals or portions of crystals which are oriented for a specific

Bragg reflection. Dark field imaging makes it possible to observe

preferred orientation in a thin film or any portion of a crystal that

contains defects (such as microtwins).49 On the JEOL 200CX changing

from bright field to dark field was accomplished with a single switch.

To study the topographical structure of thicker films a JEOL 35CF

scanning electron microscope was used. Films which were made for

optical analysis were frequently examined with this instrument. The

advantage of this technique for CdS films was that no sample

preparation was required except for sample mounting. The conductivity

of these films was high enough not to warrant coating with a conductive

film, although for COSAD films a thin carbon film had to be deposited

before these films could be examined. With this instrument the










73

presence of gross defects in the film such as pinholes or cracks could

be observed, as well as the fine grain structure of certain films. The

major disadvantage with this microscope, however, was a limiting

resolution of about 300 A which made it difficult to observe the grain

size of as-deposited thin films.

X-ray diffraction was another technique used to measure the

crystal structure and orientation of thin films. An APD 3720 computer-

controlled diffractometer (Phillips Co.) was used to carry out

diffraction experiments. Typically, diffraction was measured from two-

theta values ranging from 15 to 900 with a angular step of 0.05. Use

of the computer to store X-ray diffraction spectra on disk greatly

facilitated the determination of the change in crystal structure with

heat treatments. Changes in peak positions and intensities could be

readily determined, and by making high resolution scans (0.02/step)

over certain peaks it was possible to calculate the grain size and film

strain using certain utility programs available in the computer system.

Again, no sample preparation was necessary for this technique as

diffraction from 1 Pm films supported on silica substrates was readily

observed.



Chemical Analysis

Initially, Energy Dispersive Spectroscopy (EDS) was used to

measure the stoichiometry of thin films during both SEM and STEM

observations; however, it was found to give results that were not

quantitative. Particularly in the SEM, it was found that the results

were very dependent on sample geometry with respect to the detector. A










74

technique know as X-ray secondary fluorescence was used instead to

measure the stoichiometry of nearly every thin film that was made. The

particular instrument used for these measurements utilized a silver X-

ray tube to produce the primary X-ray beam. Tube voltages of 50 kV

were used with a current of 30 mA. One of four different secondary

targets could be selected to generate the X-rays that were used to

analyze the sample. The advantage of this technique is that the

background radiation from the X-ray tube (bremsstrahlung) is totally

removed and only the very narrow X-ray line corresponding to the

secondary target reaches the sample which results in a great reduction

of the background counts. The particular geometry of this system

limits the radiation to only one polarization which further reduces the

background count.50 Detection levels for most transition metals was

0.1 ppm. Beam size incident on the sample is 1 cm in diameter and

penetration depth is only a few microns which makes this technique

ideally suited for measuring thin films supported by substrates. The

X-rays emitted from the sample are analyzed by EDS with this system;

however, due to the above arrangements, a much more quantitative

determination could be made when the results were compared to a

standard reference.

The standard used, in this case, to determine the stoichiometry of

thin films was a piece of target material. The stoichiometry of the

target was verified by a wet chemical technique and electron probe

microanalysis (EPMA) (JEOL 730 Superprobe). For solution analysis a

portion of the target was dissolved in nitric acid and then this

solution was diluted a given amount. The amount of Cd and S in










75

solution was determined by using an induction coupled plasma (ICP)

spectrometer (Allied Analytical Systems, Waltham, MA). By comparing

these amounts to those in standard solutions the stoichiometry was

determined. For EPMA the stoichiometry of the target material was

determined by use of standards and a ZAF or Opz program.51 The ratio

of the integrated peak intensities for Cd and S in the target was then

determined by X-ray fluorescence. The ratio obtained from thin films

was compared directly to this ratio to quantitatively determine the

stoichiometry.

Film thickness could be readily determined with this technique by

measuring the intensity of the silicon K-alpha peak that was emitted by

the substrate through the film. The intensity of this line is

exponentially attenuated by the thickness of material that it passes

through and can be related by the following equation:



I = 0Io exp-at (2.1)



where Io is the intensity from a bare substrate and t is the film

thickness. From equation 2.1 a linear relationship between a function

of the intensity, F(I), and t can be obtained by55


F(I) = log10 I (2.2)
Io


The intensity from three different films of known thickness (verified

by profilometry) were measured and by plotting F(I) versus t, a master

curve was generated. The thickness of an unknown film could then be

found by solving the above equation for F(I) using the intensity from











the unknown. The x-coordinate on the curve for the unknown F(I) would

give the thickness. Film thickness of up to 3 im could accurately be

determined with this technique. Changes in density or loss of material

as a result of heat treatment could also be monitored with this

technique.

Several other analytical techniques were used to analyze the

composition of thin films. In each case the results for thin films

were compared to the results obtained from target material. The

different techniques were used to both acquire information, which the

specific technique is most suited for, and to cross-check the

composition of thin films. Auger electron spectroscopy (AES Perkin-

Elmer, Norwalk, CT) gave the least quantitative determination of the

absolute composition, but the technique was very quantitative for

determining differences between samples. X-ray photoelectron

spectroscopy (XPS Kratos,UK) gave semi-quantitative results and also

showed how the surface of thin films were altered by heat treatment, a

result which was not detected with any other technique.



Optical Measurements

By far the most extensive measurements made on thin films were

optical measurements. The availability of several spectrometers,

monochromaters, lasers, and optical hardware greatly facilitated these

measurements. The UV-VIS spectrophotometer used for many measurements

was a Perkin-Elmer Lambda 9 (Norwalk, CT). Photoluminescence was

measured with both a Princeton Applied Research Optical Multichannel

Analyzer (Princeton, NJ), and an Instruments SA Raman spectrometer









77

(Metuchen, NJ). A variety of lasers were used for different

experiments,, although most photoluminescence experiments were made with

a Spectra-Physics model 2025 Argon Ion laser (Mountain View, CA). Many

measurements were made at both room temperature and at temperatures

down to 9 K. Low temperatures were obtained with an Air Products

(Allentown, PA) closed-cycle helium cold finger and digital temperature

controller. Experiments carried out with these instruments will be

extensively detailed in the following sections.



Index of Refraction

A very unique technique for measuring the dispersion or index of

refraction as a function of wavelength was used on thin films.

Normally the index of refraction of a material is related to the

percent of transmitted light by the well known equation:



T = (1 R)2 exp (-at) (4.1)
1 R' exp (-2at)

where the reflection, R, is given by (n l)2/(n +1)2. The situation

is much more complicated, however, for a thin film supported on a

substrate with a different refractive index. The reflection that

occurs at the film/substrate interface must be dealt with as well as

the reflections that occur at the other interfaces. This leads to a

more complicated expression:



T = (1 RI) (1 R2) (1 R3) exp (-at) (4.2)
(1 R2R3) { 1 [ RIR2 + RIR3 (1 R2) ] exp (-2at) }










78

where RI, R2, and R3 are the reflectivities of the air-film, film-

substrate, and substrate-air interfaces, respectively.16 The values of

the RI, R2, and R3 are given by the same equation stated above with the

refractive indices of the three materials (air, film, and substrate)

substituted for n, respectively. Even if the value of the absorption

coefficient is known, the transmission must be measured to 0.5% to get

a 1% accuracy in the refractive index,52 which is experimentally very

difficult to do unless a specially designed spectrometer is used.

When the refractive index of the film is significantly different

from that of the substrate (as the case with CdS on silica) a very

strong wavelength dependent interference effect occurs. The effect can

be explained by examining Figure 16. When the incident light beam

passes through the thin film and encounters the interface it will be

reflected due to the differences in refractive indices. Depending on

the thickness and the refractive index of the film (the product of

these two values determines the optical path length) the reflected ray

will either constructively or destructively combine with the incident

beam. This is the same process that occurs in a Fabry-Perot

interferometer described previously. However, instead of changing the

physical distance of the cavity to change the optical path length,

resonance is achieved by scanning the wavelength of incident light. In

terms of the total transmission that is measured, a series of

interference fringes is observed, which vary with wavelength. These

fringes are known as fringes of equal chromatic order (FECO), and the

wavelengths at which maxima and minima in transmission occur is given

by the same equation used for resonance conditions in a Fabry-Perot

















Rf
/ ------


AIR
ni


Tf T8S TfTR8R;


Figure 16 Interference effects at thin film, substrate, and air
interfaces.


T,.T; RS













m X = 2 n d (4.3)

where m is the order number of the fringe, d is the thickness, and X is

the wavelength. To take advantage of this effect to determine the

index of refraction two assumptions must be made. The first is that

the order number is not a discrete number but that it can vary

continuously. Why this is important will be described shortly. The

second assumption is that the dispersion of the refractive index can be

described by the following equation




n2 = A ____ + B(44)
x(x)(44


where the square root of B gives the infinite wavelength refractive

index and A and x are constants.

To make use of this effect, a FECO spectrum is obtained at normal

incidence. A typical spectra from a 2 pm CdS film is shown in Figure

17a. Curves are drawn tangent to the maximum and minimum peaks and the

values of the wavelength of the tangent points are determined. The

sample is then rotated in the beam by 30, resulting in an increase in

the optical path length by a factor of 1/(cos 30). The assumption

that the order number is a continuous mathematical variable means that

the normal incidence spectrum can be shifted to a shorter wavelength by

increasing each order number by an amount 6m.34 This increase produces

a shift in the FECO spectra shown in Figure 17b. Again, tangent curves

are drawn and the tangent points to the interference spectrum are

determined. The difference in the pairs of tangency points are then


































0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9
WAVELENGTH (urn)


a)


1 1.2 1.4 1.6 1.E
WAVELENGTH (urn)

b)


Fringes of equal chromatic order (FECO) spectrum for 2.0 um
thick film of CdS on a silica substrate, a) Normal
incidence spectrum showing many orders; b) Expanded
abscissa scale showing shift in FECO spectrum for 30
incidence.


Figure 17









82

used in an iterative calculation to determine the values of A, B, x and

n.

The problems associated with this technique involve precise

placement of the sample at the focal point of the beam and accurate

rotation of the sample about the axis which passes through the focal

point. These problems were addressed by using a micrometer adjustable

x-positioner and a micro-rotation stage, which are shown placed in the

Lambda 9 spectrophotometer in Figure 18. Normal incidence to the film

was determined by rotation of the stage a few degrees on either side of

the initial zero and observing the shift in position of a given

interference fringe. The true zero point was taken as the position

which produced the midpoint between the two shifts. The focal point of

the beam was then determined by observing the peak value of a given

fringe as the sample holder was moved with the positioner.

The most difficult part of this technique (except for writing the

iteration program for the analysis) was the manual determination of the

value of the tangency points from the FECO spectra. The wavelength of

the tangent point must be known to a precision of 0.1 nm to obtain an

accuracy of three decimal places in the refractive index. Wavelengths

can be measured with the spectrometer to this precision, but this

precision could not be obtained by manual measurement of the tangent

point from the printout of the spectra.

The problem was alleviated by interfacing the Lambda 9 to an IBM

AT computer. Use of the computer greatly facilitated all aspects of

using the spectrometer as spectra could be stored on disk and

































I.. i


Photograph of micro-positioner stage used for acquiring
FECO spectrum, shown mounted in Lambda 9 spectrophotometer.


Figure 18












manipulated later. The BASIC program used to remotely program, run,

and accumulate data from the Lambda 9 is listed in Appendix A.

For the determination of the refractive index a particular

software package available with the AT was found to be very useful.

This software called is Asyst (McMillain Software Co.) and it is a very

powerful mathematical analysis program. To determine the refractive

index, the numbers representing the FECO spectra were imported into

Asyst and stored in a two dimensional array. Using a very short

program, the array was plotted, and the equations for the curves

tangent to the maximum and minimum were fit. Next, arrays were

generated to represent each of the tangent curves. Asyst could then be

used to very easily determine the points where the three arrays

intersected, with an accuracy of 0.1%. The intersection points were

stored in a fourth array. This process was repeated for a 30

incidence spectra. The very long iterative BASIC program that was

originally used to calculate the dispersion equation could be shortened

considerably with utility functions available in Asyst. In fact, by

manipulating the numbers as arrays, the calculation was much simpler

and much faster.

Even though the above process produced an accurate result, a

second technique, ellipsometry, was used to verify these results.

Ellipsometry is based on the change in polarization of a light beam

when it is reflected from a surface. Generally a plane polarized light

beam is made incident on the surface of a material at an angle of 45.

The resulting reflected beam is elliptically polarized and from the









85

analysis of this polarization the optical constants of the material can

be determined.

The problems associated with this technique mostly originated from

the actual instrument. The computer-controlled Gaertner ellipsometer

(Chicago, IL) is the industry standard; however, the unit available for

use in this study was a manual model which greatly reduces the capacity

for quantitative analysis. The problem does not so much have to do

with manually setting the polarizer and analyzer, but rather in reading

the analog scale to determine the node points. This was a special

problem on this unit because the meter was nonlinear; the highest

sensitivity was between a reading of 2 an 3 on a scale of 1 to 5. In

addition, the gain for the photocell had to be manually adjusted as a

node point was found. The gain was also nonlinear and there was a flat

spot at one end of the range. These two problems contributed to

difficulties in reproducing the results; however, the greatest problem

with the analysis had to do with the computer program used to calculate

the values of n and t from psi and delta. Supplied by Gaertner, this

program did not actually calculate the values of psi and delta, but

instead it looked up the values in a table, based on the values of the

two pairs of angles that were inputed. From the table values of psi

and delta the program then calculated the refractive index and

thickness by an iterative technique. One difficulty with the program

occurred if the particular combination of angles inputed were outside

values in the table. The program would hang up and the only solution

was to boot the program out. This actually happened a surprisingly

large number of times, even when similar samples were analyzed. To









86

alleviate the problems with this program, a graphical method was

developed to calculate the refractive index and thickness from the set

of angles measured with the ellipsometer. A brief description of this

method is given in Appendix B.



UV-VIS Absorption

Measurement of the fundamental optical absorption edge in

semiconductors permits direct determination of the nature of the band

gap. Optical absorption properties of thin films were measured over a

variety of temperatures with the Lambda 9 spectrophotometer operating

in one of several modes. In the transmission mode a linear, zero to

100% scale was used, which was useful measuring FECO spectra. A more

useful mode for measuring the band edge, however, was to plot the

transmitted light in terms of absorbance. Because the absorbance scale

is the natural logarithm of reciprocal transmission, features of the

band gap absorption above the band edge could be studied. In the

transmission mode unless the scale was expanded these features would

appear indistinct, because transmission above the band gap was

typically less than 1% with the thickness of films usually studied. In

the absorbance mode this 1% transmission would correspond to a value of

4.6 on a possible absorbance scale of 6.0. Another advantage of using

the absorbance scale is that the spectra obtained qualitatively

reflected the shape of the true absorption curve. As explained earlier

in this chapter, the transmission spectrum that is measured of a thin

film on a substrate is the result of a complicated combination of the

reflections which occur at the various interfaces. The transmission,









87

however, is exponentially proportional to the absorption coefficient,

which is why absorbance spectra approximate absorption spectra. The

two terms will therefore be used interchangeably when describing

spectra.

The band gap energy was taken as the point where the slope of the

absorption curve was a maximum.54 One unique feature of the Lambda 9

was that it allowed anywhere from the first to fourth derivative of a

spectra to be taken. The maximum of the first derivative of the

absorption edge was taken as the band gap energy. A calculation to

verify these results will be described in Chapter V. Basically, it

evolves solving equation 4.2 for a using the absorbance spectra, and

then plotting a2 verses energy. A direct band gap material will give a

straight line for this plot, and the extrapolation of the line to a2 =

0 gives the optical band gap of the material.54

The temperature dependence of the band gap energy was measured by

cooling the sample with the cold finger. The sample was mounted in a

ring sample holder which would then be attached to the end of the cold

finger. A radiation shield was then positioned around the cold finger

and a vacuum insulation collar with fused quartz windows was then slid

over the entire assembly. A vacuum of less than 1 micron pressure was

then obtained in a few minutes by pumping the system with a Balzer's 70

1/sec turbomolecular pump. Once this vacuum was obtained in the

collar, cooling of the sample to 9 K could be accomplished in less

than 1 hour, although most measurements were made after 2 hours of

cooling. Any intermediate temperature between 9 K and 250 K could

achieved with the digital temperature controller and heater assembly.









88

The cold finger assembly was mounted on a height-adjustable rack system

so that once a sample was cooled to low temperatures the cold finger

assembly could be moved so that measurements on different equipment

such as the OMA and Raman spectrometers could be accomplished.

All aspects of the measurements with the Lambda 9 were greatly

enhanced by interface to the AT computer. Spectra were stored as ASCII

files which later could be imported into a LOTUS worksheet (Lotus

Development Co., Cambridge, MA) for manipulation and plotting. The

BASIC program listed in Appendix A would accumulate spectra in the form

of a long column of numbers, each number representing the absorption or

transmission at a given data point. The data interval was based on the

slit width used for the analysis. The slit defines the bandpass that

determines the resolution to which features can be resolved. Usually a

data interval of one-third the slit width was used for the analysis.

Slit widths from 4 nm down to 0.5 nm were used depending on the

required resolution. As the slit width is decreased, the signal to

noise ratio is also decreased. This means that the amount of time that

is spent at a given data point must be increased. Computer control of

the spectrophotometer was indispensable for making high resolution runs

where very slow scan speeds were necessary.



Photoluminescence and Raman

Further investigations of the band structure of semiconductors can

be made by measurement of the emission bands or peaks produced by

photoluminescence and resonant Raman scattering. Measurement of these

emissions for this study were made with the two spectrometers described












above. The majority of the measurements, however, were made with the

optical multichannel analyzer (OMA) and a 0.33 meter Instruments SA

monochromater. Most measurements were made at low temperatures, using

the cold finger to cool the sample.

The detection system of the OMA spectrometer consisted of a linear

silicon photodiode array which was 1024 elements wide with the diode

centers spaced every 25 um. Approximately 700 of the center diodes

were intensified to increase their light conversion efficiency. The

diode array was connected to a multichannel analyzer and the array was

scanned by the MCA to generate a spectra. Scan rates as fast as 16.6

msec/scan could be used, although to obtain a higher number of counts,

scan rates of 500 msec were typically used. The computer system

controlling the MCA was setup to accumulate a number of scans so that

signal averaging could be used to reduce noise levels. Also, the dark

current or background noise could be automatically subtracted from a

spectrum. The diode array could be cooled to 5 C with a Peltier

cooler to further reduce the background noise level. With the

monochromater and grating combination the MCA window was about 20 nm.

This is the width of the spectra which was displayed on the screen.

The grating in the monochrometer was rotated to change the wavelength

window.

The third meter monochromater was used with a 2400 lines/mm

grating. The f-stop or light gathering capability of the monochromater

was calculated with









90

focal length

f-stop = ____________ (4.5)

grating size





where grating size is the linear dimension of the grating. With a 55

mm grating, the f-stop of the monochromater is f6. The reciprocal

linear dispersion (RLD) of a monochromater is a measure of its ability

to disperse light, and can be calculated by


RLD = X dL (4.6)
f de

where d8/dL is the angular dispersion (in rad/mm) and is approximately

equal to the number of grating lines per mm. The RLD for this

monochrometer was calculated to be 1.25 nm/mm. The bandpass or

resolution of a monochromater is usually calculated by multiplying this

number by the width of the slit used. With a linear diode array

detection system, however, the resolution is determined by the spacial

resolution of the array. Assuming that three diodes can resolve a

peak, which correspond to 75 pm, the resolution of this system was 75

Pm X 1.25 nm/mm or 0.9 nm.

A schematic of the experimental setup is shown in Figure 19 and

photographs of the system are shown in Figure 20a and Figure 20b. An

f2 lens was used to focus emitted light into the spectrometer and a 100

mm focal length lens was used to focus the laser beam on the sample.

Typically a focused spot size of 50 um was used, so with an incident

power of 1 mW, the focused power density corresponds to 400 W/cm2. As



























Fused silica window


He cryogenic cooler
sample chamber


Laser line filter


Ar laser
beam


Probe-forming lens


Spectrometer/Detector


Collection lens Slit


To vacuum pump


To optical multichannel analyzer


Schematic diagram of experimental setup used for measuring
photoluminescence.


Figure 19







































Photograph of OMA setup which was illustrated in Figure 19.


Figure 20


,. + I : 0k L+"++
i+.+ .. ... ,'40




Full Text
171
TABLE 5
Band Edge Position for Several Different Films
Determined by Maximum Slope Method
Sample
Maximum Slope Position
(nm)
Energy
(ev)
ASDP 1 A/sec
t = 0.5 p
507.8
2.442
t = 0.75 ym
508.9
2.437
t = 1.0 ym
509.8
2.432 (2.44)1
(2.37)2
ASDP 3 A/sec
508.5
2.439
ASDP 5 A/sec
501.7
2.472
ASDP LN2
496.5
2.497
ASDP 300 C
523.1
2.370
FHT 500 C
505.1
2.455 (2.46)1
5 hr.
(2.45)2
'Determined by maximum slope of absorption curve (Figure 79a)
^Determined by intercept method (Figure 79b)
Note: All films measured were approximately 1.0 ym thick unless
other wise stated.


ABSORPTION COEFFICIENT-2 (cm-2) ABSORPTION COEFFICIENT (cm-1)
(Millions) (Thousands)
191
a)
b)
Figure 73 Alternate methods for determining the value of the optical
band edge, a) plot of absorption coefficient calculated
from Equation 5.4 versus photon energy, point of maximum
slope gives the band energy; b) plot of a- versus photon
energy, extrapolation to a 0 gives band gap energy.


56
programming control over deposition parameters and constant monitoring
of deposition rate, thickness, power output and crystal oscillator
status. The analog portion of the I/O board was connected to the
pressure gauge controller and to the RF power supplies. Connection to
the power supplies permitted recording of the DC bias which develops on
the target as a result of the sputtering process. All of these process
parameters were stored on disk during a deposition run. This made it
possible to go back and review the deposition process, if a film was
later found to have anomalous properties.
A unique feature of the deposition system has to do with the
various substrate holders which could be used. For low temperature
depositions, a liquid nitrogen (LN2) cooled substrate holder was
employed. This holder is made up of two double concentric tubes in
which the center tube acts as a reservoir for LN2 and the outer tube,
which is open at the bottom to the vacuum system, acts as an insulating
vacuum jacket. An aluminum plate with a slightly recessed area to hold
the substrate is attached to the end of the inner reservoir tube with a
copper screw. To reduce the amount of contamination that would occur
on the cold substrate, an additional LN2 cooled coil was positioned
around the substrate holder assembly. This coil is referred to as the
cryoshield in Figure 10. For room temperature depositions, the same
holder was used, however, without filling the LN2 reservoir.
High temperature depositions were accomplished by replacing the
LN2 feedthrough assembly with a resistively heated substrate holder.
This holder was made by drilling holes lengthwise in a thin aluminum
block and inserting nichrome wound ceramic heaters. A Chromel-Alumel


65
thermal conductivity of the sapphire would aid in reducing heating by
the laser beam during low temperature spectroscopic measurements.
These substrates were of very high quality with no detectable flaws and
a very smooth surface finish. Some COSAD films were deposited onto
sapphire substrates to aid in X-ray chemical analysis, since the
sapphire had no X-ray lines which overlapped the lines in these films.
For other TEM analysis, films were deposited on slabs of single
crystal NaCl and on TEM grids which held carbon support films. The TEM
grid-carbon film combination was used for quick analysis of deposited
films. Once the grid-carbon film assembly was prepared, thin films
could be deposited directly onto the carbon film, and no other sample
preparation was necessary. Films of thickness from 500 to 3000 were
typically deposited for TEM analysis. The carbon support film was made
by thermally evaporating approximately 200 of amorphous carbon onto
sheets of single crystal mica. After the carbon film was floated off
the mica, 300 mesh TEM grids were carefully placed on the floating
film. The film with grids was then picked up on a thin cardboard card
and allowed to dry.
Single crystal NaCl was used as a substrate because it was found
that after heat treatment or as a result of a high temperature
deposition, thin films of CdS could not be floated off silica
substrates. Normally the 1% solution of HF would readily float films
off silica, but after any type of heat treatment above 200 C not even
a high concentration of HF was successful for floating the films.
Apparently some type of chemical bond forms between CdS and the silica
even at relatively low temperatures. Single crystal NaCl substrates


116
Figure 33 SEM micrograph of thin film deposited at 300 C,
backscattered electron image.


236
13. W. Albers, "Physical Chemsitry of Defects," Physics and Chemistry
of II-VI Compounds, ed. M. Aven and J.S. Prener, (North-Holland
Publishing Co. Amsterdam, 1967), p.137.
14. B. Segall and D.T.F. Marple, "Intrinsic Exciton Absorption,"
Physics and Chemistry of II-VI Compounds, ed. M. Aven and J.S.
Prener, (North-Holland Publishing Co. Amsterdam, 1967), p.319.
15. B. Segall, "Band Structure," Physics and Chemistry of II-VI
Compounds, ed. M. Aven and J.S. Prener, (North-Holland Publishing
Co. Amsterdam, 1967), p.3.
16. F.A. Kroger, "Luminescence and Absorption of Solid Solutions in the
Ternary System ZnS-CdS-MnS," Physica, 1_, 92 (1940).
17. R.E. Halsted, "Radiative Recombination in the Band Edge Region,"
Physics and Chemistry of II-VI Compounds, ed. M. Aven and J.S.
Prener, (North-Holland Publishing Co. Amsterdam, 1967), p.385.
18. R.J. Collins, "Mechanism and Defects Responsible for Edge Emmision
in CdS," J. Appl. Phys., 30, 1135 (1959).
19. D.G. Thomas and J.J. Hopfield, "Optical Properties of Bound
Complexes in Cadium Sulfide," Phys. Rev., 128, 2135 (1962).
20. C.H. Henry, R.A. Faulkner, and K. Nassau, "Donor-Acceptor Pair
Lines in Cadmium Sulfide," Phys. Rev., 183, 798 (1969).
21. C.H. Henry, K. Nassau, and J.W. Shiever, "Optical Studies of
Shallow Acceptors in CdS and CdSe," Phys. Rev. B, 4, 2453 (1971).
22. C.H. Henry and K. Nassau, "Lifetimes of Bound Excitons," Phys. Rev.
1, 1628 (1970).
23. D.G. Thomas and J.J. Hopfield, "Spin-Flip Raman Scattering in
Cadmium Sulfide," Phys. Rev., 175, 1021 (1968).
24. R.C. Leite, J.F. Scott, and T.C. Damen, "Multiple-Phonon Resonant
Raman Scattering in CdS," Phys. Rev. Lett., 22, 780 (1969).
25. R.C. Leite, J.F. Scott, and T.C. Damen, "Resonant Raman Effect in
Semiconductors," Phys. Rev., 188, 1285 (1969).
26. K. Bohnert, H. Kalt, and C. Klingshirn, "Intrinsic Absorptive
Optical Bistability in CdS," Appl. Phys. Lett., 43, 1088 (1983).
27. R.K. Cook and R.W. Christy, "Optical Properties of Polycrystalline
CdS Films," J. Appl. Phys.,51, 668 (1980).
28. L.I. Maissel and R. Gland, Handbook of Thin Film Technology,
(McGraw-Hill, New York, 1970), chap. 1.


30
k
0*0
WURTZITE
14
Figure 4
Band gap structure for wurtzite crystals near k=0.


Counts *IQ1
196
O
Wavglangth rm
Low resolution photoluminescence spectra of heat treated
and as-deposited thin films, showing relative intensities.
Figure 75


86
alleviate the problems with this program, a graphical method was
developed to calculate the refractive index and thickness from the set
of angles measured with the ellipsometer. A brief description of this
method is given in Appendix B.
UV-VIS Absorption
Measurement of the fundamental optical absorption edge in
semiconductors permits direct determination of the nature of the band
gap. Optical absorption properties of thin films were measured over a
variety of temperatures with the Lambda 9 spectrophotometer operating
in one of several modes. In the transmission mode a linear, zero to
100% scale was used, which was useful measuring FECO spectra. A more
useful mode for measuring the band edge, however, was to plot the
transmitted light in terms of absorbance. Because the absorbance scale
is the natural logarithm of reciprocal transmission, features of the
band gap absorption above the band edge could be studied. In the
transmission mode unless the scale was expanded these features would
appear indistinct, because transmission above the band gap was
typically less than 1% with the thickness of films usually studied. In
the absorbance mode this 1% transmission would correspond to a value of
4.6 on a possible absorbance scale of 6.0. Another advantage of using
the absorbance scale is that the spectra obtained qualitatively
reflected the shape of the true absorption curve. As explained earlier
in this chapter, the transmission spectrum that is measured of a thin
film on a substrate is the result of a complicated combination of the
reflections which occur at the various interfaces. The transmission,


190
ABSORBANCE + In A
a =
t
(5.A)
The term In A represents the losses due to reflection interference and
the value can be determined from the absorbance spectra of different
thickness films, such as Figure 64, or a close approximation can be
made by using the value of absorbance below the band edge, such as the
value at 580 nm in Figure 64. The absorption coefficient spectra as a
function of energy obtained by this calculation is shown in Figure 73a
for both an as-deposited film and a heat treated film. The value of
the optical band gap is given by either the maximum in slope of this
curve, or by plotting the square of a and extrapolating the linear
portion of the curve to the intersection of the x-axis. The value of
the x-axis intercept is the band gap energy. This plot is shown in
Figure 73b for the same two films and a listing of other films and a
comparison of the different techniques for calculating the band gap is
presented in Table 5.
The above Figures and Table when compared to the data presented in
the Bibliographical Review chapter show that the thin films produced in
this study exhibit similar results to single crystal properties. This
is another indication of the high quality of films which can be
produced by the deposition process and subsequent heat treatments that
were used in this study.
Photoluminescence and Raman Spectroscopy
Photoluminescence offers a complementary analysis to absorption
measurements. While the optical transitions involve similar states,


127
Strain determinations. Residual strain in thin films can be
manifested as both a shift and a broadening of X-ray diffraction lines.
However, broadening of X-ray lines due to strain will only occur if a
nonuniform microstrain is present (usually due to plastic deformation).
Broadening usually is a result of a very small grain size. Digital
manipulation of the diffraction data permitted determination of peak
broadening due to nonuniform strain and grain size effects by use of an
utility program available in the APD computer. The computer program,
however, does not calculate the uniform strain which would produce a
shift in peak positions. Residual uniform strain was determined by
comparing the position of the (0006) diffraction line in thin films to
the position observed in a powdered sample of CdS. By differentiating
Bragg's law, the change in lattice parameter due to a uniform strain
was calculated from the shift observed in the peak positions.^
Figure 40a shows a comparison of the broadening and peak position seen
in films deposited at two different rates and the line width and
position which results when these films are fully annealed. The tick
mark shows the position of the (0006) line of the powdered sample. The
calculated uniform strain which would produce the observed line shifts
and the effective mean nonuniform strain and effective mean grain size
that would produce the broadening are listed in Table 2. Also shown
are the values which would cause the broadening if only grain size
effects were present.
Thin films were considered to be uniformly strained, therefore the
results calculated by the APD program for nonuniform strain are not
considered relevant. The grain size effects listed in the last column


205
blue-edge emission is certainly broad enough for both bound excitons to
be present. The bound exciton is a much stronger transition, so it
should be easier to detect in photoluminescence measurements. The free
exciton emission may be too low in intensity to detect. For absorption
measurements the cross section of the free exciton is much larger than
the bound exciton, because it does not have the same momentum
constraints as the bound exciton. Therefore, the absorption spectra of
free excitons should be broad and the bound exciton should exhibit a
sharp absorption band. Estimations show that the UV-VIS
spectrophotometer may not have a high enough resolution to observe the
bound exciton absorption peaks. Photoluminescence on the other hand
does not have the same momentum constraints, therefore the emissions
will have different characteristics. Since the bound exciton has both
a higher oscillator strength and more efficient radiative conversion,
bound excitons should be more predominant in photoluminescence spectra.
The blue edge emission displays a temperature dependence similar
to optical absorption. This is shown in Figure 81. As the temperature
is increased, the photoluminescence peak shifts to longer wavelengths
(lower energy) and decreases in intensity. As with the UV-visible
spectra, the peak is still present at 40 K, but at 100 K it becomes
indistinct. The Figure also shows that with an increase in temperature
the intensity of the green edge emission peak decreases. The decrease
occurs because as the temperature is increased, more phonons become
available to cause non-radiative transitions.


154
50 nm
a)
Figure 53 TEM micrograph of C0SAD1 thin film. a) after five minute
exposure to condensed electron beam; b) SAD pattern.


61
Electrons in the plasma can be accelerated by the positive half of the
RF field, which results in both the ionization of argon atoms, and the
bombardment of the target. This bombardment results in the development
of a negative potential on the target surface, which is called the DC
bias. The argon ions in the plasma are too massive to be accelerated by
the RF field; however, they are accelerated towards the target by the
DC bias, which results in sputtering of the target material.
The configuration of diode sputtering results in exposing the
substrate and growing films to the full energy of the plasma. The
growing film is continuously bombarded by a number of energetic
particles, both charged and neutral. This bombardment can either be
enhanced or reduced by applying a potential to the substrate during
deposition. Substrate biasing can only reduce the amount of
bombardment to a certain extent, essentially because the substrate is
totally emersed in the plasma during deposition. Considerable heating
of the substrate and film occurs as a result of the bombardment, even
at moderate plasma powers.
RF planar magnetron sputtering reduces the amount of bombardment
by incorporating a strong permanent magnet behind the cathode assembly.
The configuration of a US sputter gun is shown schematically in Figure
13. Curved magnetic field lines force the electrons in the plasma into
circular orbits, which both enhances the plasma immediately above the
target, and reduces the amount of electron bombardment on the growing
film. The target is a circular disk that is secured to the magnet
housing, which comprises the cathode. The anode is comprised of a
circular cap (called the ground shield in the Figure) which fits over


207
Raman spectroscopy
The photoluminescence measurements described above were conducted
by exciting the films with a laser beam of energy much greater than the
band gap. Excited states which are generated must first drop to a
lower energy corresponding to either an impurity level, or an exciton
level by way of a non-radiative transition before a radiative
transition can occur. When a excitation energy is used that is very
close in energy to the excited state, the energy can be absorbed and
then scattered by a phonon state. The scattering by the phonon results
in the re-emission of the light at a different frequency. Because the
excitation energy is very close to the excited state level, a resonance
effect takes place and an enhancement of the scattering results. This
process is called resonant Raman scattering and in thin films it was
found to be a very pronounced effect when a laser energy near the
energy of the exciton level was used to excite the film. Resonant
Raman scattering by the longitudinal optical (LO) phonon of the 488 nm
laser line would result in a series of equally spaced peaks that were
shifted ~ 300 cm ^ from the laser line. These peaks were very intense
and could be measured with either the OMA or Raman spectrometers. A
typical spectrum of the first two LO phonon peaks which was acquired
with the OMA and plotted in terms of Raman shift from the laser line is
shown in Figure 82. This spectra was acquired with similar scan
parameters and laser intensity as photoluminescence spectra, but the
intensity of the 1L0 phonon is much greater than any photoluminescence
peak. The temperature dependence of this emission is shown in Figure
83. A similar low temperature spectrum obtained with the Raman


234
b) A wide range of ni values and d values is selected and the
Drude Equations are solved for xc and Ac.
c) A graphical comparison of the measured A is made with xc and
Ac for each set of n values over the range of d values. Since d varies
cyclically, it is not important which d values are used as long as they
span the cycle (A).
d) This allows a rapid calculation of n since the approach of the
calculation to the data can be seen in the plots.
e) n can be calculated with desired accuracy.
f) Once n is known, d can be calculated from the Drude Equations
using the well known quadratic formula, with possible solutions at 8,
-9, 2mir-0, 2imr9 where m is the integer order number. Thus the
thickness calculation cannot yield a unique value. However, if one
knows the approximate thickness, so that m may be defined, then an
accurate value can be obtained.


59
Figure 12
Schematic diagram of deposition chamber configured for
COSAD.


193
Photoluminescence spectra
As-deposited thin films. Thin films which were deposited at room
temperature were found to only give very low intensity, broad band
photoluminescence. The defect structure of the band gap that led to
the band tailing observed in the absorbance spectra of these films is
also responsible for the low intensity observed in these measurements.
These defects states provide non-radiative processes for the relaxation
of excited states, so that the photoluminescence spectra have very low
intensities. Because there are many different states with slightly
different positions in the band gap, a broadening of the luminescence
also occurs. A typical spectrum of films deposited at low rates,
obtained at room temperature with the OMA and the low resolution
grating is shown in Figure 74. This is an expanded intensity scale so
the signal to noise ratio is low. The sharp peak at 457 nm is the
laser line used to excite the sample. Usually, the three different
broad bands observed in this spectrum would occur in all films
deposited at room temperature, but the intensity of the different bands
would vary. For example, the higher intensity band centered at 535 nm
was found to be of much lower intensity in films deposited at higher
rates and it was totally absent in films deposited at 5 /sec. The
energy of the 535 nm band corresponds to direct transitions in the band
gap and is the "green-edge" emission that was discussed in the
Bibliographical Review chapter (Chapter III). The radiative
transitions from conduction to valance band or from shallow donor to
shallow acceptor impurity levels are characterized by this emission.
The two low intensity broad bands centered at 670 nm and 750 nm are


156
Figure 55 TEM micrograph of C0SAD1 thin film, a) after heat
treatment to 650 C for 30 min.; b) SAD pattern.


31
Figure 5 Energy diagram for Wannier excitons as a function of
exciton momentum K, showing "hydrogenic" states which merge
into a continuum at energies greater than Eg.^


50
Semiconductor Doped Filter Glasses
Sharp cutoff filter glasses are silica based glasses which contain
a fine dispersion of semiconductor microcrystallites. The variation of
the sharp absorption edge position is achieved by either varying the
composition and or heat treatment schedule. Great interest has been
generated recently because it is thought by many authors that the
crystal size developed in these materials by thermal treatments is on
the size order for quantum confinement effects to occur. For glasses
containing a mixture of CdS and CdSe many authors have reported a blue
shift in the absorption band edge. In addition, a blue shift of the
high energy exciton photoluminescence peak has been interpreted in
terms of quantum confinement.
Warnock and Awashalom^^ in two publications on mixed crystal
glass (those containing CdS.27/Se.73) report that the glasses which
displayed smallest size distribution (average size = 94 ) exhibited
the largest shift in the exciton photoluminescence, and this
photoluminescence was shown to decay on a time scale of only 18 psec,
which is nearly two orders of magnitude faster than the lifetime
displayed by excitons in bulk material.^ The shifts in peak position
and fast lifetime were interpreted in terms of a quantum confinement
effects.
A very recent paper by Borrelli et al.^ has shown that the blue
shift in the photoluminescence displayed by this particular mixed
crystal glass is not due to confinement effects, but rather to
compositional effects. A careful study of the crystal structure by X-
ray diffraction revealed that a change in the stoichiometry occurs


CHAPTER V
RESULTS AND DISCUSSION
Thin Film Physical Properties
The primary focus of the experimental results presented in this
section are on the variation of the microstructure, composition and
electrical properties of thin films as a function of deposition
parameters and post deposition treatments. First the properties of as-
deposited thin films will be described and then the variation of
properties with different heat treatments will be reported. Finally,
the various physical properties of COSAD thin films will be described.
The variations presented in this section will be correlated to optical
properties presented in a subsequent section.
Microstructure Characterization
Transmission electron microscopy
Room temperature depositions. A wide variety of microstructures
were obtained by altering the substrate temperature during deposition.
The great majority of thin films made for this study, however, were
deposited onto substrates held at room temperature. Several advantages
were realized by using this temperature: l)the deposition process was
greatly simplified, 2)very high quality thin films could be produced,
and 3)optimum optical properties were obtained by heat treating films
deposited at this temperature. These heat treatments, however,
96


21
When the cavity does not satisfy the above requirement, then a
linear relationship will exist between the incident and transmitted
intensity. If a material with a nonlinear refractive index is placed
in the cavity, then a positive feedback loop will occur where the
refractive index and the light intensity become mutually reinforcing.
As the incident intensity is increased, a change in refractive index
occurs which brings the device closer to resonance, which further
increases the intensity inside the cavity, which further changes the
index, etc. This continues until a saturation intensity is reached, at
which point a phase shift to resonance occurs within the cavity and the
transmitted intensity suddenly increases.
The ratio of the incident intensity to the transmitted intensity
as a function of the phase shift 6 is given by the Airy function A(0)
I.
i
A(0)
(2.18)
where
A(0) = l (2.19)
1 + F sin2(6/2)
and F is related to the reflectivities of the mirrors R by
( 1 R )2
(2.20)


145
Figure 47 XPS survey scan of furnace heat treated thin film.


134
The results for the four different analytical techniques used for
determining the composition of thin films are listed in Table 4. The
absolute values of the sulfur and cadmium concentrations are shown to
vary greatly between different techniques; however, the difference
between as-deposited and heat treated thin films is nearly the same in
each case, except for the XPS results. Films that were heat treated to
650 C were used because of their use for optical properties studies.
They were also found to show the greatest difference. The discrepancy
in the XPS results for heat treated thin films and the implications of
the results of the other techniques will be discussed below. The
primary indication of these results, however, is that as-deposited thin
films contain ~ 2% excess sulfur and once they are heat treated, this
excess increases to 4%. As indicated by the precision lattice
parameter described above, the unit cell of heat treated thin films is
very close to the ideal structure of CdS, so very little of this excess
sulfur is interstitial, or very few cadmium vacancies are present.
This means that the sulfur must be a second phase.
Analytical techniques
Use of electron probe microanalysis (EPMA) for determining the
composition of thin films was first thought to be the best method and
most quantitative, for the same reasons stated above. Not even
qualitative results, however, could be obtained. The standard ZAF or
pz^ programs used to calculate composition consistently gave results
of less than 100 weight percent for the composition of thin films.
Even a thin film program which relies on some approximations, did not


CHAPTER VI
CONCLUSIONS
1. RF magnetron sputtering of cadmium sulfide has proven to be a
valuable deposition technique for producing high optical quality thin
films. The as-deposited thin film structure can be controlled by a
variation of the deposition rate and substrate temperature. Unique
structures can be produced by deposition onto substrates held at LN2
temperatures. A low density columnar structure is produced in which
small microcrystallites are embedded in a semi-amorphous matrix.
Electron diffraction experiments indicate that the microcrystallites
are highly faulted, which leads to a degradation of the optical
properties. The crystallographic defects provide defect states in the
band gap which are indicated by the large amount of band tailing
observed in optical absorption spectra. Optical properties are further
degraded by the presence of excess cadmium. The large number defects
responsible for band tailing provide non-radiative transitions so that
band edge photoluminescence is not observed in these films.
2. High density fine grained polycrystalline films result when
sputtered material is deposited onto substrates held at room
temperatures. Many of the grains appear to be faulted, with the
presence of stacking faults and microtwins. The number of faults
increases with the deposition rate, therefore the faults are related to
the growth rate of the thin film. Electron and X-ray diffraction
218


b)
30 ran
Figure 22 TEM bright field micrographs of 1000 thick film deposited
onto carbon support film, a) low magnification showing
uniformity of grain size; b) high magnification showing
microtwins and stacking faults.


114
conclusion derived from SEM micrographs that cannot be readily
determined by TEM relates to the lack of flatness on the surface of
these films, on this size scale. This may be the result of columnar
growth on a very fine scale.
The columnar structure observed by TEM in thin films deposited at
LN2 temperatures is not readily seen by SEM, as shown in Figure 32.
The size of the structure observed in this 1.0 pm film is greater than
the grain size observed by SEM of room temperature deposited films,
albeit this is a much lower resolution micrograph so the size may be
distorted. By referring back to Figure 26 one can see that the width
of the columns is approximately 500 . The size measured here is
approximately 800 , which is similar to the differences between the
grain size measured by TF1M and SEM on room temperature deposited films.
From Figure 32 it is not possible to determine that a semi-amorphous
columnar structure is present in this thicker film, which is one
drawback of using SEM for microstructure determinations.
Other capabilities of the SEM, however, make the observation of
other physical properties possible. The nonuniformities seen in films
deposited at temperatures greater than 200 C are greatly enhanced by
imaging backscattered electrons from the film. The energy of
backscattered electrons is dependent upon composition, so the contrast
observed in the micrograph of Figure 33 is related to the compositional
nonuniformities of a film deposited at 300 C. These results correlate
well with those reported by Cook and Christy,*^ who found that
deposition above 200 C yielded poor quality thin films. The large


221
thin films. Associated with the changes in microstructure that occur
upon heat treatment to high temperatures is the appearance of exciton
levels. These levels appear as sharp absorption bands or as low
intensity blue edge photoluminescence. There is, however, considerable
broadening of the blue edge photoluminescence associated with the
exciton levels. The shape of the emission peak is similar to that
observed in single crystal platelets; however, it is ~ 5 times wider.
This broadening may be due to orientation effects or the presence of
grain boundaries which provide different environments for the
annihilation of excitons to produce the photoluminescence.
6. Heat treating films by rapid thermal anneal (RTA) produces films
which also exhibit exciton levels. The grain size of these films is
three times smaller than furnace annealed thin films, which indicates
that for excitons to be present in sputter deposited thin films, only
the annealing of certain defects is required, not large grain growth.
The actual defects which must be removed are not known, but they must
be those associated with band tailing, as the appearance of exciton
levels is coincident with the removal of band tailing. From TEM
micrographs of unheat treated films, these defects most likely are the
stacking faults associated with microtwins.
7. Thin films were found to be nonstoichiometric, particularly after
high temperature heat treatments where the sulfur content was found to
increase to approximately 4%. The increase in sulfur is obviously due
to a loss of cadmium, which has a much higher vapor pressure than
sulfur at the heat treatment temperature. Only a small portion of the
excess sulfur may exist interstitially, as the indication from other


161
b)
Figure 59 TEM micrograph of C0SAD2 thin film, a) after ten minute
exposure to condensed electron beam; b) SAD pattern.


93
Figure 21 Photographs of setup for Raman spectroscopy, a) shown are
the computer for acquiring data and the vacuum pumping
system for the cold finger; b) similar optics setup to OMA
shown. Laser beam is brought in from the left, focussed on
sample in cold finger, and the emitted light is focussed
into the first slit of the spectrometer, at the far right.


45
deposition process is RF sputtering, and several investigations of the
deposition of CdS by this technique are presented below.
RF Sputtering
Only recently has the deposition of CdS thin films by RF
sputtering been investigated to any great extent. One of the first
investigations was reported by Lagnado and Lichtensteiger. They
described some of the properties of CdS thin films produced by RF-diode
sputtering. A very strong preferred orientation of the CdS
crystallites was found to occur with the c-axis parallel to the
substrate plane. Electrical resistivity measurements at different
temperatures revealed two activation energies for conduction,
interpreted to show the activation of an unspecified trap below the
conduction band, although no data on the chemical analysis for
impurities was presented.
Recently the technique of RF-diode sputtering has received
considerable attention for producing CdS thin films for solar cell
heterojunctions.^^ The large photoconductivity of CdS makes it an
attractive material for both solar cell and sensitive photodetector
applications. A number of investigations on the RF diode-sputtering of
CdS for these applications have been reported by Martil et al.-^41
These workers have investigated the dependence of the physical,
electrical, and optical properties on both deposition parameters and
post deposition heat treatments.
The earliest publication by Martil et al.^ outlines the
dependence of deposition rate, thin film grain size and resistivity as


AXIS SPACING
128
*10
b)
Figure 40 Analysis of X-ray diffraction spectra, a) (0006) X-ray
diffraction peak of two as-deposited and one furnace heat
treated thin films showing position shift and broadening
due to strain and grain size effects; b) plot of c-axis
lattice parameter versus Nelson-Riley function for
precision lattice determination.


91
He cryogenic cooler
ample chamber
Figure 19 Schematic diagram of experimental setup used for measuring
photoluminescence.


189
takes into account the reflections that take place within the film and
the resulting interference can be written as
T =
A e
-a t
1 -Be
-2 a t
(5.1)
where
A = ( 1 -R^ ( 1 R2) ( 1 R3) ( 1 R2R3)
and
B = ( R R2 + R1R;J ) ( 1 R2 )2.
- 1
Equation 5.1 can be simplified and written as
T =
a t
B e
-a t
(5.2)
and the inverse is given by
1
T
1 at
e
B -at
e
(5.3)
If the absorption coefficient a is assumed to be very large (a ~ 10^)
then the second term in equation 5.3 is essentially zero. Since the
absorbance is defined as the natural logarithm of inverse transmission,
then the absorption coefficient can be calculated from the absorbance
spectra by applying


231
accurately to measure the film refractive index and, if the thickness
is approximately known, to measure the film thickness.
Instrument Modifications
Sources of error in an ellipsometer are numerous but only a few
have a large effect on the results.
1. Polarizer, Analyzer. Their accuracy is very adequate (0.01),
even for fine work. Their relative alignment can be checked by placing
the arms of the ellipsometer in line with each other. After removing
the compensator the positions of maximum transmission and extinction
can be checked. Angular tracking can then be checked by rotating the
polarizer.
2. Quarter wave plate compensator. Our ellipsometer has a fixed
compensator at +90. By inserting it into the beam, its position can
also be checked. If the compensator, is defective, it can still be
used with a slight modification of the equations.
3. Incidence Angle. This is the most critical adjustment in the
instrument. A variation in the angle between source and detector of
less than Io can change the refractive index in the first place. We
found that despite a well collimated laser source, our instrument was
designed to accommodate changes in the angle of incidence of more than
several degrees. This was done by using a wide angle collection system
with a 2cm x 2cm photocell. This very sloppy alignment was used in
order to collect light from samples which were not plane parallel.


228
1410 INPUT #1,D$
1420 PRINT #lf"$SS" + STR$(SS)
1430 INPUT //1,D$
1440 PRINT //I,"$DI" + STR$(DI)
1450 INPUT #1,D$
1460 PRINT //1,"$FR" + STR$(FR)
1470 INPUT //I ,D$
1480 PRINT #1,"$SL" + STR$(SL)
1490 INPUT //1,D$
1500 IP CR=0 THEN 1590
1510 PRINT #1,"$CR" + STR$(CR)
1520 INPUT //1, D$
1530 PRINT //1,"$RS" + STR$(RS)
1540 INPUT #1,D$
1550 PRINT //1,"$PE" + STR$( PE): INPUT #1,D$
1560 PRINT #1, "$PR" + STR$( PR): INPUT //1,D$
1570 PRINT //I, "$MX" + STR$ (MX) : INPUT //1,D$
1580 PRINT #1,"$MI" + STR$(MI):INPUT #l,D$:GOTO 1600
1590 PRINT #1,"$CR 0":INPUT #1,D$
1600 PRINT //I, "$RA" + STR$(RA): INPUT #1,D$
1610 PRINT CHR$(10);"CONFIRM SELECTED VALUES ON LAMBDA 9 SCREEN"
1620 INPUT "CHANGE ANY VALUES Y/N ";CHG$
1630 IF CHG$="Y" THEN 1640 ELSE 1650
1640 CLS:GOTO 1050
1650 PRINT #1,"$SC"
1660 INPUT //1, D$
1670 PRINT D$
1680 REM PRINT DATA
1690 FOR 1=1 TO XPTS+12
1700 INPUT //1,X$
1710 S$=LEFT$(X$,1)
1720 S=VAL(S$)
1730 IF S=0 THEN S=LEN(X$) ELSE 1750
1740 X1$=RIGHT$(X$,S-l):X$=X1$
1750 X(I)=VAL(X$)
1760 NEXT I
1770 INPUT "STORE DATA ON DISK Y/N ";ANS$
1775 IF ANS$="Y" THEN 1780 ELSE 1910
1780 INPUT "INSERT DATA DISK IN DRIVE A AND TYPE R ";R$
1790 IF R$="R" THEN 1800 ELSE 1780
1800 INPUT "NAME OF FILE (MAXIMUN OF 8 CHARACTERS)";FILN$
1810 IF LEN(FILN$)>8 THEN 1800
1820 IF LEN(FILN$)=0 THEN 1800
1830 OPEN "0",//2,"A:"+FILN$+".PRN"
1840 N=XPTS+11
1850 PRINT #2,AH:PRINT //2,AL:PRINT #2,DI
1860 FOR 1=1 TO 11:WRITE //2,X(I):NEXT I
1870 IF SB=0 THEN SCALE=200 ELSE 1890
1880 FOR J=ll TO N:WRITE #2,X(J)/SCALE:NEXT J:GOTO 1910
1890 SCALE=10000


APPENDIX B
ELLIPSOMETRY
When an elliptically polarized beam of light is reflected from a
surface, its polarization direction is changed by the refractive index
and absorption coefficient of the reflecting surface. If the surface
is a thin film, then the film thickness also enters into the reflection
equations (Drude Equations).
Ellipsometer instruments can be purchased with prepackaged
computer programs to calculate the film thickness, refractive index and
extinction coefficient from the ellipsometry data.
We recently acquired a Gaertner Ellipsometer and conducted tests
with the following results.
a)While the ellipsometer uses a laser source, and 0.01 degree of
resolution, for the polarizer and analyzer settings, the data was not
accurate enough to measure a standard.
b)The program supplied was adequate for substrates with vastly
different indices from the deposited films, but was totally useless for
systems where the indices were even relatively far apart, such as CdS
films on silica substrates.
Therefore, an intensive analysis of the sources of error in both
the experimental apparatus and in the calculation program was
conducted. The results showed that with some modifications in the
instrument and with a new program, the ellipsometer can be used very
230


177
are responsible for most of the band tailing are phonon assisted
transitions and with the lowering of the temperature, these transitions
are no longer possible. Band tailing is not completely reduced in the
as-deposited thin films because there are other defect states present
in the band gap (created by the crystallographic defects) which do not
require phonon scattering for transitions.
The most significant change in the absorbance spectra at low
temperature is shown to occur in films heat treated at high
temperatures. The appearance of several sharp absorption bands which
correspond to exciton transitions are observed. Figure 68a displays
these bands and other changes in the absorbance spectra of a 1.0 pm
thick film, deposited at 1 /sec, that was heat treated to 650 C for 4
minutes. Shown in Figure 68b is the absorbance spectrum from a 10 pm
thick single crystal platelet of CdS, measured under the same
conditions as the thin film. The platelet was oriented with the c-axis
perpendicular to the incident beam. Above the band edge, the two sharp
peaks observed in the thin film spectrum appear similar to the platelet
spectrum, but the features are more pronounced in the thin film
spectrum. Also, the entire curve for the thin film is shifted to lower
energy. The similarities of these band edge features, however, do
indicate the presence of exciton states in the polycrystalline thin
fi1ms.
Further proof is afforded by plotting the absorption peaks on an
energy scale, as shown in Figure 69a. The spectrum obtained in this
study can be directly compared to reflection spectrum of a bulk single
crystal CdS shown in Figure 69b. A comparison of the peak energies is


20
Figure 1
Schematic diagram of a Fabry-Perot interferometer showing
how interference of the forward and reverse beams changes
the output intensity.^


RELATIVE INTENSITY
33
WAVELENGTH IN ANGSTROMS
Low resolution photoluminescence spectrum of CdS single
crystal showing "green-edge" emission due to band edge
recombinations and "blue-edge" emission due to excitons.
Figure shows emissions are dependent upon the polarization
of the excitation source.^
Figure 6


46
a function of sputtering power, pressure, substrate temperature and
substrate bias. Thin films were deposited onto fused silica
substrates. The first effect they describe is the dependence of the
deposition rate and resistivity on the pressure of gas used. A maximum
in the deposition rate and a minimum in resistivity occurred for a
sputtering pressure of 5 pin (5 X 10"^ torr). For higher pressures they
found the rate actually decreased, and the resistivity increased. The
decrease in deposition rate at higher pressures was explained by a
backscattering mechanism which increases the probability that more
sputtered atoms will return to the target at higher pressures. The
minimum in resistivity at 5 pm pressure was claimed to be a result of
the maximum deposition rate that occurred at this pressure. Although
no direct proof was given, the authors claim the high deposition rate
decreased the number or concentration of impurities that were trapped
in the film.
The second effect described in this early paper by Martil et al.^
was the dependence of thin film grain size on the substrate temperature
and substrate bias. The grain size was determined by SEM observations.
As would be expected, the grain size of films increased from 300 to
3500 as the substrate temperature was increased from 90 C to 300 C.
The deposition rate accordingly decreased as the temperature was
increased. When the substrate bias was increased to above -100 volts,
the resulting thin films were found to be amorphous. The temperature
effects can be explained by considering that with increasing
temperature 1) the critical size for a nucleus increases, 2) the
surface diffusion coefficient increases and 3) the sticking coefficient


Finally, I would like to express my graditude to the Air Force
Office of Scientific Research (AFOSR 84-0395) for funding this research
project.


141
Figure 44 X-ray photoelectron spectroscopy (XPS) high resolution scan
of sulfur 2p peak, exhibited by an as-deposited (ASDP) and
a furnace heat treated (FHT) thin film. Heat treated thin
film peak shifted slightly to higher binding energy.
Shake-up peaks are indicated by SU and satellite peak is
marked SAT.


138
film, which would lead to an incorrect correlation factor and incorrect
estimation of the composition of thin films.
The reproducibility obtained by XRF could not be accomplished by
energy dispersive spectroscopy (EDS) obtained during SEM observations.
This was primarily due to the inability to measure the specimen current
of the electron beam used to generate secondary electrons for imaging
and X-rays for composition analysis. Widely different intensity values
were often obtained from the same sample when measured at different
times. This problem was alleviated to some degree by mounting a piece
of target material on each sample mount so that the intensities of the
X-ray lines obtained from thin films could be normalized to a standard.
A hole was drilled in the sample mounts so that the surface of the
target material would be level with the substrates and geometric
differences would be reduced. Again, a correlation factor was used to
calculate the composition of thin films based on the intensities of the
X-ray lines measured from the target material. The matrix effect that
might occur in XRE would be less pronounced in EDS, because the
electron penetration depth that generates X-rays is much less than the
X-ray line that produces the secondary fluorescence in XRE.
The absolute values of the composition obtained by Auger electron
spectroscopy (AES) are considered to be reasonable since the accuracy
of the technique is limited to +/-30% when published values of the
sensitivity factors are used.-^ However, since very similar values for
the composition of the target material were obtained, a very simple
correction factor can be applied to the sensitivity factors to give the
correct results. The corrected concentrations are given in parenthesis


82
used in an iterative calculation to determine the values of A, B, x and
n.
The problems associated with this technique involve precise
placement of the sample at the focal point of the beam and accurate
rotation of the sample about the axis which passes through the focal
point. These problems were addressed by using a micrometer adjustable
x-positioner and a micro-rotation stage, which are shown placed in the
Lambda 9 spectrophotometer in Figure 18. Normal incidence to the film
was determined by rotation of the stage a few degrees on either side of
the initial zero and observing the shift in position of a given
interference fringe. The true zero point was taken as the position
which produced the midpoint between the two shifts. The focal point of
the beam was then determined by observing the peak value of a given
fringe as the sample holder was moved with the positioner.
The most difficult part of this technique (except for writing the
iteration program for the analysis) was the manual determination of the
value of the tangency points from the FECO spectra. The wavelength of
the tangent point must be known to a precision of 0.1 nm to obtain an
accuracy of three decimal places in the refractive index. Wavelengths
can be measured with the spectrometer to this precision, but this
precision could not be obtained by manual measurement of the tangent
point from the printout of the spectra.
The problem was alleviated by interfacing the Lambda 9 to an IBM
AT computer. Use of the computer greatly facilitated all aspects of
using the spectrometer as spectra could be stored on disk and


Counts xlO'
209
Figure 83 Temperature dependence of resonant Raman peaks.


162
b)
Figure 60 TEM micrograph of C0SAD2 thin film, a) after heat
treatment to 650 C for 30 min, showing large unidentified
crystals; b) SAD pattern showing both single crystal
pattern from large crystals and diffuse pattern from thin
film.


62
GROUND
Figure 13 Cross-sectional view of RF magnetron sputter gun made by US
Guns Inc.46


118
a)
b)
Figure 34 SEM micrographs of heat treated thin films. a) heat
treated 650 for 4 min; b) heat treated 500 C for five
hours.


71
annealing COSAD films high purity compressed air was used with the same
furnace setup. Flow rates of 100 cc/m were typically used.
Temperature control was accomplished with a single setpoint temperature
controller (Love Control Co.) which provided temperature control to +/-
Io C. Two thermocouples were used, one mounted outside the tube for
control and one positioned inside the tube adjacent to the sample.
Heating rates of up to 35 C/min could be realized, and cooling rates
of 100 C/min could be obtained by sliding the muffle tube out of the
furnace hot zone. Temperatures for heat treating thin films ranged
from 200 C to 700 C, with times ranging from 1 minute up to 5 hours.
To reduce the amount of grain growth that would occur as a result
of high temperature heat treatments, some films were heated by a
technique known as rapid thermal annealing. This process utilizes high
power quartz lamps to rapidly heat the sample. Because of the low
thermal mass that is achieved with this technique, heating rates in
excess of 500 C/sec are possible. The sample is sandwiched between
two thin sheets of graphite which act as black body absorbers and help
to transfer the heat generated by the lamps to the sample. In this
particular system lamps with a total power of 5000 watts were used.
Heat treatments up to 650 C with times ranging from 10 to 60 seconds
were done under a flowing nitrogen atmosphere.
Thin Film Characterization
Microstructure
Determination of thin film microstructure was achieved using
several analytical techniques. Direct examination of the


CHAPTER I
INTRODUCTION
Optical Signal Processing
With the very intense development of optical communications in
recent years, the need for integrated optical systems for the
processing of optical signals has greatly increased. To take full
advantage of the speed and information bandwidth available at optical
frequencies, an all optical system is desired. All of today's systems
operate by the high bandwidth transmission of optical signals in
optical fibers, followed by conversion to lower bandwidth electrical
signals before any processing, such as amplification and multiplexing,
can take place. The limiting drift velocity of charge carriers in a
semiconductor and the capacitive coupling between adjacent elements
present the fundamental limit for processing speed in these systems.
Also, the serial nature by which the electronic data must be
manipulated presents another speed barrier. An alternate approach
would utilize optical bistability and optical switching demonstrated in
certain materials for all-optical modulation, detection and
multiplexing. Optical switching is achievable with materials which
display a third order nonlinear susceptibility. A third order
susceptibility leads to an intensity dependent index of refraction or
absorption. This nonlinear refractive index can exhibit onset and
decay on a very fast time scale, which makes it an attractive
1


Figure 32 SEM micrograph of thin film deposited at LN2 temperatures


Figure 14
Schematic diagram of RC decoupling circuit used protect
digital equipment from RF transients.^


ABSORBANCE
169
Figure 63 Room temperature UV-visible absorbance spectrum of films
deposited at LN2 temperature (LN2), room temperature (RT),
and 300 C (300C).


155
100 nm
Figure 54 TEM micrograph of C0SAD1 thin film, after heat treatment to
500 C for one hour.


90
focal length
f-stop = (A.5)
grating size
where grating size is the linear dimension of the grating. With a 55
mm grating, the f-stop of the monochromater is f6. The reciprocal
linear dispersion (RLD) of a monochromater is a measure of its ability
to disperse light, and can be calculated by
RLD = X (4.6)
f d0
where d0/dL is the angular dispersion (in rad/mm) and is approximately
equal to the number of grating lines per mm. The RLD for this
monochrometer was calculated to be 1.25 nm/mm. The bandpass or
resolution of a monochromater is usually calculated by multiplying this
number by the width of the slit used. With a linear diode array
detection system, however, the resolution is determined by the spacial
resolution of the array. Assuming that three diodes can resolve a
peak, which correspond to 75 pm, the resolution of this system was 75
pm X 1.25 nm/mm or 0.9 nm.
A schematic of the experimental setup is shown in Figure 19 and
photographs of the system are shown in Figure 20a and Figure 20b. An
f2 lens was used to focus emitted light into the spectrometer and a 100
mm focal length lens was used to focus the laser beam on the sample.
Typically a focused spot size of 50 urn was used, so with an incident
power of 1 mW, the focused power density corresponds to 400 W/cm^. As


29
which involved sulfur atmosphere heat treatments and electron
bombardments, showed that sulfur vacancies were the predominant native
defect and that they acted as the recombination center responsible for
the green edge emission associated with CdS luminescence.^ Other
studies which investigated impurity doping effects are described in a
following section on photoluminescence.
A detailed knowledge of the band gap structure of CdS has come
from the extensive study of exciton states. Cadmium sulfide is found
to be a direct gap semiconductor with a band gap equal to 2.59 eV at 0
K. The wurtzite lattice of the material is described by a p-like
valance band consisting of two gamma-7 states and one gamraa-9 state,
and a s-like conduction band made up of one gamma-9 state. A diagram
of the band extrema is shown in Figure 4. The three states in the
valance band are also known as the A, B, and C free exciton states.
These intrinsic exciton states are modeled as Wannier excitons; i.e.
the electron and hole behave like a hydrogen atom. The orbital
movements of the electron and hole are determined by their effective
masses within the band extreme. As shown in Figure 5, the solution for
the wave function of this model results in a series of discrete
parabolic bands below Eg which merge into a continuum at higher
energies.^ Because of the unique band structure of this material
there are a large number of possible exciton states. Any of the
exciton energy levels (i.e. n=l,2,3, etc.) can be associated with the
three primary states (A,B and C) in the valance band. In addition, any
one of these free excitons can be associated with an impurity center,


238
43. J. Warnock and D.D. Awschalom, "Quantum Size Effects in Simple
Colored Glasses," Phys. Rev. B, 32, 5529 (1985).
44. J. Warnock and D.D. Awschalom, "Picosecond Studies of Electron
Confinement in Simple Colored Glasses," Appl. Phys. Lett., 48, 425
(1986).
45. N.F. Borrelli, D.W. Hall, H.J. Holland, and D.W. Smith, "Quantum
Confinement Effects in Semiconductor Microcrystallites in Glass,"
to be published in J. Appl. Phys.
46. US Guns, Cambell, CA.
47. J.A. Thornton and A.S. Penfold, "Cylindrical Magnetron Sputtering,"
Thin Film Prosesses, ed. J.L. Vossen and W. Kern, (Academic Press,
New York, 1978).
48. J.L. Vossen and J.J. Cuomo, "Physical Methods of Film Deposition,"
Thin film Processes, ed. J.L. Vossen and W. Kern, (Academic Press,
New York, 1978). p.38
49. E.M. Clausen Jr., "The Gamma to Alpha Transformation in Thin Film
Alumina," Master's Thesis, University of Florida, (1985).
50. P.C. Baker, "Trace Element Analysis of Limestone by Secondary
Fluorescence X-ray Spectroscopy," Master's Thesis, University of
Florida, (1981).
51. J.I. Goldstein, ed. Scanning Electron Microscopy and X-ray
Microanalysis, (Plemun Press, New York, 1981).
52. R. Swaneopeol, "Determining Refractive Index and Thickness of Thin
Films From Wavelength Measurements Only," J. Opt. Soc. Amer. A, 2,
1339 (1985).
53. E. Hecht and A. Zajac, Optics, (Addison-Wesley Publishing. Co.
Reading, MA, 1979), p. 306.
54. T.S. Moss, Optical Properties of Semiconductors, (Butterworths
Scientific Publishing. London, 1959), p. 34.
55. J.E. Cline and S. Schwartz, "Determination of the Thickness of
Aluminum on Silicon by X-ray Fluorescence," J. Electrochem. Soc.,
144, 605 (1967).
56. L.I. Maissel and R. Gland, Handbook of Thin Film Technology,
(McGraw-Hill, New York, 1970), chap. 15.
57. A. Joshi, "Auger Electron Spectroscopy," Metals Handbook Volumn 10:
Materials Characterization, 9th ed, (American Society for Metals,
Cleveland, 1986), p. 549.


167
Raman scattering were utilized at both room temperature and low
temperatures to fully characterize the optical properties of thin
films.
Absorption Spectra
UV-VIS absorption was used in this study to establish the
relationship between thin film processing conditions and their effect
on the band structure of the material. The main emphasis of this study
as stated before, was to produce a thin film with the same optical
properties as those of bulk single crystal material. From the
measurement of absorption spectra, it was possible to determine the
presence of band defects such as band tailing, to determine the
temperature dependence of the band gap energy and at low temperatures,
to observe exciton transitions.
By examining the shape of the absorption edge, a qualitative
measure of the band gap structure could be determined. At energies
below the absorption edge, the sharpness of the transition from low
absorption to the edge gives an indication of the amount of band
tailing. Only transitions which conserve momentum are allowed,
therefore, for a direct band gap semiconductor, only k equals zero
excitations occur without phonon scattering. When a great many
crystallographic defects are present in a material, then the band
structure will not be well defined. The presence of defects permits
transitions at other values of k to be quantum mechanically possible,
which in the absorption spectra is manifested as a tailing of the
absorption edge at low energies.


136
produce a reasonable result. It could not be determined if there was a
problem with the software or the instrument. Even if EPMA results
could have been obtained it would have been impractical to analyze
every film produced and every heat treatment studied.
X-ray fluorescence was originally considered to be the best
technique for the general analysis of thin film composition for several
reasons: l)the intensities of the X-ray fluorescence lines should be
directly proportional to the amount of material present in the thin
film, 2)high peak intensities gave a large signal to noise ratio, 3)no
sample preparation was necessary and analysis was quick (typically 15
minutes), 4)film thickness could be readily determined and 5)the
results were found to be very reproducible. Different films that were
deposited under exactly in the same conditions and heat treated exactly
the same way were found to give nearly the same peak intensities for
the cadmium and sulfur lines. An example of this reproducibility is
shown in Figure 42 which shows peak intensity and integrated peak
intensity ratios for several identical samples. A problem with the
technique occurs if a linear correlation factor is used to calibrate
the XRF spectra obtained from thin films: the amount of sulfur appears
to be too high, when compared to the other techniques. The correlation
factor is based on the ratio of the intensities of the cadmium and
sulfur lines obtained from the target material, assuming a
stoichiometric composition for the target. One possible explanation
for the high value is that a matrix effect that alters the ratio of the
X-ray lines measured for the target material may not occur in a thin


39
CdS Thin Films
Vacuum Deposition
A great number of studies have investigated the thermal
evaporation of CdS for the deposition of thin films. Although all thin
films for this study were deposited by RF-magnetron sputtering, some of
the results of these investigations are relevant to the present work.
Many of the studies of thermal evaporation were undertaken to research
the electrical properties of CdS thin films, although some optical
properties have been studied. The problem is that most of these
studies present results which both show differences from single crystal
results and also differ from each other.^ The reasons for the
discrepancies are related to the difficulties in evaporating CdS, which
lead to problems with maintaining stoichiometry and crystal structure
in the deposited films. The difficulty in evaporating CdS, as well as
other chalcogenide compounds, is that complete dissociation of the
compound occurs during evaporation.^^ If too high a temperature is
used, then the compound will dissociate incongruently and because Cd
has a higher volatility than S, non-stoichiometric thin films result.
Source temperatures between 650 and 700 must be accurately controlled
to avoid excess cadmium.^^ Due to differences in the sticking
coefficients of Cd and S, a non-stoichiometric thin film will also
result if the an improper substrate temperature is used. Cook and
Christy^' have found that if fused quartz substrates are used at
temperatures greater than 200 C, then non-uniform films result. In
comparison, Wohlgemuth et al.^9 have found that if the substrate is


87
however, is exponentially proportional to the absorption coefficient,
which is why absorbance spectra approximate absorption spectra. The
two terms will therefore be used interchangeably when describing
spectra.
The band gap energy was taken as the point where the slope of the
absorption curve was a maximum.One unique feature of the Lambda 9
was that it allowed anywhere from the first to fourth derivative of a
spectra to be taken. The maximum of the first derivative of the
absorption edge was taken as the band gap energy. A calculation to
verify these results will be described in Chapter V. Basically, it
evolves solving equation A.2 for a using the absorbance spectra, and
then plotting verses energy. A direct band gap material will give a
n
straight line for this plot, and the extrapolation of the line to on =
0 gives the optical band gap of the material.^
The temperature dependence of the band gap energy was measured by
cooling the sample with the cold finger. The sample was mounted in a
ring sample holder which would then be attached to the end of the cold
finger. A radiation shield was then positioned around the cold finger
and a vacuum insulation collar with fused quartz windows was then slid
over the entire assembly. A vacuum of less than 1 micron pressure was
then obtained in a few minutes by pumping the system with a Balzer's 70
1/sec turbomolecular pump. Once this vacuum was obtained in the
collar, cooling of the sample to 9 K could be accomplished in less
than 1 hour, although most measurements were made after 2 hours of
cooling. Any intermediate temperature between 9 K and 250 K could
achieved with the digital temperature controller and heater assembly.


.ounts x10
197
nm
Figure 76 Same spectra as Figure 75, relative peak positions.


188
and the coefficients obtained from FECO spectra of several films. The
refractive index was calculated at a wavelength of 632 nm for
comparison to the value of refractive index obtained by ellipsometry,
listed in the last column of the Table. Table 7 shows that the FECO
method for determining refractive is only valid at long wavelengths.
Ellipsometry measurements. By using the graphical method
described in Appendix B to determine the refractive index from the
polarization angles measured with the ellipsometer, very accurate
results could be obtained. These results are useful for quantitative
comparison of different films, unfortunately this comparison could only
be made at one wavelength; i.e. 632 nm, the wavelength of the HeNe
laser used with the ellipsometer. Table 7 shows there is a significant
change in refractive index when a thin film is heat treated. This
should be expected because the density of the film changes
considerably, particulary from high temperature heat treatments. The
results show that ellipsometry is sensitive to the slight differences
in microstructure of thin films deposited at different rates. In
addition to these advantages, the technique is also very useful for
refractive index and thickness determinations on very thin films, which
cannot be measured with the FECO technique.
Absorption coefficient. A very close approximation to the
wavelength dependence of the absorption coefficient can be made by
taking the absorbance spectra and applying a few simple relationships.
First, the extended equation for the transmission of light through a
thin film supported on a substrate (equation 4.2, Chapter III) which


57
thermocouple was embedded in the core of the holder, and control of the
temperature was made by controlling the input power to the holder with
a variable autotransformer. Substrates could be heated to 300 C with
this arrangement with a temperature control of +/- 5 C.
A single planar magnetron sputter gun supplied by US Guns
(Campbell,CA) was used for sputter depositing the CdS thin films. The
gun accepts two inch diameter targets, which can range from l/16th to a
quarter of an inch in thickness. An Eratron RF power supply
(Campbell,CA) operating at 13.57 MHz with a power capability of 600
watts was used to supply RF power to the gun, through an auto load
match tuning network. The matching network is require to balance the
impedance of the gun and plasma to the 50 ohm output from the power
supply. If the impedance is not matched, then the RF power is
reflected, and the only thing that is accomplished is heating of the
heat sinks in the power supply. The utility of an auto load network is
that when conditions of the plasma change, the network will
automatically match the impedance, thereby eliminating any reflected RF
power.
High purity cadmium sulfide sputtering targets were obtained from
CVD Industries (Woburn, MA). These targets are made by a chemical
vapor deposition process so they are supplied with a bulk density of
nearly 98% of theoretical density. High bulk density reduces the
amount of outgassing during sputtering. All impurities were less than
1 ppm and the stoichiometry of the target material was slightly cadmium
rich (50.46%). Nearly all thin films for this study were made from CVD
targets; however, a different type of sputtering target was obtained


123
cracking seen in the furnace heat treated films is a result of the
large and rapid grain growth that occurs.
X-ray diffraction experiments
Diffraction spectra. Although SEM observations gave an indication
of microstructure morphology of thicker films, the technique did not
provide any crystal structure information. To determine the
crystallography of thicker films X-ray diffraction (XRD) was used. The
computer controlled diffractometer used for these measurements greatly
facilitated the determination of microstructure changes with heat
treatment, as well as permitting quantitative analysis of X-ray
spectra. Programs for calculating diffraction peak line broadening due
to strain and size effects and for calculating precision lattice
constants were available with this system.
The diffraction spectra obtained from films 8000 thick,
deposited at three different temperatures are displayed in Figure 38
and the diffraction spectra obtained after heat treating these films
are shown in Figure 39. Many of the features observed in SAD patterns
are evident in these X-ray diffraction patterns. The broad band peak
centered at ~ 20 in all patterns is due to amorphous scattering from
the substrate.
In Figure 38a, of a film deposited at LN2 temperatures, the
amorphous peak is much more intense and the intensity of the (0002)
peak is considerably reduced. This supports the results obtained from
TEM analysis that the structure of these films is highly disordered,
but there is some indication of crystallinity. After heat treatment,


74
technique know as X-ray secondary fluorescence was used instead to
measure the stoichiometry of nearly every thin film that was made. The
particular instrument used for these measurements utilized a silver X-
ray tube to produce the primary X-ray beam. Tube voltages of 50 kV
were used with a current of 30 mA. One of four different secondary
targets could be selected to generate the X-rays that were used to
analyze the sample. The advantage of this technique is that the
background radiation from the X-ray tube (bremsstrahlung) is totally
removed and only the very narrow X-ray line corresponding to the
secondary target reaches the sample which results in a great reduction
of the background counts. The particular geometry of this system
limits the radiation to only one polarization which further reduces the
background count.^ Detection levels for most transition metals was ~
0.1 ppm. Beam size incident on the sample is 1 cm in diameter and
penetration depth is only a few microns which makes this technique
ideally suited for measuring thin films supported by substrates. The
X-rays emitted from the sample are analyzed by EDS with this system;
however, due to the above arrangements, a much more quantitative
determination could be made when the results were compared to a
standard reference.
The standard used, in this case, to determine the stoichiometry of
thin films was a piece of target material. The stoichiometry of the
target was verified by a wet chemical technique and electron probe
microanalysis (EPMA) (JEOL 730 Superprobe). For solution analysis a
portion of the target was dissolved in nitric acid and then this
solution was diluted a given amount. The amount of Cd and S in


76
the unknown. The x-coordinate on the curve for the unknown F(I) would
give the thickness. Film thickness of up to 3 pm could accurately be
determined with this technique. Changes in density or loss of material
as a result of heat treatment could also be monitored with this
technique.
Several other analytical techniques were used to analyze the
composition of thin films. In each case the results for thin films
were compared to the results obtained from target material. The
different techniques were used to both acquire information, which the
specific technique is most suited for, and to cross-check the
composition of thin films. Auger electron spectroscopy (AES Perkin-
Elmer, Norwalk, CT) gave the least quantitative determination of the
absolute composition, but the technique was very quantitative for
determining differences between samples. X-ray photoelectron
spectroscopy (XPS Kratos,UK) gave semi-quantitative results and also
showed how the surface of thin films were altered by heat treatment, a
result which was not detected with any other technique.
Optical Measurements
By far the most extensive measurements made on thin films were
optical measurements. The availability of several spectrometers,
monochromaters, lasers, and optical hardware greatly facilitated these
measurements. The UV-VIS spectrophotometer used for many measurements
was a Perkin-Elmer Lambda 9 (Norwalk, CT). Photoluminescence was
measured with both a Princeton Applied Research Optical Multichannel
Analyzer (Princeton, NJ), and an Instruments SA Raman spectrometer


Figure 24 Selected area diffraction pattern of thin film. a) actual
pattern; b) indexed schematic pattern.


vlC1
202
Wavelangth (na)
Figure 79 Composite low temperature OMA spectrum showing relative
intensities of band edge and exciton emissions.


106
to right. The film is supported by a carbon film. At the far left an
individual island structure is observed. The high magnification
micrograph shown in Figure 27b indicates that the islands are mostly
amorphous, with very small microcrystallites imbedded in the amorphous
matrix. The microcrystallites are characterized by the darker contrast
of approximately 80 in size within the globular structures. The very
fine mottled structure is the background amorphous structure of the
carbon support film. The SAD pattern included in the Figure shows that
only the nearest neighbor spacing is present, and the crystal structure
is highly distorted. Referring back to Figure 27a, as the film
thickness increases to the right, the islands grow to form a columnar
structure. The columns become fairly thick before they coalesce to
form a low density continuous films, indicated at the far left in the
Figure.
High temperature depositions. Other attempts to alter the
microstructure of thin films were made by deposition onto heated
substrates. Both room temperature and LN2 deposited films reported
above display large numbers of crystallographic defects, which as
described in a later section, degrade the optical properties. By
depositing films onto substrates held at 200 C, microstructures with
fewer defects could be produced. A higher deposition temperature
should lead to fewer defects because the deposited atoms have more
energy to diffuse to more favorable positions. Temperatures above 200
C, however, produced films which were very strongly bonded to the
substrate and hence could not be removed for examination. A
representative thin film deposited at 200 C which was "floated-off" a


77
(Metuchen, NJ). A variety of lasers were used for different
experiments, although most photoluminescence experiments were made with
a Spectra-Physics model 2025 Argon Ion laser (Mountain View, CA). Many
measurements were made at both room temperature and at temperatures
down to 9 K. Low temperatures were obtained with an Air Products
(Allentown, PA) closed-cycle helium cold finger and digital temperature
controller. Experiments carried out with these instruments will be
extensively detailed in the following sections.
Index of Refraction
A very unique technique for measuring the dispersion or index of
refraction as a function of wavelength was used on thin films.
Normally the index of refraction of a material is related to the
percent of transmitted light by the well known equation:
T (1 R)2 exp (-at)
1 R^ exp (-2at)
where the reflection, R, is given by (n l)^/(n +1)^. The situation
is much more complicated, however, for a thin film supported on a
substrate with a different refractive index. The reflection that
occurs at the film/substrate interface must be dealt with as well as
the reflections that occur at the other interfaces. This leads to a
more complicated expression:
T
(1 R].) (1 R2) (1 ~ R3) exp (-at)
(1 R2R3) ( 1 [ R^R2 + R^3 (1 R2)Z ] exp (-2at) }
(4.2)


150
Figure 50 Digitized image from scanning electron microscope,
secondary electron image.


151
Figure 51 Digitized element maps. a) high magnification cadmium
X-ray map; b) high magnification sulfur X-ray map.


23
Figure 2a displays how the relative transmitted intensity and the
sharpness of the transition is related to the reflectivities and
finesse of the cavity. As the reflectivity of the mirrors is
decreased, the finesse is decreased and the transition at the critical
phase shift broadens. When the reflectivities and the finesse of the
cavity are large, then the transitions are sharp. For these
conditions, the transmitted image from a diffuse source through the
Fabry-Perot will appear as a series of sharp concentric rings, as shown
in Figure 2b.
For a nonlinear Fabry-Perot, however, the Airy function must be
slightly modified to account for the nonlinear index by
A(0)
NL
(2.21)
1 + F sin ( x I
eff
6)
where x is a constant describing the nonlinear refraction,^ an
is the effective mean intensity within the cavity. The total Fabry-
Perot fractional transmission can be written as^
( 1 R )2 ( 1 A )
(l-R(l-A))2
a(0)nl
(2.22)


S/CD RATIO
137
, FILM NUMBER
¡ZZ] PEAK INTENSITY rCsl INTEGRATED AREA
Figure 42 X-ray fluorescence sulfur to cadmium peak ratios for
several identical thin films.


139
in Table 4, and they show comparable results to the other techniques.
Typical AES spectra obtained from target material and heat treated thin
films are shown in Figure 43a and 43b. The shape of the peaks are
qualitatively the same, although the heat treated spectrum has been
shifted to higher energy due to a charging effect. The difference
between the ratios of the Cd peak-to-peak distance to the sulfur peak-
to-peak distance gives a quantitative indication of the difference in
the concentration of cadmium in the two samples. AES should be very
quantitative for measuring the differences between similar samples, so
the values obtained most likely represent the true difference, although
this technique does not indicate how the sulfur exists in the
structure.
To attempt to determine the chemical state of the sulfur, X-ray
photoelectron spectroscopy (XPS) was used. This technique is very
sensitive to changes in the binding energy of core electrons, which
reflects changes in the local chemical environment.^ The presence of
significant concentrations of free sulfur, would either shift the
sulfur lines, split, or broaden them. A high resolution scan of the
sulfur 2p transition for an as-deposited (ASDP) and a furnace heat
treated (FHT) thin film are shown in Figure 44. Both films were
deposited at room temperature and at a rate of 1 /sec. Very little
shift is observed and no significant broadening is seen. The position
of the 2p transition for free sulfur would be at a binding energy of
165 eV. The only unique feature is the two shake-up peaks, labeled SU
and both of these show very little shift in energy. The satellite peak
labeled SAT is an artifact peak and has no meaning.


49
The latest investigation to be reported by Martil et al.^
describes the effects of heat treatments on the electrical and optical
properties of sputtered films. Heat treatments were carried out under
H2 and N2 atmospheres at temperatures ranging from 100 C to 550 C and
for times ranging from 20 minutes to 5 hours. The primary effect on
the electrical properties was a two order of magnitude reduction in the
resistivity to 4 X 10" £2 cm for heat treatments at 200 C. Carrier
mobilities accordingly increased to 50 70 cm^/Vs for this treatment.
Higher temperatures were found to increase the resistivity and decrease
the mobility. The decrease in resistivity was explained by claiming
that oxygen desorption from the grain boundaries occurs at 200 C. As
previously described, oxygen is a trapping center which in this case
decreased the carrier concentration and simultaneously increased the
scattering which reduces the mobility.^ No explanation was given for
the increase in resistivity with increasing temperature, although it
could be due to a loss of cadmium.
The optical absorption edge was shown to become sharper and occur
at a higher energy for heat treatments at 200 C. Temperatures higher
than this were not reported to significantly alter the absorption edge;
however, treatments above 550 C produced an overall decrease in the
transmission. This was explained by a dissociation or reevaporation
process that occurred as a consequence of the higher temperatures. The
optical band gap was found to increase from 2.36 eV to 2.39 eV for
treatments at 200 C, which was again explained in terms of oxygen
desorption from the grain boundaries, although no chemical analysis was
reported to prove this hypothesis.


OPTICAL PROCESSES IN CADMIUM SULFIDE
THIN FILMS
By
EDWARD M. CLAUSEN, JR.
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA


225
concentration on the band structure. Heat treatments in equilibrium
sulfur or cadmium atmospheres may produce stoichiometric thin films.
It would be interesting to know if the electrical conductivity and
photoluminescence change when the excess sulfur is removed. Other post
deposition treatments which should be further investigated are heat
treatments by RTA, to determine if very small, defect free crystals
could be produced in a thin film.
A large research effort should be directed towards further
investigations of COSAD thin films, since these films appear quite
promising for quantum confinement devices. Studies of the deposition
conditions and heat treatment schedules on the microstructure
development in these films could provide both a parallel confirmation
of the effects observed in bulk filter glasses and permit a further
understanding of the confinement processes that take place in these
materials. Photoluminescence studies of thicker films would positively
identify a confinement process, by correlation of the blue shift with
particle size. If well characterize COSAD thin films could be
produced, nonlinear optical measurements are easily made because these
films on the proper substrate should act as low loss planar waveguides.
When prisms are used to couple light into and out of thin film
waveguide, the angle at which the light couples is dependent upon the
refractive index of the film. The nonlinearity of these films could be
determined by measuring a power dependent coupling angle. If these
experiments proved successful, then the next step would be either to
attempt logic gate experiments by utilizing small scale integration, or


CHAPTER III
BACKGROUND BIBLIOGRAPHICAL REVIEW
Bulk Properties of Cadmium sulfide
Nearly all of the early work on cadmium sulfide was carried out on
single crystal platelets which were made by a chemical vapor phase
growth process. The natural crystal structure of cadmium sulfide is
hexagonal wurtzite, although single crystals of the cubic zincblende
structure have been fabricated. Within either of the two crystal
structures it is possible to have regions which are made up of the
alternate crystal structure. The transition from a hexagonal to a
cubic lattice or visa versa can occur through a well known twinning
i n
mechanism,1^- in which the twinned region is bound by stacking faults.
The twinned regions can be manifested during deformation of the crystal
or under particular growth conditions, although it is difficult to
differentiate these two sources when crystals are grown from the vapor
phase. In either case, the two crystal structures do not have a center
of symmetry or inversion, which leads to the unique properties of
noncentrosymmetric crystals such as piezoelectricity, pyroelectricity,
and third order optical susceptibility.
Another important physical property of CdS is the stoichiometry of
the crystal. Very little work has been done on the defect chemistry of
CdS; however, the work done on other II-VI semiconductors such as ZnS
and CdTe indicates that the range of nonstoichiometry at room
temperature is very small, e.g. 0.01 to 0.1 %.13 Early work by Collins
28


Counts xlO
203
Wavelength (nm)
Figure 80 Low temperature OMA spectra of thin film and single crystal
platelet, showing relative peak positions. The bound 1^
and I2 excitons are labeled on the platelet
photoluminescence.


102
Figure 25 Thin film deposited at high deposition rate.
micrograph showing large number of defects; b)
corresponding SAD pattern.
a) TEM


51
during the heat treatments which develop the microcrystallites. It was
postulated by these authors that more of the selenium remains in glass,
while sulfur is more easily incorporated in the crystallites.^
Therefore, at the lower temperatures which produce the smaller
crystallites, the crystallites end up containing more sulfur, so the
optical properties of glasses containing these crystallites are blue
shifted toward the optical properties of pure CdS. True quantum
confinement effects were, however, shown to occur in a series of
experimental glasses which contained either CdS or CdSe, but not both.


170
Figure 64
Room temperature absorbance spectra of different thickness
films, deposited at 1 /sec onto room temperature
substrates.


I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
Stanley R. Bates,
Associate Engineer of
Materials Science and
Engineering
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
Timothy J. Anderson,
Professor of Chemical
Engineering
This dissertation was submitted to the Graduate Faculty of the
College of Engineering and to the Graduate School and was accepted as
partial fulfillment of the requirements for the degree of Doctor of
Philosophy.
December, 1987
Dean, College of Engineering
Dean, Graduate School


107
b)
Figure 28 Thin film deposited at high temperature, a) TEH micrograph
showing large defect free grains; b) corresponding SAD
pattern.


deposition rate:
69
Figure 15 Graph of sputtering rate versus input power density for CdS
and BK-7 targets.


174
is a useful technique for evaluating the results of different heat
treatments. Figure 66 shows the total absorbance curve and Figure 67
shows an expanded abscissa scale for heat treatments of 650 C 30 sec,
650 C 4 min, and 500 C 5 hours. Figure 66 shows a structural change
between RTA and the 500 C 5 hour heat treatment in the band tail and
all three curves show two sub-edge features, both as shoulders above
the sharp rise in absorption. Figure 67 indicates that furnace heat
treating to 650 C for 4 minutes results in nearly the same low energy
band structure as the heat treatment at 500 C for 5 hours. A rapid
thermal anneal (RTA) film displays the same curve shape at high values
of absorbance, but as indicated in Figure 67, there still is some band
tailing present.
Low temperature UV-VIS absorption
At sufficiently low temperatures, many features of the absorption
spectrum appear more clearly and can be related to the band structure
in these films. Spectra were be obtained at temperatures down to 9 K
by cooling a sample with the closed loop helium refrigerator. With a
decrease in temperature several changes in the absorbance curve are
observed. First the band edge shifts to higher energy, both due to a
change in the lattice parameter and to the temperature dependence of
the band energy. A plot of the band gap energy as a function of
temperature is displayed in Figure 71b.
Second, the amount of band tailing is shown to significantly
decrease in heat treated films; however, the decrease is not as
predominant in as-deposited thin films. The indirect transitions which


223
9. Shift of the absorption edge in COSAD films appears to be mostly a
chemical shift and not due to quantum confinement. Heat treatments to
the same temperatures and times, but in different atmospheres result in
different shifts of the band edge. Heat treatments in an oxidizing
atmosphere produced a larger shift in the band edge than heat
treatments in an inert atmosphere. This could be due to the formation
of cadmium oxide, which has a larger band gap energy than cadmium
sulfide. The large shift of the band edge, however, cannot be entirely
due to the formation of an oxide, as the edge observed in films treated
in inert atmospheres is still considerably shifted from the energy of
pure CdS. There may be other unknown chemical effects or some
confinement process occurring. Another possible reason for the shift
could be due to surface states, which would be enhanced by the large
difference in dielectric constant at the interface and by the large
stress created by the high curvature of the spherical particles.
10. The broad shoulder on the top of the absorption edge, which
appears similar to the room temperature exciton shoulder observed in
pure films of CdS is of unknown origin in COSAD films. The shoulder
does not change shape with decreases in temperature, although there
still could be considerable orientational broadening even at low
temperatures. Unfortunately no photoluminescence was observed in COSAD
films primarily due to a dilution effect, arising from the thickness of
the films and the relatively small amount to CdS. Photoluminescence
would have indicated the presence of excitons.


109
section. The TEM micrographs that were obtained of a heat treated film
are shown in Figures 29a and 29b, and the SAD pattern for this film is
shown in Figure 30. A film with an original thickness of 1000 was
deposited on a NaCl substrate and then heat treated to 650 C for 4
minutes. By dissolving the substrate the film was captured on a TEM
carrier grid. The micrographs in Figure 29 show the film to be very
porous with an interconnected grain network. This high porosity
results when a large amount of grain growth occurs in a very thin film
because there simply is not enough material to produce a continuous
structure. The grains within the network display an average size of
2000 and many dislocations, stacking faults and bend contours are
indicated by the variation in contrast observed within grains. These
defects might be intrinsic or they may have occurred when the film was
"floated-off" the substrate. It is interesting to note that very few
if any microtwins can be observed in this structure. This result
indicates further that these defects are growth related. The SAD
pattern shown in Figure 30 displays a single crystal pattern with a
basal plane zone axis, which means that the preferred orientation
observed in as-deposited films is maintained after heat treatment,
although this orientation may have been enhanced somewhat because the
film was heated on a crystalline substrate.
Scanning electron microscopy
As-deposited thin films. The microstructure of thin films was
determined by SEM for several reasons. As explained above, most heat
treated thin films could not be removed from their substrates for TEM


xlO
201
Figure 78 High resolution OMA spectrum of heat treated thin film at
9K, showing exciton photoluminescence.


35
The exciton states were first extensively studied and
characterized by Thomas and Hopfield.19 They have shown that within
the higher energy band a number of sharp luminescence lines occur which
correspond to transitions of both free excitons (A, B, and C) and
excitons bound to neutral donors or acceptors. A designation of 1^ was
given to excitons which are bound to neutral acceptors and I2 was given
to the excitons bound to neutral donors. These two peaks are labeled
in Figure 7. The distinctions between the various transitions were
made by using the Zeeman effect. When a strong magnetic field is
imposed on the sample, many of the luminescent lines split due to the
spin moments of the ground and excited states. From the group theory
of bound complexes,^ they were able to assign the transitions to the
different defect states.
A later study by Henry, Faulkner, and Nassau showed for the first
time donor-acceptor pair lines in the photoluminescence spectra of
CdS.^O These pair lines are narrowly spaced transitions which were
observed in the green-edge emission band and correspond to closely
spaced donor-acceptor pair-recombination bands. Again the confirmation
of these lines was made by Zeeman experiments. The significance of the
study is that for the first time direct spectroscopic evidence for the
existence of these states was made. This is important point because in
the study by Thomas and Hopfield it was assumed that these states must
exist based on the Zeeman experiments, but they had no direct evidence.
Henry, Nassua, and Shiever studied the impurity doping of CdS and
showed that Na and Li are the only shallow acceptors that can act as
substitutional impurities.^ These shallow acceptors give rise to the


FLUORESCENCE
34
WAVELENGTH Cnm)
Figure 7 High resolution photoluminescence spectrum of CdS single
crystal at 4.2 K, showing bound exciton peaks for
excitation perpendicular to the c-axis. Peaks labeled P
are observed with parallel excitation.^


192
they are not subject to the same selection rules. For direct band gap
semiconductors, radiative recombinations generally occur near k=0.
Just as all the different defect states in the band gap lead to
discrete lines, or bands, or tailing in absorption spectra, they can
also lead to photoluminescence spectra which are indicative of these
defects. As described earlier, the defect states that are of the
greatest interest to this study are the exciton states and
photoluminescence is an excellent technique for studying their
occurrence and behavior.
Use of the optical multichannel analyzer (OMA) and the Raman
spectrometers permitted a wide variation in the resolution and spectral
window over which the photoluminescence and Raman scattering of thin
films could be measured. For analysis of certain thin films which
displayed very low intensity, wide band photoluminescence, the OMA
could be used with a very low dispersion grating (152 lines/mm). While
giving very poor spectral resolution this arrangement still permitted
detection of very low light levels. To measure high intensity, narrow
bandwidth peaks, the low dispersion grating was replaced with a very
high dispersion 2400 lines/mm grating. As explained in the
Experimental Method chapter (Chapter IV) this grating permitted a
bandpass resolution of 0.9 nm, but a spectral window of only 20 nm. To
assure calibration and permit an even higher bandpass resolution, the
Raman spectrometer was used. By controlling the slit widths of the
monochromater, resolutions down to 0.05 nm could be obtained.


172
scattering losses. For the LN2 deposited film, a large amount of band
tailing is indicated by the higher absorbance below the band edge and
higher absorbance above the band edge indicates the presence of
uncompensated defects. The overall transmission of this film is
reduced because of the presence of excess cadmium, which is the reason
for the dark color of these films. Excess cadmium is also most likely
responsible for the high absorbance above the band gap. A close
relationship between the number of defects observed by TEM and the
amount of band tailing is observed in these films. Band tailing is an
indication of the structure of the band gap and the presence of band
gap defects, which appear to be directly dependent on the number of
crystallographic defects.
The band tailing-crystal defect relationship is further indicated
when the absorbance of films heat treated to different temperatures is
measured. The lower heat treatment temperatures at 200 and 350 C
shown in Figure 65 do not indicate much of a change in the amount of
band tailing; however, the heat treatment to 500 C has reduced nearly
all band tailing, which reveals a distinct absorption band below the
band edge. This is an interesting result, because this is the
temperature at which grain growth was shown to drastically increase.
Examination of Figure 65 also shows that the higher temperature heat
treatment results in a slight shift of the absorption edge to higher
energy. This might be related to the slight change in composition that
occurs at the higher heat treatment temperature.
Because there is such a strong relationship between band tailing
and crystal structure, examining the onset of absorption in thin films


219
experiments indicate the film has a very strong preferred orientation,
with the basal plane parallel to the substrate plane. It is not clear
if this orientation is nucleation or growth controlled. The close-
packed plane is the slow growth direction, which would indicate the
preferred orientation is controlled by nucleation. This same
orientation, however, was observed over a substrate temperature range
of nearly 500 K and with various substrate materials. It is unusual
that a nucleation controlled process would extend over such a large
temperature range, although with this material the nucleation range may
be very wide. A combined effect may actually control the process if
the crystallites can be modeled as thin disks. Once the initial
orientation is nucleated, only a small step in the z-direction could
cause a large increase in growth in the lateral direction. X-ray line
shifts indicate films deposited at high rates contain large residual
tensile stresses, which leads to their exfoliation when exposed to the
atmosphere. Optical absorption spectra indicate some band tailing to
be present in room temperature deposited films, which is related to the
crystallographic defects that are present. The band structure in these
films is defined well enough so that band edge photoluminescence is
exhibited; however, the large number of defects present in the band gap
leads to a decrease in the photoluminescence yield.
3. When CdS is deposited onto substrates held at high temperatures a
reduction in the number of crystallographic defects is observed. With
higher substrate temperatures, adsorbed atoms have more energy to move
around and find favorable positions with which to bond. The preferred
orientation is still maintained, however, other hexagonal orientations


75
solution was determined by using an induction coupled plasma (ICP)
spectrometer (Allied Analytical Systems, Waltham, MA). By comparing
these amounts to those in standard solutions the stoichiometry was
determined. For EPMA the stoichiometry of the target material was
determined by use of standards and a ZAF or (ppz program. The ratio
of the integrated peak intensities for Cd and S in the target was then
determined by X-ray fluorescence. The ratio obtained from thin films
was compared directly to this ratio to quantitatively determine the
stoichiometry.
Film thickness could be readily determined with this technique by
measuring the intensity of the silicon K-alpha peak that was emitted by
the substrate through the film. The intensity of this line is
exponentially attenuated by the thickness of material that it passes
through and can be related by the following equation:
I = IQ expat (2.1)
where IQ is the intensity from a bare substrate and t is the film
thickness. From equation 2.1 a linear relationship between a function
of the intensity, F(l), and t can be obtained by-^
F(I) = log10 1 (2.2)
*o
The intensity from three different films of known thickness (verified
by profilometry) were measured and by plotting F(l) versus t, a master
curve was generated. The thickness of an unknown film could then be
found by solving the above equation for F(l) using the intensity from


184
excitons peaks have broadened considerably. The disappearance of the A
exciton peak at 100 K is consistent with the calculated binding
energies of excitons, as the A exciton has a binding energy of 8 meV,^
which corresponds to 92 K. Above this temperature a sharp transition
should not be possible and we see that the peak broadens into a
shoulder. It is interesting to note, however, that even at 300 K, as
shown in Figure 68, an exciton-like shoulder is still present. Figure
71a also shows the temperature dependence of the band edge, and a plot
of this variation as a function of temperature is shown in Figure 71b.
In the Bibliographic Review chapter (Chapter III), the description
ft
of the exciton states in CdS characterized the states in terms of
crystallographic orientation and therefore their observation is
dependent on crystal orientation with respect to the polarization of
the probe beam. The fact that the grains in the films studied here
have a strong preferred orientation is one reason why all three
excitons are observed. In CdS single crystal platelets, all three
excitons are observed when the E-field is perpendicular to the c-axis,
but only the B and C excitons are observed with the E-field parallel to
the c-axis. Although a circularly polarized beam is used in the Lambda
9, the beam is correctly oriented at any one point on the film because
the orientation of the grains can be considered to be circular about
the c-axis pole. TEM and X-ray diffraction results indicate that many
grains are oriented with the basal plane of the unit cell parallel to
the film and substrate plane, so one direction vector of the unit cell
is perpendicular to the film plane. The other two direction vectors
which fully describe the unit cell and crystal orientation are randomly


195
associated with deep impurity level luminescence. The relative
intensities of these two bands could be altered with slight changes in
the geometry of the detection system. The width of these bands made it
difficult to center the grating so that the diode array was evenly
illuminated. This is one example of problems associated with a linear
diode array detection system. If the array is not evenly illuminated
the relative intensities displayed in the emission spectra will not be
proportional.
Films deposited at LN2 temperatures only displayed the longer
wavelength bands and at an even lower intensity level. The green edge
emission is not observed in these films because the band structure is
not well defined, due to the large number of crystallographic defects.
The long wavelength bands are observed because deep level traps do not
need a well established band gap to cause radiative transitions. They
can be likened to impurity color centers, which are associated with a
single impurity atom, or in this case with a vacancy or interstitial
atom associated with an impurity. The identity of this impurity is
unknown, but some authors have reasoned that it could be a halogen atom
such as chlorine.33,34
Heat treated thin films. A very significant change in the
photoluminescence of thin films occurs in films which are heat treated
at high temperatures, even for relatively short times. The intensity
of the green edge emission increases over 5x for the same excitation
intensity and the peak position shifts to a higher energy. The
relative intensity of an as-deposited thin film and a film heat treated
to 500 C for 30 minutes are shown in Figure 75 and the relative peak


Ill
Figure 30 Selected area diffraction pattern of thin film shown in
Figure 29.


22
a)
b)
Figure 2 Descriptions of the operation of a Fabry-Perot. a) plot of
the Airy function as a function of the phase shift 6,
reflectivities r, and the finesse F; b) transmitted image
from a high finesse Fabry-Perot.-^


133
TABLE 3
Concentrations in atomic percent of cadmium and sulfur
found in target material
Analytical
Technique
Cd concentration
S concentration
ICP
50.96 +/- 0.09
49.04 +/- 0.12
EPMA
50.33 +/- 0.21
49.67 +/- 0.21
XPS
51
49
AES
60.7
39.3


144
Figure 46 XPS survey scan of as-deposited thin film.


158
be seen, crystals of both size distribution are lit up. The dark field
image also shows that several of the large crystals are twinned,
indicated by the change in contrast within a crystal. Another
abnormality exhibited by the higher temperature heat treated films was
the additional appearance of large faceted shaped crystals, shown in
Figure 57. The crystallographic identity of these crystals could not
be determined; however, they were only found in certain areas of the
film, and they may correspond to some type of contamination.
COSAD films which were made by controlling the alternating
deposition process with a computer controlled stepping motor could be
produced with very low ratios of CdS in the glass. Alternating layers
of material could be produced by stopping the substrate over a source
for a given amount of time. This permitted the deposition of several
monolayers in each pass. Ratios which resulted in about 5% CdS
dispersed in the glass (designated C0SAD2) resulted in structures such
as the one displayed in Figure 58a. The extremely fine dark structure
in the micrograph is most likely due to CdS microcrystallites; however,
the accompanying SAD pattern (Figure 58b) indicates the structure to be
amorphous. When the electron beam is condensed on this type of film a
phase separation is shown to occur, but as exhibited in Figure 59a it
grows to a smaller size than shown in Figure 53a. The accompanying SAD
pattern in Figure 59b also indicates that the structure is still
amorphous.
When this type of film is heat treated to 650 C, two different
structures result. First, large semi-faceted shaped crystals are
exhibited as shown in Figure 60a. Although a single crystal


135
TABLE 4
Comparison of
used to
results from different analytical techniques
determine the composition of thin films
Analytical
As Deposited
Atomic %
Heat Treated
Atomic %
Technique
Cd
S
Cd
S
EDS
47.7
52.3
46.8
53.2
XRE
44.6
55.4
43.1
56.9
XPS
50.6
49.4
70.9
29.1
AES
37.3(47.6)
62.7(52.4)
35.7(46.0)
64.3(5'


101
reflections appear to be missing; however, upon close examination all
the spacings for hexagonal wurtzite can be found. The lines for these
spacings are too faint to show up in the SAD pattern reproduced here.
The orientation seen here is not dependent upon the substrate since
similar films were made on both amorphous and crystalline substrates.
These results are consistent with the early work on thermally
evaporated CdS.-^ jt is not clear why this specific orientation
results because the close packed plane (i.e. (0002) hexagonal plane) is
the slow growth plane. This means that the orientation is more closely
related to the nucleation rate of the thin film rather than the growth
rate, although this orientation was observed with all substrates
materials, deposition rates, and at all substrate temperatures.
The grain size seen here is smaller than the 500 size reported
by Martil et al.^-^. Grain size determinations by SEM of the films
studied here do show a different size (see SEM section below) but,
another reason for the difference can be attributed to differences in
the sputtering technique used to produce the films studied here. As
previously described, planar magnetron sputtering results in a lower
film and substrate heating due to electron bombardment, therefore a
smaller grain size should result. Higher deposition rates were found
to increase the substrate temperature. However, even with a deposition
rate of 5 /sec the substrate temperature was found to only increase to
35 C. Analysis of Figure 25 shows that no significant increase in
grain size has resulted from the increased deposition rate; however,
the micrograph does indicate that many more microstructural defects are
present. The higher supersaturation of the vapor results in additions


129
TABLE 2
Calculated strain and size effects which would produce the
X-ray line shifts and broadening observed in diffraction patterns
Sample
Effective
Nonuniform
Mean Strain^-
%
Effective
Mean Size^
A
Calculated
Uniform
Strain^
%
Size
Effects
Only^
A
ASDP 1 A/sec
0.189
1148.7
0.16
252.7
ASDP 5 A/sec
0.179
1983.6
0.39
284.3
FHT 650 C
0.041
2528.0
0.08
1005.2
RTA 650 C
0.122
837.8
0.12
332.6
Key: ASDP as-deposited; FHT furnace heat treated;
RTA rapid thermal anneal. Heat treated films were deposited at
1 A/sec
^Calculated with utility program available in APD computer. Mean
values are given which would combine to cause the observed broadening.
^Calculated from shift in X-ray line position.
JCalculated with utility program available in APD computer. Values of
grain size which would produce the observed broadening.


211
spectrometer and plotted in wavenumbers is displayed in Figure 84. The
same laser power was used to excite the sample, but as can be seen a
much lower intensity is measured. This indicates how much lower the
light gathering ability of the Raman is compared to the OMA. The
spectra however, also show that the resonant Raman effect is strong.
Laser powers for normal Raman spectroscopy typical are in the range of
a few watts. Even with this high power the Raman scattered peaks are
very low in intensity. The laser power used to excite the samples
measured in this study was only one milliwatt! The high intensity of
the 1L0 peak is due to a strong interaction with the exciton state.
This is further proof of the existence of exciton levels in the
polycrystalline thin films.
COSAD Optical Properties
The microstructure analysis described in the physical properties
section has indicated that it is possible to produce structures with
finely dispersed CdS crystallites in a glass matrix thin film. The
important question about these structures is whether quantum
confinement occurs, which would be indicated by a shift in the optical
absorption edge to higher energies. Unfortunately, a shift could also
be produced by a change in composition. Since C0SAD1 films were made
with a high CdS content, which once heat treated, leads to structures
too large for quantum confinement, it would be expected that any shift
in the band edge would be due to compositional effects. On the other
hand C0SAD2 films should exhibit a shift due to quantum confinement,
because of the small particle size displayed in these films.


185
oriented, but should be rotated about the c-axis direction vector. It
is therefore highly probable that a grain will have the proper
orientation to the circularly polarized beam and all three exciton
transitions should be measured.
Index of refraction
Fringes of equal chromatic order. Measurement of fringes of equal
chromatic order (FECO) was a very useful technique for determining the
dispersion of the refractive index at wavelengths longer than the
absorption edge. Unfortunately, equation 4.4 which was used to model
the dispersion could not be used to calculate the refractive index at
the absorption edge or at shorter wavelengths. This is because the
equation over-estimates the large change in absorption coefficient that
takes place near the band edge. As shown in Figure 9 (Chapter III) the
refractive index for bulk CdS is found to peak at the absorption edge
and then decrease at shorter wavelengths. The problem with equation
4.4 is that it indicates that the refractive index converges to
infinity at short wavelengths. This is shown graphically in Figure 72a
and 72b, which are plots of equation 4.4, using the coefficients
obtained from the FECO spectra of two different films. Figure 72b
shows the calculated dispersion near the absorption edge. The Figures
show that the curves converge at long wavelengths and give nearly the
same value for the long wavelength limit of the refractive index.
However, even at a wavelength 100 nm longer than the absorption edge,
the refractive index is overestimated. Table 7 is a listing of the
refractive index calculated at different wavelengths using equation 4.4


79
Figure 16
Interference effects at thin film, substrate, and air
interfaces.


163
diffraction pattern could be obtained from these crystals (shown in
Figure 60b), they could not be identified. Again these large crystals
were found to be unevenly distributed on the film and therefore
considered to be due to contamination. At very high magnification, the
second structure of these films can be observed. Very small spherical
microcrystallites, of 50 approximate size are shown in Figure 61.
Only a very diffuse diffraction pattern shown in Figure 60b could be
obtained, so it is difficult to identify the microcrystallites as CdS,
although the pattern does resemble the pattern of the semi-amorphous
LN2 film described above. A large number of these microcrystallites is
seen, so a stronger pattern should result. This particular film,
however, is relatively thick (~ 3000 ), so it only appears there are
many microcrystallites. The pattern is diffuse because the glass
matrix has a much larger volume fraction compared to the
microcrystallites.
Composition
Very little compositional analysis was carried out on COSAD films,
primarily because most of the films made were for TEM analysis, and
therefore were too thin for quantitative analysis. The composition of
a few of the phase separated particles discussed above however, were
determined by EDS while imaging the film in the STEM mode of the JEOL
200 CX electron microscope. The particles were found to contain
cadmium and sulfur, but the intensity levels for these two peaks were
too low to get an accurate determination of the stoichiometry. This
was particulary true of the particles displayed in Figure 61. A


2
phenomenon for optical signal processing. With the proper material,
all optical operations could conceivably be carried out in a monolithic
thin film, which would act as a guiding medium with both active and
passive regions. An optical communications fiber could be coupled
directly to this thin film, thereby fully utilizing the speed and
bandwidth of the optical signal.
A complementary application of an integrated optics technology
which utilizes optical switching would be in the area of high speed
logic operations for the next generation of computers. Logic gate
operation has been demonstrated with several materials which exhibit a
nonlinear optical susceptibility and optical bistability. A few of
these gates have been shown to switch on a subnanosecond time frame,
which is competitive with present day high speed electronic systems.
The primary advantage of the optical gate is the possibility of
parallel processing on the fundamental logic cell level, which adds
tremendous speed advantages over electronic systems. Most optical
computer designs today are based on integrated optics, in which the
active regions consist of arrays of bistable devices arranged with a
high spatial density. A high density of gates is possible because
optical gates are not subject to capacitive coupling, thus making
possible massive parallel processing without the connection problems
encountered in today's electronic systems.
Nonlinear Optical Materials
A large number of architectures have been proposed for both
computer systems and multiplexing circuits based on optical switching.


168
Room temperature UV-VIS absorption
The position of the room temperature band gap absorption edge that
was measured for as-deposited thin films was found to vary slightly
with film thickness and deposition rate, but mostly with substrate
deposition temperature. Films ranged in color from straw yellow to
dark orange to nearly black, as the deposition temperature was varied
from 300 C to LN2. Typical spectra of films deposited at 300 C, room
temperature, and LN2 using an absorbance scale are shown in Figure 63.
The variation in absorbance spectra with film thickness is shown in
Figure 64. As previously described, absorbance spectra show features
above the band gap with more detail than transmission spectra. By
taking the maximum slope of the absorbance, an approximate value of the
band gap absorption edge is given and as can be seen in Figure 63, this
position for the three curves varies considerably. A listing of the
absorption edges determined by this technique for different deposition
conditions is given in Table 5. From the Table, the maximum slope
position for films deposited under the same conditions, but with
different thicknesses varies only slightly and as shown in Figure 64
the shape of the absorbance curves is similar. In contrast, the shape
and position of the three spectrum in Figure 63 of similar thickness
films deposited at different temperatures is altered considerably. The
film deposited at 300 C shows very flat absorption above the band edge
indicating a well developed band gap, but the position of the edge is
considerably shifted to longer wavelengths which is characteristic of a
composition change. Also, because of the inhomogeneity of these films
there is a reduction in transmission at long wavelengths due to


233
The various polarizer and analyzer readings fall into four sets of
readings called zones, two zones with the fast axis of the compensator
set at +45 and two zones at -45. There is one independent set of
readings for each zone, giving four independent sets. However, both
the polarizer and analyzer may be rotated by 180 without affecting the
results, thus yielding 16 sets of readings, which can grow to 32 sets
if the compensator is rotated by 180.
Depending upon which set of readings one is making, the
calculation of x and A may be different. Furthermore, Brewster's angle
(tan phase by 180 at this point. When working at ellipsometer angles of 50
and 70, certain film substrate configurations can develop which cross
Brewster's angle. This leads to the necessity to change the x> 4
definitions.
Calculations
The equations are well known Drude Equations which will not be
reproduced here. The reader is referred to Azzam and Bashara,
Ellipsometry and Polarized Light, North Holland 1977 and reference
number 64. A two-step approach which was developed to solve the Drude
equations to determine the refractive index and thickness will however,
be outlined here. In the first step, ni is calculated using a Lotus
routine for plotting, as follows:
a) The data is used to calculate x and A.


b)
30 nm
Figure 27 Thin film deposited at LN2 temperatures. a) low
magnification TEM micrograph showing section of film
increasing in thickness from left to right; b) high
magnification TEM micrograph showing island structure.


48
study; the grain size varied between 500 and 3000-4000 as the
substrate temperature was increased from 60 to 300 C. This report,
however described the sputtering pressure dependence of the
crytstallinity. The sputtering pressure determines how much structural
damage occurs due to ion bombardment. At low pressures, bombardment is
enhanced by a large self-bias that develops on the substrate, and an
amorphous structure results as previously described. At higher
pressures, the crystallinity was also found to decrease, which was
explained by the authors as due to a porous structure that develops
from trapped gases.
Optical properties were found to be a function of both substrate
bias and sputtering pressure. A maximum in the optical band gap (2.36
eV) was found to occur for a floating substrate bias, whereas the
minimum in the band gap (2.30 eV) occurred for a -110 volt bias. The
band gap was found to increase with an increase in sputtering pressure
and became nearly constant for pressures above 10 pm. The pressure
dependence of the refractive index however contradicts the pressure
dependence of the band gap. The index of refraction was found to be a
maximum with 5 pm pressure, and it decreased with increasing pressure.
This was explained in terms of the porous structure which occurred at
higher pressures; however, if the refractive index is reduced at higher
sputtering pressures due to a more porous, less crystalline structure,
then the band gap energy should also decrease. An increase in the
number of crystalline defects should cause tailing of the band gap,
which is seen as a decrease in the gap energy.^9


97
inevitably increased the grain size of the thin films from values
obtained with cooled or room temperature substrates.
The resulting microstructure of thin films deposited at room
temperature is found to be independent of the substrate material. TEM
micrographs of three representative thin films are shown in Figures 22,
and 23. These micrographs show the structure of films 1000 in
thickness that were deposited at 1 /sec onto a carbon support film
(Figure 22), a silica slide (Figure 23a), and a NaCl substrate (Figure
23b). All three films display polygonal shaped grains with an average
grain size of 250 . Many grains are highly faulted showing a defect
structure of microtwins and stacking faults. Due to the extremely
small size of these grains the microtwin orientation cannot be
determined. Another predominant feature displayed by the
microstructure of these thin films is the large number of Moire
fringes, indicated by the highly parallel lines with an average spacing
of 10 to 15 . These patterns occur when two crystals overlap each
other with a specific orientation. When a large number are observed in
a polycrystalline film it is an indication that the grains of the film
have a preferred orientation, although the orientation cannot be
directly determined.
By performing diffraction experiments, the orientation could be
determined. The grains were found to be oriented with the basal plane
of the hexagonal lattice parallel to the substrate plane. This is
indicated by the strong cubic reflections observed in the selected area
diffraction pattern (SAD) shown in Figure 24a. In comparison to the
indexed schematic pattern shown in Figure 24b, many of the hexagonal


24
where the intensity absorption per pass A is given by
A = 1
-ad
e
(2.23)
and a is the absorption coefficient. A second equation can be written
which is parametric in Ieff for the Fabry-Perot transmission:^
ad (1-R)(1-A) Ieff
A ( 1 R ( 1 A ) )
o
(2.24)
The condition for optical bistability can be determined by
simultaneously solving equations 2.22 and 2.24. A graphical solution
of these equations which shows the criterion for optical bistability is
shown in Figure 3.
The critical intensity Ic for the onset of bistability is given by
the intensity IQ which gives more than one intersection with the line
and curve. This is the intensity at which the saturation of the
feedback occurs. Once this saturation is achieved, it is found that if
the input intensity is reduced, the output intensity does not drop
until a finite decrease in the input has occurred. In other words, a
hysteresis effect is observed. The switching between the two intensity
levels can be considered as a change in logic state. The nonlinear
Fabry-Perot interferometer therefore can be used as an optical logic


38
Nonlinear Susceptibility of CdS
Even with the obvious advantages of using CdS for nonlinear
optical applications, very few groups have investigated this material.
The primary amount of work in this area on CdS has been carried out by
Dagenais. ^ As would be expected from the large oscillator
strength of the bound exciton, a very large nonlinear refractive index
results when this level is saturated, and the decay time is very short.
By use of a Fabry-Perot arrangement and a narrow bandpass tunable dye
laser, Dagenais reported a cw saturation intensity for this level of
only 58 W/cm^, which corresponds to a nonlinear index of refraction of
1 X 10^ cm^/W.^ In a later publication, Dagenais and Sharfin^ report
that by using a high finesse Fabry-Perot, a saturation intensity of
only 26 W/cm^ is required, which for the experimental setup corresponds
to a nonlinear refractive index of 2 X 10^ cm^/W. They also reported
a switch up and switch down time of one and two nanoseconds
respectively. These are the largest values of a nonlinear refractive
index ever to be reported.
Other work on CdS has been done by Bohnert, Kalt, and
Klingshirn. 0 They did not, however, study nonlinearity by exciton
saturation, but rather by the formation of an electron-hole plasma.
High intensity laser pulses of energies just above the band gap energy
were used to study this effect. By measuring the temporal line shape
of the transmitted laser pulse they were able to determine the
renormalization of the band gap due to the formation of an electron-
hole plasma. Because this is a much smaller effect, intensities of 120
kW/cm^ were required to produce a change in the transmitted pulse.


157
200 nm
Figure 56 Dark field TEM image of C0SAD1 thin film shown in Figure
55a.


ABSORBANCE
215
Figure 87 Low temperature absorbance spectrum of C0SAD1 thin film.


Optical Measurements 76
Index of Refraction
UV-VIS Absorption 86
Photoluminescence and Raman 88
V RESULTS AND DISCUSSION 96
Thin Film Physical Properties 96
Microstructure Characterization 96
Transmission electron microscopy 96
Scanning electron microscopy 109
X-ray diffraction experiments 123
Compositional Analysis 132
Stoichiometry determination of target material 132
Analytical techniques 134
Co-Sputter Alternating Deposition 149
Microstructure 149
Composition 163
Thin Film Optical Properties 165
Absorption Spectra 167
Room temperature UV-VIS absorption 168
Low temperature UV-VIS absorption 174
Index of refraction 185
Photoluminescence and Raman Spectroscopy .... 190
Photoluminescence spectra 193
Raman spectroscopy 207
COSAD Optical Properties 211
VI CONCLUSIONS 218
VII FUTURE WORK 224
APPENDICES
A BASIC PROGRAM FOR LAMBDA 9 SPECTROPHOTOMETER ... 227
B ELLIPSOMETRY 230
BIBLIOGRAPHY 235
BIOGRAPHICAL SKETCH 240
vi


ABSORBANCE
176
Figure 67 Same designations as Figure 66, with expanded abscissa
scale.


94
indicated in the schematic, the laser beam was passed through a
variable neutral density filter and a sharp cutoff spike filter. The
variable density filter was used to adjust the power of the incident
laser beam before being focused on the sample. The spike filter was an
indispensible item which was used to remove plasma lines from the laser
beam. Plasma lines originate from the argon plasma that is used to
generate the laser beam. The lines are incoherent radiation, but they
are very sharp transitions which can be mistaken as photoluminescent
transitions as they occur over the entire visible wavelength range. A
given spike filter passes only a very narrow wavelength range of light
which corresponds to the laser line, therefore, a spike filter can only
be used with its designated laser line. These filters are rather
expensive so spike filters for only the 488 nm and 457 nm lines were
available. The argon laser emits five other lines that occur at 514
nm, 496 nm, 476 nm, 465 nm, and 454 nm. Plasma lines were removed from
the other laser lines by using a spatial filter. This filter consists
simply of a grating and it works by diffracting the laser line at a
different angle from the plasma lines. The spacial filter only works
if a high number grating is used or if the laser beam can travel
several feet before being focused.
To verify that the OMA was not missing any features of the
photoluminescence, spectra were measured with the Instruments SA Raman
spectrometer. This spectrometer utilized a double pass one meter
monochromater with a photon counting photomultiplier tube and a digital
discriminator. Photographs of the experimental setup are shown in
Figures 21a and 21b. A simple lens system consisting of two piano-


16
nonlinear contribution to the refractive index.5 The nonlinear index
of refraction for this process is expressed by
n2(S) =
e P ^4 x
h a)
(2m*) (
h
2 n
15 n2 c
X
(2.14)
where x is the relaxation time, and c is the speed of light. The
advantage of this process for nonlinear applications is that the
effective nonlinear refractive index is inversely proportional to the
band gap energy, so for small band gap materials this process leads to
a very large effect. As indicated above, the broadening results in an
effect which occurs below the band gap energy, so absorption losses are
reduced. The disadvantage of utilizing this process is the same as for
the other two processes; in some materials there is no fast mechanism
for decay of the excited state.
The final process that will lead to an electronic nonlinear
refractive index in certain semiconductors is the saturation of bound
exciton levels. These particular defect states are characterized by a
very narrow transition linewidth, which is comparable to atomic
resonances. A bound exciton is an associated electron-hole pair that
is bound to an impurity site. The oscillator strength of the bound
exciton, which is related to the polarizability, is extremely large in
comparison to oscillator strengths of molecules.^ In addition, there
is a very high density of oscillators, which contributes to a very
large nonlinear effect. The absorption transition of the bound exciton


INCIDENT LASER POWER (W)
199
Figure 77 Graph of band edge luminescence peak position versus
incident power.
522


63
the gun assembly. Most of the plasma is constrained to the area
immediately above the target, so a large reduction in substrate
bombardment and substrate heating is realized. The substrate and
growing film, however, can still be bombarded by energetic neutral
particles, which has been shown to be a major cause of substrate
heating.^ Still the heating is reduced and as will be pointed out in
the Results section (Chapter 5), a considerable difference in the as
deposited film properties occurs with RF planar magnetron sputtering,
compared to RF diode sputtering.
Thin Film Deposition
Prior to deposition of thin films, all substrates were subjected
to a three phase cleaning procedure. The first phase of the process
was an ultrasonic bath in DI water for 10 minutes. After this
treatment the substrates were removed from the water and rinsed with
isopropyl alcohol (IPA). A second ultrasonic cleaning was then carried
out for 10 minutes in IPA. The final phase of the cleaning procedure
was a 15 minute treatment in a vapor degreaser, which employed IPA as
the solvent. Substrates were slowly removed from the vapor, which
allowed the condensed vapor droplets to evaporate. To check for
cleanliness substrates were examined by edge illumination against a
black background. This permitted observation of any contaminates on
the surface of the substrate in addition to any small flaws in the
surface.
To deposit thin films, substrates were mounted on a substrate
holder and positioned in the deposition system immediately after


125
Figure 39a shows the amorphous peak to be greatly reduced and
crystallization has taken place with the strong cubic preferred
orientation.
Figure 38b shows that films deposited at room temperature exhibit
a strong cubic preferred orientation which is shown by diffraction at
(0001) where 1=2,A,6. The other hexagonal lines seen in SAD patterns
are not as readily observable in this pattern, particularly the other
two lines which are closely spaced to the (0002) peak. Those two lines
were clearly resolved in SAD patterns, but appear to be totally absent
here. This shows how much more sensitive electron diffraction is to
the local crystal structure. When this film is heat treated, the
preferred orientation is maintained as shown in Figure 39b. All
diffraction peaks become much narrower and more intense, as an
indication of grain growth. Figure 39c indicates that when this film
is heat treated to an even higher temperature, the orientation is
maintained, but the intensity of other hexagonal diffraction lines is
increased.
The most predominant hexagonal lines are seen in the XRD pattern
obtained from a film deposited at 300 C, shown in Figure 38c. Again a
preferred orientation is indicated, but other hexagonal lines are shown
to occur in a different ratio to the cubic lines, which indicates there
is another orientation present. This second orientation was not
observed in the TEM analysis because the film examined was deposited at
200 C. It appears this orientation only occurs at higher
temperatures.


INDEX OF REFRACTION INDEX OF REFRACTION
186
a)
b)
Figure 72 Index of refraction calculated from FECO spectrum, a)
showing long wavelength conversion; b) variation near the
band gap.


OPTICAL PROCESSES IN CADMIUM SULFIDE
THIN FILMS
By
EDWARD M. CLAUSEN, JR.
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA

Copyright 1987
by
Edward M. Clausen, Jr.

ACKNOWLEDGMENTS
Not enough can be said for the people who have contributed to the
start and particulary the completion of this dissertation. It would
have been very difficult for me to get through this project without
their help.
I would like to express my deepest appreciation and gratitude to
Dr. Joseph H. Simmons, my academic adviser, who supplied much
encouragement, direction, assistance and the opportunity to work on
this project. His insight and ability to help me solve problems proved
to be an extremely valuable asset, although his confidence in my
abilities to "get the job done" was invaluable. I would also like to
express my appreciation to the members of my committee, Dr. Paul
Holloway, Dr. Robert Dehoff, Dr. Stan Bates, Dr. Tim Anderson and Dr.
Ramakant Srivastava for their suggestions, comments and guidance. This
is certainly one of the best combinations of abilities and expertise
for a committee and I thank the members for their help and enthusiasm.
My closest friends Guy Latorre, Richard Robinson and B.G. Potter
deserve special thanks for their personal support and reassurance. My
roommate and friend, Steve Wallace deserves the most thanks and credit
for having to endure the more trying times of this project. I would
especially like to thank my mother and father for their support
throughout all of my endeavors, and my closest and dearest friend Laura
Harmsen for her inspiration and emotional support.
iii

Finally, I would like to express my graditude to the Air Force
Office of Scientific Research (AFOSR 84-0395) for funding this research
project.

TABLE OF CONTENTS
PAGE
ACKNOWLEDGMENTS iii
ABSTRACT vii
CHAPTERS
I INTRODUCTION 1
Optical Signal Processing 1
Nonlinear Optical Materials 2
Thin Films 5
II THEORY OF NONLINEAR OPTICS 8
Nonlinear Optical Susceptibility 8
Optical Bistability 19
III BACKGROUND BIBLIOGRAPHICAL REVIEW 28
Bulk Properties of Cadmium Sulfide 28
Photoluminescence of CdS 32
Resonant Raman Scattering in CdS 37
Nonlinear Susceptibility 38
CdS Thin Films 39
Vacuum Deposition 39
RF Sputtering 45
Semiconductor Doped Filter Glasses 50
IV EXPERIMENTAL METHOD 52
Vacuum Deposition System 52
RF Magnetron Sputtering 60
Thin Film Deposition 63
Substrate materials 64
Substrate temperatures 66
Deposition rates 68
Co-Sputter Alternating Deposition (COSAD) ... 68
Post Deposition Treatments 70
Thin Film Characterization 71
Microstructure 71
Chemical Analysis 73
v

Optical Measurements 76
Index of Refraction
UV-VIS Absorption 86
Photoluminescence and Raman 88
V RESULTS AND DISCUSSION 96
Thin Film Physical Properties 96
Microstructure Characterization 96
Transmission electron microscopy 96
Scanning electron microscopy 109
X-ray diffraction experiments 123
Compositional Analysis 132
Stoichiometry determination of target material 132
Analytical techniques 134
Co-Sputter Alternating Deposition 149
Microstructure 149
Composition 163
Thin Film Optical Properties 165
Absorption Spectra 167
Room temperature UV-VIS absorption 168
Low temperature UV-VIS absorption 174
Index of refraction 185
Photoluminescence and Raman Spectroscopy .... 190
Photoluminescence spectra 193
Raman spectroscopy 207
COSAD Optical Properties 211
VI CONCLUSIONS 218
VII FUTURE WORK 224
APPENDICES
A BASIC PROGRAM FOR LAMBDA 9 SPECTROPHOTOMETER ... 227
B ELLIPSOMETRY 230
BIBLIOGRAPHY 235
BIOGRAPHICAL SKETCH 240
vi

Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
OPTICAL PROCESSES IN CADMIUM SULFIDE
THIN FILMS
by
Edward M. Clausen, Jr.
December, 1987
Chairman: Joseph H. Simmons
Major Department: Materials Science and Engineering
The semiconductor, cadmium sulfide, has received much attention
for its optical, electronic and piezoelectric properties. Recently,
several investigators have demonstrated that its Wannier excitons hold
great promise for applications in nonlinear optics. However, the
results showed, as expected by the current theoretical thinking, that
the nonlinear optical behavior and the exciton and band gap energy
configurations are dependent upon the microstructure of the samples.
Since most applications in computer logic, or communications require
waveguide geometries, the object of this dissertation has been the
study of thin films.
The investigation presented here, therefore concerns the study of
the effect of microstructure and preparation conditions on the optical
properties of CdS thin films. High optical quality films were produced
by RF magnetron sputtering with a variety of microstructures and
crystallographic characteristics controlled by the deposition process
vii

and subsequent heat treatments. An in-depth study of the thin film
microstructure revealed the relationship between crystallographic
defects and band gap defects which lead to band tailing absorption.
Optical characterization related exciton photoluminescence, absorption,
resonant Raman scattering and band edge photoluminescence and
absorption to the microstructure of films. For the first time,
absorption bands and photoluminescence associated with exciton states
were observed in a polycrystalline thin film.
A supplementary study investigated composite films, consisting of
CdS crystals in a glass matrix, formed by a novel co-sputter deposition
process, in which alternating layers of CdS and a borosilicate glass
were deposited to form a thin film. A wide variation in the structure
of the deposited film was obtained by changing the amount of deposited
CdS and by post deposition heat treatments. Low concentrations produced
CdS microcrystallites as small as 70 , small enough for quantum
confinement processes to affect the band energy. For all films
produced, a shift in the band absorption edge to higher energies was
observed; however, it was determined that this shift must be partially
associated with a chemical shift. Possible formation of CdO could
raise the band gap energy, although this alone could not produce the
shifts observed in the films with the smallest particles, and therefore
a confinement process might exist.
viii

CHAPTER I
INTRODUCTION
Optical Signal Processing
With the very intense development of optical communications in
recent years, the need for integrated optical systems for the
processing of optical signals has greatly increased. To take full
advantage of the speed and information bandwidth available at optical
frequencies, an all optical system is desired. All of today's systems
operate by the high bandwidth transmission of optical signals in
optical fibers, followed by conversion to lower bandwidth electrical
signals before any processing, such as amplification and multiplexing,
can take place. The limiting drift velocity of charge carriers in a
semiconductor and the capacitive coupling between adjacent elements
present the fundamental limit for processing speed in these systems.
Also, the serial nature by which the electronic data must be
manipulated presents another speed barrier. An alternate approach
would utilize optical bistability and optical switching demonstrated in
certain materials for all-optical modulation, detection and
multiplexing. Optical switching is achievable with materials which
display a third order nonlinear susceptibility. A third order
susceptibility leads to an intensity dependent index of refraction or
absorption. This nonlinear refractive index can exhibit onset and
decay on a very fast time scale, which makes it an attractive
1

2
phenomenon for optical signal processing. With the proper material,
all optical operations could conceivably be carried out in a monolithic
thin film, which would act as a guiding medium with both active and
passive regions. An optical communications fiber could be coupled
directly to this thin film, thereby fully utilizing the speed and
bandwidth of the optical signal.
A complementary application of an integrated optics technology
which utilizes optical switching would be in the area of high speed
logic operations for the next generation of computers. Logic gate
operation has been demonstrated with several materials which exhibit a
nonlinear optical susceptibility and optical bistability. A few of
these gates have been shown to switch on a subnanosecond time frame,
which is competitive with present day high speed electronic systems.
The primary advantage of the optical gate is the possibility of
parallel processing on the fundamental logic cell level, which adds
tremendous speed advantages over electronic systems. Most optical
computer designs today are based on integrated optics, in which the
active regions consist of arrays of bistable devices arranged with a
high spatial density. A high density of gates is possible because
optical gates are not subject to capacitive coupling, thus making
possible massive parallel processing without the connection problems
encountered in today's electronic systems.
Nonlinear Optical Materials
A large number of architectures have been proposed for both
computer systems and multiplexing circuits based on optical switching.

3
While this field has greatly advanced, there is, however, a very great
need for suitable materials and systems. Many materials exhibit a
third order nonlinear susceptibility,^ but very few have the
characteristics necessary for an integrated optics system. A few of
the semiconductors which have been investigated include InSb, GaAs,
GaP, CdS, and CdTe. Most of these semiconductor materials which show
optical switching have been investigated in bulk form. Only the
multiple quantum well (MQW) structures made from gallium arsenide are
the notable exception and are made in thin film form.^ This material
is perhaps the most promising today for use in optical signal
processing systems, primarily because its nonlinearity occurs at the
same wavelength as the semiconductor lasers which are currently
available. For an integrated optical system this is an essential
consideration because most of the processes which produce the nonlinear
susceptibility require laser light of energy near the band gap of the
material.
Equally important considerations, however, include the value of
the nonlinear coefficient, the switching speed, and the absorption
coefficient, since these values determine how much power is required to
switch the device, and how fast recovery will be. Low power operation
is essential for any large scale integration, although the figure of
merit (FOM) most often quoted is given by
FOM = n2 (1.1)
x a
where ri2 is the nonlinear index, x is the switch-off time, and a is the
absorption coefficient. Obviously the larger the FOM the more

4
attractive a system is. MQW gallium arsenide has an additional
advantage of room temperature operation, but suffers from a slow
switch-off time and very large absorption. There are other materials
which have been shown to exhibit a much larger nonlinear effect than
MQW gallium arsenide. A material which has been demonstrated to exhibit
one of the largest nonlinear coefficients is cadmium sulfide (CdS).
The large coefficient was obtained by saturating a bound exciton level
in the band gap.^ The mechanism which leads to the large exciton
saturation effect in CdS is central to this dissertation and will be
described in a subsequent section. However, only a very few groups
have looked at CdS, and no one has either investigated the bound
exciton saturation mechanism in thin films or explored the possible
applications in integrated optics.
The most important consideration for any practical nonlinear
application is the temperature at which the nonlinear process
predominates. To the present day the largest nonlinear coefficients are
only measured at very low temperatures. For example, the large
coefficient obtained by saturating the bound exciton level in CdS
occurs at 2 K. Since the binding energy of the bound exciton only
corresponds to a few millielectron volts, the state is thermally
annihilated at higher temperatures, and the large nonlinear effect
disappears.
The electronic structure of the band gap, however, may be altered
to permit access of exciton levels at higher temperatures by
controlling the physical size of the material. The phenomenon by which
this occurs is known as quantum confinement. When the crystal size of

5
a material is on the order of the radius of the exciton state, new
boundary conditions can distort the translational motion of the
exciton, its binding energy and the individual orbits of the electron
and hole. At this size the electron and hole interactions with the
crystal surface begin to govern the electronic properties of the
semiconductor. Quantum confinement also occurs in the MQW structures
of GaAs; however the exciton is only confined in one direction. A much
stronger effect occurs when the confinement is in two or three
dimensions.
Thin Films
A study of CdS thin films was chosen for a number of reasons: 1)
very few semiconductors which display a nonlinear susceptibility have
been tested in a thin film form, despite the potentially dominating
role in applications. 2) Cadmium sulfide displays one of the largest
excitonic saturation effects, and thus the influence of film structure
due to deposition conditions or subsequent treatments could be
investigated. 3) Quantum confinement effects and their interaction with
the perturbation of the exciton absorption process are of great
interest to the future of nonlinear optical developments and
applications. Cadmium sulfide promises to offer a means of studying
both the effects and their interplay with the development of the thin
film structure. 4) Since the energies of the exciton states correspond
to wavelengths in the visible part of the spectrum, the experimental
optics for the measurement of these states is simplified, and many

6
investigative tools become available for following the underlying
processes.
The study of any thin film for this application should start with
examining the properties of the bulk material which contribute to the
specific origin for the nonlinear effect. In the case of CdS this
means looking at the electronic states of the material which lead to
the presence of excitons. These states have been thoroughly examined
for more than 30 years and they are probably the best understood in
this material. Exciton states have been shown to occur in thin
epitaxial films, but no one has investigated the presence of these
states in polycrystalline thin films, nor have the effects of
preparation conditions on the excitonic transitions of a thin films
been investigated. The objective of this study therefore is to
determine how exciton states would occur in polycrystalline thin films
of the material, how they are affected by structure and formation
conditions, and how size variations and grain boundary structures might
affect their energy level structure.
A supplementary part of this study will investigate the
possibility of producing thin film structures consisting of a glass
matrix with small isolated crystals whose sizes matched those needed to
develop quantum confinement effects. The investigation will use
spectroscopic techniques to determine if quantum confinement effects
can be induced on the exciton states in the material. The objective is
to produce a thin film of semiconductor doped filter glass.
The process of quantum confinement has been and still is under
investigation in bulk semiconductor doped filter glasses, which have

7
received considerable attention lately for nonlinear optics
application. These are crown base glasses which contain one or two
percent of CdS or mixtures of CdS and CdSe. It has been postulated
that the semiconductor crystals exist in the glass matrix as finely
dispersed microcrystallites, which are small enough to permit quantum
confinement effects to occur. This effect is still not well understood
and has not been clearly demonstrated. Many authors have observed
energy shifts that may be due to compositional effects rather than
microstructure size. However, it appears theoretically that with
sufficient confinement, the exciton state will be accessible at room
temperature.

CHAPTER II
THEORY OF NONLINEAR OPTICS
Nonlinear Optical Susceptibility
The nonlinear refractive index which is observed in certain
semiconductor materials is a result of an electronic polarization which
is induced by the interaction with a monochromatic radiation field.
The susceptibilities of the polarization determine the values of the
experimentally measured optical properties. To understand the
relationships between the susceptibilities and the optical properties
we must first consider the electro-magnetic field, E, which is given by
E(t) = E())e-i)t + E*( The resulting polarization has frequency components at all multiples of
+/-U), but considering only those that occur at oj
PU) = x(1)E(w) + x(2)[E]E(w) + X(3)[E]2E(to) + . (2.2)
The first term in the series, x^) is the linear susceptibility and by
using first order perturbation theory,^ the linear dispersion of the
refractive index below the band gap can be calculated. The higher
8

9
order terms in the series are the nonlinear susceptibilities, and
although their magnitudes are much smaller than the first term, under
very high field intensities a number of different effects can be
observed. For non-centrosymmetric crystals (i.e. crystals without an
inversion center), under appropriate conditions, the second order term
is manifested as two different effects. The first is a quadratic
variation of the refractive index with applied voltage, which is known
as the Kerr effect. The second is the generation of a second harmonic
radiation field. Second harmonic generation is a very useful effect
for doubling the frequency of a laser beam. Both of these effects have
many important applications; however, the primary interest of this
study is the effects which lead to the third order term One
consequence of the third order term is the generation of a third
harmonic radiation field. For the current topic of this study,
however, the manifestation of this term as a nonlinear refractive index
is of primary interest. The relationship between the third order
susceptibility and the nonlinear refractive index can be understood by
first considering basic dielectric theory for the displacement of a
charge in response to an applied electric field:
D(w) = E((jj) + 4tP(o)) = e E(oj) (2.3)
where D(w) is the frequency dependence of the displacement and e is the
complex dielectric constant. The variable, e can be defined as

10
e = (n + ) 2 (2.4)
2oj
where ca/2w is the extinction coefficient. By combining these two
equations a relation between the polarization and the index of
refraction can be written.
(n + ica )2= 1 + 4tt P( 2w
The nonlinear susceptibility can be defined by expansion of the
refractive index in terms of the intensity I, of the radiation inside
the sample:^
n = n-^ + n2l + + (2.6)
Next we assume that the extinction coefficient is very small compared
to n. Then by using equations 2.5 and 2.6 and expanding the terms,
n2 = n2 + 2nin2I + (n2l)2 = 1 + 4tt X(1) + 4tt X(3) [E]2. (2.7)
Finally we assume n2 is much smaller than n^ and by comparing
coefficients of [E]^ an j

11
and
1
+ 4tt
n
2tt x
(3)
(2.8)
(2.9)
There are essentially four electronic processes which can produce
a reactive nonlinear susceptibility in semiconductors. These are known
as 1) the induced free-carrier plasma, 2) the dynamic Burstein-Moss
effect, 3) the direct saturation of interband excitations, and 4) the
saturation of exciton absorption.^ Of these 1 and 2 are the most
commonly studied, 3 occurs in most direct band gap semiconductors, and
4 is the most promising for high speed operations. All four processes
can occur in some materials. The most dominating process which is
observed depends somewhat on the material, but mostly on the particular
experimental setup and measurement temperature. As shown in Table 1,
different processes result in widely different values of the reported
nonlinear index and saturation intensity.
The four processes listed above basically describe how the
transitions between different levels in a semiconductor and the
saturation of those levels result in the observed nonlinear
susceptibility. Depending on the band structure of a material and the
particular wavelength of light used for the analysis, one of the four
processes will dominate. The one process that can occur in nearly
every semiconductor, however, is the induced free-carrier plasma.
Assuming that photo induced transitions produce electron-hole pairs,
the number of these free carriers will be intensity dependent.

12
TABLE 1
Listing of Nonlinear Optical Values for Certain
Semiconductor Materials
Material
Electronic
Proces
Is (W/cm2)
n.2 (cm2/W)
toff (sec)
FOM
GaAs
(MQWS)
FES
150
10-4
10"8
1
Bulk
DBM
500
InSb
DIS
2000
6X10-5
10"6
0.06
CdS
BES
58,26
10"4,10"2
10"9
2
FCP
1.2X105
10"10
Key for Electronic Processes:
FES Free Exciton Saturation
DBM Dynamic Burstein-Moss
DIS Direct Interband Saturation
BES Bound Exciton Saturation
FCP Free Carrier Plasma

13
Depending on the recombination time and the absorption coefficient
a, the steady state density of free carriers will be given by
a i t
N = (2.10)
h oj
Once these carriers are formed, they are allowed to diffuse and form an
electron-hole plasma. The plasma will respond to an applied electric
field and the resulting undamped oscillations will produce a
polarization which can be related to the index of refraction through
the dielectric constant^
2 ( 4-irNe2 .
n = ( e ). (2.11)
* 2
m cu
By use of equation 1.10, the nonlinear refractive index for a plasma
can be written as
n2(P)
- 2 IT
2
e a
R
3
t
n m
(2.12)

14
where n is the refractive index of the material without the plasma and
m" is the effective mass. The transient susceptibility is determined
by the time it takes for the free carriers to build up;^ however, this
can be a very short time. This process will also occur at room
temperature. The disadvantage of this process for nonlinear optical
applications is that the effect is very small because there is no
coupling between the states, and therefore no resonance effects take
place. The process also can suffer from a very slow recovery time
because the recombination time in certain semiconductors is on the
order of Msec.^
Another process that is based on an intensity dependent free
carrier concentration is the dynamic Burstien-Moss or blocking effect.
Again a steady state density of charge carriers is given by equation
2.10; however, the origin of the absorption is not considered
explicitly. At low temperatures the carriers are assumed to thermalize
by a phonon scattering process so that they fill the bottom of the
conduction band. The top of the valance band becomes empty and the
shift of the effective band gap to higher energy becomes intensity
dependent. In association with this shift there must be an intensity
dependent contribution to the refractive index.^ This is because the
filled conduction band effectively blocks absorptive transitions, and
the blocked transitions no longer contribute to polarization and
refraction. Also, some type of unspecified coupling takes place
between excited states, so that the effect is enhanced somewhat. The
nonlinear refractive index for this process is given by

15
n (BM) = 2 11 ( 6 P )2 N (2.13)
3 n h u) h (ojg o) I
where N is the free carrier density given by equation 2.10, P is a
momentum matrix element, and ojq is the effective band gap. For
nonlinear optical applications the advantage of this process is that
the occupied states are closer to resonance so that a larger n2
results. There also is the possibility that this process can occur at
energies below the band gap, as saturation of excited carriers can be
produced not by direct optical absorption, but by scattering from other
excited states.^ The one disadvantage of the process that is similar
to the induced plasma process is that the interband relaxations
required for decay of the state can be very slow. In some materials a
faster decay process can occur by scattering to intraband transitions,
which effectively relaxes the system by transferring the population to
other states.^
Another mechanism for the nonlinearity observed in some materials
is by a direct interband saturation process. This process assumes that
the band structure in a direct gap semiconductor can be modeled as a
set of uncoupled two level systems which are homogeneously broadened by
a dephasing time T2. Homogeneous broadening means that the individual
transitions are indistinguishable. The T2_Lorentzian broadening
results in absorption below the band gap and excitation into the T2-
broadened "band-tail" is assumed to be responsible for the nonlinear
refraction.^ At some high level of intensity the two level system
should become saturated, and associated with this saturation is a

16
nonlinear contribution to the refractive index.5 The nonlinear index
of refraction for this process is expressed by
n2(S) =
e P ^4 x
h a)
(2m*) (
h
2 n
15 n2 c
X
(2.14)
where x is the relaxation time, and c is the speed of light. The
advantage of this process for nonlinear applications is that the
effective nonlinear refractive index is inversely proportional to the
band gap energy, so for small band gap materials this process leads to
a very large effect. As indicated above, the broadening results in an
effect which occurs below the band gap energy, so absorption losses are
reduced. The disadvantage of utilizing this process is the same as for
the other two processes; in some materials there is no fast mechanism
for decay of the excited state.
The final process that will lead to an electronic nonlinear
refractive index in certain semiconductors is the saturation of bound
exciton levels. These particular defect states are characterized by a
very narrow transition linewidth, which is comparable to atomic
resonances. A bound exciton is an associated electron-hole pair that
is bound to an impurity site. The oscillator strength of the bound
exciton, which is related to the polarizability, is extremely large in
comparison to oscillator strengths of molecules.^ In addition, there
is a very high density of oscillators, which contributes to a very
large nonlinear effect. The absorption transition of the bound exciton

17
is modeled as a saturable two level system, inhomogeneously broadened
by a T2 dephasing time. The transition linewidth is inhomogeneously
broadened because excitons bound at different locations see different
environments.^ An inhomogeneously broadened system is described as a
distribution of groups or classes of transitions, and within each class
the transitions are assumed to be identical (homogeneously broadened).
The saturation of the inhomogeneously broadened system does not depend
on the homogeneous lineshape function, but rather on the linewidth of
these "homogeneous packets". This means that the saturation intensity
is inversely dependent upon the dephasing time T2 as shown by
I
s
2 2
n n h v Av
cj) X
(2.15)
where
Av = (m T2)-'*'
(2.16)
and is the ratio of the radiative lifetime to the spontaneous decay
time, which is usually taken as equal to one. In semiconductor systems
the dephasing time is on the order of 0.1 psec,^ which means that very
low saturation intensities are required to saturate bound exciton
transitions. The importance of the saturation intensity will be
described in a following section; however, a small value indicates that
the nonlinear refractive index is very large, as the two are inversely
proportional (see equation 2.25). As shown in Table 1, in CdS a
saturation intensity as small as 26 W/cm2 has been measured, which
corresponds to n2 value of 1.3 X 10"2 cm2/W.10 This is the largest n2

18
value ever reported. Additional advantages of bound exciton saturation
are that the state decays by a radiative transition and the lifetime is
on the order of 500 psec. This would make for a very fast, low power
switch. Also, since the excitons are bound to defect sites, there are
no carrier diffusion problems, which in the other three processes tend
to wash out the effect. The one primary disadvantage of utilizing this
process is that the strongest exciton resonance occurs at 2 K. As the
temperature is increased, the transition broadens, thereby requiring a
larger saturation intensity and hence a smaller x\2 is observed. In
addition, since the exciton binding energy is only a few millielectron
volts, the state is thermally annihilated at higher temperatures.
As previously described the process of quantum confinement could
be used to access exciton levels at higher temperatures if the physical
size of the material could be made small enough. Multiple quantum well
structures produce quantum confinement in one direction because the
structure is made up of alternating layers in which the layers act as
infinite potential wells and the layer thickness is smaller than the
exciton radius. Although the exciton is not confined in the other two
directions, the effect is strong enough that nonlinearity can be
observed at room temperature. The effect would be larger if
confinement was made in the other two directions.

19
Optical Bistability
The primary means for measuring nonlinearity in materials is
through an internal feedback device known as a Fabry-Perot
interferometer. The saturation intensity of such a system is the
intensity at which the gain of the feedback saturates. This is the
point at which optical bistability occurs, and from the saturation
intensity and the interferometer parameters the nonlinear index of
refraction can be determined.
The Fabry-Perot consists of a cavity formed by two plane parallel,
highly reflecting mirrors. The transmission of monchromatic light
through the device is determined by the optical path length of the
cavity. If the cavity is not tuned to the wavelength of the light,
then a transmission of ~ 1 % results. When the optical path length is
exactly equal to an integer number of wavelengths, then a resonance
effect occurs and the output intensity from the device reaches nearly
100 % of the input intensity. A diagram of this process is shown in
Figure 1. The optical path length is determined by the physical length
d, times the refractive index n of the material within the cavity. The
condition for resonance therefore is given by
2 n d = m A
(2.17)
where m is the integer order number.

20
Figure 1
Schematic diagram of a Fabry-Perot interferometer showing
how interference of the forward and reverse beams changes
the output intensity.^

21
When the cavity does not satisfy the above requirement, then a
linear relationship will exist between the incident and transmitted
intensity. If a material with a nonlinear refractive index is placed
in the cavity, then a positive feedback loop will occur where the
refractive index and the light intensity become mutually reinforcing.
As the incident intensity is increased, a change in refractive index
occurs which brings the device closer to resonance, which further
increases the intensity inside the cavity, which further changes the
index, etc. This continues until a saturation intensity is reached, at
which point a phase shift to resonance occurs within the cavity and the
transmitted intensity suddenly increases.
The ratio of the incident intensity to the transmitted intensity
as a function of the phase shift 6 is given by the Airy function A(0)
I.
i
A(0)
(2.18)
where
A(0) = l (2.19)
1 + F sin2(6/2)
and F is related to the reflectivities of the mirrors R by
( 1 R )2
(2.20)

22
a)
b)
Figure 2 Descriptions of the operation of a Fabry-Perot. a) plot of
the Airy function as a function of the phase shift 6,
reflectivities r, and the finesse F; b) transmitted image
from a high finesse Fabry-Perot.-^

23
Figure 2a displays how the relative transmitted intensity and the
sharpness of the transition is related to the reflectivities and
finesse of the cavity. As the reflectivity of the mirrors is
decreased, the finesse is decreased and the transition at the critical
phase shift broadens. When the reflectivities and the finesse of the
cavity are large, then the transitions are sharp. For these
conditions, the transmitted image from a diffuse source through the
Fabry-Perot will appear as a series of sharp concentric rings, as shown
in Figure 2b.
For a nonlinear Fabry-Perot, however, the Airy function must be
slightly modified to account for the nonlinear index by
A(0)
NL
(2.21)
1 + F sin ( x I
eff
6)
where x is a constant describing the nonlinear refraction,^ an
is the effective mean intensity within the cavity. The total Fabry-
Perot fractional transmission can be written as^
( 1 R )2 ( 1 A )
(l-R(l-A))2
a(0)nl
(2.22)

24
where the intensity absorption per pass A is given by
A = 1
-ad
e
(2.23)
and a is the absorption coefficient. A second equation can be written
which is parametric in Ieff for the Fabry-Perot transmission:^
ad (1-R)(1-A) Ieff
A ( 1 R ( 1 A ) )
o
(2.24)
The condition for optical bistability can be determined by
simultaneously solving equations 2.22 and 2.24. A graphical solution
of these equations which shows the criterion for optical bistability is
shown in Figure 3.
The critical intensity Ic for the onset of bistability is given by
the intensity IQ which gives more than one intersection with the line
and curve. This is the intensity at which the saturation of the
feedback occurs. Once this saturation is achieved, it is found that if
the input intensity is reduced, the output intensity does not drop
until a finite decrease in the input has occurred. In other words, a
hysteresis effect is observed. The switching between the two intensity
levels can be considered as a change in logic state. The nonlinear
Fabry-Perot interferometer therefore can be used as an optical logic

25
Figure 3 Graphical solution to the Airy function showing the
critical phase shift required for the onset of
bistability.

26
gate. The great interest for logic gate applications is that the
switching between the two levels can occur on a subnanosecond time
period in some materials.
The saturation or critical intensity Ic is found to be inversely
proportional to the nonlinear refractive index and is given by
I =
c
1
3
1
M
where
X a
(2.25)
(2.26)
and p is a figure of merit value for the cavity, relating the
reflectivities of the two mirrors and the attenuation of light as
passes through the cavity. Equation 2.25 indicates that for fixed
cavity conditions, a small saturation intensity corresponds to a large
nonlinear refractive index. The total index of refraction is given by
n = n^ + An
(2.27)
where
An = n2lc.
(2.28)

27
The value of nt is what actually determines the phase shift but as
indicated by equation 1.1, the figure of merit for nonlinear optical
applications also includes the switching times and the absorption
coefficient.

CHAPTER III
BACKGROUND BIBLIOGRAPHICAL REVIEW
Bulk Properties of Cadmium sulfide
Nearly all of the early work on cadmium sulfide was carried out on
single crystal platelets which were made by a chemical vapor phase
growth process. The natural crystal structure of cadmium sulfide is
hexagonal wurtzite, although single crystals of the cubic zincblende
structure have been fabricated. Within either of the two crystal
structures it is possible to have regions which are made up of the
alternate crystal structure. The transition from a hexagonal to a
cubic lattice or visa versa can occur through a well known twinning
i n
mechanism,1^- in which the twinned region is bound by stacking faults.
The twinned regions can be manifested during deformation of the crystal
or under particular growth conditions, although it is difficult to
differentiate these two sources when crystals are grown from the vapor
phase. In either case, the two crystal structures do not have a center
of symmetry or inversion, which leads to the unique properties of
noncentrosymmetric crystals such as piezoelectricity, pyroelectricity,
and third order optical susceptibility.
Another important physical property of CdS is the stoichiometry of
the crystal. Very little work has been done on the defect chemistry of
CdS; however, the work done on other II-VI semiconductors such as ZnS
and CdTe indicates that the range of nonstoichiometry at room
temperature is very small, e.g. 0.01 to 0.1 %.13 Early work by Collins
28

29
which involved sulfur atmosphere heat treatments and electron
bombardments, showed that sulfur vacancies were the predominant native
defect and that they acted as the recombination center responsible for
the green edge emission associated with CdS luminescence.^ Other
studies which investigated impurity doping effects are described in a
following section on photoluminescence.
A detailed knowledge of the band gap structure of CdS has come
from the extensive study of exciton states. Cadmium sulfide is found
to be a direct gap semiconductor with a band gap equal to 2.59 eV at 0
K. The wurtzite lattice of the material is described by a p-like
valance band consisting of two gamma-7 states and one gamraa-9 state,
and a s-like conduction band made up of one gamma-9 state. A diagram
of the band extrema is shown in Figure 4. The three states in the
valance band are also known as the A, B, and C free exciton states.
These intrinsic exciton states are modeled as Wannier excitons; i.e.
the electron and hole behave like a hydrogen atom. The orbital
movements of the electron and hole are determined by their effective
masses within the band extreme. As shown in Figure 5, the solution for
the wave function of this model results in a series of discrete
parabolic bands below Eg which merge into a continuum at higher
energies.^ Because of the unique band structure of this material
there are a large number of possible exciton states. Any of the
exciton energy levels (i.e. n=l,2,3, etc.) can be associated with the
three primary states (A,B and C) in the valance band. In addition, any
one of these free excitons can be associated with an impurity center,

30
k
0*0
WURTZITE
14
Figure 4
Band gap structure for wurtzite crystals near k=0.

31
Figure 5 Energy diagram for Wannier excitons as a function of
exciton momentum K, showing "hydrogenic" states which merge
into a continuum at energies greater than Eg.^

32
forming a bound exciton complex, which will have a lower energy than
the corresponding free exciton.
Of all the II-VI semiconductor materials, the exciton states in
bulk CdS have been studied the most and are perhaps the best
understood. Reflection, absorption, and luminescence studies dating
back to the mid-fifties have investigated the exciton states in this
material. Excitation of the states can be accomplished by either
electron bombardment or by photon absorption. When the emission is due
to the latter process it is known as photoluminescence and the results
reported for CdS are detailed below.
Photoluminescence of CdS
When CdS is excited by photons of energy greater than the band gap
the characteristic luminescence which results form the decay of excited
states is shown to consist of two primary emission bands. The first,
known as the "green-edge emission" is due to the edge emission of
various states in the band gap, i.e. shallow donor-acceptor
recombinations. Studies of the edge emission of CdS were made by
Kroger as early as 1940. At a slightly higher energy, a band known
as the "blue-edge emission" occurs and is due to emission from free and
bound exciton complexes.^ A typical low resolution spectrum
displaying these two bands is shown Figure 6, and a high resolution
spectrum of a portion of the blue band is shown in Figure 7. The
intensity of the bands is dependent upon the polarization of the
incident light with respect to the c-axis of the crystal.

RELATIVE INTENSITY
33
WAVELENGTH IN ANGSTROMS
Low resolution photoluminescence spectrum of CdS single
crystal showing "green-edge" emission due to band edge
recombinations and "blue-edge" emission due to excitons.
Figure shows emissions are dependent upon the polarization
of the excitation source.^
Figure 6

FLUORESCENCE
34
WAVELENGTH Cnm)
Figure 7 High resolution photoluminescence spectrum of CdS single
crystal at 4.2 K, showing bound exciton peaks for
excitation perpendicular to the c-axis. Peaks labeled P
are observed with parallel excitation.^

35
The exciton states were first extensively studied and
characterized by Thomas and Hopfield.19 They have shown that within
the higher energy band a number of sharp luminescence lines occur which
correspond to transitions of both free excitons (A, B, and C) and
excitons bound to neutral donors or acceptors. A designation of 1^ was
given to excitons which are bound to neutral acceptors and I2 was given
to the excitons bound to neutral donors. These two peaks are labeled
in Figure 7. The distinctions between the various transitions were
made by using the Zeeman effect. When a strong magnetic field is
imposed on the sample, many of the luminescent lines split due to the
spin moments of the ground and excited states. From the group theory
of bound complexes,^ they were able to assign the transitions to the
different defect states.
A later study by Henry, Faulkner, and Nassau showed for the first
time donor-acceptor pair lines in the photoluminescence spectra of
CdS.^O These pair lines are narrowly spaced transitions which were
observed in the green-edge emission band and correspond to closely
spaced donor-acceptor pair-recombination bands. Again the confirmation
of these lines was made by Zeeman experiments. The significance of the
study is that for the first time direct spectroscopic evidence for the
existence of these states was made. This is important point because in
the study by Thomas and Hopfield it was assumed that these states must
exist based on the Zeeman experiments, but they had no direct evidence.
Henry, Nassua, and Shiever studied the impurity doping of CdS and
showed that Na and Li are the only shallow acceptors that can act as
substitutional impurities.^ These shallow acceptors give rise to the

36
1^ bound exciton that was described by Hopfield and Thomas. Usually
two 1^ lines are observed, and by varying the doping level these
authors proved the lines to only be due to Na and Li. High purity
crystals grown in clean reactor tubes were found to only exhibit the
Ii(Li) line. When Na was added a considerable broadening of the 1^
line occurred, but as successive runs were made in the same tube, these
authors showed the Na line could be resolved. Doping with K, Rb, or
Cs, only resulted in a sharp I^(Li) line, and P was found to give a
complex shallow acceptor. The identity of the shallow donor level
responsible for the I2 bound exciton could not be determined; however,
the authors showed by donor-acceptor pair line splitting that the donor
was not a native double donor such as a cadmium vacancy, or a sulfur
interstitial. The authors reasoned that the donor may be Na or Li
interstitials because of the small size of these atoms, and when
crystals are heavily doped, they become highly compensated.
Unfortunately they were unable to prove the identity of the donor.
Later work by Henry and Nassau involved measuring the
spectroscopic lifetimes of the two bound exciton complexes.^2 They
were basing their work on another study by Thomas and Hopfield which
showed the measured oscillator strength of the bound excitons to be
very large, which meant that the radiative decay of the weakly bound
exciton would be very fast. Thomas and Hopfield determined an
oscillator strength of 9 +/-2 corresponding to a radiative lifetime of
0.4 +/-0.1 nsec.^ This oscillator strength is about 10^ greater than
the strength of a free exciton.^ Henry and Nassau with their
experimental setup were able to measure a radiative lifetime for the I2

37
exciton to be 0.5+/-0.1 nsec. The very fast decay time of this state
is what makes CdS so attractive for nonlinear optical application.
Resonant Raman Scattering in CdS
Further studies of exciton levels in CdS have involved the
measurement of multiple phonon scattering from exciton states. Leite,
Scott, and Damen found that the scattering of longitudinal optical (LO)
phonons was enhanced when the scattering-phonon frequency coincided
with that of excitons.^ They were able to show up to nine orders of
resonant Raman scattering occurring at frequencies shifted less than 1%
from multiples of the 305 cm'^ line (the first LO line). These authors
were not able to determine the identity of the exciton state (i.e. free
or bound) which was acting as the intermediate state for the resonant
scattering in this study, butin a latter paper by Leite, Scott, and
Damen free excitons were proven to be the primary intermediate state.^
They also showed that bound excitons participated as intermediates by
observing phonon sideband features on the photoluminescence of the
bound excitons. The 1^ exciton was found to be a stronger resonant
state compared to the I2 bound exciton.
In a more recent paper by Mashshenko,^ the temperature dependence
of the LO and 2L0 lines associated with the A exciton were
investigated. At 77 K the A-LO phonon predominates the emission, but
as the temperature is increased to 110 K, this line decreases and the
A-2L0 line increases. This increase in the 2L0 line indicates that an
increase in the probability of a two-phonon process occurs at high
temperatures.

38
Nonlinear Susceptibility of CdS
Even with the obvious advantages of using CdS for nonlinear
optical applications, very few groups have investigated this material.
The primary amount of work in this area on CdS has been carried out by
Dagenais. ^ As would be expected from the large oscillator
strength of the bound exciton, a very large nonlinear refractive index
results when this level is saturated, and the decay time is very short.
By use of a Fabry-Perot arrangement and a narrow bandpass tunable dye
laser, Dagenais reported a cw saturation intensity for this level of
only 58 W/cm^, which corresponds to a nonlinear index of refraction of
1 X 10^ cm^/W.^ In a later publication, Dagenais and Sharfin^ report
that by using a high finesse Fabry-Perot, a saturation intensity of
only 26 W/cm^ is required, which for the experimental setup corresponds
to a nonlinear refractive index of 2 X 10^ cm^/W. They also reported
a switch up and switch down time of one and two nanoseconds
respectively. These are the largest values of a nonlinear refractive
index ever to be reported.
Other work on CdS has been done by Bohnert, Kalt, and
Klingshirn. 0 They did not, however, study nonlinearity by exciton
saturation, but rather by the formation of an electron-hole plasma.
High intensity laser pulses of energies just above the band gap energy
were used to study this effect. By measuring the temporal line shape
of the transmitted laser pulse they were able to determine the
renormalization of the band gap due to the formation of an electron-
hole plasma. Because this is a much smaller effect, intensities of 120
kW/cm^ were required to produce a change in the transmitted pulse.

39
CdS Thin Films
Vacuum Deposition
A great number of studies have investigated the thermal
evaporation of CdS for the deposition of thin films. Although all thin
films for this study were deposited by RF-magnetron sputtering, some of
the results of these investigations are relevant to the present work.
Many of the studies of thermal evaporation were undertaken to research
the electrical properties of CdS thin films, although some optical
properties have been studied. The problem is that most of these
studies present results which both show differences from single crystal
results and also differ from each other.^ The reasons for the
discrepancies are related to the difficulties in evaporating CdS, which
lead to problems with maintaining stoichiometry and crystal structure
in the deposited films. The difficulty in evaporating CdS, as well as
other chalcogenide compounds, is that complete dissociation of the
compound occurs during evaporation.^^ If too high a temperature is
used, then the compound will dissociate incongruently and because Cd
has a higher volatility than S, non-stoichiometric thin films result.
Source temperatures between 650 and 700 must be accurately controlled
to avoid excess cadmium.^^ Due to differences in the sticking
coefficients of Cd and S, a non-stoichiometric thin film will also
result if the an improper substrate temperature is used. Cook and
Christy^' have found that if fused quartz substrates are used at
temperatures greater than 200 C, then non-uniform films result. In
comparison, Wohlgemuth et al.^9 have found that if the substrate is

40
cooled to LN2 temperatures, then an amorphous film results which is Cd
rich.
The optical properties of CdS thin films are central to this
study; however, the reported results for vacuum vapor deposited thin
films are found to vary greatly. Most notably is the variation in the
reported optical band gap and absorption coefficient, and the
calculated index of refraction. Some of these differences can be
realized by examining Figure 8 and Figure 9 which show the refractive
index and absorption coefficient obtained by several authors. y
Referring to Figure 8 the results for single crystal CdS obtained by
Cardona and Harbeke-^ are given by curve a. Curve b is for
polycrystalline film deposited onto fused silica at 180 C by Khawaja
and Tomlin. Curve c corresponds to a polycrystalline film also
deposited on silica at 180 C by Wohlgemuth et al.^9 and curve d is for
an amorphous film deposited at LN2 temperatures by Wohlgemuth et al.
The same authors correspond to the same curve letters of the reported
absorption coefficients shown in Figure 9. As shown in Figure 8 the
results by Khawaja and Tomlin show the closest resemblance to single
crystal results, while the Wohlgemuth et al. results show a marked
difference. In contrast, as shown in Figure 9, the absorption
coefficient of polycrystalline films deposited by Wohlgemuth et al.
model single crystal results best. The highest quality films in these
studies were deposited at high temperatures, although a more recent
study by Cook and Christy*^ reports similar results for room
temperature depositions. It is obvious that subtle differences in the
deposition technique result in a wide variation of thin film

41
M)
Figure 8 Variation of index of refraction reported by several
i ? Q
authors. *

Ul
igutre 9
fion of the
Variatio^g
authors-
absorption
coefficient
reported

43
properties. This is probably due to differences in thin film
microstructures, which in the above studies were only determined by X-
ray diffraction. No direct measurements (i.e. electron microscopy)
were used to examine thin films.
Although some of the optical properties of evaporated thin films
have been investigated, albeit there are differences in the results,
apparently very few studies have been made of the photoluminescence of
thin films produced by this technique. Christmann et al. c describe
evaporation deposited epitaxial CdS films which displayed both green
and blue edge luminescence. This is one of the first studies to relate
both the morphology and composition of thin films to the observed
photoluminescence. Thin films were deposited on cleaved surfaces of
SrF2 at temperatures from 210 to 310 C. Variation of the
supersaturation by controlling either the source temperature or the
substrate temperature resulted in three basic morphologies. At low
supersaturations, smooth CdS thin films were produced which only
displayed broad band green photoluminescence. By decreasing the
substrate temperature or increasing the source temperature, films were
produced which displayed a structure with many hexagonal flat tops.
These films showed green edge emissions and very low intensity blue
edge emissions. Finally, at very high supersaturations, films which
displayed a morphology with many hexagonal pyramids were found to show
very intense blue edge, or bound exciton photoluminescence.
Through cathodoluminescence and microprobe studies it was
determined that the hexagonal pyramids were cadmium rich, and they did
not contribute to the luminescence. Only the areas adjacent to the

44
hexagonal pyramids were found to show bound exciton luminescence. The
pyramids were thought to have nucleated from Cd droplets, and as they
grew, the adjacent areas were depleted of cadmium. The required donor
states for I2 bound excitons were therefore provided by cadmium
vacancies. Other films were found to be uniformly slightly cadmium
rich, and it was thought that cadmium interstitials were responsible
for the broad band photoluminescence.
Humenberger et al.^3 also reported thin films which displayed
bound exciton luminescence. These films, however, were made by a hot-
wall epitaxial technique, which is similar to the vapor phase technique
used to make bulk single crystal platelets. Films were deposited on
BaF2 substrates. The surface morphology was found to be smooth with a
low density of hexagonal flat tops. No compositional data were given
for thin films; however, acceptor states were provided by indium doping
thin films during the deposition process. Free carrier concentrations
as a result of the indium doping were measured to be on the order of
1017 to 10^ cm-^.-^
The above two studies are the first to show by photoluminescence
the presence of exciton levels in thin films. Studies of several other
thin film deposition techniques such as chemical bath,-^ or spray
O c
pyrolisisj:> have reported photoluminescence, but none have shown
exciton emissions. To see these transitions, it is apparent that very
high quality thin films are required, and it is obvious that standard
vacuum techniques or other techniques such as those described here are
not capable of producing thin films of the necessary degree of quality.
A deposition technique which permits greater control over the

45
deposition process is RF sputtering, and several investigations of the
deposition of CdS by this technique are presented below.
RF Sputtering
Only recently has the deposition of CdS thin films by RF
sputtering been investigated to any great extent. One of the first
investigations was reported by Lagnado and Lichtensteiger. They
described some of the properties of CdS thin films produced by RF-diode
sputtering. A very strong preferred orientation of the CdS
crystallites was found to occur with the c-axis parallel to the
substrate plane. Electrical resistivity measurements at different
temperatures revealed two activation energies for conduction,
interpreted to show the activation of an unspecified trap below the
conduction band, although no data on the chemical analysis for
impurities was presented.
Recently the technique of RF-diode sputtering has received
considerable attention for producing CdS thin films for solar cell
heterojunctions.^^ The large photoconductivity of CdS makes it an
attractive material for both solar cell and sensitive photodetector
applications. A number of investigations on the RF diode-sputtering of
CdS for these applications have been reported by Martil et al.-^41
These workers have investigated the dependence of the physical,
electrical, and optical properties on both deposition parameters and
post deposition heat treatments.
The earliest publication by Martil et al.^ outlines the
dependence of deposition rate, thin film grain size and resistivity as

46
a function of sputtering power, pressure, substrate temperature and
substrate bias. Thin films were deposited onto fused silica
substrates. The first effect they describe is the dependence of the
deposition rate and resistivity on the pressure of gas used. A maximum
in the deposition rate and a minimum in resistivity occurred for a
sputtering pressure of 5 pin (5 X 10"^ torr). For higher pressures they
found the rate actually decreased, and the resistivity increased. The
decrease in deposition rate at higher pressures was explained by a
backscattering mechanism which increases the probability that more
sputtered atoms will return to the target at higher pressures. The
minimum in resistivity at 5 pm pressure was claimed to be a result of
the maximum deposition rate that occurred at this pressure. Although
no direct proof was given, the authors claim the high deposition rate
decreased the number or concentration of impurities that were trapped
in the film.
The second effect described in this early paper by Martil et al.^
was the dependence of thin film grain size on the substrate temperature
and substrate bias. The grain size was determined by SEM observations.
As would be expected, the grain size of films increased from 300 to
3500 as the substrate temperature was increased from 90 C to 300 C.
The deposition rate accordingly decreased as the temperature was
increased. When the substrate bias was increased to above -100 volts,
the resulting thin films were found to be amorphous. The temperature
effects can be explained by considering that with increasing
temperature 1) the critical size for a nucleus increases, 2) the
surface diffusion coefficient increases and 3) the sticking coefficient

47
for a material decreases.28 The appearance of an amorphous structure
with high substrate bias was explained by the authors as a result of
structural damage that occurred due to ion bombardment (which was
increased by the high negative bias).
The second report published by Martil et al.39 further explored
the effects of substrate temperature and bias on the electrical
properties of CdS thin films. They found that the resistivity
increased from 10 to 108 Q cm as the temperature was increased from 60
C to 250 C, which they claimed was due to a change in stoichiometry
(i.e. a loss of cadmium with increasing temperatures), although no
chemical analysis was presented to back up this claim. A minimum in
resistivity was also found with a substrate bias of -50 volts, or with
a floating substrate (which developed a self bias of -28 volts). This
effect was explained by the ion bombardment that results from these
bias voltages preferentially resputting oxygen and other impurities
from the growing film. Oxygen has been shown to be a acceptor-like
trapping center, located 0.9 eV below the conduction band.^2 Again no
chemical analysis was reported to prove that a decrease in oxygen
occurs. Also the variation in activation energy for conduction that
occurs with temperature as seen by Lagnado and Lichtensteiger3^ was not
explored by these authors.
The third publication by Martil et al.^ reports the influence of
the above sputtering parameters on the optical properties of thin
films. The structural changes which occur under different conditions
were also further explored. Increasing the substrate temperature
resulted in similar structural changes to those reported in the earlier

48
study; the grain size varied between 500 and 3000-4000 as the
substrate temperature was increased from 60 to 300 C. This report,
however described the sputtering pressure dependence of the
crytstallinity. The sputtering pressure determines how much structural
damage occurs due to ion bombardment. At low pressures, bombardment is
enhanced by a large self-bias that develops on the substrate, and an
amorphous structure results as previously described. At higher
pressures, the crystallinity was also found to decrease, which was
explained by the authors as due to a porous structure that develops
from trapped gases.
Optical properties were found to be a function of both substrate
bias and sputtering pressure. A maximum in the optical band gap (2.36
eV) was found to occur for a floating substrate bias, whereas the
minimum in the band gap (2.30 eV) occurred for a -110 volt bias. The
band gap was found to increase with an increase in sputtering pressure
and became nearly constant for pressures above 10 pm. The pressure
dependence of the refractive index however contradicts the pressure
dependence of the band gap. The index of refraction was found to be a
maximum with 5 pm pressure, and it decreased with increasing pressure.
This was explained in terms of the porous structure which occurred at
higher pressures; however, if the refractive index is reduced at higher
sputtering pressures due to a more porous, less crystalline structure,
then the band gap energy should also decrease. An increase in the
number of crystalline defects should cause tailing of the band gap,
which is seen as a decrease in the gap energy.^9

49
The latest investigation to be reported by Martil et al.^
describes the effects of heat treatments on the electrical and optical
properties of sputtered films. Heat treatments were carried out under
H2 and N2 atmospheres at temperatures ranging from 100 C to 550 C and
for times ranging from 20 minutes to 5 hours. The primary effect on
the electrical properties was a two order of magnitude reduction in the
resistivity to 4 X 10" £2 cm for heat treatments at 200 C. Carrier
mobilities accordingly increased to 50 70 cm^/Vs for this treatment.
Higher temperatures were found to increase the resistivity and decrease
the mobility. The decrease in resistivity was explained by claiming
that oxygen desorption from the grain boundaries occurs at 200 C. As
previously described, oxygen is a trapping center which in this case
decreased the carrier concentration and simultaneously increased the
scattering which reduces the mobility.^ No explanation was given for
the increase in resistivity with increasing temperature, although it
could be due to a loss of cadmium.
The optical absorption edge was shown to become sharper and occur
at a higher energy for heat treatments at 200 C. Temperatures higher
than this were not reported to significantly alter the absorption edge;
however, treatments above 550 C produced an overall decrease in the
transmission. This was explained by a dissociation or reevaporation
process that occurred as a consequence of the higher temperatures. The
optical band gap was found to increase from 2.36 eV to 2.39 eV for
treatments at 200 C, which was again explained in terms of oxygen
desorption from the grain boundaries, although no chemical analysis was
reported to prove this hypothesis.

50
Semiconductor Doped Filter Glasses
Sharp cutoff filter glasses are silica based glasses which contain
a fine dispersion of semiconductor microcrystallites. The variation of
the sharp absorption edge position is achieved by either varying the
composition and or heat treatment schedule. Great interest has been
generated recently because it is thought by many authors that the
crystal size developed in these materials by thermal treatments is on
the size order for quantum confinement effects to occur. For glasses
containing a mixture of CdS and CdSe many authors have reported a blue
shift in the absorption band edge. In addition, a blue shift of the
high energy exciton photoluminescence peak has been interpreted in
terms of quantum confinement.
Warnock and Awashalom^^ in two publications on mixed crystal
glass (those containing CdS.27/Se.73) report that the glasses which
displayed smallest size distribution (average size = 94 ) exhibited
the largest shift in the exciton photoluminescence, and this
photoluminescence was shown to decay on a time scale of only 18 psec,
which is nearly two orders of magnitude faster than the lifetime
displayed by excitons in bulk material.^ The shifts in peak position
and fast lifetime were interpreted in terms of a quantum confinement
effects.
A very recent paper by Borrelli et al.^ has shown that the blue
shift in the photoluminescence displayed by this particular mixed
crystal glass is not due to confinement effects, but rather to
compositional effects. A careful study of the crystal structure by X-
ray diffraction revealed that a change in the stoichiometry occurs

51
during the heat treatments which develop the microcrystallites. It was
postulated by these authors that more of the selenium remains in glass,
while sulfur is more easily incorporated in the crystallites.^
Therefore, at the lower temperatures which produce the smaller
crystallites, the crystallites end up containing more sulfur, so the
optical properties of glasses containing these crystallites are blue
shifted toward the optical properties of pure CdS. True quantum
confinement effects were, however, shown to occur in a series of
experimental glasses which contained either CdS or CdSe, but not both.

CHAPTER IV
EXPERIMENTAL METHOD
Vacuum Deposition System
Thin films for this study were made with a specially designed
vacuum system. A schematic of the deposition chamber is shown in
Figure 10, and a picture of the complete system is shown in Figure 11.
The deposition chamber consists of a rectangularly shaped stainless
steel box which measures 12" X 18" X 18". This chamber was custom
built by MDC Vacuum Corp. (Haywood, CA.). Numerous ports and
feedthroughs on the chamber permit a wide variation of deposition
configurations. The vacuum pumping system utilizes a 330 1/sec
turbomolecular pump (Balzers, Hudson, NH), a molecular sieve trap (MDC
Vacuum Corp.), and a 300 1/min two-stage mechanical pump (Sargent
Welch, Skokie, IL). With the molecular sieve trap activated, pressures
in the high 10"^ torr range were possible using the mechanical pump
only. Pumpdown from atmosphere pressure to 1X10"^ torr could be
accomplished in 2 hours with use of the turbopump. Vacuum gauging was
performed with a Leybold Heraues (East Syracuse, NY) model CM 330
combined Penning discharge and thermocouple gauge controller.
Pressures for sputter deposition were controlled by using a
micrometer adjustable throttle valve (Sputtered Films Inc. Santa
Barbara, CA.) and a mass flow meter/controller (Matheson Gas Products
Norcross, GA). High purity argon gas (99.9995%) was used as the
52

53
CRYSTAL
THICKNESS
MONITOR
cmo
O
LN2 COOLED
SUBSTRATE HOLDER
1
CAPACITANCE
MANOMETER
SHUTTER
mill ""'"'ll,,,
TURBOMOLECULAR PUMP
Figure 10 Schematic diagram of sputter deposition chamber. Computer
figure shown to indicate computer control of system.

54
Figure 11
Photograph of complete deposition system showing
sputtering chamber and instrumentation.

55
sputtering gas. Initially the thermocouple vacuum gauge was calibrated
with a capacitance manometer (MKS Instruments Inc. Burlington, MA) for
different settings of the throttle valve and mass flow controller.
Based on the previous work on CdS sputtered films, a pressure of five
microns (5X10"^ torr) was chosen and the system was calibrated for this
pressure. To obtain this pressure an argon flow rate of 20 cc/m was
needed.
Film thickness during deposition was monitored with a Leybold-
Heraeus IC 6000 deposition controller. This instrument utilizes a
quartz crystal oscillator microbalance for determination of film
thickness. All parameters of calibration and deposition control are
software programmable with this unit, either via the front panel
controls, or through an RS-232 communications link to an external
computer. The IC 6000 permits monitoring of up to six different films,
and by using a sample and hold program, two different films can be
monitored and controlled simultaneously. Close loop control of
deposition rates was achieved by connecting the IC 6000 thickness
monitor to the RF power supplies.
For semi-automatic control and constant monitoring of the
deposition process a Zenith Z-158 personal computer with 512K RAM and a
20 megabyte hard disk was interfaced to the system. A Dascon 1 I/O
board (Metrabyte Corp.) which provides four channels of analog and 12
bits of digital input/output was installed in the computer. With this
interface board and with RS-232 communications to the IC 6000
deposition controller, it was possible to monitor nearly every aspect
of the deposition process. The RS-232 communications permitted direct

56
programming control over deposition parameters and constant monitoring
of deposition rate, thickness, power output and crystal oscillator
status. The analog portion of the I/O board was connected to the
pressure gauge controller and to the RF power supplies. Connection to
the power supplies permitted recording of the DC bias which develops on
the target as a result of the sputtering process. All of these process
parameters were stored on disk during a deposition run. This made it
possible to go back and review the deposition process, if a film was
later found to have anomalous properties.
A unique feature of the deposition system has to do with the
various substrate holders which could be used. For low temperature
depositions, a liquid nitrogen (LN2) cooled substrate holder was
employed. This holder is made up of two double concentric tubes in
which the center tube acts as a reservoir for LN2 and the outer tube,
which is open at the bottom to the vacuum system, acts as an insulating
vacuum jacket. An aluminum plate with a slightly recessed area to hold
the substrate is attached to the end of the inner reservoir tube with a
copper screw. To reduce the amount of contamination that would occur
on the cold substrate, an additional LN2 cooled coil was positioned
around the substrate holder assembly. This coil is referred to as the
cryoshield in Figure 10. For room temperature depositions, the same
holder was used, however, without filling the LN2 reservoir.
High temperature depositions were accomplished by replacing the
LN2 feedthrough assembly with a resistively heated substrate holder.
This holder was made by drilling holes lengthwise in a thin aluminum
block and inserting nichrome wound ceramic heaters. A Chromel-Alumel

57
thermocouple was embedded in the core of the holder, and control of the
temperature was made by controlling the input power to the holder with
a variable autotransformer. Substrates could be heated to 300 C with
this arrangement with a temperature control of +/- 5 C.
A single planar magnetron sputter gun supplied by US Guns
(Campbell,CA) was used for sputter depositing the CdS thin films. The
gun accepts two inch diameter targets, which can range from l/16th to a
quarter of an inch in thickness. An Eratron RF power supply
(Campbell,CA) operating at 13.57 MHz with a power capability of 600
watts was used to supply RF power to the gun, through an auto load
match tuning network. The matching network is require to balance the
impedance of the gun and plasma to the 50 ohm output from the power
supply. If the impedance is not matched, then the RF power is
reflected, and the only thing that is accomplished is heating of the
heat sinks in the power supply. The utility of an auto load network is
that when conditions of the plasma change, the network will
automatically match the impedance, thereby eliminating any reflected RF
power.
High purity cadmium sulfide sputtering targets were obtained from
CVD Industries (Woburn, MA). These targets are made by a chemical
vapor deposition process so they are supplied with a bulk density of
nearly 98% of theoretical density. High bulk density reduces the
amount of outgassing during sputtering. All impurities were less than
1 ppm and the stoichiometry of the target material was slightly cadmium
rich (50.46%). Nearly all thin films for this study were made from CVD
targets; however, a different type of sputtering target was obtained

58
from EM Chemicals Inc. (Hawthorne, NY) for testing. This target was
made by a hot pressing technique, which results in a lower bulk density
and a higher impurity content. Impurity information was not given by
the company for these targets; however, thin films that were sputtered
from these targets were found to contain much higher levels of iron
than the levels found in films sputtered from CVD targets.
Targets that were used for COSAD were made from a piece of glass
obtained from Schott Glass Industries (Duryea, PA). The glass was a
special type of borosilicate crown glass (#8329) which is similar in
composition to a BK-7 glass, but it is processed in such a way so that
all the volatile impurities in the glass are removed. This glass is
sold by Schott as a special type of over coating glass used for thermal
evaporation. Targets were made by first sectioning the as supplied
cylinder of glass into 3/16 inch discs from which two inch blanks were
cut. Both sides of a blank were polished to 600 grit before being used
as a sputtering target. After it was found that a high power level was
required to sputter this glass, a 1/I6th inch copper plate was bonded
to one blank to improve thermal contact to the sputter gun. The
bonding was accomplished with an indium tin solder.
For producing COSAD films, the deposition chamber was reconfigured
to accept a second RF planar magnetron sputter gun, for sputtering the
glass target. This gun was used with a second 600 watt Eratron supply
and the two power supplies were driven by a common oscillator to phase
match the two plasmas. In addition, a set of baffling plates were
mounted inside the chamber as shown in Figure 12. The center baffle
plate was used to isolate the two plasmas. A pair of three inch

59
Figure 12
Schematic diagram of deposition chamber configured for
COSAD.

60
diameter apertures centered over the two sputter guns were cut in the
top plate to provide access to the two plasma sources. A dual
substrate holder was rotated in the plane above the two apertures, so
that the substrate was alternatingly exposed to one source and then the
other. The dual holder provided the capability of producing two films
during a single deposition run. Initially a DC motor with a reducing
gear drive was used to rotate the substrates with a linear speed. A
more sophisticated direct drive stepping motor was later added to the
system to permit variable exposure to the two plasma sources. The
stepping motor was interfaced with the Dascon I/O board in the Z-158
computer via an external digital control circuit.
One disadvantage of the COSAD configuration was that substrates
could not be heated or cooled and the temperature of the substrate
could not be monitored during a deposition. The utility of this
particular system configuration, however, is that single component
films could still be produced, simply by positioning the substrate
holders over the respective apertures, and exposing the substrate to
deposit a film.
RF Magnetron Sputtering
As described in the Bibliographic Review section, the only sputter
deposition of CdS was carried out by RF-diode sputtering. In this
configuration two parallel plates are used. The target is attached to
the cathode plate, and substrate is attached to the anode plate, which
can either be at ground or floating potential. A plasma is generated
between the two plates by applying RF power to the cathode plate.

61
Electrons in the plasma can be accelerated by the positive half of the
RF field, which results in both the ionization of argon atoms, and the
bombardment of the target. This bombardment results in the development
of a negative potential on the target surface, which is called the DC
bias. The argon ions in the plasma are too massive to be accelerated by
the RF field; however, they are accelerated towards the target by the
DC bias, which results in sputtering of the target material.
The configuration of diode sputtering results in exposing the
substrate and growing films to the full energy of the plasma. The
growing film is continuously bombarded by a number of energetic
particles, both charged and neutral. This bombardment can either be
enhanced or reduced by applying a potential to the substrate during
deposition. Substrate biasing can only reduce the amount of
bombardment to a certain extent, essentially because the substrate is
totally emersed in the plasma during deposition. Considerable heating
of the substrate and film occurs as a result of the bombardment, even
at moderate plasma powers.
RF planar magnetron sputtering reduces the amount of bombardment
by incorporating a strong permanent magnet behind the cathode assembly.
The configuration of a US sputter gun is shown schematically in Figure
13. Curved magnetic field lines force the electrons in the plasma into
circular orbits, which both enhances the plasma immediately above the
target, and reduces the amount of electron bombardment on the growing
film. The target is a circular disk that is secured to the magnet
housing, which comprises the cathode. The anode is comprised of a
circular cap (called the ground shield in the Figure) which fits over

62
GROUND
Figure 13 Cross-sectional view of RF magnetron sputter gun made by US
Guns Inc.46

63
the gun assembly. Most of the plasma is constrained to the area
immediately above the target, so a large reduction in substrate
bombardment and substrate heating is realized. The substrate and
growing film, however, can still be bombarded by energetic neutral
particles, which has been shown to be a major cause of substrate
heating.^ Still the heating is reduced and as will be pointed out in
the Results section (Chapter 5), a considerable difference in the as
deposited film properties occurs with RF planar magnetron sputtering,
compared to RF diode sputtering.
Thin Film Deposition
Prior to deposition of thin films, all substrates were subjected
to a three phase cleaning procedure. The first phase of the process
was an ultrasonic bath in DI water for 10 minutes. After this
treatment the substrates were removed from the water and rinsed with
isopropyl alcohol (IPA). A second ultrasonic cleaning was then carried
out for 10 minutes in IPA. The final phase of the cleaning procedure
was a 15 minute treatment in a vapor degreaser, which employed IPA as
the solvent. Substrates were slowly removed from the vapor, which
allowed the condensed vapor droplets to evaporate. To check for
cleanliness substrates were examined by edge illumination against a
black background. This permitted observation of any contaminates on
the surface of the substrate in addition to any small flaws in the
surface.
To deposit thin films, substrates were mounted on a substrate
holder and positioned in the deposition system immediately after

64
cleaning. The deposition system was pumped down to a pressure of < 1.0
X 106 torr before backfilling with the sputtering gas. The IC-6000
was usually programmed for a fast ramp up to the desired power level
and then a rate controlled presputter was initiated before the shutter
was opened. If a sputtering target was newly installed, a presputter
burn-in of 30 minutes was carried out; otherwise the target was
presputtered for 5 minutes before exposing the substrate to the plasma.
The IC-6000 automatically controlled the deposition rate and thickness.
Deposition rate was varied between 1 to 5 /sec and films for optical
analysis were typically made 1 pm thick, although the film thickness
for other analysis was varied.
Substrate materials
Several different types of substrates were used to support thin
films, depending on how the films were to be analyzed. Standard
substrates for optical analysis were made of optical quality fused
silica, typically one inch square and 1 mm thick. These substrates
were found to have very few surface flaws and were essentially
transparent over the optical region in which films were analyzed. The
low index of refraction (n=1.46) of these substrates was also necessary
for measuring the refractive index of the film and for planar waveguide
measurements. Some films were also deposited on silica for TEM
analysis. Self-supporting TEM specimens were prepared by floating
these films off the silica with a 1% HE solution.
Other films for optical analysis were deposited onto single
crystal sapphire substrates. This was done to determine if the high

65
thermal conductivity of the sapphire would aid in reducing heating by
the laser beam during low temperature spectroscopic measurements.
These substrates were of very high quality with no detectable flaws and
a very smooth surface finish. Some COSAD films were deposited onto
sapphire substrates to aid in X-ray chemical analysis, since the
sapphire had no X-ray lines which overlapped the lines in these films.
For other TEM analysis, films were deposited on slabs of single
crystal NaCl and on TEM grids which held carbon support films. The TEM
grid-carbon film combination was used for quick analysis of deposited
films. Once the grid-carbon film assembly was prepared, thin films
could be deposited directly onto the carbon film, and no other sample
preparation was necessary. Films of thickness from 500 to 3000 were
typically deposited for TEM analysis. The carbon support film was made
by thermally evaporating approximately 200 of amorphous carbon onto
sheets of single crystal mica. After the carbon film was floated off
the mica, 300 mesh TEM grids were carefully placed on the floating
film. The film with grids was then picked up on a thin cardboard card
and allowed to dry.
Single crystal NaCl was used as a substrate because it was found
that after heat treatment or as a result of a high temperature
deposition, thin films of CdS could not be floated off silica
substrates. Normally the 1% solution of HF would readily float films
off silica, but after any type of heat treatment above 200 C not even
a high concentration of HF was successful for floating the films.
Apparently some type of chemical bond forms between CdS and the silica
even at relatively low temperatures. Single crystal NaCl substrates

66
were also required for TEM analysis of COSAD films, since the HE
solutions normally used to lift CdS off silica slides would strongly
attack these films.
Substrate temperatures
Several different substrate temperatures were used to investigate
the effect of deposition temperature on thin film grain size. With
the LN2 feedthrough assembly that was described above, substrates could
be cooled to LN2 temperatures in about 20 minutes. For some
depositions a low temperature thermal contact paste was use to mount
the substrate in the holder, although most frequently, standard hold
down clips were used. An Iron-Constantan thermocouple was initially
used to monitor the temperature of the substrate holder during a
deposition. To use a digital meter to measure the output from this
thermocouple during a plasma deposition, an RC circuit had to be used
to decouple the RF signal that was imposed on the DC thermocouple
signal. The RC circuit is essentially a low band pass filter. A
schematic diagram of the circuit is shown in Figure 14. If this filter
is not used, a very large RF signal can be induced in the lead wires,
which will destroy most D/A converters used in digital meters.
For depositing films at room temperature, the LN2 feedthrough
assembly (without LN2 in the reservoir) was used to hold substrates.
These depositions were very close to room temperature, since it was
found that even with the highest deposition rates, the holder would
only heat up to 35 C. When substrate temperatures greater than this
were required, the LN2 assembly was replaced with the resistively
heated holder. Temperatures as high as 300 C could be obtained in

Figure 14
Schematic diagram of RC decoupling circuit used protect
digital equipment from RF transients.^

68
about 10 minutes with this holder, although most depositions were made
at 200 C. The unique feature of this holder is that it permitted
outgassing of the substrates prior to deposition. This was done by
heating to 200 C for 30 minutes in high vacuum.
Deposition rates
A variation in deposition rates was also studied to determine its
effect on thin film properties. For pure films of CdS, rates from 1
/sec to 5 /sec were used. These rates were monitored and controlled
by the IC 6000 deposition controller. Figure 15 shows a graph of the
sputtering rate versus power density for both CdS and BK-7 targets. As
shown, the glass target used for making COSAD films is much more
difficult to sputter, as an input of 275 watts (13.5 W/cm^) only
resulted in a deposition rate of 2 /sec. Power levels greater than
300 watts could only be used with the BK-7 glass target which had a
copper backing plate bounded to the back of the target. Even at 500
watts, however, a deposition rate of only 4 /sec could be obtained.
At this power level, considerable heating of the substrate was found to
occur, even when the substrate was rotated during COSAD runs.
Co-Sputter Alternating Deposition (COSAD)
By using the dual sputter gun configuration described above, a
very unique type of thin film could be produced. The acronym describes
the process by which these films were made; by rotating the substrate
above the two sputter guns, an alternating layer of glass and the CdS
was deposited, which was repeated continuously. Initially the two guns

deposition rate:
69
Figure 15 Graph of sputtering rate versus input power density for CdS
and BK-7 targets.

70
were set up to sputter deposit at a static rate of 3 /sec. This is
the maximum rate that could be used with the glass target without
causing excessive heating. The circle over which the substrate was
rotated was eight inches in diameter, so with the three inch diameter
apertures used with the guns, the duty cycle was rather low, only 0.12
for each gun. With a rotation speed of 20 RPM, the resulting total
dynamic deposition rate is less than 1 /sec. One possible way to
increase this rate would have been to reduce the rotation speed. This,
however, would also mean that the substrate would be spending more time
over areas which were not depositing film and were actually depositing
impurities.
The use of the computer controlled stepping motor to rotate the
substrate holder alleviated the problem with the linear drive system.
With this setup is was possible to increase the amount of time the
substrate spent over a given source. At the same time, it reduced the
amount of impurities introduced into the film because the stepping
motor could be driven full speed between the two sources, which was
about 120 RPM. By simply altering the computer program, this system
permitted easy variation of deposited structure.
Post Deposition Treatments
To improve many of the physical properties of thin films a post
deposition heat treatment had to be instituted. Generally, heat
treatments on CdS thin films were made in a fused silica muffle tube
furnace under a flowing argon atmosphere. The argon used for heat
treatments was of the same grade as the gas used for sputtering. For

71
annealing COSAD films high purity compressed air was used with the same
furnace setup. Flow rates of 100 cc/m were typically used.
Temperature control was accomplished with a single setpoint temperature
controller (Love Control Co.) which provided temperature control to +/-
Io C. Two thermocouples were used, one mounted outside the tube for
control and one positioned inside the tube adjacent to the sample.
Heating rates of up to 35 C/min could be realized, and cooling rates
of 100 C/min could be obtained by sliding the muffle tube out of the
furnace hot zone. Temperatures for heat treating thin films ranged
from 200 C to 700 C, with times ranging from 1 minute up to 5 hours.
To reduce the amount of grain growth that would occur as a result
of high temperature heat treatments, some films were heated by a
technique known as rapid thermal annealing. This process utilizes high
power quartz lamps to rapidly heat the sample. Because of the low
thermal mass that is achieved with this technique, heating rates in
excess of 500 C/sec are possible. The sample is sandwiched between
two thin sheets of graphite which act as black body absorbers and help
to transfer the heat generated by the lamps to the sample. In this
particular system lamps with a total power of 5000 watts were used.
Heat treatments up to 650 C with times ranging from 10 to 60 seconds
were done under a flowing nitrogen atmosphere.
Thin Film Characterization
Microstructure
Determination of thin film microstructure was achieved using
several analytical techniques. Direct examination of the

72
microstructure was made by electron microscopy. The various TEM
specimens that were described in a previous section were examined in a
JEOL 200 CX analytical microscope (Japanese Electron Optics Laboratory,
Boston, MA.). This instrument was used in both transmission (TEM) and
scanning transmission (STEM) modes to directly observe and measure the
ultra-fine structure of thin films. Selected area diffraction (SAD)
mode was used to conduct electron diffraction experiments for
determination of crystal structure and orientation. In the SAD mode it
was possible to obtain diffraction information from areas as small as 5
um. The line to line resolution observed in the TEM mode was 8 .
Another technique used for determination of crystal orientation was
dark field imaging. By tilting the primary beam in the microscope, the
contrast that is observed in a dark field image is only due to those
crystals or portions of crystals which are oriented for a specific
Bragg reflection. Dark field imaging makes it possible to observe
preferred orientation in a thin film or any portion of a crystal that
contains defects (such as microtwins).^ On the JEOL 200CX changing
from bright field to dark field was accomplished with a single switch.
To study the topographical structure of thicker films a JEOL 35CF
scanning electron microscope was used. Films which were made for
optical analysis were frequently examined with this instrument. The
advantage of this technique for CdS films was that no sample
preparation was required except for sample mounting. The conductivity
of these films was high enough not to warrant coating with a conductive
film, although for COSAD films a thin carbon film had to be deposited
before these films could be examined. With this instrument the

73
presence of gross defects in the film such as pinholes or cracks could
be observed, as well as the fine grain structure of certain films. The
major disadvantage with this microscope, however, was a limiting
resolution of about 300 which made it difficult to observe the grain
size of as-deposited thin films.
X-ray diffraction was another technique used to measure the
crystal structure and orientation of thin films. An APD 3720 computer-
controlled diffractometer (Phillips Co.) was used to carry out
diffraction experiments. Typically, diffraction was measured from two-
theta values ranging from 15 to 90 with a angular step of 0.05. Use
of the computer to store X-ray diffraction spectra on disk greatly
facilitated the determination of the change in crystal structure with
heat treatments. Changes in peak positions and intensities could be
readily determined, and by making high resolution scans (0.02/step)
over certain peaks it was possible to calculate the grain size and film
strain using certain utility programs available in the computer system.
Again, no sample preparation was necessary for this technique as
diffraction from 1 pm films supported on silica substrates was readily
observed.
Chemical Analysis
Initially, Energy Dispersive Spectroscopy (EDS) was used to
measure the stoichiometry of thin films during both SEM and STEM
observations; however, it was found to give results that were not
quantitative. Particularly in the SEM, it was found that the results
were very dependent on sample geometry with respect to the detector. A

74
technique know as X-ray secondary fluorescence was used instead to
measure the stoichiometry of nearly every thin film that was made. The
particular instrument used for these measurements utilized a silver X-
ray tube to produce the primary X-ray beam. Tube voltages of 50 kV
were used with a current of 30 mA. One of four different secondary
targets could be selected to generate the X-rays that were used to
analyze the sample. The advantage of this technique is that the
background radiation from the X-ray tube (bremsstrahlung) is totally
removed and only the very narrow X-ray line corresponding to the
secondary target reaches the sample which results in a great reduction
of the background counts. The particular geometry of this system
limits the radiation to only one polarization which further reduces the
background count.^ Detection levels for most transition metals was ~
0.1 ppm. Beam size incident on the sample is 1 cm in diameter and
penetration depth is only a few microns which makes this technique
ideally suited for measuring thin films supported by substrates. The
X-rays emitted from the sample are analyzed by EDS with this system;
however, due to the above arrangements, a much more quantitative
determination could be made when the results were compared to a
standard reference.
The standard used, in this case, to determine the stoichiometry of
thin films was a piece of target material. The stoichiometry of the
target was verified by a wet chemical technique and electron probe
microanalysis (EPMA) (JEOL 730 Superprobe). For solution analysis a
portion of the target was dissolved in nitric acid and then this
solution was diluted a given amount. The amount of Cd and S in

75
solution was determined by using an induction coupled plasma (ICP)
spectrometer (Allied Analytical Systems, Waltham, MA). By comparing
these amounts to those in standard solutions the stoichiometry was
determined. For EPMA the stoichiometry of the target material was
determined by use of standards and a ZAF or (ppz program. The ratio
of the integrated peak intensities for Cd and S in the target was then
determined by X-ray fluorescence. The ratio obtained from thin films
was compared directly to this ratio to quantitatively determine the
stoichiometry.
Film thickness could be readily determined with this technique by
measuring the intensity of the silicon K-alpha peak that was emitted by
the substrate through the film. The intensity of this line is
exponentially attenuated by the thickness of material that it passes
through and can be related by the following equation:
I = IQ expat (2.1)
where IQ is the intensity from a bare substrate and t is the film
thickness. From equation 2.1 a linear relationship between a function
of the intensity, F(l), and t can be obtained by-^
F(I) = log10 1 (2.2)
*o
The intensity from three different films of known thickness (verified
by profilometry) were measured and by plotting F(l) versus t, a master
curve was generated. The thickness of an unknown film could then be
found by solving the above equation for F(l) using the intensity from

76
the unknown. The x-coordinate on the curve for the unknown F(I) would
give the thickness. Film thickness of up to 3 pm could accurately be
determined with this technique. Changes in density or loss of material
as a result of heat treatment could also be monitored with this
technique.
Several other analytical techniques were used to analyze the
composition of thin films. In each case the results for thin films
were compared to the results obtained from target material. The
different techniques were used to both acquire information, which the
specific technique is most suited for, and to cross-check the
composition of thin films. Auger electron spectroscopy (AES Perkin-
Elmer, Norwalk, CT) gave the least quantitative determination of the
absolute composition, but the technique was very quantitative for
determining differences between samples. X-ray photoelectron
spectroscopy (XPS Kratos,UK) gave semi-quantitative results and also
showed how the surface of thin films were altered by heat treatment, a
result which was not detected with any other technique.
Optical Measurements
By far the most extensive measurements made on thin films were
optical measurements. The availability of several spectrometers,
monochromaters, lasers, and optical hardware greatly facilitated these
measurements. The UV-VIS spectrophotometer used for many measurements
was a Perkin-Elmer Lambda 9 (Norwalk, CT). Photoluminescence was
measured with both a Princeton Applied Research Optical Multichannel
Analyzer (Princeton, NJ), and an Instruments SA Raman spectrometer

77
(Metuchen, NJ). A variety of lasers were used for different
experiments, although most photoluminescence experiments were made with
a Spectra-Physics model 2025 Argon Ion laser (Mountain View, CA). Many
measurements were made at both room temperature and at temperatures
down to 9 K. Low temperatures were obtained with an Air Products
(Allentown, PA) closed-cycle helium cold finger and digital temperature
controller. Experiments carried out with these instruments will be
extensively detailed in the following sections.
Index of Refraction
A very unique technique for measuring the dispersion or index of
refraction as a function of wavelength was used on thin films.
Normally the index of refraction of a material is related to the
percent of transmitted light by the well known equation:
T (1 R)2 exp (-at)
1 R^ exp (-2at)
where the reflection, R, is given by (n l)^/(n +1)^. The situation
is much more complicated, however, for a thin film supported on a
substrate with a different refractive index. The reflection that
occurs at the film/substrate interface must be dealt with as well as
the reflections that occur at the other interfaces. This leads to a
more complicated expression:
T
(1 R].) (1 R2) (1 ~ R3) exp (-at)
(1 R2R3) ( 1 [ R^R2 + R^3 (1 R2)Z ] exp (-2at) }
(4.2)

78
where R^, R2, and R3 are the reflectivities of the air-film, film-
substrate, and substrate-air interfaces, respectively.16 The values of
the Rl, R2, and R3 are given by the same equation stated above with the
refractive indices of the three materials (air, film, and substrate)
substituted for n, respectively. Even if the value of the absorption
coefficient is known, the transmission must be measured to 0.5% to get
a 1% accuracy in the refractive index,which is experimentally very
difficult to do unless a specially designed spectrometer is used.
When the refractive index of the film is significantly different
from that of the substrate (as the case with CdS on silica) a very
strong wavelength dependent interference effect occurs. The effect can
be explained by examining Figure 16. When the incident light beam
passes through the thin film and encounters the interface it will be
reflected due to the differences in refractive indices. Depending on
the thickness and the refractive index of the film (the product of
these two values determines the optical path length) the reflected ray
will either constructively or destructively combine with the incident
beam. This is the same process that occurs in a Fabry-Perot
interferometer described previously. However, instead of changing the
physical distance of the cavity to change the optical path length,
resonance is achieved by scanning the wavelength of incident light. In
terms of the total transmission that is measured, a series of
interference fringes is observed, which vary with wavelength. These
fringes are known as fringes of equal chromatic order (FECO), and the
wavelengths at which maxima and minima in transmission occur is given
by the same equation used for resonance conditions in a Fabry-Perot

79
Figure 16
Interference effects at thin film, substrate, and air
interfaces.

80
m A = 2 n d (4.3)
where m is the order number of the fringe, d is the thickness, and A is
the wavelength. To take advantage of this effect to determine the
index of refraction two assumptions must be made. The first is that
the order number is not a discrete number but that it can vary
continuously. Why this is important will be described shortly. The
second assumption is that the dispersion of the refractive index can be
described by the following equation
n
2 =
+
B
(4.4)
where the square root of B gives the infinite wavelength refractive
index and A and x are constants.
To make use of this effect, a FECO spectrum is obtained at normal
incidence. A typical spectra from a 2 pm CdS film is shown in Figure
17a. Curves are drawn tangent to the maximum and minimum peaks and the
values of the wavelength of the tangent points are determined. The
sample is then rotated in the beam by 30, resulting in an increase in
the optical path length by a factor of l/(cos 30). The assumption
that the order number is a continuous mathematical variable means that
the normal incidence spectrum can be shifted to a shorter wavelength by
increasing each order number by an amount 6m. ^4 This increase produces
a shift in the FECO spectra shown in Figure 17b. Again, tangent curves
are drawn and the tangent points to the interference spectrum are
determined. The difference in the pairs of tangency points are then

TRANSMISSION TRANSMISSION
81
a)
b)
Figure 17 Fringes of equal chromatic order (FECO) spectrum for 2.0 um
thick film of CdS on a silica substrate, a) Normal
incidence spectrum showing many orders; b) Expanded
abscissa scale showing shift in FECO spectrum for 30
incidence.

82
used in an iterative calculation to determine the values of A, B, x and
n.
The problems associated with this technique involve precise
placement of the sample at the focal point of the beam and accurate
rotation of the sample about the axis which passes through the focal
point. These problems were addressed by using a micrometer adjustable
x-positioner and a micro-rotation stage, which are shown placed in the
Lambda 9 spectrophotometer in Figure 18. Normal incidence to the film
was determined by rotation of the stage a few degrees on either side of
the initial zero and observing the shift in position of a given
interference fringe. The true zero point was taken as the position
which produced the midpoint between the two shifts. The focal point of
the beam was then determined by observing the peak value of a given
fringe as the sample holder was moved with the positioner.
The most difficult part of this technique (except for writing the
iteration program for the analysis) was the manual determination of the
value of the tangency points from the FECO spectra. The wavelength of
the tangent point must be known to a precision of 0.1 nm to obtain an
accuracy of three decimal places in the refractive index. Wavelengths
can be measured with the spectrometer to this precision, but this
precision could not be obtained by manual measurement of the tangent
point from the printout of the spectra.
The problem was alleviated by interfacing the Lambda 9 to an IBM
AT computer. Use of the computer greatly facilitated all aspects of
using the spectrometer as spectra could be stored on disk and

83
Figure 18 Photograph of micro-positioner stage used for acquiring
FECO spectrum, shown mounted in Lambda 9 spectrophotometer.

84
manipulated later. The BASIC program used to remotely program, run,
and accumulate data from the Lambda 9 is listed in Appendix A.
For the determination of the refractive index a particular
software package available with the AT was found to be very useful.
This software called is Asyst (McMillain Software Co.) and it is a very
powerful mathematical analysis program. To determine the refractive
index, the numbers representing the FECO spectra were imported into
Asyst and stored in a two dimensional array. Using a very short
program, the array was plotted, and the equations for the curves
tangent to the maximum and minimum were fit. Next, arrays were
generated to represent each of the tangent curves. Asyst could then be
used to very easily determine the points where the three arrays
intersected, with an accuracy of 0.1%. The intersection points were
stored in a fourth array. This process was repeated for a 30
incidence spectra. The very long iterative BASIC program that was
originally used to calculate the dispersion equation could be shortened
considerably with utility functions available in Asyst. In fact, by
manipulating the numbers as arrays, the calculation was much simpler
and much faster.
Even though the above process produced an accurate result, a
second technique, ellipsometry, was used to verify these results.
Ellipsometry is based on the change in polarization of a light beam
when it is reflected from a surface. Generally a plane polarized light
beam is made incident on the surface of a material at an angle of 45.
The resulting reflected beam is elliptically polarized and from the

85
analysis of this polarization the optical constants of the material can
be determined.
The problems associated with this technique mostly originated from
the actual instrument. The computer-controlled Gaertner ellipsometer
(Chicago, IL) is the industry standard; however, the unit available for
use in this study was a manual model which greatly reduces the capacity
for quantitative analysis. The problem does not so much have to do
with manually setting the polarizer and analyzer, but rather in reading
the analog scale to determine the node points. This was a special
problem on this unit because the meter was nonlinear; the highest
sensitivity was between a reading of 2 an 3 on a scale of 1 to 5. In
addition, the gain for the photocell had to be manually adjusted as a
node point was found. The gain was also nonlinear and there was a flat
spot at one end of the range. These two problems contributed to
difficulties in reproducing the results; however, the greatest problem
with the analysis had to do with the computer program used to calculate
the values of n and t from psi and delta. Supplied by Gaertner, this
program did not actually calculate the values of psi and delta, but
instead it looked up the values in a table, based on the values of the
two pairs of angles that were inputed. From the table values of psi
and delta the program then calculated the refractive index and
thickness by an iterative technique. One difficulty with the program
occurred if the particular combination of angles inputed were outside
values in the table. The program would hang up and the only solution
was to boot the program out. This actually happened a surprisingly
large number of times, even when similar samples were analyzed. To

86
alleviate the problems with this program, a graphical method was
developed to calculate the refractive index and thickness from the set
of angles measured with the ellipsometer. A brief description of this
method is given in Appendix B.
UV-VIS Absorption
Measurement of the fundamental optical absorption edge in
semiconductors permits direct determination of the nature of the band
gap. Optical absorption properties of thin films were measured over a
variety of temperatures with the Lambda 9 spectrophotometer operating
in one of several modes. In the transmission mode a linear, zero to
100% scale was used, which was useful measuring FECO spectra. A more
useful mode for measuring the band edge, however, was to plot the
transmitted light in terms of absorbance. Because the absorbance scale
is the natural logarithm of reciprocal transmission, features of the
band gap absorption above the band edge could be studied. In the
transmission mode unless the scale was expanded these features would
appear indistinct, because transmission above the band gap was
typically less than 1% with the thickness of films usually studied. In
the absorbance mode this 1% transmission would correspond to a value of
4.6 on a possible absorbance scale of 6.0. Another advantage of using
the absorbance scale is that the spectra obtained qualitatively
reflected the shape of the true absorption curve. As explained earlier
in this chapter, the transmission spectrum that is measured of a thin
film on a substrate is the result of a complicated combination of the
reflections which occur at the various interfaces. The transmission,

87
however, is exponentially proportional to the absorption coefficient,
which is why absorbance spectra approximate absorption spectra. The
two terms will therefore be used interchangeably when describing
spectra.
The band gap energy was taken as the point where the slope of the
absorption curve was a maximum.One unique feature of the Lambda 9
was that it allowed anywhere from the first to fourth derivative of a
spectra to be taken. The maximum of the first derivative of the
absorption edge was taken as the band gap energy. A calculation to
verify these results will be described in Chapter V. Basically, it
evolves solving equation A.2 for a using the absorbance spectra, and
then plotting verses energy. A direct band gap material will give a
n
straight line for this plot, and the extrapolation of the line to on =
0 gives the optical band gap of the material.^
The temperature dependence of the band gap energy was measured by
cooling the sample with the cold finger. The sample was mounted in a
ring sample holder which would then be attached to the end of the cold
finger. A radiation shield was then positioned around the cold finger
and a vacuum insulation collar with fused quartz windows was then slid
over the entire assembly. A vacuum of less than 1 micron pressure was
then obtained in a few minutes by pumping the system with a Balzer's 70
1/sec turbomolecular pump. Once this vacuum was obtained in the
collar, cooling of the sample to 9 K could be accomplished in less
than 1 hour, although most measurements were made after 2 hours of
cooling. Any intermediate temperature between 9 K and 250 K could
achieved with the digital temperature controller and heater assembly.

88
The cold finger assembly was mounted on a height-adjustable rack system
so that once a sample was cooled to low temperatures the cold finger
assembly could be moved so that measurements on different equipment
such as the OMA and Raman spectrometers could be accomplished.
All aspects of the measurements with the Lambda 9 were greatly
enhanced by interface to the AT computer. Spectra were stored as ASCII
files which later could be imported into a LOTUS worksheet (Lotus
Development Co., Cambridge, MA) for manipulation and plotting. The
BASIC program listed in Appendix A would accumulate spectra in the form
of a long column of numbers, each number representing the absorption or
transmission at a given data point. The data interval was based on the
slit width used for the analysis. The slit defines the bandpass that
determines the resolution to which features can be resolved. Usually a
data interval of one-third the slit width was used for the analysis.
Slit widths from 4 nm down to 0.5 nm were used depending on the
required resolution. As the slit width is decreased, the signal to
noise ratio is also decreased. This means that the amount of time that
is spent at a given data point must be increased. Computer control of
the spectrophotometer was indispensible for making high resolution runs
where very slow scan speeds were necessary.
Photoluminescence and Raman
Further investigations of the band structure of semiconductors can
be made by measurement of the emission bands or peaks produced by
photoluminescence and resonant Raman scattering. Measurement of these
emissions for this study were made with the two spectrometers described

89
above. The majority of the measurements, however, were made with the
optical multichannel analyzer (OMA) and a 0.33 meter Instruments SA
monochromater. Most measurements were made at low temperatures, using
the cold finger to cool the sample.
The detection system of the OMA spectrometer consisted of a linear
silicon photodiode array which was 1024 elements wide with the diode
centers spaced every 25 pm. Approximately 700 of the center diodes
were intensified to increase their light conversion efficiency. The
diode array was connected to a multichannel analyzer and the array was
scanned by the MCA to generate a spectra. Scan rates as fast as 16.6
msec/scan could be used, although to obtain a higher number of counts,
scan rates of 500 msec were typically used. The computer system
controlling the MCA was setup to accumulate a number of scans so that
signal averaging could be used to reduce noise levels. Also, the dark
current or background noise could be automatically subtracted from a
spectrum. The diode array could be cooled to 5 C with a Peltier
cooler to further reduce the background noise level. With the
monochromater and grating combination the MCA window was about 20 nm.
This is the width of the spectra which was displayed on the screen.
The grating in the monochrometer was rotated to change the wavelength
window.
The third meter monochromater was used with a 2400 lines/mm
grating. The f-stop or light gathering capability of the monochromater
was calculated with

90
focal length
f-stop = (A.5)
grating size
where grating size is the linear dimension of the grating. With a 55
mm grating, the f-stop of the monochromater is f6. The reciprocal
linear dispersion (RLD) of a monochromater is a measure of its ability
to disperse light, and can be calculated by
RLD = X (4.6)
f d0
where d0/dL is the angular dispersion (in rad/mm) and is approximately
equal to the number of grating lines per mm. The RLD for this
monochrometer was calculated to be 1.25 nm/mm. The bandpass or
resolution of a monochromater is usually calculated by multiplying this
number by the width of the slit used. With a linear diode array
detection system, however, the resolution is determined by the spacial
resolution of the array. Assuming that three diodes can resolve a
peak, which correspond to 75 pm, the resolution of this system was 75
pm X 1.25 nm/mm or 0.9 nm.
A schematic of the experimental setup is shown in Figure 19 and
photographs of the system are shown in Figure 20a and Figure 20b. An
f2 lens was used to focus emitted light into the spectrometer and a 100
mm focal length lens was used to focus the laser beam on the sample.
Typically a focused spot size of 50 urn was used, so with an incident
power of 1 mW, the focused power density corresponds to 400 W/cm^. As

91
He cryogenic cooler
ample chamber
Figure 19 Schematic diagram of experimental setup used for measuring
photoluminescence.

92
Figure 20
Photograph of OMA setup which was illustrated in Figure 19.

93
Figure 21 Photographs of setup for Raman spectroscopy, a) shown are
the computer for acquiring data and the vacuum pumping
system for the cold finger; b) similar optics setup to OMA
shown. Laser beam is brought in from the left, focussed on
sample in cold finger, and the emitted light is focussed
into the first slit of the spectrometer, at the far right.

94
indicated in the schematic, the laser beam was passed through a
variable neutral density filter and a sharp cutoff spike filter. The
variable density filter was used to adjust the power of the incident
laser beam before being focused on the sample. The spike filter was an
indispensible item which was used to remove plasma lines from the laser
beam. Plasma lines originate from the argon plasma that is used to
generate the laser beam. The lines are incoherent radiation, but they
are very sharp transitions which can be mistaken as photoluminescent
transitions as they occur over the entire visible wavelength range. A
given spike filter passes only a very narrow wavelength range of light
which corresponds to the laser line, therefore, a spike filter can only
be used with its designated laser line. These filters are rather
expensive so spike filters for only the 488 nm and 457 nm lines were
available. The argon laser emits five other lines that occur at 514
nm, 496 nm, 476 nm, 465 nm, and 454 nm. Plasma lines were removed from
the other laser lines by using a spatial filter. This filter consists
simply of a grating and it works by diffracting the laser line at a
different angle from the plasma lines. The spacial filter only works
if a high number grating is used or if the laser beam can travel
several feet before being focused.
To verify that the OMA was not missing any features of the
photoluminescence, spectra were measured with the Instruments SA Raman
spectrometer. This spectrometer utilized a double pass one meter
monochromater with a photon counting photomultiplier tube and a digital
discriminator. Photographs of the experimental setup are shown in
Figures 21a and 21b. A simple lens system consisting of two piano-

95
convex lenses was used to focus the light emitted from the sample into
the monochromater. Other than this difference, the setup was the same
as the OMA. With 1800 lines/mm gratings the RLD of the monochromater
was calculated to be 0.24 nm/mm. The width of the four slits used in
this instrument determine the resolution, so with the smallest
practical slit of 10 um, a resolution of 0.024 nm could be realized.
The small linear dimension of the gratings (125 mm) with respect to the
long local length however, reduced the light gathering capability to an
f8. This turned out to be a very serious limitation for using the
spectrometer to measure photoluminescence, as integration times of up
to 5 sec per data point were required. A typical run was made using
slit widths of 100 um, which gave nearly four times the resolution of
the OMA, and data was collected every 0.05 nm for 2 sec. For a 20 nm
window this meant that a single scan would take 15 minutes. At least
three scans were required to reduce noise levels by signal averaging,
which meant that for only one wavelength window, 45 minutes were
required to acquire data. To make the accumulation of data easier, the
Z-158 computer was interfaced to the spectrometer controller, and is
shown in Figure 21b. The BASIC program to run the spectrometer was so
long however, that it had to be compiled to reduce running time.

CHAPTER V
RESULTS AND DISCUSSION
Thin Film Physical Properties
The primary focus of the experimental results presented in this
section are on the variation of the microstructure, composition and
electrical properties of thin films as a function of deposition
parameters and post deposition treatments. First the properties of as-
deposited thin films will be described and then the variation of
properties with different heat treatments will be reported. Finally,
the various physical properties of COSAD thin films will be described.
The variations presented in this section will be correlated to optical
properties presented in a subsequent section.
Microstructure Characterization
Transmission electron microscopy
Room temperature depositions. A wide variety of microstructures
were obtained by altering the substrate temperature during deposition.
The great majority of thin films made for this study, however, were
deposited onto substrates held at room temperature. Several advantages
were realized by using this temperature: l)the deposition process was
greatly simplified, 2)very high quality thin films could be produced,
and 3)optimum optical properties were obtained by heat treating films
deposited at this temperature. These heat treatments, however,
96

97
inevitably increased the grain size of the thin films from values
obtained with cooled or room temperature substrates.
The resulting microstructure of thin films deposited at room
temperature is found to be independent of the substrate material. TEM
micrographs of three representative thin films are shown in Figures 22,
and 23. These micrographs show the structure of films 1000 in
thickness that were deposited at 1 /sec onto a carbon support film
(Figure 22), a silica slide (Figure 23a), and a NaCl substrate (Figure
23b). All three films display polygonal shaped grains with an average
grain size of 250 . Many grains are highly faulted showing a defect
structure of microtwins and stacking faults. Due to the extremely
small size of these grains the microtwin orientation cannot be
determined. Another predominant feature displayed by the
microstructure of these thin films is the large number of Moire
fringes, indicated by the highly parallel lines with an average spacing
of 10 to 15 . These patterns occur when two crystals overlap each
other with a specific orientation. When a large number are observed in
a polycrystalline film it is an indication that the grains of the film
have a preferred orientation, although the orientation cannot be
directly determined.
By performing diffraction experiments, the orientation could be
determined. The grains were found to be oriented with the basal plane
of the hexagonal lattice parallel to the substrate plane. This is
indicated by the strong cubic reflections observed in the selected area
diffraction pattern (SAD) shown in Figure 24a. In comparison to the
indexed schematic pattern shown in Figure 24b, many of the hexagonal

b)
30 ran
Figure 22 TEM bright field micrographs of 1000 thick film deposited
onto carbon support film, a) low magnification showing
uniformity of grain size; b) high magnification showing
microtwins and stacking faults.

99
Figure 23 TEM micrographs of thin films deposited onto alternate
substrates, a) silica glass slide; b) NaCl slab.

Figure 24 Selected area diffraction pattern of thin film. a) actual
pattern; b) indexed schematic pattern.

101
reflections appear to be missing; however, upon close examination all
the spacings for hexagonal wurtzite can be found. The lines for these
spacings are too faint to show up in the SAD pattern reproduced here.
The orientation seen here is not dependent upon the substrate since
similar films were made on both amorphous and crystalline substrates.
These results are consistent with the early work on thermally
evaporated CdS.-^ jt is not clear why this specific orientation
results because the close packed plane (i.e. (0002) hexagonal plane) is
the slow growth plane. This means that the orientation is more closely
related to the nucleation rate of the thin film rather than the growth
rate, although this orientation was observed with all substrates
materials, deposition rates, and at all substrate temperatures.
The grain size seen here is smaller than the 500 size reported
by Martil et al.^-^. Grain size determinations by SEM of the films
studied here do show a different size (see SEM section below) but,
another reason for the difference can be attributed to differences in
the sputtering technique used to produce the films studied here. As
previously described, planar magnetron sputtering results in a lower
film and substrate heating due to electron bombardment, therefore a
smaller grain size should result. Higher deposition rates were found
to increase the substrate temperature. However, even with a deposition
rate of 5 /sec the substrate temperature was found to only increase to
35 C. Analysis of Figure 25 shows that no significant increase in
grain size has resulted from the increased deposition rate; however,
the micrograph does indicate that many more microstructural defects are
present. The higher supersaturation of the vapor results in additions

102
Figure 25 Thin film deposited at high deposition rate.
micrograph showing large number of defects; b)
corresponding SAD pattern.
a) TEM

103
of atoms onto non-favorable positions, which increase the number of
defects during growth. Higher deposition rates should increase the
amount of stress induced in the film. Strain determinations by X-ray
line broadening measurements will be described in a following section.
Low temperature depositions. Although high quality thin films
could be produced at room temperature, some films were deposited onto
substrates cooled to LN2 temperatures to determine if a grain size
small enough for quantum confinement could be produced. Wolgehmuth et
al.29 showed that evaporated material deposited at this temperature was
found to be amorphous as determined by X-ray diffraction. In that work
a high supersaturation was used to produce these films which may have
also contributed to the amorphous structure. A much lower deposition
rate was used in this study to help promote crystal growth, so that
combined with the low temperature, a very small grain size would
result. Figure 26a shows the micrograph of a film deposited at 3 /sec
onto a carbon support film which was attached to a substrate cooled to
80 K. The "grain" structure that is observed is not due to individual
grains but rather to a columnar structure with low density areas
indicated by the low contrast between the columns. The SAD pattern
shown in Figure 26b indicates that the film is somewhat crystalline,
but highly faulted as evidenced by the width of the rings. The three
rings corresponding to the cubic reflections are still present
indicating that only the nearest neighbor spacing is present. No
hexagonal lines can be found in SAD patterns of these films.
An indication of how a film grows on a cooled substrate is shown
in Figure 27a. This section of a film increases in thickness from left

104
b)
Figure 26 Thin film deposited at LN2 temperatures, a) TEH micrograph
showing low density columnar structure; b) corresponding
SAD pattern indicating a semi-amorphous structure.

b)
30 nm
Figure 27 Thin film deposited at LN2 temperatures. a) low
magnification TEM micrograph showing section of film
increasing in thickness from left to right; b) high
magnification TEM micrograph showing island structure.

106
to right. The film is supported by a carbon film. At the far left an
individual island structure is observed. The high magnification
micrograph shown in Figure 27b indicates that the islands are mostly
amorphous, with very small microcrystallites imbedded in the amorphous
matrix. The microcrystallites are characterized by the darker contrast
of approximately 80 in size within the globular structures. The very
fine mottled structure is the background amorphous structure of the
carbon support film. The SAD pattern included in the Figure shows that
only the nearest neighbor spacing is present, and the crystal structure
is highly distorted. Referring back to Figure 27a, as the film
thickness increases to the right, the islands grow to form a columnar
structure. The columns become fairly thick before they coalesce to
form a low density continuous films, indicated at the far left in the
Figure.
High temperature depositions. Other attempts to alter the
microstructure of thin films were made by deposition onto heated
substrates. Both room temperature and LN2 deposited films reported
above display large numbers of crystallographic defects, which as
described in a later section, degrade the optical properties. By
depositing films onto substrates held at 200 C, microstructures with
fewer defects could be produced. A higher deposition temperature
should lead to fewer defects because the deposited atoms have more
energy to diffuse to more favorable positions. Temperatures above 200
C, however, produced films which were very strongly bonded to the
substrate and hence could not be removed for examination. A
representative thin film deposited at 200 C which was "floated-off" a

107
b)
Figure 28 Thin film deposited at high temperature, a) TEH micrograph
showing large defect free grains; b) corresponding SAD
pattern.

108
silica substrate and its corresponding SAD pattern are shown in Figures
28a and 28b. The absence of any contrast in many of the grains
indicates a highly ordered crystal structure. The grains are again
polygonal in shape with an average size of 800 . The SAD pattern
shows that the preferred orientation is still maintained, but several
of the other hexagonal reflections are now more predominant. Although
fewer crystallographic defects are observed in these films, the "bulk"
structure was found to be very nonuniform. Variation in color of the
film across the substrate indicated a variation in composition and/or
thickness, which made these films unsuitable for optical studies. Films
deposited at temperatures greater than 200 C were found to be even
more nonuniform. The nonuniformities result from nonuniform thermal
contact because silica substrates are such poor conductors of heat.
Thermal contact paste was not used due to concerns about contamination
from the paste. The nonuniformities encountered at high deposition
temperatures will be detailed more thoroughly in a following section on
the scanning electron microscopy of thin films.
Heat treated thin films. Once the optical properties of as-
deposited thin films were determined, it became apparent that to obtain
optimum optical properties some type of thermal treatment had to be
conducted on the films to remove the defect structure produced by the
deposition process. However, due to difficulties with the increased
bonding strength of thin films to substrates as a result of even mild
heat treatments, very few TEH micrographs were obtained. The primary
technique used for observing the microstructure of heat treated thin
films was by SEM, and the results are presented in the following

109
section. The TEM micrographs that were obtained of a heat treated film
are shown in Figures 29a and 29b, and the SAD pattern for this film is
shown in Figure 30. A film with an original thickness of 1000 was
deposited on a NaCl substrate and then heat treated to 650 C for 4
minutes. By dissolving the substrate the film was captured on a TEM
carrier grid. The micrographs in Figure 29 show the film to be very
porous with an interconnected grain network. This high porosity
results when a large amount of grain growth occurs in a very thin film
because there simply is not enough material to produce a continuous
structure. The grains within the network display an average size of
2000 and many dislocations, stacking faults and bend contours are
indicated by the variation in contrast observed within grains. These
defects might be intrinsic or they may have occurred when the film was
"floated-off" the substrate. It is interesting to note that very few
if any microtwins can be observed in this structure. This result
indicates further that these defects are growth related. The SAD
pattern shown in Figure 30 displays a single crystal pattern with a
basal plane zone axis, which means that the preferred orientation
observed in as-deposited films is maintained after heat treatment,
although this orientation may have been enhanced somewhat because the
film was heated on a crystalline substrate.
Scanning electron microscopy
As-deposited thin films. The microstructure of thin films was
determined by SEM for several reasons. As explained above, most heat
treated thin films could not be removed from their substrates for TEM

110
a)
500 nra
Figure 29 Thin film deposited on NaCl substrate and subsequently heat
treated, a) TEM micrograph showing porous network; b) TEM
micrograph showing dislocations and stacking faults.

Ill
Figure 30 Selected area diffraction pattern of thin film shown in
Figure 29.

112
examination. Also, films of useful thickness for optical studies were
much too thick for TEM. Finally, no specimen preparation was required
to examine the films, other than mounting the substrate with film on a
specimen holder. This afforded the ability to examine any type of film
regardless of thickness or treatment. The limited resolution of the
SEM, initially, was a problem, especially for as-deposited thin films
with very small grain size. However, optimization of the technique
yielded high magnification SEM micrographs (86,000X with 400
resolution).
The typical low magnification structure seen in 1.0 pm thick films
deposited at room temperature is shown in Figure 31a. The micrograph
shows these films to be of very high quality with very few large
defects such as pin holes or cracks. A high magnification micrograph
typical of these films are shown in Figure 31b. By making line
intercept measurements on Figure 31b, and assuming that the change in
contrast from point to point is due to a grain boundary, an average
grain size of 480 is measured. Although this number is only
approximate, this size is nearly twice that measured by TEM
observations and is slightly less than the size observed by Martil et
al. The difference in size between TEM and SEM observations of the
films studied here is greater than the difference in magnification
calibration between the two instruments, therefore the difference is
most likely due to the fact that the grain size observed at the surface
of a thin film is not the same as that observed in the interior. Also,
because the SEM micrographs were obtained near the resolution limit of
the microscope, there is considerable distortion of the image. The one

113
b)
Figure 31
SEM micrographs of
magnification; b)
structure.
as-deposited thin film. a) low
high magnification showing grain

114
conclusion derived from SEM micrographs that cannot be readily
determined by TEM relates to the lack of flatness on the surface of
these films, on this size scale. This may be the result of columnar
growth on a very fine scale.
The columnar structure observed by TEM in thin films deposited at
LN2 temperatures is not readily seen by SEM, as shown in Figure 32.
The size of the structure observed in this 1.0 pm film is greater than
the grain size observed by SEM of room temperature deposited films,
albeit this is a much lower resolution micrograph so the size may be
distorted. By referring back to Figure 26 one can see that the width
of the columns is approximately 500 . The size measured here is
approximately 800 , which is similar to the differences between the
grain size measured by TF1M and SEM on room temperature deposited films.
From Figure 32 it is not possible to determine that a semi-amorphous
columnar structure is present in this thicker film, which is one
drawback of using SEM for microstructure determinations.
Other capabilities of the SEM, however, make the observation of
other physical properties possible. The nonuniformities seen in films
deposited at temperatures greater than 200 C are greatly enhanced by
imaging backscattered electrons from the film. The energy of
backscattered electrons is dependent upon composition, so the contrast
observed in the micrograph of Figure 33 is related to the compositional
nonuniformities of a film deposited at 300 C. These results correlate
well with those reported by Cook and Christy,*^ who found that
deposition above 200 C yielded poor quality thin films. The large

Figure 32 SEM micrograph of thin film deposited at LN2 temperatures

116
Figure 33 SEM micrograph of thin film deposited at 300 C,
backscattered electron image.

117
differences in sticking coefficients of cadmium and sulfur at high
temperatures is responsible for this result.
Heat treated thin films. Furnace heat treatments similar to those
carried out by Martil et al.^ were used to study the grain growth of
thin films, in addition to heat treatments by rapid thermal annealing.
Optimum film properties were obtained by furnace heat treating to 500
C for 5 hours, or to 650 C for 4 minutes. Typical microstructures
obtained of 8000 thick films subjected to these heat treatments are
shown in Figures 34a and 34b. The grains appear isotropic with an
average size (determined by a line intercept method) of 2850 for the
500 C treatment and 3000 for the 650 C treatment. These grain
sizes are similar to the size observed in the porous network displayed
in the TEM micrograph of a very thin film heated to these temperatures
(Figure 28). Thermal shock, mismatch of the film and substrate
expansion coefficients, or excessive grain growth could be responsible
for the cracking of films which is sometimes observed when films are
subjected to fast furnace heating rates. This cracking is shown in
Figure 35.
From other heat treatments at 200 and 350 C for times of 5 hours,
it was found that the grain size exhibited two regions of growth. This
is shown in Figure 36, which is a plot of average grain size versus
reciprocal temperature, for constant time. The two regions of growth
exhibited in the plot are a classical indication that two different
mechanisms are responsible for grain growth in these films. At low
temperatures the grain growth is slow with a nearly constant rate.
This is an indication that grain growth occurs only by grain boundary

118
a)
b)
Figure 34 SEM micrographs of heat treated thin films. a) heat
treated 650 for 4 min; b) heat treated 500 C for five
hours.

119
Figure 35 Low magnification SEM micrograph of thin film shown in
Figure 34a.

GRAIN SIZE (nm)
120
(1/D
Figure 36 Plot of average grain size versus heat treatment
temperature.

121
diffusion, which is a low energy process. At higher temperatures,
enhanced diffusion of impurities at the grain boundaries results in a
large increase in grain growth. Grain growth is therefore controlled
by the activation energy for impurity diffusion. The curve appears to
flatten out at the highest temperatures, which is possibly due to two
effects. First, since the film is of a finite thickness, there must be
a limit on the maximum size of the grains. Generally the largest grain
size obtainable should correspond to the thickness of the film, but
when a film is in contact with a substrate, this constraint will depend
upon the interface energy between the film and substrate. The more
likely effect, however, is due to a loss of material at higher
temperatures. Thin films were found to totally evaporate at 800 C in
a very short time, even though the sublimation temperature of bulk CdS
is 925 C. At 650 C for long times a loss of material was also found
to occur which would disrupt the grain growth process.
Although some of the optical properties were optimized by heat
treatments to 650 C, the larger grains and cracking of these films
increased the amount of light scattering. Rapid thermal annealing
(RTA) permitted high temperature treatments for very short times, which
resulted in very little grain growth and no cracking of the film.
Figure 37 shows a high magnification micrograph of a film that received
an RTA treatment at 650 C for 30 sec. The film displays an average
grain size of only 900 , which is considerably smaller than the size
obtained by furnace heat treatments to this temperature. Since this
film was subjected to a very large thermal stress, it appears that the

122
Figure 37 SEM micrograph of thin film subject to rapid thermal
anneal (RTA) heat treatment.

123
cracking seen in the furnace heat treated films is a result of the
large and rapid grain growth that occurs.
X-ray diffraction experiments
Diffraction spectra. Although SEM observations gave an indication
of microstructure morphology of thicker films, the technique did not
provide any crystal structure information. To determine the
crystallography of thicker films X-ray diffraction (XRD) was used. The
computer controlled diffractometer used for these measurements greatly
facilitated the determination of microstructure changes with heat
treatment, as well as permitting quantitative analysis of X-ray
spectra. Programs for calculating diffraction peak line broadening due
to strain and size effects and for calculating precision lattice
constants were available with this system.
The diffraction spectra obtained from films 8000 thick,
deposited at three different temperatures are displayed in Figure 38
and the diffraction spectra obtained after heat treating these films
are shown in Figure 39. Many of the features observed in SAD patterns
are evident in these X-ray diffraction patterns. The broad band peak
centered at ~ 20 in all patterns is due to amorphous scattering from
the substrate.
In Figure 38a, of a film deposited at LN2 temperatures, the
amorphous peak is much more intense and the intensity of the (0002)
peak is considerably reduced. This supports the results obtained from
TEM analysis that the structure of these films is highly disordered,
but there is some indication of crystallinity. After heat treatment,

INTENSITY INltNSIlY INTENSITY
124
a)
b)
c)
Figure 38 X-ray diffraction patterns of as-deposited thin films, a)
deposited at LN9 temperatures; b) deposited at room
temperature; c) deposited at 300 C.

125
Figure 39a shows the amorphous peak to be greatly reduced and
crystallization has taken place with the strong cubic preferred
orientation.
Figure 38b shows that films deposited at room temperature exhibit
a strong cubic preferred orientation which is shown by diffraction at
(0001) where 1=2,A,6. The other hexagonal lines seen in SAD patterns
are not as readily observable in this pattern, particularly the other
two lines which are closely spaced to the (0002) peak. Those two lines
were clearly resolved in SAD patterns, but appear to be totally absent
here. This shows how much more sensitive electron diffraction is to
the local crystal structure. When this film is heat treated, the
preferred orientation is maintained as shown in Figure 39b. All
diffraction peaks become much narrower and more intense, as an
indication of grain growth. Figure 39c indicates that when this film
is heat treated to an even higher temperature, the orientation is
maintained, but the intensity of other hexagonal diffraction lines is
increased.
The most predominant hexagonal lines are seen in the XRD pattern
obtained from a film deposited at 300 C, shown in Figure 38c. Again a
preferred orientation is indicated, but other hexagonal lines are shown
to occur in a different ratio to the cubic lines, which indicates there
is another orientation present. This second orientation was not
observed in the TEM analysis because the film examined was deposited at
200 C. It appears this orientation only occurs at higher
temperatures.

INTENSITY INTENSITY INTENSITY
126
a)
DEGREES TWO THETA
c)
X-ray diffraction patterns of heat treated thin films from
Figure 38; a) film of Figure 38a, heat treated 500 C, one
hour; b) film of Figure 38b, same heat treatment; c) film
of Figure 38b, heat treated 720 C, ten minutes.
Figure 39

127
Strain determinations. Residual strain in thin films can be
manifested as both a shift and a broadening of X-ray diffraction lines.
However, broadening of X-ray lines due to strain will only occur if a
nonuniform microstrain is present (usually due to plastic deformation).
Broadening usually is a result of a very small grain size. Digital
manipulation of the diffraction data permitted determination of peak
broadening due to nonuniform strain and grain size effects by use of an
utility program available in the APD computer. The computer program,
however, does not calculate the uniform strain which would produce a
shift in peak positions. Residual uniform strain was determined by
comparing the position of the (0006) diffraction line in thin films to
the position observed in a powdered sample of CdS. By differentiating
Bragg's law, the change in lattice parameter due to a uniform strain
was calculated from the shift observed in the peak positions.^
Figure 40a shows a comparison of the broadening and peak position seen
in films deposited at two different rates and the line width and
position which results when these films are fully annealed. The tick
mark shows the position of the (0006) line of the powdered sample. The
calculated uniform strain which would produce the observed line shifts
and the effective mean nonuniform strain and effective mean grain size
that would produce the broadening are listed in Table 2. Also shown
are the values which would cause the broadening if only grain size
effects were present.
Thin films were considered to be uniformly strained, therefore the
results calculated by the APD program for nonuniform strain are not
considered relevant. The grain size effects listed in the last column

AXIS SPACING
128
*10
b)
Figure 40 Analysis of X-ray diffraction spectra, a) (0006) X-ray
diffraction peak of two as-deposited and one furnace heat
treated thin films showing position shift and broadening
due to strain and grain size effects; b) plot of c-axis
lattice parameter versus Nelson-Riley function for
precision lattice determination.

129
TABLE 2
Calculated strain and size effects which would produce the
X-ray line shifts and broadening observed in diffraction patterns
Sample
Effective
Nonuniform
Mean Strain^-
%
Effective
Mean Size^
A
Calculated
Uniform
Strain^
%
Size
Effects
Only^
A
ASDP 1 A/sec
0.189
1148.7
0.16
252.7
ASDP 5 A/sec
0.179
1983.6
0.39
284.3
FHT 650 C
0.041
2528.0
0.08
1005.2
RTA 650 C
0.122
837.8
0.12
332.6
Key: ASDP as-deposited; FHT furnace heat treated;
RTA rapid thermal anneal. Heat treated films were deposited at
1 A/sec
^Calculated with utility program available in APD computer. Mean
values are given which would combine to cause the observed broadening.
^Calculated from shift in X-ray line position.
JCalculated with utility program available in APD computer. Values of
grain size which would produce the observed broadening.

130
of the Table do, however, correlate closely to grain size observed by
TEM for as-deposited thin films. The shift of the higher rate film to
a smaller two-theta value indicates the presence of a tensile stress.
Higher stress levels in higher rate films may be due to growth related
defects and the inclusion of impurities. The higher stress level in
these films causes the film to exfoliate within several hours after
exposure to the atmosphere. Figure 41a shows an optical micrograph of
a film exhibiting the onset of this exfoliation, and Figure 41b
displays the appearance of a film after several days of exposure. The
appearance of the exfoliation shown in Figure 41a is characteristic of
a tensile stress. Exposure to atmosphere leads to the absorption of
water on the film surface, which apparently increases the stress to a
value large enough to cause the film to break away from the substrate.
Some type of stress related corrosion process may also contribute to
this result. Lower deposition rate films were found to be stable with
exposure to the atmosphere indefinitely, so X-ray line shift due to
uniform strain was expected to be less.
Precision lattice constant. A calculation of the precision
lattice constant of heat treated thin films was made from the positions
of diffraction peaks measured from thin films. Due to the strong
preferred orientation only the c-axis lattice constant could be
determined and because small two-theta values had to be used, the
calculated lattice parameter was plotted versus the Nelson-Riley
function. The plot shown in Figure 40b indicates an extrapolated
lattice parameter of co=6.707 , which is 0.09% less than the JCPDS
card file value of 6.713 .^3

131
b)
Figure 41 Optical micrographs of exfoliated thin film, 100X
magnification, a) after four hours exposure to atmosphere;
b) after two days exposure.

132
Compositional Analysis
Stoichiometry determination of target material
The first parameter needed to establish the composition
stoichiometry of thin films was the composition of the target material
from which films were made. Once this composition was determined, the
target material could be used as a standard for all other analytical
techniques. Both induction coupled plasma (ICP) and electron probe
microanalysis (EPMA) were used to determine the stoichiometry of the
target material. These two techniques are by far the most quantitative
because the results are determined by comparison to known standards.
Additional measurements were made by standardless techniques which rely
on sensitivity factors for quantitative analysis, such as: X-ray
photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES).
The manufacturer of the target material claimed the composition to be
slightly sulfur rich, but as can be seen Table 3, both ICP and EPMA
data indicate the target material to be slightly cadmium rich, although
the variation is within a few standard deviations of each other. In
either case the results indicate the target material to be
stoichiometric to 0.50 %, which for the other analytical techniques
used to measure thin films, is an acceptable variation. XPS gives a
result that is very close to the excepted value; however, AES shows a
very large difference from the other results which demonstrates that
the technique is not useful for determining absolute concentrations,
but as described below, the technique is very sensitive for determining
differences between samples.

133
TABLE 3
Concentrations in atomic percent of cadmium and sulfur
found in target material
Analytical
Technique
Cd concentration
S concentration
ICP
50.96 +/- 0.09
49.04 +/- 0.12
EPMA
50.33 +/- 0.21
49.67 +/- 0.21
XPS
51
49
AES
60.7
39.3

134
The results for the four different analytical techniques used for
determining the composition of thin films are listed in Table 4. The
absolute values of the sulfur and cadmium concentrations are shown to
vary greatly between different techniques; however, the difference
between as-deposited and heat treated thin films is nearly the same in
each case, except for the XPS results. Films that were heat treated to
650 C were used because of their use for optical properties studies.
They were also found to show the greatest difference. The discrepancy
in the XPS results for heat treated thin films and the implications of
the results of the other techniques will be discussed below. The
primary indication of these results, however, is that as-deposited thin
films contain ~ 2% excess sulfur and once they are heat treated, this
excess increases to 4%. As indicated by the precision lattice
parameter described above, the unit cell of heat treated thin films is
very close to the ideal structure of CdS, so very little of this excess
sulfur is interstitial, or very few cadmium vacancies are present.
This means that the sulfur must be a second phase.
Analytical techniques
Use of electron probe microanalysis (EPMA) for determining the
composition of thin films was first thought to be the best method and
most quantitative, for the same reasons stated above. Not even
qualitative results, however, could be obtained. The standard ZAF or
pz^ programs used to calculate composition consistently gave results
of less than 100 weight percent for the composition of thin films.
Even a thin film program which relies on some approximations, did not

135
TABLE 4
Comparison of
used to
results from different analytical techniques
determine the composition of thin films
Analytical
As Deposited
Atomic %
Heat Treated
Atomic %
Technique
Cd
S
Cd
S
EDS
47.7
52.3
46.8
53.2
XRE
44.6
55.4
43.1
56.9
XPS
50.6
49.4
70.9
29.1
AES
37.3(47.6)
62.7(52.4)
35.7(46.0)
64.3(5'

136
produce a reasonable result. It could not be determined if there was a
problem with the software or the instrument. Even if EPMA results
could have been obtained it would have been impractical to analyze
every film produced and every heat treatment studied.
X-ray fluorescence was originally considered to be the best
technique for the general analysis of thin film composition for several
reasons: l)the intensities of the X-ray fluorescence lines should be
directly proportional to the amount of material present in the thin
film, 2)high peak intensities gave a large signal to noise ratio, 3)no
sample preparation was necessary and analysis was quick (typically 15
minutes), 4)film thickness could be readily determined and 5)the
results were found to be very reproducible. Different films that were
deposited under exactly in the same conditions and heat treated exactly
the same way were found to give nearly the same peak intensities for
the cadmium and sulfur lines. An example of this reproducibility is
shown in Figure 42 which shows peak intensity and integrated peak
intensity ratios for several identical samples. A problem with the
technique occurs if a linear correlation factor is used to calibrate
the XRF spectra obtained from thin films: the amount of sulfur appears
to be too high, when compared to the other techniques. The correlation
factor is based on the ratio of the intensities of the cadmium and
sulfur lines obtained from the target material, assuming a
stoichiometric composition for the target. One possible explanation
for the high value is that a matrix effect that alters the ratio of the
X-ray lines measured for the target material may not occur in a thin

S/CD RATIO
137
, FILM NUMBER
¡ZZ] PEAK INTENSITY rCsl INTEGRATED AREA
Figure 42 X-ray fluorescence sulfur to cadmium peak ratios for
several identical thin films.

138
film, which would lead to an incorrect correlation factor and incorrect
estimation of the composition of thin films.
The reproducibility obtained by XRF could not be accomplished by
energy dispersive spectroscopy (EDS) obtained during SEM observations.
This was primarily due to the inability to measure the specimen current
of the electron beam used to generate secondary electrons for imaging
and X-rays for composition analysis. Widely different intensity values
were often obtained from the same sample when measured at different
times. This problem was alleviated to some degree by mounting a piece
of target material on each sample mount so that the intensities of the
X-ray lines obtained from thin films could be normalized to a standard.
A hole was drilled in the sample mounts so that the surface of the
target material would be level with the substrates and geometric
differences would be reduced. Again, a correlation factor was used to
calculate the composition of thin films based on the intensities of the
X-ray lines measured from the target material. The matrix effect that
might occur in XRE would be less pronounced in EDS, because the
electron penetration depth that generates X-rays is much less than the
X-ray line that produces the secondary fluorescence in XRE.
The absolute values of the composition obtained by Auger electron
spectroscopy (AES) are considered to be reasonable since the accuracy
of the technique is limited to +/-30% when published values of the
sensitivity factors are used.-^ However, since very similar values for
the composition of the target material were obtained, a very simple
correction factor can be applied to the sensitivity factors to give the
correct results. The corrected concentrations are given in parenthesis

139
in Table 4, and they show comparable results to the other techniques.
Typical AES spectra obtained from target material and heat treated thin
films are shown in Figure 43a and 43b. The shape of the peaks are
qualitatively the same, although the heat treated spectrum has been
shifted to higher energy due to a charging effect. The difference
between the ratios of the Cd peak-to-peak distance to the sulfur peak-
to-peak distance gives a quantitative indication of the difference in
the concentration of cadmium in the two samples. AES should be very
quantitative for measuring the differences between similar samples, so
the values obtained most likely represent the true difference, although
this technique does not indicate how the sulfur exists in the
structure.
To attempt to determine the chemical state of the sulfur, X-ray
photoelectron spectroscopy (XPS) was used. This technique is very
sensitive to changes in the binding energy of core electrons, which
reflects changes in the local chemical environment.^ The presence of
significant concentrations of free sulfur, would either shift the
sulfur lines, split, or broaden them. A high resolution scan of the
sulfur 2p transition for an as-deposited (ASDP) and a furnace heat
treated (FHT) thin film are shown in Figure 44. Both films were
deposited at room temperature and at a rate of 1 /sec. Very little
shift is observed and no significant broadening is seen. The position
of the 2p transition for free sulfur would be at a binding energy of
165 eV. The only unique feature is the two shake-up peaks, labeled SU
and both of these show very little shift in energy. The satellite peak
labeled SAT is an artifact peak and has no meaning.

140
b)
Figure 43 Auger electron spectroscopy survey scans. a) Target
material; b) heat treated thin film.

141
Figure 44 X-ray photoelectron spectroscopy (XPS) high resolution scan
of sulfur 2p peak, exhibited by an as-deposited (ASDP) and
a furnace heat treated (FHT) thin film. Heat treated thin
film peak shifted slightly to higher binding energy.
Shake-up peaks are indicated by SU and satellite peak is
marked SAT.

142
As indicated in Table 4, there is significant differences between
as-deposited and heat treated thin films. The concentration
calculations in XPS are based on quantification factors and on
integrated peak intensities. Since the factors are constants, the
result presented in Table 4 indicate that the integrated area of the
cadmium peak has nearly doubled in heat treated films as compared to as
deposited. A high resolution scan of the cadmium 3p5 and 3p3
transitions is shown in Figure 45. Surprisingly there is a significant
shift in the peak position in addition to a broadening. By comparison
to published XPS spectra on cadmium compounds-^ this shift could
possibly correspond to cadmium bonded to oxygen. The survey scans
displayed in Figure 46 and Figure 47 exhibit oxygen Is peaks in both
films and the peak appears to be more intense in the heat treated film
(compare the height of the 0 Is peak to the Cd 3p3 peak). Figure 48a
displays a high resolution scan of the oxygen peak and Figure 48b shows
the same scan after a one minute sputter etch, which removes
approximately 100 . The large reduction in the intensity of this peak
after sputtering is an indication that a thin oxidized surface layer is
present. Unfortunately, a high resolution scan of the Cd 3d peaks was
not acquired after the sputter etch to determine if the shift in the
peak position is due to the oxidized surface layer. Analysis was
performed on a second XPS instrument (Perkin-Elmer) which was equipped
with a sputter gun. The results for a heat treated film before and
after a one minute sputter etch are shown in Figure 49. Several
differences between this spectrum and the spectrum obtained with the
Kratos XPS can be observed. First, the 3d peaks in Figure 49 are

143
Figure 45 XPS high resolution scan of cadmium 3d3 and 3d5 peaks of an
as-deposited (ASDP) and furnace heat treated (FHT) thin
film.

144
Figure 46 XPS survey scan of as-deposited thin film.

145
Figure 47 XPS survey scan of furnace heat treated thin film.

146
CTS
a)
b)
Figure 48 XPS high resolution scans of oxygen Is peak, a) furnace
heat treated thin film; b) after one minute sputter etch.

MC E )^E
147
XPS high resolution scan of cadmium 3d3 and 3d5 peaks
obtained from Perkin-Elmer instrument. Spectrum are of a
heat treated thin film, as-received and after a one minute
sputter etch.
Figure 49

148
nearly twice as wide as the peaks seen in Figure 45. Second, the peaks
are shifted to lower binding energy and third, a very definite doublet
peak is observed. The first two differences may be due to charging
effects. Charging was not observed in the Kratos spectrum (Figure 45)
because a low energy electron flood gun was used to compensate the
build-up of static charge. The clear resolution of a doublet that
was not resolved in Figure 45 is, however, an anomalous result. Also,
unexpectedly, the Cd peaks have shifted in the opposite direction after
the sputter etch. This may be due to preferential sputtering of the
sulfur, so that more metallic cadmium is present. It may also be due
to a change in grounding potential, which would effect the static
charge built-up on the sample.
Apparently XPS is not sensitive enough to determine if the small
amount of excess sulfur present in thin films exists as a distributed
second phase. The final technique that was used to determine this
distribution is X-ray mapping. Although this type of X-ray analysis by
EPMA would have been the most quantitative, the probe size used in the
EPMA is not small enough to spatially resolve the grain boundaries of
thin films, which is where the sulfur is most likely to exist. By
using EDS X-ray mapping with the SEM, high spatial resolution could be
obtained. Simultaneous maps of sulfur and cadmium were acquired at a
magnification of 20,000X, which represented about 150 grains. A low
energy electron beam of 10KV was used to reduce to volume of material
sampled and a high density 256 x 256 image, with 8 bit resolution, and
a 200 msec dwell time was used to generate the X-ray maps. Even at
this high magnification, if the cadmium and sulfur were evenly

149
distributed, an image with very little contrast should result. The
area sampled is displayed in Figure 50, which is a digitized SEM image.
The contrast that is observed is due to the grain boundaries of the
surface grains and can be compared to the contrast observed in a normal
SEM micrograph, shown in Figure 39. Figure 51a shows that the cadmium
map has a low even contrast, but the sulfur map shown in Figure 51b is
highly mottled, indicating an uneven distribution of sulfur. This
indicates that the sulfur is distributed as a second phase, at the
grain boundaries.
Co-Sputter Alternating Deposition
Microstructure
The purpose of making COSAD films was to produce a two phase
structure, in which very small crystallites of CdS are embedded in a
glass matrix. The intent of this phase of the research was to
determine if a structure could be produced in a thin film with small
semiconductor microcrystallites isolated in a dielectric matrix with
the crystals small enough for quantum confinement. Thin films made by
COSAD were therefore primarily analyzed by transmission electron
microscopy, since the required size of the crystallites was less than
100 . By changing the ratio of deposited CdS to glass different
structures could be produced. The most stable films were those made
with a small ratio of CdS to glass, while films made with equal ratios
were found to phase separate during examination in the TEM. An example
of the two extremes of the variation that could be produced will be
described here.

150
Figure 50 Digitized image from scanning electron microscope,
secondary electron image.

151
Figure 51 Digitized element maps. a) high magnification cadmium
X-ray map; b) high magnification sulfur X-ray map.

152
COSAD films with equal proportions of semiconductor and glass were
the first to be produced. These films will be referred to as C0SAD1.
A representative film which was deposited onto a carbon support film is
shown in Figure 52a. The structure appears very indistinct and appears
to be amorphous, but the SAD pattern shown in Figure 52b indicates
there is some crystallinity in the structure. If the electron beam is
condensed on this type of film, the structure is found to phase
separate very quickly, as displayed in Figure 53a. The SAD pattern in
53b shows that a significant amount of crystallization has occurred and
the pattern indicates a hexagonal CdS non-oriented crystal structure.
If this type of film is furnace heat treated a significant change
in microstructure also occurs. Heat treating to 500 C results in
total phase separation and the growth of spherical shaped CdS
crystallites, with an average size of 300 . This structure is shown
in Figure 54. By heating this type of film to 650 C, these
crystallites are found to grow and form a fairly evenly dispersed
bimodal size distribution microstructure, which is shown in Figure 55a.
The average size of the larger spherical crystals is 700 A and the
smaller crystallites have an average size of 200 . The SAD pattern
for this structure is shown in Figure 55b. All the reflections for
hexagonal CdS are present, in addition to some reflections for an
unidentified phase. Because these reflections are so closely spaced in
the pattern, it was not possible to determine their source by dark
field imaging. A typical dark field image obtained from the inner most
diffraction rings is shown in Figure 56. The bright crystals in the
image are those which are properly oriented for diffraction and as can

a)
50 run
b)
Figure 52 TEH micrograph of C0SAD1 thin film, a) as-deposited on
carbon support film; b) corresponding SAD pattern.

154
50 nm
a)
Figure 53 TEM micrograph of C0SAD1 thin film. a) after five minute
exposure to condensed electron beam; b) SAD pattern.

155
100 nm
Figure 54 TEM micrograph of C0SAD1 thin film, after heat treatment to
500 C for one hour.

156
Figure 55 TEM micrograph of C0SAD1 thin film, a) after heat
treatment to 650 C for 30 min.; b) SAD pattern.

157
200 nm
Figure 56 Dark field TEM image of C0SAD1 thin film shown in Figure
55a.

158
be seen, crystals of both size distribution are lit up. The dark field
image also shows that several of the large crystals are twinned,
indicated by the change in contrast within a crystal. Another
abnormality exhibited by the higher temperature heat treated films was
the additional appearance of large faceted shaped crystals, shown in
Figure 57. The crystallographic identity of these crystals could not
be determined; however, they were only found in certain areas of the
film, and they may correspond to some type of contamination.
COSAD films which were made by controlling the alternating
deposition process with a computer controlled stepping motor could be
produced with very low ratios of CdS in the glass. Alternating layers
of material could be produced by stopping the substrate over a source
for a given amount of time. This permitted the deposition of several
monolayers in each pass. Ratios which resulted in about 5% CdS
dispersed in the glass (designated C0SAD2) resulted in structures such
as the one displayed in Figure 58a. The extremely fine dark structure
in the micrograph is most likely due to CdS microcrystallites; however,
the accompanying SAD pattern (Figure 58b) indicates the structure to be
amorphous. When the electron beam is condensed on this type of film a
phase separation is shown to occur, but as exhibited in Figure 59a it
grows to a smaller size than shown in Figure 53a. The accompanying SAD
pattern in Figure 59b also indicates that the structure is still
amorphous.
When this type of film is heat treated to 650 C, two different
structures result. First, large semi-faceted shaped crystals are
exhibited as shown in Figure 60a. Although a single crystal

159
* * T a V .
* ***, at
§ A' * 3*?
*: \- ,*## J
l^^v VI *1
* * Ll ^ -
200 nm
Figure 57 TEM micrograph of heat treated C0SAD1 thin film, showing
large unidentified crystals.

160
a)
100 nm
b)
Figure 58 TEM micrograph of C0SAD2 thin film. a) as-deposited; b)
SAD pattern indicating amorphous structure.

161
b)
Figure 59 TEM micrograph of C0SAD2 thin film, a) after ten minute
exposure to condensed electron beam; b) SAD pattern.

162
b)
Figure 60 TEM micrograph of C0SAD2 thin film, a) after heat
treatment to 650 C for 30 min, showing large unidentified
crystals; b) SAD pattern showing both single crystal
pattern from large crystals and diffuse pattern from thin
film.

163
diffraction pattern could be obtained from these crystals (shown in
Figure 60b), they could not be identified. Again these large crystals
were found to be unevenly distributed on the film and therefore
considered to be due to contamination. At very high magnification, the
second structure of these films can be observed. Very small spherical
microcrystallites, of 50 approximate size are shown in Figure 61.
Only a very diffuse diffraction pattern shown in Figure 60b could be
obtained, so it is difficult to identify the microcrystallites as CdS,
although the pattern does resemble the pattern of the semi-amorphous
LN2 film described above. A large number of these microcrystallites is
seen, so a stronger pattern should result. This particular film,
however, is relatively thick (~ 3000 ), so it only appears there are
many microcrystallites. The pattern is diffuse because the glass
matrix has a much larger volume fraction compared to the
microcrystallites.
Composition
Very little compositional analysis was carried out on COSAD films,
primarily because most of the films made were for TEM analysis, and
therefore were too thin for quantitative analysis. The composition of
a few of the phase separated particles discussed above however, were
determined by EDS while imaging the film in the STEM mode of the JEOL
200 CX electron microscope. The particles were found to contain
cadmium and sulfur, but the intensity levels for these two peaks were
too low to get an accurate determination of the stoichiometry. This
was particulary true of the particles displayed in Figure 61. A

50 run
Figure 61 High magnification TEH micrograph of heat treated C0SAD2
thin film showing fine dispersion of microcrystallites.

165
typical EDS X-ray spectrum obtained while operating the JEOL 200 CX in
the STEM mode is shown in Figure 62. This spectrum was acquired from
the film displayed in Figure 61 and it shows that the particles are
CdS. Except for silicon, the other constituents of the glass matrix
(e.g. Ca and K) could not be determined with this technique because of
their low concentration and the thickness of the film. The copper peak
originates from the TEM support grid.
The compositions of thicker films made for optical analysis were
determined by XRF. One difficulty encountered in using this technique
was that the potassium Ka line overlapped the cadmium L3 line, so the
amount of potassium could only be roughly determined. For COSAD films
deposited on silica substrates the total amount of silicon also could
not be determined, due to fluorescence of the substrate. The relative
amounts of cadmium and sulfur, however, could be determined and it was
found that peak intensity ratios very close to those of as deposited
pure CdS films were obtained with COSAD films with both high and low
amounts of CdS. A few COSAD films were deposited on single crystal
sapphire to determine the composition separately from that of the
substrate.
Thin Film Optical Properties
Now that the physical properties of thin films have been fully
described, the results that are central to this dissertation will be
presented in this section. These results describe how the changes in
microstructure produced by various treatments affect the optical
properties. UV-visible absorption, photoluminescence, and resonant

COUNTS
166
1 M UI 1 1 LU n
2.6 5.2 7.8 10.
ENERGY CKqV)
Figure 62 EDS X-ray spectrum of C0SAD1 film.

167
Raman scattering were utilized at both room temperature and low
temperatures to fully characterize the optical properties of thin
films.
Absorption Spectra
UV-VIS absorption was used in this study to establish the
relationship between thin film processing conditions and their effect
on the band structure of the material. The main emphasis of this study
as stated before, was to produce a thin film with the same optical
properties as those of bulk single crystal material. From the
measurement of absorption spectra, it was possible to determine the
presence of band defects such as band tailing, to determine the
temperature dependence of the band gap energy and at low temperatures,
to observe exciton transitions.
By examining the shape of the absorption edge, a qualitative
measure of the band gap structure could be determined. At energies
below the absorption edge, the sharpness of the transition from low
absorption to the edge gives an indication of the amount of band
tailing. Only transitions which conserve momentum are allowed,
therefore, for a direct band gap semiconductor, only k equals zero
excitations occur without phonon scattering. When a great many
crystallographic defects are present in a material, then the band
structure will not be well defined. The presence of defects permits
transitions at other values of k to be quantum mechanically possible,
which in the absorption spectra is manifested as a tailing of the
absorption edge at low energies.

168
Room temperature UV-VIS absorption
The position of the room temperature band gap absorption edge that
was measured for as-deposited thin films was found to vary slightly
with film thickness and deposition rate, but mostly with substrate
deposition temperature. Films ranged in color from straw yellow to
dark orange to nearly black, as the deposition temperature was varied
from 300 C to LN2. Typical spectra of films deposited at 300 C, room
temperature, and LN2 using an absorbance scale are shown in Figure 63.
The variation in absorbance spectra with film thickness is shown in
Figure 64. As previously described, absorbance spectra show features
above the band gap with more detail than transmission spectra. By
taking the maximum slope of the absorbance, an approximate value of the
band gap absorption edge is given and as can be seen in Figure 63, this
position for the three curves varies considerably. A listing of the
absorption edges determined by this technique for different deposition
conditions is given in Table 5. From the Table, the maximum slope
position for films deposited under the same conditions, but with
different thicknesses varies only slightly and as shown in Figure 64
the shape of the absorbance curves is similar. In contrast, the shape
and position of the three spectrum in Figure 63 of similar thickness
films deposited at different temperatures is altered considerably. The
film deposited at 300 C shows very flat absorption above the band edge
indicating a well developed band gap, but the position of the edge is
considerably shifted to longer wavelengths which is characteristic of a
composition change. Also, because of the inhomogeneity of these films
there is a reduction in transmission at long wavelengths due to

ABSORBANCE
169
Figure 63 Room temperature UV-visible absorbance spectrum of films
deposited at LN2 temperature (LN2), room temperature (RT),
and 300 C (300C).

170
Figure 64
Room temperature absorbance spectra of different thickness
films, deposited at 1 /sec onto room temperature
substrates.

171
TABLE 5
Band Edge Position for Several Different Films
Determined by Maximum Slope Method
Sample
Maximum Slope Position
(nm)
Energy
(ev)
ASDP 1 A/sec
t = 0.5 p
507.8
2.442
t = 0.75 ym
508.9
2.437
t = 1.0 ym
509.8
2.432 (2.44)1
(2.37)2
ASDP 3 A/sec
508.5
2.439
ASDP 5 A/sec
501.7
2.472
ASDP LN2
496.5
2.497
ASDP 300 C
523.1
2.370
FHT 500 C
505.1
2.455 (2.46)1
5 hr.
(2.45)2
'Determined by maximum slope of absorption curve (Figure 79a)
^Determined by intercept method (Figure 79b)
Note: All films measured were approximately 1.0 ym thick unless
other wise stated.

172
scattering losses. For the LN2 deposited film, a large amount of band
tailing is indicated by the higher absorbance below the band edge and
higher absorbance above the band edge indicates the presence of
uncompensated defects. The overall transmission of this film is
reduced because of the presence of excess cadmium, which is the reason
for the dark color of these films. Excess cadmium is also most likely
responsible for the high absorbance above the band gap. A close
relationship between the number of defects observed by TEM and the
amount of band tailing is observed in these films. Band tailing is an
indication of the structure of the band gap and the presence of band
gap defects, which appear to be directly dependent on the number of
crystallographic defects.
The band tailing-crystal defect relationship is further indicated
when the absorbance of films heat treated to different temperatures is
measured. The lower heat treatment temperatures at 200 and 350 C
shown in Figure 65 do not indicate much of a change in the amount of
band tailing; however, the heat treatment to 500 C has reduced nearly
all band tailing, which reveals a distinct absorption band below the
band edge. This is an interesting result, because this is the
temperature at which grain growth was shown to drastically increase.
Examination of Figure 65 also shows that the higher temperature heat
treatment results in a slight shift of the absorption edge to higher
energy. This might be related to the slight change in composition that
occurs at the higher heat treatment temperature.
Because there is such a strong relationship between band tailing
and crystal structure, examining the onset of absorption in thin films

ABSORBANCE
173
WAVELENGTH (nm)
Figure 65 Room temperature absorbance spectra comparing as-deposited
(ASDP) and heat treated thins. Temperature of heat
treatment shown; all treatments for five hours.

174
is a useful technique for evaluating the results of different heat
treatments. Figure 66 shows the total absorbance curve and Figure 67
shows an expanded abscissa scale for heat treatments of 650 C 30 sec,
650 C 4 min, and 500 C 5 hours. Figure 66 shows a structural change
between RTA and the 500 C 5 hour heat treatment in the band tail and
all three curves show two sub-edge features, both as shoulders above
the sharp rise in absorption. Figure 67 indicates that furnace heat
treating to 650 C for 4 minutes results in nearly the same low energy
band structure as the heat treatment at 500 C for 5 hours. A rapid
thermal anneal (RTA) film displays the same curve shape at high values
of absorbance, but as indicated in Figure 67, there still is some band
tailing present.
Low temperature UV-VIS absorption
At sufficiently low temperatures, many features of the absorption
spectrum appear more clearly and can be related to the band structure
in these films. Spectra were be obtained at temperatures down to 9 K
by cooling a sample with the closed loop helium refrigerator. With a
decrease in temperature several changes in the absorbance curve are
observed. First the band edge shifts to higher energy, both due to a
change in the lattice parameter and to the temperature dependence of
the band energy. A plot of the band gap energy as a function of
temperature is displayed in Figure 71b.
Second, the amount of band tailing is shown to significantly
decrease in heat treated films; however, the decrease is not as
predominant in as-deposited thin films. The indirect transitions which

ABSORBANCE
175
WAVELENGTH (nm)
Absorbance spectra of different heat treatments. Furnace
heat treatment 500 C, five hours (FHT 500); furnace heat
treatment 650 C, four minutes (FHT 650); rapid thermal
anneal heat treatment, 650 C, 30 seconds (RTA 650). All
films deposited at 1 /sec.
Figure 66

ABSORBANCE
176
Figure 67 Same designations as Figure 66, with expanded abscissa
scale.

177
are responsible for most of the band tailing are phonon assisted
transitions and with the lowering of the temperature, these transitions
are no longer possible. Band tailing is not completely reduced in the
as-deposited thin films because there are other defect states present
in the band gap (created by the crystallographic defects) which do not
require phonon scattering for transitions.
The most significant change in the absorbance spectra at low
temperature is shown to occur in films heat treated at high
temperatures. The appearance of several sharp absorption bands which
correspond to exciton transitions are observed. Figure 68a displays
these bands and other changes in the absorbance spectra of a 1.0 pm
thick film, deposited at 1 /sec, that was heat treated to 650 C for 4
minutes. Shown in Figure 68b is the absorbance spectrum from a 10 pm
thick single crystal platelet of CdS, measured under the same
conditions as the thin film. The platelet was oriented with the c-axis
perpendicular to the incident beam. Above the band edge, the two sharp
peaks observed in the thin film spectrum appear similar to the platelet
spectrum, but the features are more pronounced in the thin film
spectrum. Also, the entire curve for the thin film is shifted to lower
energy. The similarities of these band edge features, however, do
indicate the presence of exciton states in the polycrystalline thin
fi1ms.
Further proof is afforded by plotting the absorption peaks on an
energy scale, as shown in Figure 69a. The spectrum obtained in this
study can be directly compared to reflection spectrum of a bulk single
crystal CdS shown in Figure 69b. A comparison of the peak energies is

ABSORBANCE ABSORBANCE
a)
b)
Variation of absorbance spectra with temperature, a)
spectrum observed at room temperature and 9 K; b) low
temperature spectrum of thin film (TF) and single crystal
platelet (SC).
Figure 68

179
a)
2.50 2. 52 2. 54 2. 56 2. 58 2. 60 2. 62 2. 64
PHOTON EN6RCY UV)
b)
Figure 69 Comparison of low temperature spectra. a) absorbance
spectrum of thin film, with energy abscissa scale; b)
reflection spectrum of single crystal, 4.2 K.^

180
listed in Table 6. The absolute values of the peak energies are
slightly different, but the energy difference between the A, B and C
levels is nearly the same in both spectra. The difference in absolute
values may be attributed to the same source which produces the
difference in the absorption band edge, displayed in Figure 68b. The
peaks are much sharper in the reflection spectra because the technique
is very sensitive to changes in reflectivity and also because the
reflection spectrum was taken at 4.2 K. The broader peaks observed
here may be due to temperature broadening, as well as orientational
broadening in the polycrystalline films.
The observation of exciton absorption peaks was made in all films
heat treated to above 500 C. With the large grain growth that occurs
with high temperature furnace heat treatments, it is not surprising
that exciton levels should occur in a polycrystalline thin film,
because even with a grain size of 3000 , the band gap should be well
established. An interesting result however, is indicated by Figure 70
which shows exciton levels to be present in a RTA film in which the
grain size is only 900 . The absorption peaks are less intense and
slightly shifted, but they are definitely present. This indicates that
large grain growth is not required to produce polycrystalline thin
films which display exciton levels. What is needed is only the
annealing of certain defects.
Another interesting observation is the variation of the exciton
peaks with temperature. Figure 71a shows that only a slight shift and
decrease in intensity occurs when the temperature is increased to 40
K, but at 100 K the A exciton peak has become a shoulder, the B and C

181
TABLE 6
Comparison of Exciton Reflection and Absorption
Peak positions
Peak Position
Energy
Peak Position
Energy
Exciton
CdS Platelet
Delta
CdS Thin Film
Delta
A
2.555
0.015
2.545
0.013
B
2.570
0.080
2.558
0.072
C
2.630
2.620
Note: All energy values given in electron volts, eV.

ABSORBANCE
182
Figure 70 Low temperature absorbance spectra of furnace heat treated
(FHT) and rapid thermal anneal (RTA) thin film.

BAND GAP ENERGY
183
b)
Figure 71 Variation of band gap energy with temperature. a) position
of absorbance spectrum with temperature; b) plot of band
gap energy versus temperature.

184
excitons peaks have broadened considerably. The disappearance of the A
exciton peak at 100 K is consistent with the calculated binding
energies of excitons, as the A exciton has a binding energy of 8 meV,^
which corresponds to 92 K. Above this temperature a sharp transition
should not be possible and we see that the peak broadens into a
shoulder. It is interesting to note, however, that even at 300 K, as
shown in Figure 68, an exciton-like shoulder is still present. Figure
71a also shows the temperature dependence of the band edge, and a plot
of this variation as a function of temperature is shown in Figure 71b.
In the Bibliographic Review chapter (Chapter III), the description
ft
of the exciton states in CdS characterized the states in terms of
crystallographic orientation and therefore their observation is
dependent on crystal orientation with respect to the polarization of
the probe beam. The fact that the grains in the films studied here
have a strong preferred orientation is one reason why all three
excitons are observed. In CdS single crystal platelets, all three
excitons are observed when the E-field is perpendicular to the c-axis,
but only the B and C excitons are observed with the E-field parallel to
the c-axis. Although a circularly polarized beam is used in the Lambda
9, the beam is correctly oriented at any one point on the film because
the orientation of the grains can be considered to be circular about
the c-axis pole. TEM and X-ray diffraction results indicate that many
grains are oriented with the basal plane of the unit cell parallel to
the film and substrate plane, so one direction vector of the unit cell
is perpendicular to the film plane. The other two direction vectors
which fully describe the unit cell and crystal orientation are randomly

185
oriented, but should be rotated about the c-axis direction vector. It
is therefore highly probable that a grain will have the proper
orientation to the circularly polarized beam and all three exciton
transitions should be measured.
Index of refraction
Fringes of equal chromatic order. Measurement of fringes of equal
chromatic order (FECO) was a very useful technique for determining the
dispersion of the refractive index at wavelengths longer than the
absorption edge. Unfortunately, equation 4.4 which was used to model
the dispersion could not be used to calculate the refractive index at
the absorption edge or at shorter wavelengths. This is because the
equation over-estimates the large change in absorption coefficient that
takes place near the band edge. As shown in Figure 9 (Chapter III) the
refractive index for bulk CdS is found to peak at the absorption edge
and then decrease at shorter wavelengths. The problem with equation
4.4 is that it indicates that the refractive index converges to
infinity at short wavelengths. This is shown graphically in Figure 72a
and 72b, which are plots of equation 4.4, using the coefficients
obtained from the FECO spectra of two different films. Figure 72b
shows the calculated dispersion near the absorption edge. The Figures
show that the curves converge at long wavelengths and give nearly the
same value for the long wavelength limit of the refractive index.
However, even at a wavelength 100 nm longer than the absorption edge,
the refractive index is overestimated. Table 7 is a listing of the
refractive index calculated at different wavelengths using equation 4.4

INDEX OF REFRACTION INDEX OF REFRACTION
186
a)
b)
Figure 72 Index of refraction calculated from FECO spectrum, a)
showing long wavelength conversion; b) variation near the
band gap.

187
TABLE 7
Comparison of Refractive Index Determined by
FECO method and Ellipsometry
Sample
FECO
Refractive Index
Ellipsometry
Refractive Index
ASDP 1 A/s
3.036
2.667
FHT 200 C
3.120
2.827
5 hr.
FHT 350 C
3.098
2.805
5 hr.
FHT 500 C
2.874
2.448
5 hr.
Note: All heat treated films deposited at 1 /sec.

188
and the coefficients obtained from FECO spectra of several films. The
refractive index was calculated at a wavelength of 632 nm for
comparison to the value of refractive index obtained by ellipsometry,
listed in the last column of the Table. Table 7 shows that the FECO
method for determining refractive is only valid at long wavelengths.
Ellipsometry measurements. By using the graphical method
described in Appendix B to determine the refractive index from the
polarization angles measured with the ellipsometer, very accurate
results could be obtained. These results are useful for quantitative
comparison of different films, unfortunately this comparison could only
be made at one wavelength; i.e. 632 nm, the wavelength of the HeNe
laser used with the ellipsometer. Table 7 shows there is a significant
change in refractive index when a thin film is heat treated. This
should be expected because the density of the film changes
considerably, particulary from high temperature heat treatments. The
results show that ellipsometry is sensitive to the slight differences
in microstructure of thin films deposited at different rates. In
addition to these advantages, the technique is also very useful for
refractive index and thickness determinations on very thin films, which
cannot be measured with the FECO technique.
Absorption coefficient. A very close approximation to the
wavelength dependence of the absorption coefficient can be made by
taking the absorbance spectra and applying a few simple relationships.
First, the extended equation for the transmission of light through a
thin film supported on a substrate (equation 4.2, Chapter III) which

189
takes into account the reflections that take place within the film and
the resulting interference can be written as
T =
A e
-a t
1 -Be
-2 a t
(5.1)
where
A = ( 1 -R^ ( 1 R2) ( 1 R3) ( 1 R2R3)
and
B = ( R R2 + R1R;J ) ( 1 R2 )2.
- 1
Equation 5.1 can be simplified and written as
T =
a t
B e
-a t
(5.2)
and the inverse is given by
1
T
1 at
e
B -at
e
(5.3)
If the absorption coefficient a is assumed to be very large (a ~ 10^)
then the second term in equation 5.3 is essentially zero. Since the
absorbance is defined as the natural logarithm of inverse transmission,
then the absorption coefficient can be calculated from the absorbance
spectra by applying

190
ABSORBANCE + In A
a =
t
(5.A)
The term In A represents the losses due to reflection interference and
the value can be determined from the absorbance spectra of different
thickness films, such as Figure 64, or a close approximation can be
made by using the value of absorbance below the band edge, such as the
value at 580 nm in Figure 64. The absorption coefficient spectra as a
function of energy obtained by this calculation is shown in Figure 73a
for both an as-deposited film and a heat treated film. The value of
the optical band gap is given by either the maximum in slope of this
curve, or by plotting the square of a and extrapolating the linear
portion of the curve to the intersection of the x-axis. The value of
the x-axis intercept is the band gap energy. This plot is shown in
Figure 73b for the same two films and a listing of other films and a
comparison of the different techniques for calculating the band gap is
presented in Table 5.
The above Figures and Table when compared to the data presented in
the Bibliographical Review chapter show that the thin films produced in
this study exhibit similar results to single crystal properties. This
is another indication of the high quality of films which can be
produced by the deposition process and subsequent heat treatments that
were used in this study.
Photoluminescence and Raman Spectroscopy
Photoluminescence offers a complementary analysis to absorption
measurements. While the optical transitions involve similar states,

ABSORPTION COEFFICIENT-2 (cm-2) ABSORPTION COEFFICIENT (cm-1)
(Millions) (Thousands)
191
a)
b)
Figure 73 Alternate methods for determining the value of the optical
band edge, a) plot of absorption coefficient calculated
from Equation 5.4 versus photon energy, point of maximum
slope gives the band energy; b) plot of a- versus photon
energy, extrapolation to a 0 gives band gap energy.

192
they are not subject to the same selection rules. For direct band gap
semiconductors, radiative recombinations generally occur near k=0.
Just as all the different defect states in the band gap lead to
discrete lines, or bands, or tailing in absorption spectra, they can
also lead to photoluminescence spectra which are indicative of these
defects. As described earlier, the defect states that are of the
greatest interest to this study are the exciton states and
photoluminescence is an excellent technique for studying their
occurrence and behavior.
Use of the optical multichannel analyzer (OMA) and the Raman
spectrometers permitted a wide variation in the resolution and spectral
window over which the photoluminescence and Raman scattering of thin
films could be measured. For analysis of certain thin films which
displayed very low intensity, wide band photoluminescence, the OMA
could be used with a very low dispersion grating (152 lines/mm). While
giving very poor spectral resolution this arrangement still permitted
detection of very low light levels. To measure high intensity, narrow
bandwidth peaks, the low dispersion grating was replaced with a very
high dispersion 2400 lines/mm grating. As explained in the
Experimental Method chapter (Chapter IV) this grating permitted a
bandpass resolution of 0.9 nm, but a spectral window of only 20 nm. To
assure calibration and permit an even higher bandpass resolution, the
Raman spectrometer was used. By controlling the slit widths of the
monochromater, resolutions down to 0.05 nm could be obtained.

193
Photoluminescence spectra
As-deposited thin films. Thin films which were deposited at room
temperature were found to only give very low intensity, broad band
photoluminescence. The defect structure of the band gap that led to
the band tailing observed in the absorbance spectra of these films is
also responsible for the low intensity observed in these measurements.
These defects states provide non-radiative processes for the relaxation
of excited states, so that the photoluminescence spectra have very low
intensities. Because there are many different states with slightly
different positions in the band gap, a broadening of the luminescence
also occurs. A typical spectrum of films deposited at low rates,
obtained at room temperature with the OMA and the low resolution
grating is shown in Figure 74. This is an expanded intensity scale so
the signal to noise ratio is low. The sharp peak at 457 nm is the
laser line used to excite the sample. Usually, the three different
broad bands observed in this spectrum would occur in all films
deposited at room temperature, but the intensity of the different bands
would vary. For example, the higher intensity band centered at 535 nm
was found to be of much lower intensity in films deposited at higher
rates and it was totally absent in films deposited at 5 /sec. The
energy of the 535 nm band corresponds to direct transitions in the band
gap and is the "green-edge" emission that was discussed in the
Bibliographical Review chapter (Chapter III). The radiative
transitions from conduction to valance band or from shallow donor to
shallow acceptor impurity levels are characterized by this emission.
The two low intensity broad bands centered at 670 nm and 750 nm are

Count*
194
Figure 74 Low resolution photoluminescence spectrum acquired with
optical multichannel analyzer (OMA).

195
associated with deep impurity level luminescence. The relative
intensities of these two bands could be altered with slight changes in
the geometry of the detection system. The width of these bands made it
difficult to center the grating so that the diode array was evenly
illuminated. This is one example of problems associated with a linear
diode array detection system. If the array is not evenly illuminated
the relative intensities displayed in the emission spectra will not be
proportional.
Films deposited at LN2 temperatures only displayed the longer
wavelength bands and at an even lower intensity level. The green edge
emission is not observed in these films because the band structure is
not well defined, due to the large number of crystallographic defects.
The long wavelength bands are observed because deep level traps do not
need a well established band gap to cause radiative transitions. They
can be likened to impurity color centers, which are associated with a
single impurity atom, or in this case with a vacancy or interstitial
atom associated with an impurity. The identity of this impurity is
unknown, but some authors have reasoned that it could be a halogen atom
such as chlorine.33,34
Heat treated thin films. A very significant change in the
photoluminescence of thin films occurs in films which are heat treated
at high temperatures, even for relatively short times. The intensity
of the green edge emission increases over 5x for the same excitation
intensity and the peak position shifts to a higher energy. The
relative intensity of an as-deposited thin film and a film heat treated
to 500 C for 30 minutes are shown in Figure 75 and the relative peak

Counts *IQ1
196
O
Wavglangth rm
Low resolution photoluminescence spectra of heat treated
and as-deposited thin films, showing relative intensities.
Figure 75

.ounts x10
197
nm
Figure 76 Same spectra as Figure 75, relative peak positions.

198
positions are shown in Figure 76. Both of these changes occur because
the band gap is better defined after the annealing heat treatment.
With this change in band structure, the band edges are sharper, which
shifts the energy and increases the probability of a radiative
transition. Figure 75 also shows that the intensity of the laser line
has increased significantly, which is most likely due to an increase in
scattering of the laser line.
All films which were heat treated to high temperatures displayed a
high intensity edge emission peak and the peak position was found to be
nearly the same. One difficulty with determining peak position,
however, was that absorption of the laser line would cause enough
heating of the film to shift the position of the peak to lower energy.
A graph of the shift in peak position with incident laser power is
displayed in Figure 77. At very low powers only a small shift occurs,
but at higher incident powers the shift is significant. At even higher
incident powers the focussed laser beam would cause vaporization of the
thin film. Very low incident laser powers were therefore used to study
the room temperature photoluminescence of thin films, although low
power excitation reduced the intensity of the emission.
The problem with power dependent peak shift was reduced
considerably by cooling the thin film with the cold finger and
acquiring photoluminescence spectra at low temperature. As with UV-
visible spectroscopy, several significant changes in thin film
photoluminescence are observed when the film is cooled to 9 K. First,
the green edge emission band increases in intensity and shifts to a
higher energy corresponding to the increase in band gap energy with

INCIDENT LASER POWER (W)
199
Figure 77 Graph of band edge luminescence peak position versus
incident power.
522

200
decrease in temperature. Second, the peak position becomes less
sensitive to incident power levels, i.e. no shift occurs unless very
high powers are used. Finally, the most significant change which
corresponds to the exciton absorption peaks seen in the UV-visible
spectra, is the appearance of the "blue-edge" emission peak. Because
of the narrow bandwidth of this peak the high resolution grating had to
be used in the OMA monochromater. A typical photoluminescence spectrum
of the blue edge emission exhibited by a thin film which also displayed
the measurable absorption peaks is shown in Figure 78 and a composite
spectra showing the relative intensities of the two emission bands is
shown in Figure 79. The sharp peak at 492.3 nm in Figure 78
corresponds to one of the exciton transitions, although it is not clear
which one. The first two absorption peaks in the absorbance spectrum
separated 5 3 nm should exhibit two photoluminescence peaks, but only
one can be seen here. The photoluminescence spectrum obtained from the
same single crystal platelet used in the absorption measurements
(Figure 68b) is shown in Figure 80, compared with the blue edge
emission measured in the film described above. The platelet displays
both the 1^ and I2 bound exciton peaks and as can be seen, the
transitions are much sharper and shifted to energies higher than the
photoluminescence exhibited by the thin film. The shift in position is
identical to that seen in the absorption edge displayed in Figure 67b.
The shape of the thin film photoluminescence is similar to the
platelet; the high energy side is steeper. The peak width of the thin
film emission is nearly five times wider. In other work on both on
bulk single crystal CdS and CdS epitaxial thin films, exciton

xlO
201
Figure 78 High resolution OMA spectrum of heat treated thin film at
9K, showing exciton photoluminescence.

vlC1
202
Wavelangth (na)
Figure 79 Composite low temperature OMA spectrum showing relative
intensities of band edge and exciton emissions.

Counts xlO
203
Wavelength (nm)
Figure 80 Low temperature OMA spectra of thin film and single crystal
platelet, showing relative peak positions. The bound 1^
and I2 excitons are labeled on the platelet
photoluminescence.

204
transitions were exhibited by very sharp emission bands, although these
measurements were made at lower temperatures. The measurements of
single crystal platelets made here show that the exciton emissions are
just as sharp at 9 K. This rules out the possibility that the
emissions measured from thin films are temperature broadened. One
possible reason for the observed broadening may be orientational
broadening or the presence of grain boundaries. All materials which
have displayed sharp exciton photoluminescence have been essentially
single crystals. The impurity centers near grain boundaries which
excitons are bound to may vary in energy slightly from those impurity
centers within the interior of grains. This might produce the
broadening of the exciton photoluminescence observed in polycrystalline
thin films.
The photoluminescence spectra displayed here exhibits another
difference from the absorbance spectra. The three absorbance bands
were characterized as free exciton transitions, based on their position
and shape. The blue edge emission exhibited by thin films, however, is
more characteristic of bound exciton transitions. As previously
explained, these are excitons which are bound to impurity sites.
Usually the impurity is a neutral acceptor or neutral donor, which give
rise to an 1^ or an I2 bound exciton, respectively. It is certainly
reasonable to expect that these types of impurity centers exist in the
thin films studied here. Sulfur interstitial atoms or cadmium vacancy
atoms could act as donor impurity sites, which correlates well with the
single peak exhibited in the blue-edge emission of thin films.
However, compared to the single crystal platelet photoluminescence, the

205
blue-edge emission is certainly broad enough for both bound excitons to
be present. The bound exciton is a much stronger transition, so it
should be easier to detect in photoluminescence measurements. The free
exciton emission may be too low in intensity to detect. For absorption
measurements the cross section of the free exciton is much larger than
the bound exciton, because it does not have the same momentum
constraints as the bound exciton. Therefore, the absorption spectra of
free excitons should be broad and the bound exciton should exhibit a
sharp absorption band. Estimations show that the UV-VIS
spectrophotometer may not have a high enough resolution to observe the
bound exciton absorption peaks. Photoluminescence on the other hand
does not have the same momentum constraints, therefore the emissions
will have different characteristics. Since the bound exciton has both
a higher oscillator strength and more efficient radiative conversion,
bound excitons should be more predominant in photoluminescence spectra.
The blue edge emission displays a temperature dependence similar
to optical absorption. This is shown in Figure 81. As the temperature
is increased, the photoluminescence peak shifts to longer wavelengths
(lower energy) and decreases in intensity. As with the UV-visible
spectra, the peak is still present at 40 K, but at 100 K it becomes
indistinct. The Figure also shows that with an increase in temperature
the intensity of the green edge emission peak decreases. The decrease
occurs because as the temperature is increased, more phonons become
available to cause non-radiative transitions.

Counts xlO
206
9. 0 K
Wavslength (nn)
Figure 81 Temperature dependence of blue edge photoluminescence.

207
Raman spectroscopy
The photoluminescence measurements described above were conducted
by exciting the films with a laser beam of energy much greater than the
band gap. Excited states which are generated must first drop to a
lower energy corresponding to either an impurity level, or an exciton
level by way of a non-radiative transition before a radiative
transition can occur. When a excitation energy is used that is very
close in energy to the excited state, the energy can be absorbed and
then scattered by a phonon state. The scattering by the phonon results
in the re-emission of the light at a different frequency. Because the
excitation energy is very close to the excited state level, a resonance
effect takes place and an enhancement of the scattering results. This
process is called resonant Raman scattering and in thin films it was
found to be a very pronounced effect when a laser energy near the
energy of the exciton level was used to excite the film. Resonant
Raman scattering by the longitudinal optical (LO) phonon of the 488 nm
laser line would result in a series of equally spaced peaks that were
shifted ~ 300 cm ^ from the laser line. These peaks were very intense
and could be measured with either the OMA or Raman spectrometers. A
typical spectrum of the first two LO phonon peaks which was acquired
with the OMA and plotted in terms of Raman shift from the laser line is
shown in Figure 82. This spectra was acquired with similar scan
parameters and laser intensity as photoluminescence spectra, but the
intensity of the 1L0 phonon is much greater than any photoluminescence
peak. The temperature dependence of this emission is shown in Figure
83. A similar low temperature spectrum obtained with the Raman

Count* xlO
208
O
Figure 82
Longitudinal optical (LO) resonant Raman peaks measured
with OMA.

Counts xlO'
209
Figure 83 Temperature dependence of resonant Raman peaks.

INTENSITY
(Thousand*)
(Thousands)
WAVENUMBER (cm-1)
Figure 84
Same LO peaks displayed in Figure 82, shown here measured
with the Raman spectrometer.

211
spectrometer and plotted in wavenumbers is displayed in Figure 84. The
same laser power was used to excite the sample, but as can be seen a
much lower intensity is measured. This indicates how much lower the
light gathering ability of the Raman is compared to the OMA. The
spectra however, also show that the resonant Raman effect is strong.
Laser powers for normal Raman spectroscopy typical are in the range of
a few watts. Even with this high power the Raman scattered peaks are
very low in intensity. The laser power used to excite the samples
measured in this study was only one milliwatt! The high intensity of
the 1L0 peak is due to a strong interaction with the exciton state.
This is further proof of the existence of exciton levels in the
polycrystalline thin films.
COSAD Optical Properties
The microstructure analysis described in the physical properties
section has indicated that it is possible to produce structures with
finely dispersed CdS crystallites in a glass matrix thin film. The
important question about these structures is whether quantum
confinement occurs, which would be indicated by a shift in the optical
absorption edge to higher energies. Unfortunately, a shift could also
be produced by a change in composition. Since C0SAD1 films were made
with a high CdS content, which once heat treated, leads to structures
too large for quantum confinement, it would be expected that any shift
in the band edge would be due to compositional effects. On the other
hand C0SAD2 films should exhibit a shift due to quantum confinement,
because of the small particle size displayed in these films.

ABSORBANCE
212
Figure 85
Room temperature absorbance spectrum of C0SAD1 thin film.
As-deposited (ASDP) and after heat treatment to 650 C, 60
minutes (FHT).

213
A typical absorbance spectrum of an as-deposited C0SAD1 film is
shown in Figure 85. The high absorption tail at short wavelengths is
due to the glass matrix. This type of absorption is an indication of a
nonstoichiometric oxygen content and has been shown to occur in other
glasses which were sputter deposited with an argon plasma.^ The
spectrum of the as-deposited COSAD film shown in the Figure exhibits no
appearance of a band edge, which is inconsistent with the
microstructural analysis that indicated a semi-crystalline structure.
Also shown in Figure 85 is the absorbance spectrum of a C0SAD1 type
film heat treated to 650 C in an air atmosphere. This spectrum, at
first, certainly appears to be the result of quantum confinement; the
band edge has shifted to 460 nm (2.70 eV) and the exciton shoulder is
more pronounced. Microstructure analysis on this type of film however,
indicated a crystallite size much too large for quantum confinement,
which means that this shift in band edge energy is most likely a
crystal composition effect. Further proof that this is a chemical
shift is shown in Figure 86 which displays the spectrum of a C0SAD1
type film heat treated in an argon atmosphere. The band edge shift is
less and the exciton shoulder is less pronounced.
It is probably not correct to call the feature on the absorption
edge an exciton shoulder, as the shape of this feature does not change
considerably as the temperature is decreased. In pure films of CdS the
room temperature shoulder resolved into two sharp absorption bands at
low temperatures. As shown in Figure 87 the shoulder does not
appreciably change shape at 9 K and there is only a small shift in the
band edge.

ABSORBANCE
214
WAVELENGTH (nm)
Figure 86 Room temperature absorbance spectrum of C0SAD1 thin film
heat treated in air (AR) and argon (AG), at 600 C, for 30
minutes.

ABSORBANCE
215
Figure 87 Low temperature absorbance spectrum of C0SAD1 thin film.

216
A much different absorbance spectrum is displayed by the C0SAD2
type films. Shown in Figure 88 are the spectra of both an as-deposited
and a heat treated thin film. Unlike C0SAD1 films this type of film
shows the appearance of a band edge which is shifted to a very high
energy, even in the non-heat treated state. This is an interesting
result, because although the microstructure of this type of film is
very small, it appeared to be amorphous. Heat treatment to 600 C in
an argon atmosphere produced only a small change in the edge position,
but it was still shifted to a higher energy than C0SAD1 films. The
large shift in band energy must be related to a confinement process.
Another interesting result is indicated by the absorbance scale, which
is an order of magnitude less than the scale used to measure C0SAD1
films. This is an indication that there is considerably less CdS in
the structure of C0SAD2 films.
Unfortunately no photoluminescence spectra could be obtained from
either type of COSAD film. This may be due to a dilution effect, due
to both a low CdS concentration in these films and a small thickness.
Also, because the band edge was shifted to such a high energy in the
C0SAD2 films it was not possible to excite the sample, even when using
the highest energy argon laser line.

ABSORBANCE
217
WAVELENGTH (nm)
Figure 88 Room temperature absorbance spectrum of C0SAD2 thin film.
As-deposited (ASDP) and after furnace heat treatment at
650 C for ten minutes (FHT).

CHAPTER VI
CONCLUSIONS
1. RF magnetron sputtering of cadmium sulfide has proven to be a
valuable deposition technique for producing high optical quality thin
films. The as-deposited thin film structure can be controlled by a
variation of the deposition rate and substrate temperature. Unique
structures can be produced by deposition onto substrates held at LN2
temperatures. A low density columnar structure is produced in which
small microcrystallites are embedded in a semi-amorphous matrix.
Electron diffraction experiments indicate that the microcrystallites
are highly faulted, which leads to a degradation of the optical
properties. The crystallographic defects provide defect states in the
band gap which are indicated by the large amount of band tailing
observed in optical absorption spectra. Optical properties are further
degraded by the presence of excess cadmium. The large number defects
responsible for band tailing provide non-radiative transitions so that
band edge photoluminescence is not observed in these films.
2. High density fine grained polycrystalline films result when
sputtered material is deposited onto substrates held at room
temperatures. Many of the grains appear to be faulted, with the
presence of stacking faults and microtwins. The number of faults
increases with the deposition rate, therefore the faults are related to
the growth rate of the thin film. Electron and X-ray diffraction
218

219
experiments indicate the film has a very strong preferred orientation,
with the basal plane parallel to the substrate plane. It is not clear
if this orientation is nucleation or growth controlled. The close-
packed plane is the slow growth direction, which would indicate the
preferred orientation is controlled by nucleation. This same
orientation, however, was observed over a substrate temperature range
of nearly 500 K and with various substrate materials. It is unusual
that a nucleation controlled process would extend over such a large
temperature range, although with this material the nucleation range may
be very wide. A combined effect may actually control the process if
the crystallites can be modeled as thin disks. Once the initial
orientation is nucleated, only a small step in the z-direction could
cause a large increase in growth in the lateral direction. X-ray line
shifts indicate films deposited at high rates contain large residual
tensile stresses, which leads to their exfoliation when exposed to the
atmosphere. Optical absorption spectra indicate some band tailing to
be present in room temperature deposited films, which is related to the
crystallographic defects that are present. The band structure in these
films is defined well enough so that band edge photoluminescence is
exhibited; however, the large number of defects present in the band gap
leads to a decrease in the photoluminescence yield.
3. When CdS is deposited onto substrates held at high temperatures a
reduction in the number of crystallographic defects is observed. With
higher substrate temperatures, adsorbed atoms have more energy to move
around and find favorable positions with which to bond. The preferred
orientation is still maintained, however, other hexagonal orientations

220
can be observed because other growth planes may become active at
higher temperatures. With the decrease in the number of
crystallographic defects, band tailing is reduced indicating a better
defined band gap; however, a shift in the absorption edge is observed.
The shift is due to a change in composition, as these films are found
to be nonstoichiometric. A higher sulfur content in these films occurs
because the sticking coefficient of cadmium is reduced at higher
substrate temperatures. Due to large nonuniformities in the
composition across the film area a degradation in the optical
properties at wavelengths longer than the band edge occurs.
4. To produce the highest quality thin films for optical applications
a post deposition heat treatment must be instituted to remove defects
created by the deposition and growth processes. Heat treatments of
less than 500 C cause a small change in grain size which appears to be
linear with the increase in heat treatment temperature. Low
temperature treatments also lead to a small change in the optical
properties. Band tailing is still present and photoluminescence yield
is still low. With heat treatments to 500 C and above, a dramatic
increase in grain size occurs. Higher temperatures may provide the
activation energy required for impurity assited diffusion at the grain
boundaries. Associated with this grain growth is a reduction in the
amount of band tailing and a large increase in the photoluminescence
yield. These results are due to a removal of crystallographic defects
with grain growth, which are related to defects in the band gap.
5. For the first time absorption bands and photoluminescence
associated with exciton states have been observed in polycrystalline

221
thin films. Associated with the changes in microstructure that occur
upon heat treatment to high temperatures is the appearance of exciton
levels. These levels appear as sharp absorption bands or as low
intensity blue edge photoluminescence. There is, however, considerable
broadening of the blue edge photoluminescence associated with the
exciton levels. The shape of the emission peak is similar to that
observed in single crystal platelets; however, it is ~ 5 times wider.
This broadening may be due to orientation effects or the presence of
grain boundaries which provide different environments for the
annihilation of excitons to produce the photoluminescence.
6. Heat treating films by rapid thermal anneal (RTA) produces films
which also exhibit exciton levels. The grain size of these films is
three times smaller than furnace annealed thin films, which indicates
that for excitons to be present in sputter deposited thin films, only
the annealing of certain defects is required, not large grain growth.
The actual defects which must be removed are not known, but they must
be those associated with band tailing, as the appearance of exciton
levels is coincident with the removal of band tailing. From TEM
micrographs of unheat treated films, these defects most likely are the
stacking faults associated with microtwins.
7. Thin films were found to be nonstoichiometric, particularly after
high temperature heat treatments where the sulfur content was found to
increase to approximately 4%. The increase in sulfur is obviously due
to a loss of cadmium, which has a much higher vapor pressure than
sulfur at the heat treatment temperature. Only a small portion of the
excess sulfur may exist interstitially, as the indication from other

222
chalcogeni.de compounds is that this type of defect would be limited to
less than 1%. Calculations show that if all the excess sulfur were
present at the grain boundaries as a thin layer, then this amount of
sulfur would be six to seven monolayers, which corresponds to a grain
boundary layer 14 thick. For an average grain size of 3000 , this
is not an unreasonable thickness for grain boundaries. The uneven
contrast observed in X-ray maps of sulfur in heat treated films
indicates that there is an uneven distribution of sulfur and therefore
it may exist as small pockets, instead of a thin evenly distributed
second phase. The presents of this second phase does not effect the
optical properties significantly, but it may be responsible for the
large resistivity observed in high temperature heat treated thin films.
8. The limited study on COSAD films indicates that it is possible to
produce a structure where very fine particles of a semiconductor are
embedded in a glass matrix. By variation of the amount of CdS that is
co-deposited a wide variation in the structure of the film can be
accomplished. High amounts of CdS produce a film which readily phase
separates during examination in the TEM. When this type of film is
heat treated to high temperatures a very distinct two phase structure
results where the CdS is dispersed as a bimodal distribution of
spherical particles. The larger particles have an average size of 500
, and the smaller particles are sized at 200 . When very low
concentrations of CdS are co-deposited, then a structure which does not
phase separate results. After heat treatment, very small particles of
less than 70 result.

223
9. Shift of the absorption edge in COSAD films appears to be mostly a
chemical shift and not due to quantum confinement. Heat treatments to
the same temperatures and times, but in different atmospheres result in
different shifts of the band edge. Heat treatments in an oxidizing
atmosphere produced a larger shift in the band edge than heat
treatments in an inert atmosphere. This could be due to the formation
of cadmium oxide, which has a larger band gap energy than cadmium
sulfide. The large shift of the band edge, however, cannot be entirely
due to the formation of an oxide, as the edge observed in films treated
in inert atmospheres is still considerably shifted from the energy of
pure CdS. There may be other unknown chemical effects or some
confinement process occurring. Another possible reason for the shift
could be due to surface states, which would be enhanced by the large
difference in dielectric constant at the interface and by the large
stress created by the high curvature of the spherical particles.
10. The broad shoulder on the top of the absorption edge, which
appears similar to the room temperature exciton shoulder observed in
pure films of CdS is of unknown origin in COSAD films. The shoulder
does not change shape with decreases in temperature, although there
still could be considerable orientational broadening even at low
temperatures. Unfortunately no photoluminescence was observed in COSAD
films primarily due to a dilution effect, arising from the thickness of
the films and the relatively small amount to CdS. Photoluminescence
would have indicated the presence of excitons.

CHAPTER VI
FUTURE WORK
There are several aspects of cadmium sulfide and COSAD thin films
which require further study and investigation.
Obviously, the first study is to make a nonlinear optical
measurement to determine if the exciton states in the pure films of CdS
can be saturated to produce an effect that is comparable to the large
nonlinear refractive index observed in single crystal platelets. Since
the films are of high enough optical quality to act as thin film Fabry-
Perot interferometers, a very simple intensity saturation experiment
could be conducted.The only difficulty in this experiment is the
need for a high resolution tunable dye laser which can be operated in
the wavelength region of 488 nm, the wavelength required for exciton
saturation.^ A shorter wavelength, high speed pulsed laser could also
be used to measure the nonlinear absorption, (due to a free carrier
plasma) by measuring the temporal waveform of a transmitted pulse.^
If this pulsed laser could be used with the dye laser, the change in
the waveform as the laser is tuned near the exciton level would provide
valuable information. Finally, if a nonlinear intensity saturation was
observed, then a pump-probe experiment could be carried out to
determine the switching speed of the thin film Fabry-Perot. * 1
Further work on the stoichiometry as a function of heat treatment
atmosphere should be carried out to determine the influence of sulfur
224

225
concentration on the band structure. Heat treatments in equilibrium
sulfur or cadmium atmospheres may produce stoichiometric thin films.
It would be interesting to know if the electrical conductivity and
photoluminescence change when the excess sulfur is removed. Other post
deposition treatments which should be further investigated are heat
treatments by RTA, to determine if very small, defect free crystals
could be produced in a thin film.
A large research effort should be directed towards further
investigations of COSAD thin films, since these films appear quite
promising for quantum confinement devices. Studies of the deposition
conditions and heat treatment schedules on the microstructure
development in these films could provide both a parallel confirmation
of the effects observed in bulk filter glasses and permit a further
understanding of the confinement processes that take place in these
materials. Photoluminescence studies of thicker films would positively
identify a confinement process, by correlation of the blue shift with
particle size. If well characterize COSAD thin films could be
produced, nonlinear optical measurements are easily made because these
films on the proper substrate should act as low loss planar waveguides.
When prisms are used to couple light into and out of thin film
waveguide, the angle at which the light couples is dependent upon the
refractive index of the film. The nonlinearity of these films could be
determined by measuring a power dependent coupling angle. If these
experiments proved successful, then the next step would be either to
attempt logic gate experiments by utilizing small scale integration, or

226
examine a different semiconductor to determine if a confinement process
could be induced.

APPENDIX A
BASIC PROGRAM USED FOR EXTERNAL CONTROL OF LAMBDA 9
1000 REM LAMBDA 9 COMMUNICATIONS PROGRAM
1010 REM VER. UWIS6.BAS
1015 REM WRITE COMMAND USED TO OUTPUT DATA TO FILE NAME *.PRN
1020 REM FOR ASYST PROGRAM INPUT AND LOTUS WORKSHEET
1030 CLEAR:CLOSE:KEY OFF:CLS
1040 DIM X(3000)
1045 REM ARRAY MUST BE LARGE ENOUGH TO ACCOMMODATE ENTIRE SCAN
1050 INPUT "TRANSMISSION OR ABSORBANCE SCAN T/A ";ANS$
1060 IF ANS$="T" THEN 1070 ELSE 1080
1070 SB=0:PRINT "TRANSMISSION SCAN"¡GOTO 1090
1080 SB=5:PRINT "ABSORBANCE SCAN"
1090 INPUT "SELECT ABSCISSA MAX ";AH
1100 INPUT "SELECT ABSCISSA MIN ";AL
1110 INPUT "SELECT SCAN SPEED ";SS
1120 INPUT "SELECT DATA INTERVAL ";DI
1130 INPUT "SELECT RESPONSE ";FR
1140 INPUT "SELECT SLIT WIDTH ";SL
1150 SL=SL*100:REM CONVERT SLIT SETTING
1160 REM CALCULATE NUMBER OF DATA POINTS
1170 XPTS=(AH-AL)/DI
1180 INPUT "SELECT NORMAL (1) OR EXTENDED RANGE (2) "jRA
1190 INPUT "SELECT CHART REGISTRATION: OFF(O).SERIAL DASH (1),
OVERLAY DASH (2)";CR
1200 IF CR=0 THEN 1260
1210 INPUT "SELECT PRINTER FUNCTION: OFF (0),GRID + SCALE (l) ";PR
1220 INPUT "SELECT PEN LINE MODE (1-4) ";PE
1230 INPUT "SELECT RECORDER FORMAT (nm/cra) ";RS
1240 INPUT "SELECT ORDIANTE MAXIMUM ";MX
1250 INPUT "SELECT ORDINATE MINIMUM ";MI
1260 IF CHG$="Y" THEN 1350
1270 REM OPEN COMMUNICATIONS PORT
1280 OPEN "COMI: 4800 E, 7,1" AS//1
1290 PRINT //I, "$RE 0"
1300 PRINT It 1, CHR$ (17)
1310 INPUT //I ,D$
1320 PRINT //1, "$PA 2"
1330 INPUT tfl,D$
1340 PRINT D$
1350 REM SET UP INSTRUMENT
1360 PRINT //I,"$SB" + STR$(SB)
1370 INPUT //I ,D$
1380 PRINT //I, "$AH" + STR$(AH)
1390 INPUT //1, D$
1400 PRINT //I, "$AL" + STR$ (AL)
227

228
1410 INPUT #1,D$
1420 PRINT #lf"$SS" + STR$(SS)
1430 INPUT //1,D$
1440 PRINT //I,"$DI" + STR$(DI)
1450 INPUT #1,D$
1460 PRINT //1,"$FR" + STR$(FR)
1470 INPUT //I ,D$
1480 PRINT #1,"$SL" + STR$(SL)
1490 INPUT //1,D$
1500 IP CR=0 THEN 1590
1510 PRINT #1,"$CR" + STR$(CR)
1520 INPUT //1, D$
1530 PRINT //1,"$RS" + STR$(RS)
1540 INPUT #1,D$
1550 PRINT //1,"$PE" + STR$( PE): INPUT #1,D$
1560 PRINT #1, "$PR" + STR$( PR): INPUT //1,D$
1570 PRINT //I, "$MX" + STR$ (MX) : INPUT //1,D$
1580 PRINT #1,"$MI" + STR$(MI):INPUT #l,D$:GOTO 1600
1590 PRINT #1,"$CR 0":INPUT #1,D$
1600 PRINT //I, "$RA" + STR$(RA): INPUT #1,D$
1610 PRINT CHR$(10);"CONFIRM SELECTED VALUES ON LAMBDA 9 SCREEN"
1620 INPUT "CHANGE ANY VALUES Y/N ";CHG$
1630 IF CHG$="Y" THEN 1640 ELSE 1650
1640 CLS:GOTO 1050
1650 PRINT #1,"$SC"
1660 INPUT //1, D$
1670 PRINT D$
1680 REM PRINT DATA
1690 FOR 1=1 TO XPTS+12
1700 INPUT //1,X$
1710 S$=LEFT$(X$,1)
1720 S=VAL(S$)
1730 IF S=0 THEN S=LEN(X$) ELSE 1750
1740 X1$=RIGHT$(X$,S-l):X$=X1$
1750 X(I)=VAL(X$)
1760 NEXT I
1770 INPUT "STORE DATA ON DISK Y/N ";ANS$
1775 IF ANS$="Y" THEN 1780 ELSE 1910
1780 INPUT "INSERT DATA DISK IN DRIVE A AND TYPE R ";R$
1790 IF R$="R" THEN 1800 ELSE 1780
1800 INPUT "NAME OF FILE (MAXIMUN OF 8 CHARACTERS)";FILN$
1810 IF LEN(FILN$)>8 THEN 1800
1820 IF LEN(FILN$)=0 THEN 1800
1830 OPEN "0",//2,"A:"+FILN$+".PRN"
1840 N=XPTS+11
1850 PRINT #2,AH:PRINT //2,AL:PRINT #2,DI
1860 FOR 1=1 TO 11:WRITE //2,X(I):NEXT I
1870 IF SB=0 THEN SCALE=200 ELSE 1890
1880 FOR J=ll TO N:WRITE #2,X(J)/SCALE:NEXT J:GOTO 1910
1890 SCALE=10000

229
1900 FOR J=ll TO N:WRITE #2,X(J)/SCALE:NEXT J
1910 PRINT //1,"$MA": INPUT //1,D$: PRINT D$ : CLOSE It2
1920 INPUT "MAKE ANOTHER RUN OR STOP R/S ",ANS$
1930 IF ANS$="R" THEN 1940 ELSE 1960
1940 INPUT "ENTER NEW VALUES OR USE CURRENT SETUP N/C";ANS$
1950 IF ANS$="N" THEN 1030 ELSE 1290
1960 STOP:END

APPENDIX B
ELLIPSOMETRY
When an elliptically polarized beam of light is reflected from a
surface, its polarization direction is changed by the refractive index
and absorption coefficient of the reflecting surface. If the surface
is a thin film, then the film thickness also enters into the reflection
equations (Drude Equations).
Ellipsometer instruments can be purchased with prepackaged
computer programs to calculate the film thickness, refractive index and
extinction coefficient from the ellipsometry data.
We recently acquired a Gaertner Ellipsometer and conducted tests
with the following results.
a)While the ellipsometer uses a laser source, and 0.01 degree of
resolution, for the polarizer and analyzer settings, the data was not
accurate enough to measure a standard.
b)The program supplied was adequate for substrates with vastly
different indices from the deposited films, but was totally useless for
systems where the indices were even relatively far apart, such as CdS
films on silica substrates.
Therefore, an intensive analysis of the sources of error in both
the experimental apparatus and in the calculation program was
conducted. The results showed that with some modifications in the
instrument and with a new program, the ellipsometer can be used very
230

231
accurately to measure the film refractive index and, if the thickness
is approximately known, to measure the film thickness.
Instrument Modifications
Sources of error in an ellipsometer are numerous but only a few
have a large effect on the results.
1. Polarizer, Analyzer. Their accuracy is very adequate (0.01),
even for fine work. Their relative alignment can be checked by placing
the arms of the ellipsometer in line with each other. After removing
the compensator the positions of maximum transmission and extinction
can be checked. Angular tracking can then be checked by rotating the
polarizer.
2. Quarter wave plate compensator. Our ellipsometer has a fixed
compensator at +90. By inserting it into the beam, its position can
also be checked. If the compensator, is defective, it can still be
used with a slight modification of the equations.
3. Incidence Angle. This is the most critical adjustment in the
instrument. A variation in the angle between source and detector of
less than Io can change the refractive index in the first place. We
found that despite a well collimated laser source, our instrument was
designed to accommodate changes in the angle of incidence of more than
several degrees. This was done by using a wide angle collection system
with a 2cm x 2cm photocell. This very sloppy alignment was used in
order to collect light from samples which were not plane parallel.

232
Here a modification was necessary. We placed a small aperture at
the entrance to the collection optics. Then we placed the detection
photocell at the end of a 30cm long tube, behind a second small
aperture. The two apertures, separated by 30cm reduced the acceptance
angle of the detector system to: 0.2 degrees. With the arm of the
detection, this was further lowered to 0.05 degrees.
In order to accept non planar or parallel samples, the stage
was adjusted to tilt until the reflected light was aligned
with the detection system.
A. Multiple reflections. Imperfect substrates and thin
substrates can produce multiple reflection beams, arising from
substrate striae or the bottom surface. These should be masked and
only the top surface reflection should be received at the detector.
Using the above alignment procedures, two standards were run every
day and the angle was adjusted when necessary. Care was taken not to
change the alignment until the standards were again measured. An
accuracy of the Ath place in index could be obtained.
Data Analysis
Ellipsometry equations for the calculation of index and thickness,
require a measurement of the quantities x and A, as shown below.
However, the instrument yields polarizer Pi and analyzer Ai readings.
These readings can be used to calculate the values of x and A, however,
this operation is not straight forward.

233
The various polarizer and analyzer readings fall into four sets of
readings called zones, two zones with the fast axis of the compensator
set at +45 and two zones at -45. There is one independent set of
readings for each zone, giving four independent sets. However, both
the polarizer and analyzer may be rotated by 180 without affecting the
results, thus yielding 16 sets of readings, which can grow to 32 sets
if the compensator is rotated by 180.
Depending upon which set of readings one is making, the
calculation of x and A may be different. Furthermore, Brewster's angle
(tan phase by 180 at this point. When working at ellipsometer angles of 50
and 70, certain film substrate configurations can develop which cross
Brewster's angle. This leads to the necessity to change the x> 4
definitions.
Calculations
The equations are well known Drude Equations which will not be
reproduced here. The reader is referred to Azzam and Bashara,
Ellipsometry and Polarized Light, North Holland 1977 and reference
number 64. A two-step approach which was developed to solve the Drude
equations to determine the refractive index and thickness will however,
be outlined here. In the first step, ni is calculated using a Lotus
routine for plotting, as follows:
a) The data is used to calculate x and A.

234
b) A wide range of ni values and d values is selected and the
Drude Equations are solved for xc and Ac.
c) A graphical comparison of the measured A is made with xc and
Ac for each set of n values over the range of d values. Since d varies
cyclically, it is not important which d values are used as long as they
span the cycle (A).
d) This allows a rapid calculation of n since the approach of the
calculation to the data can be seen in the plots.
e) n can be calculated with desired accuracy.
f) Once n is known, d can be calculated from the Drude Equations
using the well known quadratic formula, with possible solutions at 8,
-9, 2mir-0, 2imr9 where m is the integer order number. Thus the
thickness calculation cannot yield a unique value. However, if one
knows the approximate thickness, so that m may be defined, then an
accurate value can be obtained.

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42, 116 (1977).
62. B.D. Cullity, Elements of X-ray Diffraction, (Addison-Wesley
Publishing Co., Reading, MA, 1978), p. 458.
63 B.D. Cullity, Elements of X-ray Diffraction, (Addison-Wesley Pub.
Co., Inc., Reading, MA, 1978), chap. 11.
64. E. Elizalde and F. Rueda, "On the Determination of the Optical
Constants n(A) and a(A) of Thin Supported Films," Elect. Opt.,
122, 45 (1984).
65. K.A. Simmons and J.H. Simmons "Graphical Method for Solution of
Ellipsometry Data" to be published in J. Appl. Phys.

BIOGRAPHICAL SKETCH
Edward M. Clausen, Jr. was born December 11, 1958, in Chicago,
Illinois. He graduated from Eastlake North High School in 1977 and
began his college career at Cleveland State University, in Cleveland,
Ohio. After transferring to the University of Florida, in Gainesville,
Florida, he received his Bachelor of Science degree in Materials
Science and Engineering in August of 1982.
Edward began his graduate studies at the University of Florida in
January of 1983, and received a Master's degree in Materials Science
and Engineering in May of 1985. He is currently working to complete
the requirements of his doctoral degree in Materials Science and
Engineering.
240

I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
Joseph H. Simmons, Chairman
Professor of Materials
Science and Engineering
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
Paul H. Holloway,
Professor of Materials
Science and Engineering
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
Robert T. Dehoff,
Professor of Materials
Science and Engineering

I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
Stanley R. Bates,
Associate Engineer of
Materials Science and
Engineering
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
Timothy J. Anderson,
Professor of Chemical
Engineering
This dissertation was submitted to the Graduate Faculty of the
College of Engineering and to the Graduate School and was accepted as
partial fulfillment of the requirements for the degree of Doctor of
Philosophy.
December, 1987
Dean, College of Engineering
Dean, Graduate School



58
from EM Chemicals Inc. (Hawthorne, NY) for testing. This target was
made by a hot pressing technique, which results in a lower bulk density
and a higher impurity content. Impurity information was not given by
the company for these targets; however, thin films that were sputtered
from these targets were found to contain much higher levels of iron
than the levels found in films sputtered from CVD targets.
Targets that were used for COSAD were made from a piece of glass
obtained from Schott Glass Industries (Duryea, PA). The glass was a
special type of borosilicate crown glass (#8329) which is similar in
composition to a BK-7 glass, but it is processed in such a way so that
all the volatile impurities in the glass are removed. This glass is
sold by Schott as a special type of over coating glass used for thermal
evaporation. Targets were made by first sectioning the as supplied
cylinder of glass into 3/16 inch discs from which two inch blanks were
cut. Both sides of a blank were polished to 600 grit before being used
as a sputtering target. After it was found that a high power level was
required to sputter this glass, a 1/I6th inch copper plate was bonded
to one blank to improve thermal contact to the sputter gun. The
bonding was accomplished with an indium tin solder.
For producing COSAD films, the deposition chamber was reconfigured
to accept a second RF planar magnetron sputter gun, for sputtering the
glass target. This gun was used with a second 600 watt Eratron supply
and the two power supplies were driven by a common oscillator to phase
match the two plasmas. In addition, a set of baffling plates were
mounted inside the chamber as shown in Figure 12. The center baffle
plate was used to isolate the two plasmas. A pair of three inch


ABSORBANCE
212
Figure 85
Room temperature absorbance spectrum of C0SAD1 thin film.
As-deposited (ASDP) and after heat treatment to 650 C, 60
minutes (FHT).


200
decrease in temperature. Second, the peak position becomes less
sensitive to incident power levels, i.e. no shift occurs unless very
high powers are used. Finally, the most significant change which
corresponds to the exciton absorption peaks seen in the UV-visible
spectra, is the appearance of the "blue-edge" emission peak. Because
of the narrow bandwidth of this peak the high resolution grating had to
be used in the OMA monochromater. A typical photoluminescence spectrum
of the blue edge emission exhibited by a thin film which also displayed
the measurable absorption peaks is shown in Figure 78 and a composite
spectra showing the relative intensities of the two emission bands is
shown in Figure 79. The sharp peak at 492.3 nm in Figure 78
corresponds to one of the exciton transitions, although it is not clear
which one. The first two absorption peaks in the absorbance spectrum
separated 5 3 nm should exhibit two photoluminescence peaks, but only
one can be seen here. The photoluminescence spectrum obtained from the
same single crystal platelet used in the absorption measurements
(Figure 68b) is shown in Figure 80, compared with the blue edge
emission measured in the film described above. The platelet displays
both the 1^ and I2 bound exciton peaks and as can be seen, the
transitions are much sharper and shifted to energies higher than the
photoluminescence exhibited by the thin film. The shift in position is
identical to that seen in the absorption edge displayed in Figure 67b.
The shape of the thin film photoluminescence is similar to the
platelet; the high energy side is steeper. The peak width of the thin
film emission is nearly five times wider. In other work on both on
bulk single crystal CdS and CdS epitaxial thin films, exciton


92
Figure 20
Photograph of OMA setup which was illustrated in Figure 19.


187
TABLE 7
Comparison of Refractive Index Determined by
FECO method and Ellipsometry
Sample
FECO
Refractive Index
Ellipsometry
Refractive Index
ASDP 1 A/s
3.036
2.667
FHT 200 C
3.120
2.827
5 hr.
FHT 350 C
3.098
2.805
5 hr.
FHT 500 C
2.874
2.448
5 hr.
Note: All heat treated films deposited at 1 /sec.


104
b)
Figure 26 Thin film deposited at LN2 temperatures, a) TEH micrograph
showing low density columnar structure; b) corresponding
SAD pattern indicating a semi-amorphous structure.


95
convex lenses was used to focus the light emitted from the sample into
the monochromater. Other than this difference, the setup was the same
as the OMA. With 1800 lines/mm gratings the RLD of the monochromater
was calculated to be 0.24 nm/mm. The width of the four slits used in
this instrument determine the resolution, so with the smallest
practical slit of 10 um, a resolution of 0.024 nm could be realized.
The small linear dimension of the gratings (125 mm) with respect to the
long local length however, reduced the light gathering capability to an
f8. This turned out to be a very serious limitation for using the
spectrometer to measure photoluminescence, as integration times of up
to 5 sec per data point were required. A typical run was made using
slit widths of 100 um, which gave nearly four times the resolution of
the OMA, and data was collected every 0.05 nm for 2 sec. For a 20 nm
window this meant that a single scan would take 15 minutes. At least
three scans were required to reduce noise levels by signal averaging,
which meant that for only one wavelength window, 45 minutes were
required to acquire data. To make the accumulation of data easier, the
Z-158 computer was interfaced to the spectrometer controller, and is
shown in Figure 21b. The BASIC program to run the spectrometer was so
long however, that it had to be compiled to reduce running time.


149
distributed, an image with very little contrast should result. The
area sampled is displayed in Figure 50, which is a digitized SEM image.
The contrast that is observed is due to the grain boundaries of the
surface grains and can be compared to the contrast observed in a normal
SEM micrograph, shown in Figure 39. Figure 51a shows that the cadmium
map has a low even contrast, but the sulfur map shown in Figure 51b is
highly mottled, indicating an uneven distribution of sulfur. This
indicates that the sulfur is distributed as a second phase, at the
grain boundaries.
Co-Sputter Alternating Deposition
Microstructure
The purpose of making COSAD films was to produce a two phase
structure, in which very small crystallites of CdS are embedded in a
glass matrix. The intent of this phase of the research was to
determine if a structure could be produced in a thin film with small
semiconductor microcrystallites isolated in a dielectric matrix with
the crystals small enough for quantum confinement. Thin films made by
COSAD were therefore primarily analyzed by transmission electron
microscopy, since the required size of the crystallites was less than
100 . By changing the ratio of deposited CdS to glass different
structures could be produced. The most stable films were those made
with a small ratio of CdS to glass, while films made with equal ratios
were found to phase separate during examination in the TEM. An example
of the two extremes of the variation that could be produced will be
described here.


26
gate. The great interest for logic gate applications is that the
switching between the two levels can occur on a subnanosecond time
period in some materials.
The saturation or critical intensity Ic is found to be inversely
proportional to the nonlinear refractive index and is given by
I =
c
1
3
1
M
where
X a
(2.25)
(2.26)
and p is a figure of merit value for the cavity, relating the
reflectivities of the two mirrors and the attenuation of light as
passes through the cavity. Equation 2.25 indicates that for fixed
cavity conditions, a small saturation intensity corresponds to a large
nonlinear refractive index. The total index of refraction is given by
n = n^ + An
(2.27)
where
An = n2lc.
(2.28)


BIOGRAPHICAL SKETCH
Edward M. Clausen, Jr. was born December 11, 1958, in Chicago,
Illinois. He graduated from Eastlake North High School in 1977 and
began his college career at Cleveland State University, in Cleveland,
Ohio. After transferring to the University of Florida, in Gainesville,
Florida, he received his Bachelor of Science degree in Materials
Science and Engineering in August of 1982.
Edward began his graduate studies at the University of Florida in
January of 1983, and received a Master's degree in Materials Science
and Engineering in May of 1985. He is currently working to complete
the requirements of his doctoral degree in Materials Science and
Engineering.
240


112
examination. Also, films of useful thickness for optical studies were
much too thick for TEM. Finally, no specimen preparation was required
to examine the films, other than mounting the substrate with film on a
specimen holder. This afforded the ability to examine any type of film
regardless of thickness or treatment. The limited resolution of the
SEM, initially, was a problem, especially for as-deposited thin films
with very small grain size. However, optimization of the technique
yielded high magnification SEM micrographs (86,000X with 400
resolution).
The typical low magnification structure seen in 1.0 pm thick films
deposited at room temperature is shown in Figure 31a. The micrograph
shows these films to be of very high quality with very few large
defects such as pin holes or cracks. A high magnification micrograph
typical of these films are shown in Figure 31b. By making line
intercept measurements on Figure 31b, and assuming that the change in
contrast from point to point is due to a grain boundary, an average
grain size of 480 is measured. Although this number is only
approximate, this size is nearly twice that measured by TEM
observations and is slightly less than the size observed by Martil et
al. The difference in size between TEM and SEM observations of the
films studied here is greater than the difference in magnification
calibration between the two instruments, therefore the difference is
most likely due to the fact that the grain size observed at the surface
of a thin film is not the same as that observed in the interior. Also,
because the SEM micrographs were obtained near the resolution limit of
the microscope, there is considerable distortion of the image. The one


83
Figure 18 Photograph of micro-positioner stage used for acquiring
FECO spectrum, shown mounted in Lambda 9 spectrophotometer.


131
b)
Figure 41 Optical micrographs of exfoliated thin film, 100X
magnification, a) after four hours exposure to atmosphere;
b) after two days exposure.


ABSORBANCE ABSORBANCE
a)
b)
Variation of absorbance spectra with temperature, a)
spectrum observed at room temperature and 9 K; b) low
temperature spectrum of thin film (TF) and single crystal
platelet (SC).
Figure 68


117
differences in sticking coefficients of cadmium and sulfur at high
temperatures is responsible for this result.
Heat treated thin films. Furnace heat treatments similar to those
carried out by Martil et al.^ were used to study the grain growth of
thin films, in addition to heat treatments by rapid thermal annealing.
Optimum film properties were obtained by furnace heat treating to 500
C for 5 hours, or to 650 C for 4 minutes. Typical microstructures
obtained of 8000 thick films subjected to these heat treatments are
shown in Figures 34a and 34b. The grains appear isotropic with an
average size (determined by a line intercept method) of 2850 for the
500 C treatment and 3000 for the 650 C treatment. These grain
sizes are similar to the size observed in the porous network displayed
in the TEM micrograph of a very thin film heated to these temperatures
(Figure 28). Thermal shock, mismatch of the film and substrate
expansion coefficients, or excessive grain growth could be responsible
for the cracking of films which is sometimes observed when films are
subjected to fast furnace heating rates. This cracking is shown in
Figure 35.
From other heat treatments at 200 and 350 C for times of 5 hours,
it was found that the grain size exhibited two regions of growth. This
is shown in Figure 36, which is a plot of average grain size versus
reciprocal temperature, for constant time. The two regions of growth
exhibited in the plot are a classical indication that two different
mechanisms are responsible for grain growth in these films. At low
temperatures the grain growth is slow with a nearly constant rate.
This is an indication that grain growth occurs only by grain boundary


ABSORBANCE
175
WAVELENGTH (nm)
Absorbance spectra of different heat treatments. Furnace
heat treatment 500 C, five hours (FHT 500); furnace heat
treatment 650 C, four minutes (FHT 650); rapid thermal
anneal heat treatment, 650 C, 30 seconds (RTA 650). All
films deposited at 1 /sec.
Figure 66


Count* xlO
208
O
Figure 82
Longitudinal optical (LO) resonant Raman peaks measured
with OMA.


ABSORBANCE
214
WAVELENGTH (nm)
Figure 86 Room temperature absorbance spectrum of C0SAD1 thin film
heat treated in air (AR) and argon (AG), at 600 C, for 30
minutes.


40
cooled to LN2 temperatures, then an amorphous film results which is Cd
rich.
The optical properties of CdS thin films are central to this
study; however, the reported results for vacuum vapor deposited thin
films are found to vary greatly. Most notably is the variation in the
reported optical band gap and absorption coefficient, and the
calculated index of refraction. Some of these differences can be
realized by examining Figure 8 and Figure 9 which show the refractive
index and absorption coefficient obtained by several authors. y
Referring to Figure 8 the results for single crystal CdS obtained by
Cardona and Harbeke-^ are given by curve a. Curve b is for
polycrystalline film deposited onto fused silica at 180 C by Khawaja
and Tomlin. Curve c corresponds to a polycrystalline film also
deposited on silica at 180 C by Wohlgemuth et al.^9 and curve d is for
an amorphous film deposited at LN2 temperatures by Wohlgemuth et al.
The same authors correspond to the same curve letters of the reported
absorption coefficients shown in Figure 9. As shown in Figure 8 the
results by Khawaja and Tomlin show the closest resemblance to single
crystal results, while the Wohlgemuth et al. results show a marked
difference. In contrast, as shown in Figure 9, the absorption
coefficient of polycrystalline films deposited by Wohlgemuth et al.
model single crystal results best. The highest quality films in these
studies were deposited at high temperatures, although a more recent
study by Cook and Christy*^ reports similar results for room
temperature depositions. It is obvious that subtle differences in the
deposition technique result in a wide variation of thin film


213
A typical absorbance spectrum of an as-deposited C0SAD1 film is
shown in Figure 85. The high absorption tail at short wavelengths is
due to the glass matrix. This type of absorption is an indication of a
nonstoichiometric oxygen content and has been shown to occur in other
glasses which were sputter deposited with an argon plasma.^ The
spectrum of the as-deposited COSAD film shown in the Figure exhibits no
appearance of a band edge, which is inconsistent with the
microstructural analysis that indicated a semi-crystalline structure.
Also shown in Figure 85 is the absorbance spectrum of a C0SAD1 type
film heat treated to 650 C in an air atmosphere. This spectrum, at
first, certainly appears to be the result of quantum confinement; the
band edge has shifted to 460 nm (2.70 eV) and the exciton shoulder is
more pronounced. Microstructure analysis on this type of film however,
indicated a crystallite size much too large for quantum confinement,
which means that this shift in band edge energy is most likely a
crystal composition effect. Further proof that this is a chemical
shift is shown in Figure 86 which displays the spectrum of a C0SAD1
type film heat treated in an argon atmosphere. The band edge shift is
less and the exciton shoulder is less pronounced.
It is probably not correct to call the feature on the absorption
edge an exciton shoulder, as the shape of this feature does not change
considerably as the temperature is decreased. In pure films of CdS the
room temperature shoulder resolved into two sharp absorption bands at
low temperatures. As shown in Figure 87 the shoulder does not
appreciably change shape at 9 K and there is only a small shift in the
band edge.


5
a material is on the order of the radius of the exciton state, new
boundary conditions can distort the translational motion of the
exciton, its binding energy and the individual orbits of the electron
and hole. At this size the electron and hole interactions with the
crystal surface begin to govern the electronic properties of the
semiconductor. Quantum confinement also occurs in the MQW structures
of GaAs; however the exciton is only confined in one direction. A much
stronger effect occurs when the confinement is in two or three
dimensions.
Thin Films
A study of CdS thin films was chosen for a number of reasons: 1)
very few semiconductors which display a nonlinear susceptibility have
been tested in a thin film form, despite the potentially dominating
role in applications. 2) Cadmium sulfide displays one of the largest
excitonic saturation effects, and thus the influence of film structure
due to deposition conditions or subsequent treatments could be
investigated. 3) Quantum confinement effects and their interaction with
the perturbation of the exciton absorption process are of great
interest to the future of nonlinear optical developments and
applications. Cadmium sulfide promises to offer a means of studying
both the effects and their interplay with the development of the thin
film structure. 4) Since the energies of the exciton states correspond
to wavelengths in the visible part of the spectrum, the experimental
optics for the measurement of these states is simplified, and many


15
n (BM) = 2 11 ( 6 P )2 N (2.13)
3 n h u) h (ojg o) I
where N is the free carrier density given by equation 2.10, P is a
momentum matrix element, and ojq is the effective band gap. For
nonlinear optical applications the advantage of this process is that
the occupied states are closer to resonance so that a larger n2
results. There also is the possibility that this process can occur at
energies below the band gap, as saturation of excited carriers can be
produced not by direct optical absorption, but by scattering from other
excited states.^ The one disadvantage of the process that is similar
to the induced plasma process is that the interband relaxations
required for decay of the state can be very slow. In some materials a
faster decay process can occur by scattering to intraband transitions,
which effectively relaxes the system by transferring the population to
other states.^
Another mechanism for the nonlinearity observed in some materials
is by a direct interband saturation process. This process assumes that
the band structure in a direct gap semiconductor can be modeled as a
set of uncoupled two level systems which are homogeneously broadened by
a dephasing time T2. Homogeneous broadening means that the individual
transitions are indistinguishable. The T2_Lorentzian broadening
results in absorption below the band gap and excitation into the T2-
broadened "band-tail" is assumed to be responsible for the nonlinear
refraction.^ At some high level of intensity the two level system
should become saturated, and associated with this saturation is a


19
Optical Bistability
The primary means for measuring nonlinearity in materials is
through an internal feedback device known as a Fabry-Perot
interferometer. The saturation intensity of such a system is the
intensity at which the gain of the feedback saturates. This is the
point at which optical bistability occurs, and from the saturation
intensity and the interferometer parameters the nonlinear index of
refraction can be determined.
The Fabry-Perot consists of a cavity formed by two plane parallel,
highly reflecting mirrors. The transmission of monchromatic light
through the device is determined by the optical path length of the
cavity. If the cavity is not tuned to the wavelength of the light,
then a transmission of ~ 1 % results. When the optical path length is
exactly equal to an integer number of wavelengths, then a resonance
effect occurs and the output intensity from the device reaches nearly
100 % of the input intensity. A diagram of this process is shown in
Figure 1. The optical path length is determined by the physical length
d, times the refractive index n of the material within the cavity. The
condition for resonance therefore is given by
2 n d = m A
(2.17)
where m is the integer order number.


11
and
1
+ 4tt
n
2tt x
(3)
(2.8)
(2.9)
There are essentially four electronic processes which can produce
a reactive nonlinear susceptibility in semiconductors. These are known
as 1) the induced free-carrier plasma, 2) the dynamic Burstein-Moss
effect, 3) the direct saturation of interband excitations, and 4) the
saturation of exciton absorption.^ Of these 1 and 2 are the most
commonly studied, 3 occurs in most direct band gap semiconductors, and
4 is the most promising for high speed operations. All four processes
can occur in some materials. The most dominating process which is
observed depends somewhat on the material, but mostly on the particular
experimental setup and measurement temperature. As shown in Table 1,
different processes result in widely different values of the reported
nonlinear index and saturation intensity.
The four processes listed above basically describe how the
transitions between different levels in a semiconductor and the
saturation of those levels result in the observed nonlinear
susceptibility. Depending on the band structure of a material and the
particular wavelength of light used for the analysis, one of the four
processes will dominate. The one process that can occur in nearly
every semiconductor, however, is the induced free-carrier plasma.
Assuming that photo induced transitions produce electron-hole pairs,
the number of these free carriers will be intensity dependent.


CHAPTER II
THEORY OF NONLINEAR OPTICS
Nonlinear Optical Susceptibility
The nonlinear refractive index which is observed in certain
semiconductor materials is a result of an electronic polarization which
is induced by the interaction with a monochromatic radiation field.
The susceptibilities of the polarization determine the values of the
experimentally measured optical properties. To understand the
relationships between the susceptibilities and the optical properties
we must first consider the electro-magnetic field, E, which is given by
E(t) = E())e-i)t + E*( The resulting polarization has frequency components at all multiples of
+/-U), but considering only those that occur at oj
PU) = x(1)E(w) + x(2)[E]E(w) + X(3)[E]2E(to) + . (2.2)
The first term in the series, x^) is the linear susceptibility and by
using first order perturbation theory,^ the linear dispersion of the
refractive index below the band gap can be calculated. The higher
8


198
positions are shown in Figure 76. Both of these changes occur because
the band gap is better defined after the annealing heat treatment.
With this change in band structure, the band edges are sharper, which
shifts the energy and increases the probability of a radiative
transition. Figure 75 also shows that the intensity of the laser line
has increased significantly, which is most likely due to an increase in
scattering of the laser line.
All films which were heat treated to high temperatures displayed a
high intensity edge emission peak and the peak position was found to be
nearly the same. One difficulty with determining peak position,
however, was that absorption of the laser line would cause enough
heating of the film to shift the position of the peak to lower energy.
A graph of the shift in peak position with incident laser power is
displayed in Figure 77. At very low powers only a small shift occurs,
but at higher incident powers the shift is significant. At even higher
incident powers the focussed laser beam would cause vaporization of the
thin film. Very low incident laser powers were therefore used to study
the room temperature photoluminescence of thin films, although low
power excitation reduced the intensity of the emission.
The problem with power dependent peak shift was reduced
considerably by cooling the thin film with the cold finger and
acquiring photoluminescence spectra at low temperature. As with UV-
visible spectroscopy, several significant changes in thin film
photoluminescence are observed when the film is cooled to 9 K. First,
the green edge emission band increases in intensity and shifts to a
higher energy corresponding to the increase in band gap energy with


INTENSITY INTENSITY INTENSITY
126
a)
DEGREES TWO THETA
c)
X-ray diffraction patterns of heat treated thin films from
Figure 38; a) film of Figure 38a, heat treated 500 C, one
hour; b) film of Figure 38b, same heat treatment; c) film
of Figure 38b, heat treated 720 C, ten minutes.
Figure 39


204
transitions were exhibited by very sharp emission bands, although these
measurements were made at lower temperatures. The measurements of
single crystal platelets made here show that the exciton emissions are
just as sharp at 9 K. This rules out the possibility that the
emissions measured from thin films are temperature broadened. One
possible reason for the observed broadening may be orientational
broadening or the presence of grain boundaries. All materials which
have displayed sharp exciton photoluminescence have been essentially
single crystals. The impurity centers near grain boundaries which
excitons are bound to may vary in energy slightly from those impurity
centers within the interior of grains. This might produce the
broadening of the exciton photoluminescence observed in polycrystalline
thin films.
The photoluminescence spectra displayed here exhibits another
difference from the absorbance spectra. The three absorbance bands
were characterized as free exciton transitions, based on their position
and shape. The blue edge emission exhibited by thin films, however, is
more characteristic of bound exciton transitions. As previously
explained, these are excitons which are bound to impurity sites.
Usually the impurity is a neutral acceptor or neutral donor, which give
rise to an 1^ or an I2 bound exciton, respectively. It is certainly
reasonable to expect that these types of impurity centers exist in the
thin films studied here. Sulfur interstitial atoms or cadmium vacancy
atoms could act as donor impurity sites, which correlates well with the
single peak exhibited in the blue-edge emission of thin films.
However, compared to the single crystal platelet photoluminescence, the


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235


and subsequent heat treatments. An in-depth study of the thin film
microstructure revealed the relationship between crystallographic
defects and band gap defects which lead to band tailing absorption.
Optical characterization related exciton photoluminescence, absorption,
resonant Raman scattering and band edge photoluminescence and
absorption to the microstructure of films. For the first time,
absorption bands and photoluminescence associated with exciton states
were observed in a polycrystalline thin film.
A supplementary study investigated composite films, consisting of
CdS crystals in a glass matrix, formed by a novel co-sputter deposition
process, in which alternating layers of CdS and a borosilicate glass
were deposited to form a thin film. A wide variation in the structure
of the deposited film was obtained by changing the amount of deposited
CdS and by post deposition heat treatments. Low concentrations produced
CdS microcrystallites as small as 70 , small enough for quantum
confinement processes to affect the band energy. For all films
produced, a shift in the band absorption edge to higher energies was
observed; however, it was determined that this shift must be partially
associated with a chemical shift. Possible formation of CdO could
raise the band gap energy, although this alone could not produce the
shifts observed in the films with the smallest particles, and therefore
a confinement process might exist.
viii


113
b)
Figure 31
SEM micrographs of
magnification; b)
structure.
as-deposited thin film. a) low
high magnification showing grain


85
analysis of this polarization the optical constants of the material can
be determined.
The problems associated with this technique mostly originated from
the actual instrument. The computer-controlled Gaertner ellipsometer
(Chicago, IL) is the industry standard; however, the unit available for
use in this study was a manual model which greatly reduces the capacity
for quantitative analysis. The problem does not so much have to do
with manually setting the polarizer and analyzer, but rather in reading
the analog scale to determine the node points. This was a special
problem on this unit because the meter was nonlinear; the highest
sensitivity was between a reading of 2 an 3 on a scale of 1 to 5. In
addition, the gain for the photocell had to be manually adjusted as a
node point was found. The gain was also nonlinear and there was a flat
spot at one end of the range. These two problems contributed to
difficulties in reproducing the results; however, the greatest problem
with the analysis had to do with the computer program used to calculate
the values of n and t from psi and delta. Supplied by Gaertner, this
program did not actually calculate the values of psi and delta, but
instead it looked up the values in a table, based on the values of the
two pairs of angles that were inputed. From the table values of psi
and delta the program then calculated the refractive index and
thickness by an iterative technique. One difficulty with the program
occurred if the particular combination of angles inputed were outside
values in the table. The program would hang up and the only solution
was to boot the program out. This actually happened a surprisingly
large number of times, even when similar samples were analyzed. To


220
can be observed because other growth planes may become active at
higher temperatures. With the decrease in the number of
crystallographic defects, band tailing is reduced indicating a better
defined band gap; however, a shift in the absorption edge is observed.
The shift is due to a change in composition, as these films are found
to be nonstoichiometric. A higher sulfur content in these films occurs
because the sticking coefficient of cadmium is reduced at higher
substrate temperatures. Due to large nonuniformities in the
composition across the film area a degradation in the optical
properties at wavelengths longer than the band edge occurs.
4. To produce the highest quality thin films for optical applications
a post deposition heat treatment must be instituted to remove defects
created by the deposition and growth processes. Heat treatments of
less than 500 C cause a small change in grain size which appears to be
linear with the increase in heat treatment temperature. Low
temperature treatments also lead to a small change in the optical
properties. Band tailing is still present and photoluminescence yield
is still low. With heat treatments to 500 C and above, a dramatic
increase in grain size occurs. Higher temperatures may provide the
activation energy required for impurity assited diffusion at the grain
boundaries. Associated with this grain growth is a reduction in the
amount of band tailing and a large increase in the photoluminescence
yield. These results are due to a removal of crystallographic defects
with grain growth, which are related to defects in the band gap.
5. For the first time absorption bands and photoluminescence
associated with exciton states have been observed in polycrystalline


152
COSAD films with equal proportions of semiconductor and glass were
the first to be produced. These films will be referred to as C0SAD1.
A representative film which was deposited onto a carbon support film is
shown in Figure 52a. The structure appears very indistinct and appears
to be amorphous, but the SAD pattern shown in Figure 52b indicates
there is some crystallinity in the structure. If the electron beam is
condensed on this type of film, the structure is found to phase
separate very quickly, as displayed in Figure 53a. The SAD pattern in
53b shows that a significant amount of crystallization has occurred and
the pattern indicates a hexagonal CdS non-oriented crystal structure.
If this type of film is furnace heat treated a significant change
in microstructure also occurs. Heat treating to 500 C results in
total phase separation and the growth of spherical shaped CdS
crystallites, with an average size of 300 . This structure is shown
in Figure 54. By heating this type of film to 650 C, these
crystallites are found to grow and form a fairly evenly dispersed
bimodal size distribution microstructure, which is shown in Figure 55a.
The average size of the larger spherical crystals is 700 A and the
smaller crystallites have an average size of 200 . The SAD pattern
for this structure is shown in Figure 55b. All the reflections for
hexagonal CdS are present, in addition to some reflections for an
unidentified phase. Because these reflections are so closely spaced in
the pattern, it was not possible to determine their source by dark
field imaging. A typical dark field image obtained from the inner most
diffraction rings is shown in Figure 56. The bright crystals in the
image are those which are properly oriented for diffraction and as can


232
Here a modification was necessary. We placed a small aperture at
the entrance to the collection optics. Then we placed the detection
photocell at the end of a 30cm long tube, behind a second small
aperture. The two apertures, separated by 30cm reduced the acceptance
angle of the detector system to: 0.2 degrees. With the arm of the
detection, this was further lowered to 0.05 degrees.
In order to accept non planar or parallel samples, the stage
was adjusted to tilt until the reflected light was aligned
with the detection system.
A. Multiple reflections. Imperfect substrates and thin
substrates can produce multiple reflection beams, arising from
substrate striae or the bottom surface. These should be masked and
only the top surface reflection should be received at the detector.
Using the above alignment procedures, two standards were run every
day and the angle was adjusted when necessary. Care was taken not to
change the alignment until the standards were again measured. An
accuracy of the Ath place in index could be obtained.
Data Analysis
Ellipsometry equations for the calculation of index and thickness,
require a measurement of the quantities x and A, as shown below.
However, the instrument yields polarizer Pi and analyzer Ai readings.
These readings can be used to calculate the values of x and A, however,
this operation is not straight forward.


179
a)
2.50 2. 52 2. 54 2. 56 2. 58 2. 60 2. 62 2. 64
PHOTON EN6RCY UV)
b)
Figure 69 Comparison of low temperature spectra. a) absorbance
spectrum of thin film, with energy abscissa scale; b)
reflection spectrum of single crystal, 4.2 K.^


146
CTS
a)
b)
Figure 48 XPS high resolution scans of oxygen Is peak, a) furnace
heat treated thin film; b) after one minute sputter etch.


14
where n is the refractive index of the material without the plasma and
m" is the effective mass. The transient susceptibility is determined
by the time it takes for the free carriers to build up;^ however, this
can be a very short time. This process will also occur at room
temperature. The disadvantage of this process for nonlinear optical
applications is that the effect is very small because there is no
coupling between the states, and therefore no resonance effects take
place. The process also can suffer from a very slow recovery time
because the recombination time in certain semiconductors is on the
order of Msec.^
Another process that is based on an intensity dependent free
carrier concentration is the dynamic Burstien-Moss or blocking effect.
Again a steady state density of charge carriers is given by equation
2.10; however, the origin of the absorption is not considered
explicitly. At low temperatures the carriers are assumed to thermalize
by a phonon scattering process so that they fill the bottom of the
conduction band. The top of the valance band becomes empty and the
shift of the effective band gap to higher energy becomes intensity
dependent. In association with this shift there must be an intensity
dependent contribution to the refractive index.^ This is because the
filled conduction band effectively blocks absorptive transitions, and
the blocked transitions no longer contribute to polarization and
refraction. Also, some type of unspecified coupling takes place
between excited states, so that the effect is enhanced somewhat. The
nonlinear refractive index for this process is given by


226
examine a different semiconductor to determine if a confinement process
could be induced.


132
Compositional Analysis
Stoichiometry determination of target material
The first parameter needed to establish the composition
stoichiometry of thin films was the composition of the target material
from which films were made. Once this composition was determined, the
target material could be used as a standard for all other analytical
techniques. Both induction coupled plasma (ICP) and electron probe
microanalysis (EPMA) were used to determine the stoichiometry of the
target material. These two techniques are by far the most quantitative
because the results are determined by comparison to known standards.
Additional measurements were made by standardless techniques which rely
on sensitivity factors for quantitative analysis, such as: X-ray
photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES).
The manufacturer of the target material claimed the composition to be
slightly sulfur rich, but as can be seen Table 3, both ICP and EPMA
data indicate the target material to be slightly cadmium rich, although
the variation is within a few standard deviations of each other. In
either case the results indicate the target material to be
stoichiometric to 0.50 %, which for the other analytical techniques
used to measure thin films, is an acceptable variation. XPS gives a
result that is very close to the excepted value; however, AES shows a
very large difference from the other results which demonstrates that
the technique is not useful for determining absolute concentrations,
but as described below, the technique is very sensitive for determining
differences between samples.


36
1^ bound exciton that was described by Hopfield and Thomas. Usually
two 1^ lines are observed, and by varying the doping level these
authors proved the lines to only be due to Na and Li. High purity
crystals grown in clean reactor tubes were found to only exhibit the
Ii(Li) line. When Na was added a considerable broadening of the 1^
line occurred, but as successive runs were made in the same tube, these
authors showed the Na line could be resolved. Doping with K, Rb, or
Cs, only resulted in a sharp I^(Li) line, and P was found to give a
complex shallow acceptor. The identity of the shallow donor level
responsible for the I2 bound exciton could not be determined; however,
the authors showed by donor-acceptor pair line splitting that the donor
was not a native double donor such as a cadmium vacancy, or a sulfur
interstitial. The authors reasoned that the donor may be Na or Li
interstitials because of the small size of these atoms, and when
crystals are heavily doped, they become highly compensated.
Unfortunately they were unable to prove the identity of the donor.
Later work by Henry and Nassau involved measuring the
spectroscopic lifetimes of the two bound exciton complexes.^2 They
were basing their work on another study by Thomas and Hopfield which
showed the measured oscillator strength of the bound excitons to be
very large, which meant that the radiative decay of the weakly bound
exciton would be very fast. Thomas and Hopfield determined an
oscillator strength of 9 +/-2 corresponding to a radiative lifetime of
0.4 +/-0.1 nsec.^ This oscillator strength is about 10^ greater than
the strength of a free exciton.^ Henry and Nassau with their
experimental setup were able to measure a radiative lifetime for the I2


88
The cold finger assembly was mounted on a height-adjustable rack system
so that once a sample was cooled to low temperatures the cold finger
assembly could be moved so that measurements on different equipment
such as the OMA and Raman spectrometers could be accomplished.
All aspects of the measurements with the Lambda 9 were greatly
enhanced by interface to the AT computer. Spectra were stored as ASCII
files which later could be imported into a LOTUS worksheet (Lotus
Development Co., Cambridge, MA) for manipulation and plotting. The
BASIC program listed in Appendix A would accumulate spectra in the form
of a long column of numbers, each number representing the absorption or
transmission at a given data point. The data interval was based on the
slit width used for the analysis. The slit defines the bandpass that
determines the resolution to which features can be resolved. Usually a
data interval of one-third the slit width was used for the analysis.
Slit widths from 4 nm down to 0.5 nm were used depending on the
required resolution. As the slit width is decreased, the signal to
noise ratio is also decreased. This means that the amount of time that
is spent at a given data point must be increased. Computer control of
the spectrophotometer was indispensible for making high resolution runs
where very slow scan speeds were necessary.
Photoluminescence and Raman
Further investigations of the band structure of semiconductors can
be made by measurement of the emission bands or peaks produced by
photoluminescence and resonant Raman scattering. Measurement of these
emissions for this study were made with the two spectrometers described


GRAIN SIZE (nm)
120
(1/D
Figure 36 Plot of average grain size versus heat treatment
temperature.


216
A much different absorbance spectrum is displayed by the C0SAD2
type films. Shown in Figure 88 are the spectra of both an as-deposited
and a heat treated thin film. Unlike C0SAD1 films this type of film
shows the appearance of a band edge which is shifted to a very high
energy, even in the non-heat treated state. This is an interesting
result, because although the microstructure of this type of film is
very small, it appeared to be amorphous. Heat treatment to 600 C in
an argon atmosphere produced only a small change in the edge position,
but it was still shifted to a higher energy than C0SAD1 films. The
large shift in band energy must be related to a confinement process.
Another interesting result is indicated by the absorbance scale, which
is an order of magnitude less than the scale used to measure C0SAD1
films. This is an indication that there is considerably less CdS in
the structure of C0SAD2 films.
Unfortunately no photoluminescence spectra could be obtained from
either type of COSAD film. This may be due to a dilution effect, due
to both a low CdS concentration in these films and a small thickness.
Also, because the band edge was shifted to such a high energy in the
C0SAD2 films it was not possible to excite the sample, even when using
the highest energy argon laser line.


142
As indicated in Table 4, there is significant differences between
as-deposited and heat treated thin films. The concentration
calculations in XPS are based on quantification factors and on
integrated peak intensities. Since the factors are constants, the
result presented in Table 4 indicate that the integrated area of the
cadmium peak has nearly doubled in heat treated films as compared to as
deposited. A high resolution scan of the cadmium 3p5 and 3p3
transitions is shown in Figure 45. Surprisingly there is a significant
shift in the peak position in addition to a broadening. By comparison
to published XPS spectra on cadmium compounds-^ this shift could
possibly correspond to cadmium bonded to oxygen. The survey scans
displayed in Figure 46 and Figure 47 exhibit oxygen Is peaks in both
films and the peak appears to be more intense in the heat treated film
(compare the height of the 0 Is peak to the Cd 3p3 peak). Figure 48a
displays a high resolution scan of the oxygen peak and Figure 48b shows
the same scan after a one minute sputter etch, which removes
approximately 100 . The large reduction in the intensity of this peak
after sputtering is an indication that a thin oxidized surface layer is
present. Unfortunately, a high resolution scan of the Cd 3d peaks was
not acquired after the sputter etch to determine if the shift in the
peak position is due to the oxidized surface layer. Analysis was
performed on a second XPS instrument (Perkin-Elmer) which was equipped
with a sputter gun. The results for a heat treated film before and
after a one minute sputter etch are shown in Figure 49. Several
differences between this spectrum and the spectrum obtained with the
Kratos XPS can be observed. First, the 3d peaks in Figure 49 are


54
Figure 11
Photograph of complete deposition system showing
sputtering chamber and instrumentation.


119
Figure 35 Low magnification SEM micrograph of thin film shown in
Figure 34a.


3
While this field has greatly advanced, there is, however, a very great
need for suitable materials and systems. Many materials exhibit a
third order nonlinear susceptibility,^ but very few have the
characteristics necessary for an integrated optics system. A few of
the semiconductors which have been investigated include InSb, GaAs,
GaP, CdS, and CdTe. Most of these semiconductor materials which show
optical switching have been investigated in bulk form. Only the
multiple quantum well (MQW) structures made from gallium arsenide are
the notable exception and are made in thin film form.^ This material
is perhaps the most promising today for use in optical signal
processing systems, primarily because its nonlinearity occurs at the
same wavelength as the semiconductor lasers which are currently
available. For an integrated optical system this is an essential
consideration because most of the processes which produce the nonlinear
susceptibility require laser light of energy near the band gap of the
material.
Equally important considerations, however, include the value of
the nonlinear coefficient, the switching speed, and the absorption
coefficient, since these values determine how much power is required to
switch the device, and how fast recovery will be. Low power operation
is essential for any large scale integration, although the figure of
merit (FOM) most often quoted is given by
FOM = n2 (1.1)
x a
where ri2 is the nonlinear index, x is the switch-off time, and a is the
absorption coefficient. Obviously the larger the FOM the more


TRANSMISSION TRANSMISSION
81
a)
b)
Figure 17 Fringes of equal chromatic order (FECO) spectrum for 2.0 um
thick film of CdS on a silica substrate, a) Normal
incidence spectrum showing many orders; b) Expanded
abscissa scale showing shift in FECO spectrum for 30
incidence.


110
a)
500 nra
Figure 29 Thin film deposited on NaCl substrate and subsequently heat
treated, a) TEM micrograph showing porous network; b) TEM
micrograph showing dislocations and stacking faults.


Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
OPTICAL PROCESSES IN CADMIUM SULFIDE
THIN FILMS
by
Edward M. Clausen, Jr.
December, 1987
Chairman: Joseph H. Simmons
Major Department: Materials Science and Engineering
The semiconductor, cadmium sulfide, has received much attention
for its optical, electronic and piezoelectric properties. Recently,
several investigators have demonstrated that its Wannier excitons hold
great promise for applications in nonlinear optics. However, the
results showed, as expected by the current theoretical thinking, that
the nonlinear optical behavior and the exciton and band gap energy
configurations are dependent upon the microstructure of the samples.
Since most applications in computer logic, or communications require
waveguide geometries, the object of this dissertation has been the
study of thin films.
The investigation presented here, therefore concerns the study of
the effect of microstructure and preparation conditions on the optical
properties of CdS thin films. High optical quality films were produced
by RF magnetron sputtering with a variety of microstructures and
crystallographic characteristics controlled by the deposition process
vii


BAND GAP ENERGY
183
b)
Figure 71 Variation of band gap energy with temperature. a) position
of absorbance spectrum with temperature; b) plot of band
gap energy versus temperature.


12
TABLE 1
Listing of Nonlinear Optical Values for Certain
Semiconductor Materials
Material
Electronic
Proces
Is (W/cm2)
n.2 (cm2/W)
toff (sec)
FOM
GaAs
(MQWS)
FES
150
10-4
10"8
1
Bulk
DBM
500
InSb
DIS
2000
6X10-5
10"6
0.06
CdS
BES
58,26
10"4,10"2
10"9
2
FCP
1.2X105
10"10
Key for Electronic Processes:
FES Free Exciton Saturation
DBM Dynamic Burstein-Moss
DIS Direct Interband Saturation
BES Bound Exciton Saturation
FCP Free Carrier Plasma


229
1900 FOR J=ll TO N:WRITE #2,X(J)/SCALE:NEXT J
1910 PRINT //1,"$MA": INPUT //1,D$: PRINT D$ : CLOSE It2
1920 INPUT "MAKE ANOTHER RUN OR STOP R/S ",ANS$
1930 IF ANS$="R" THEN 1940 ELSE 1960
1940 INPUT "ENTER NEW VALUES OR USE CURRENT SETUP N/C";ANS$
1950 IF ANS$="N" THEN 1030 ELSE 1290
1960 STOP:END


Copyright 1987
by
Edward M. Clausen, Jr.


ABSORBANCE
217
WAVELENGTH (nm)
Figure 88 Room temperature absorbance spectrum of C0SAD2 thin film.
As-deposited (ASDP) and after furnace heat treatment at
650 C for ten minutes (FHT).


9
order terms in the series are the nonlinear susceptibilities, and
although their magnitudes are much smaller than the first term, under
very high field intensities a number of different effects can be
observed. For non-centrosymmetric crystals (i.e. crystals without an
inversion center), under appropriate conditions, the second order term
is manifested as two different effects. The first is a quadratic
variation of the refractive index with applied voltage, which is known
as the Kerr effect. The second is the generation of a second harmonic
radiation field. Second harmonic generation is a very useful effect
for doubling the frequency of a laser beam. Both of these effects have
many important applications; however, the primary interest of this
study is the effects which lead to the third order term One
consequence of the third order term is the generation of a third
harmonic radiation field. For the current topic of this study,
however, the manifestation of this term as a nonlinear refractive index
is of primary interest. The relationship between the third order
susceptibility and the nonlinear refractive index can be understood by
first considering basic dielectric theory for the displacement of a
charge in response to an applied electric field:
D(w) = E((jj) + 4tP(o)) = e E(oj) (2.3)
where D(w) is the frequency dependence of the displacement and e is the
complex dielectric constant. The variable, e can be defined as


143
Figure 45 XPS high resolution scan of cadmium 3d3 and 3d5 peaks of an
as-deposited (ASDP) and furnace heat treated (FHT) thin
film.


160
a)
100 nm
b)
Figure 58 TEM micrograph of C0SAD2 thin film. a) as-deposited; b)
SAD pattern indicating amorphous structure.


64
cleaning. The deposition system was pumped down to a pressure of < 1.0
X 106 torr before backfilling with the sputtering gas. The IC-6000
was usually programmed for a fast ramp up to the desired power level
and then a rate controlled presputter was initiated before the shutter
was opened. If a sputtering target was newly installed, a presputter
burn-in of 30 minutes was carried out; otherwise the target was
presputtered for 5 minutes before exposing the substrate to the plasma.
The IC-6000 automatically controlled the deposition rate and thickness.
Deposition rate was varied between 1 to 5 /sec and films for optical
analysis were typically made 1 pm thick, although the film thickness
for other analysis was varied.
Substrate materials
Several different types of substrates were used to support thin
films, depending on how the films were to be analyzed. Standard
substrates for optical analysis were made of optical quality fused
silica, typically one inch square and 1 mm thick. These substrates
were found to have very few surface flaws and were essentially
transparent over the optical region in which films were analyzed. The
low index of refraction (n=1.46) of these substrates was also necessary
for measuring the refractive index of the film and for planar waveguide
measurements. Some films were also deposited on silica for TEM
analysis. Self-supporting TEM specimens were prepared by floating
these films off the silica with a 1% HE solution.
Other films for optical analysis were deposited onto single
crystal sapphire substrates. This was done to determine if the high


13
Depending on the recombination time and the absorption coefficient
a, the steady state density of free carriers will be given by
a i t
N = (2.10)
h oj
Once these carriers are formed, they are allowed to diffuse and form an
electron-hole plasma. The plasma will respond to an applied electric
field and the resulting undamped oscillations will produce a
polarization which can be related to the index of refraction through
the dielectric constant^
2 ( 4-irNe2 .
n = ( e ). (2.11)
* 2
m cu
By use of equation 1.10, the nonlinear refractive index for a plasma
can be written as
n2(P)
- 2 IT
2
e a
R
3
t
n m
(2.12)


55
sputtering gas. Initially the thermocouple vacuum gauge was calibrated
with a capacitance manometer (MKS Instruments Inc. Burlington, MA) for
different settings of the throttle valve and mass flow controller.
Based on the previous work on CdS sputtered films, a pressure of five
microns (5X10"^ torr) was chosen and the system was calibrated for this
pressure. To obtain this pressure an argon flow rate of 20 cc/m was
needed.
Film thickness during deposition was monitored with a Leybold-
Heraeus IC 6000 deposition controller. This instrument utilizes a
quartz crystal oscillator microbalance for determination of film
thickness. All parameters of calibration and deposition control are
software programmable with this unit, either via the front panel
controls, or through an RS-232 communications link to an external
computer. The IC 6000 permits monitoring of up to six different films,
and by using a sample and hold program, two different films can be
monitored and controlled simultaneously. Close loop control of
deposition rates was achieved by connecting the IC 6000 thickness
monitor to the RF power supplies.
For semi-automatic control and constant monitoring of the
deposition process a Zenith Z-158 personal computer with 512K RAM and a
20 megabyte hard disk was interfaced to the system. A Dascon 1 I/O
board (Metrabyte Corp.) which provides four channels of analog and 12
bits of digital input/output was installed in the computer. With this
interface board and with RS-232 communications to the IC 6000
deposition controller, it was possible to monitor nearly every aspect
of the deposition process. The RS-232 communications permitted direct


44
hexagonal pyramids were found to show bound exciton luminescence. The
pyramids were thought to have nucleated from Cd droplets, and as they
grew, the adjacent areas were depleted of cadmium. The required donor
states for I2 bound excitons were therefore provided by cadmium
vacancies. Other films were found to be uniformly slightly cadmium
rich, and it was thought that cadmium interstitials were responsible
for the broad band photoluminescence.
Humenberger et al.^3 also reported thin films which displayed
bound exciton luminescence. These films, however, were made by a hot-
wall epitaxial technique, which is similar to the vapor phase technique
used to make bulk single crystal platelets. Films were deposited on
BaF2 substrates. The surface morphology was found to be smooth with a
low density of hexagonal flat tops. No compositional data were given
for thin films; however, acceptor states were provided by indium doping
thin films during the deposition process. Free carrier concentrations
as a result of the indium doping were measured to be on the order of
1017 to 10^ cm-^.-^
The above two studies are the first to show by photoluminescence
the presence of exciton levels in thin films. Studies of several other
thin film deposition techniques such as chemical bath,-^ or spray
O c
pyrolisisj:> have reported photoluminescence, but none have shown
exciton emissions. To see these transitions, it is apparent that very
high quality thin films are required, and it is obvious that standard
vacuum techniques or other techniques such as those described here are
not capable of producing thin films of the necessary degree of quality.
A deposition technique which permits greater control over the


ACKNOWLEDGMENTS
Not enough can be said for the people who have contributed to the
start and particulary the completion of this dissertation. It would
have been very difficult for me to get through this project without
their help.
I would like to express my deepest appreciation and gratitude to
Dr. Joseph H. Simmons, my academic adviser, who supplied much
encouragement, direction, assistance and the opportunity to work on
this project. His insight and ability to help me solve problems proved
to be an extremely valuable asset, although his confidence in my
abilities to "get the job done" was invaluable. I would also like to
express my appreciation to the members of my committee, Dr. Paul
Holloway, Dr. Robert Dehoff, Dr. Stan Bates, Dr. Tim Anderson and Dr.
Ramakant Srivastava for their suggestions, comments and guidance. This
is certainly one of the best combinations of abilities and expertise
for a committee and I thank the members for their help and enthusiasm.
My closest friends Guy Latorre, Richard Robinson and B.G. Potter
deserve special thanks for their personal support and reassurance. My
roommate and friend, Steve Wallace deserves the most thanks and credit
for having to endure the more trying times of this project. I would
especially like to thank my mother and father for their support
throughout all of my endeavors, and my closest and dearest friend Laura
Harmsen for her inspiration and emotional support.
iii


72
microstructure was made by electron microscopy. The various TEM
specimens that were described in a previous section were examined in a
JEOL 200 CX analytical microscope (Japanese Electron Optics Laboratory,
Boston, MA.). This instrument was used in both transmission (TEM) and
scanning transmission (STEM) modes to directly observe and measure the
ultra-fine structure of thin films. Selected area diffraction (SAD)
mode was used to conduct electron diffraction experiments for
determination of crystal structure and orientation. In the SAD mode it
was possible to obtain diffraction information from areas as small as 5
um. The line to line resolution observed in the TEM mode was 8 .
Another technique used for determination of crystal orientation was
dark field imaging. By tilting the primary beam in the microscope, the
contrast that is observed in a dark field image is only due to those
crystals or portions of crystals which are oriented for a specific
Bragg reflection. Dark field imaging makes it possible to observe
preferred orientation in a thin film or any portion of a crystal that
contains defects (such as microtwins).^ On the JEOL 200CX changing
from bright field to dark field was accomplished with a single switch.
To study the topographical structure of thicker films a JEOL 35CF
scanning electron microscope was used. Films which were made for
optical analysis were frequently examined with this instrument. The
advantage of this technique for CdS films was that no sample
preparation was required except for sample mounting. The conductivity
of these films was high enough not to warrant coating with a conductive
film, although for COSAD films a thin carbon film had to be deposited
before these films could be examined. With this instrument the


INTENSITY
(Thousand*)
(Thousands)
WAVENUMBER (cm-1)
Figure 84
Same LO peaks displayed in Figure 82, shown here measured
with the Raman spectrometer.


73
presence of gross defects in the film such as pinholes or cracks could
be observed, as well as the fine grain structure of certain films. The
major disadvantage with this microscope, however, was a limiting
resolution of about 300 which made it difficult to observe the grain
size of as-deposited thin films.
X-ray diffraction was another technique used to measure the
crystal structure and orientation of thin films. An APD 3720 computer-
controlled diffractometer (Phillips Co.) was used to carry out
diffraction experiments. Typically, diffraction was measured from two-
theta values ranging from 15 to 90 with a angular step of 0.05. Use
of the computer to store X-ray diffraction spectra on disk greatly
facilitated the determination of the change in crystal structure with
heat treatments. Changes in peak positions and intensities could be
readily determined, and by making high resolution scans (0.02/step)
over certain peaks it was possible to calculate the grain size and film
strain using certain utility programs available in the computer system.
Again, no sample preparation was necessary for this technique as
diffraction from 1 pm films supported on silica substrates was readily
observed.
Chemical Analysis
Initially, Energy Dispersive Spectroscopy (EDS) was used to
measure the stoichiometry of thin films during both SEM and STEM
observations; however, it was found to give results that were not
quantitative. Particularly in the SEM, it was found that the results
were very dependent on sample geometry with respect to the detector. A


17
is modeled as a saturable two level system, inhomogeneously broadened
by a T2 dephasing time. The transition linewidth is inhomogeneously
broadened because excitons bound at different locations see different
environments.^ An inhomogeneously broadened system is described as a
distribution of groups or classes of transitions, and within each class
the transitions are assumed to be identical (homogeneously broadened).
The saturation of the inhomogeneously broadened system does not depend
on the homogeneous lineshape function, but rather on the linewidth of
these "homogeneous packets". This means that the saturation intensity
is inversely dependent upon the dephasing time T2 as shown by
I
s
2 2
n n h v Av
cj) X
(2.15)
where
Av = (m T2)-'*'
(2.16)
and is the ratio of the radiative lifetime to the spontaneous decay
time, which is usually taken as equal to one. In semiconductor systems
the dephasing time is on the order of 0.1 psec,^ which means that very
low saturation intensities are required to saturate bound exciton
transitions. The importance of the saturation intensity will be
described in a following section; however, a small value indicates that
the nonlinear refractive index is very large, as the two are inversely
proportional (see equation 2.25). As shown in Table 1, in CdS a
saturation intensity as small as 26 W/cm2 has been measured, which
corresponds to n2 value of 1.3 X 10"2 cm2/W.10 This is the largest n2


4
attractive a system is. MQW gallium arsenide has an additional
advantage of room temperature operation, but suffers from a slow
switch-off time and very large absorption. There are other materials
which have been shown to exhibit a much larger nonlinear effect than
MQW gallium arsenide. A material which has been demonstrated to exhibit
one of the largest nonlinear coefficients is cadmium sulfide (CdS).
The large coefficient was obtained by saturating a bound exciton level
in the band gap.^ The mechanism which leads to the large exciton
saturation effect in CdS is central to this dissertation and will be
described in a subsequent section. However, only a very few groups
have looked at CdS, and no one has either investigated the bound
exciton saturation mechanism in thin films or explored the possible
applications in integrated optics.
The most important consideration for any practical nonlinear
application is the temperature at which the nonlinear process
predominates. To the present day the largest nonlinear coefficients are
only measured at very low temperatures. For example, the large
coefficient obtained by saturating the bound exciton level in CdS
occurs at 2 K. Since the binding energy of the bound exciton only
corresponds to a few millielectron volts, the state is thermally
annihilated at higher temperatures, and the large nonlinear effect
disappears.
The electronic structure of the band gap, however, may be altered
to permit access of exciton levels at higher temperatures by
controlling the physical size of the material. The phenomenon by which
this occurs is known as quantum confinement. When the crystal size of


237
29. J.H. Wohlgemuth, D.E. Brodie, and P.C. Eastman, "A Comparison of
the Optical Properties of Crystalline and Amorphous CdS," Can. J.
Phys., D8, 581 (1975).
30. M. Cardona and G. Harbeke, "Optical Properties and Band Structure
of Wurtzite-Type Crystals and Rutile," Phys. Rev., 137, A1467
(1965).
31. E. Khawaja and S.G. Tomlin, "The Optical Constants Of Thin
Evaporated Films of Cadmium and Zinc Sulphides," J. Phys. D, 8,
581 (1975).
32. M.H. Christmann, K.A. Jones, and K.H. Olsen, "Formation of
Hexagonal Flat Tops on the Surface of Heteroepitaxial (0001) CdS
Films," J. Appl. Phys., 45, 4295 (9174).
33. J. Humenberger, G. Linnert, and K. Lischka, "Hot-Wall Epitaxy of
CdS Thin Films and Their Photolumescence," Thin Solid Films, 121,
75 (1984).
34. M. Gracia-Jimenz, G. Martinez, J.L. Martinez, E. Gomez, and A.
Zehe, "Photoluminescence of Chemical Deposited CdS Films," J.
Electrochem. Soc., 131, 2974 (1984).
35. B.J. Feldman and J.A. Duisman, "Room-Temperature Photoluminescence
In Sprayed-Pyrolyzed CdS," Appl. Phys. Lett., 37, 1092 (1980).
36. I. Lagnado and M. Lichtensteiger, "RF-Sputtered Cadmium Sulfide
Thin Crystals," J. Vac. Sci. Technol., 7, 318 (1970).
37. J.A. Thornton, D.G. Cornog, W.W. Anderson, and J.E. Philips, Proc.
of the 16th IEEE Photovoltaic Specialists Conf., 737 (1982).
38. I. Matril, G. Gonzalez-Diaz, F. Sanchez-Quesada, and M. Rodigiez-
Vidal, "Deposition Dependence of RF Sputtered CdS Films," Thin
Solid Films, 90, 253 (1982).
39. I. Matril, G. Gonzalez-Diaz, and F. Sanchez-Quesada, "Temperature
and Bias Effects on the Electrical Properties of CdS Thin Films
Prepared by RF Sputtering," Thin Solid Films, 114, 327 (1984).
40. I. Matril, G. Gonzalez-Diaz, F. Sanchez-Quesada, and M. Rodigiez-
Vidal, "Structural and Optical Properties of RF Sputtered CdS Thin
Films," Thin Solid Films, 121, 85 (1984).
41. I. Martil, G. Gonzalea-Diaz, and F. Sanchez-Quesada, "Heat
Treatments of RF Sputtered Films for Solar Cell Applications," Sol.
Eng. Mat., 12, 345 (1985).
42. F.B. Michwletti and P. Mark, "Ambient-Sensitive Photoelectronic
Behavior of CdS Sintered Layers," J. Appl. Phys., 39 5274 (1968).


7
received considerable attention lately for nonlinear optics
application. These are crown base glasses which contain one or two
percent of CdS or mixtures of CdS and CdSe. It has been postulated
that the semiconductor crystals exist in the glass matrix as finely
dispersed microcrystallites, which are small enough to permit quantum
confinement effects to occur. This effect is still not well understood
and has not been clearly demonstrated. Many authors have observed
energy shifts that may be due to compositional effects rather than
microstructure size. However, it appears theoretically that with
sufficient confinement, the exciton state will be accessible at room
temperature.


CHAPTER VI
FUTURE WORK
There are several aspects of cadmium sulfide and COSAD thin films
which require further study and investigation.
Obviously, the first study is to make a nonlinear optical
measurement to determine if the exciton states in the pure films of CdS
can be saturated to produce an effect that is comparable to the large
nonlinear refractive index observed in single crystal platelets. Since
the films are of high enough optical quality to act as thin film Fabry-
Perot interferometers, a very simple intensity saturation experiment
could be conducted.The only difficulty in this experiment is the
need for a high resolution tunable dye laser which can be operated in
the wavelength region of 488 nm, the wavelength required for exciton
saturation.^ A shorter wavelength, high speed pulsed laser could also
be used to measure the nonlinear absorption, (due to a free carrier
plasma) by measuring the temporal waveform of a transmitted pulse.^
If this pulsed laser could be used with the dye laser, the change in
the waveform as the laser is tuned near the exciton level would provide
valuable information. Finally, if a nonlinear intensity saturation was
observed, then a pump-probe experiment could be carried out to
determine the switching speed of the thin film Fabry-Perot. * 1
Further work on the stoichiometry as a function of heat treatment
atmosphere should be carried out to determine the influence of sulfur
224


ABSORBANCE
173
WAVELENGTH (nm)
Figure 65 Room temperature absorbance spectra comparing as-deposited
(ASDP) and heat treated thins. Temperature of heat
treatment shown; all treatments for five hours.


89
above. The majority of the measurements, however, were made with the
optical multichannel analyzer (OMA) and a 0.33 meter Instruments SA
monochromater. Most measurements were made at low temperatures, using
the cold finger to cool the sample.
The detection system of the OMA spectrometer consisted of a linear
silicon photodiode array which was 1024 elements wide with the diode
centers spaced every 25 pm. Approximately 700 of the center diodes
were intensified to increase their light conversion efficiency. The
diode array was connected to a multichannel analyzer and the array was
scanned by the MCA to generate a spectra. Scan rates as fast as 16.6
msec/scan could be used, although to obtain a higher number of counts,
scan rates of 500 msec were typically used. The computer system
controlling the MCA was setup to accumulate a number of scans so that
signal averaging could be used to reduce noise levels. Also, the dark
current or background noise could be automatically subtracted from a
spectrum. The diode array could be cooled to 5 C with a Peltier
cooler to further reduce the background noise level. With the
monochromater and grating combination the MCA window was about 20 nm.
This is the width of the spectra which was displayed on the screen.
The grating in the monochrometer was rotated to change the wavelength
window.
The third meter monochromater was used with a 2400 lines/mm
grating. The f-stop or light gathering capability of the monochromater
was calculated with


47
for a material decreases.28 The appearance of an amorphous structure
with high substrate bias was explained by the authors as a result of
structural damage that occurred due to ion bombardment (which was
increased by the high negative bias).
The second report published by Martil et al.39 further explored
the effects of substrate temperature and bias on the electrical
properties of CdS thin films. They found that the resistivity
increased from 10 to 108 Q cm as the temperature was increased from 60
C to 250 C, which they claimed was due to a change in stoichiometry
(i.e. a loss of cadmium with increasing temperatures), although no
chemical analysis was presented to back up this claim. A minimum in
resistivity was also found with a substrate bias of -50 volts, or with
a floating substrate (which developed a self bias of -28 volts). This
effect was explained by the ion bombardment that results from these
bias voltages preferentially resputting oxygen and other impurities
from the growing film. Oxygen has been shown to be a acceptor-like
trapping center, located 0.9 eV below the conduction band.^2 Again no
chemical analysis was reported to prove that a decrease in oxygen
occurs. Also the variation in activation energy for conduction that
occurs with temperature as seen by Lagnado and Lichtensteiger3^ was not
explored by these authors.
The third publication by Martil et al.^ reports the influence of
the above sputtering parameters on the optical properties of thin
films. The structural changes which occur under different conditions
were also further explored. Increasing the substrate temperature
resulted in similar structural changes to those reported in the earlier


103
of atoms onto non-favorable positions, which increase the number of
defects during growth. Higher deposition rates should increase the
amount of stress induced in the film. Strain determinations by X-ray
line broadening measurements will be described in a following section.
Low temperature depositions. Although high quality thin films
could be produced at room temperature, some films were deposited onto
substrates cooled to LN2 temperatures to determine if a grain size
small enough for quantum confinement could be produced. Wolgehmuth et
al.29 showed that evaporated material deposited at this temperature was
found to be amorphous as determined by X-ray diffraction. In that work
a high supersaturation was used to produce these films which may have
also contributed to the amorphous structure. A much lower deposition
rate was used in this study to help promote crystal growth, so that
combined with the low temperature, a very small grain size would
result. Figure 26a shows the micrograph of a film deposited at 3 /sec
onto a carbon support film which was attached to a substrate cooled to
80 K. The "grain" structure that is observed is not due to individual
grains but rather to a columnar structure with low density areas
indicated by the low contrast between the columns. The SAD pattern
shown in Figure 26b indicates that the film is somewhat crystalline,
but highly faulted as evidenced by the width of the rings. The three
rings corresponding to the cubic reflections are still present
indicating that only the nearest neighbor spacing is present. No
hexagonal lines can be found in SAD patterns of these films.
An indication of how a film grows on a cooled substrate is shown
in Figure 27a. This section of a film increases in thickness from left


60
diameter apertures centered over the two sputter guns were cut in the
top plate to provide access to the two plasma sources. A dual
substrate holder was rotated in the plane above the two apertures, so
that the substrate was alternatingly exposed to one source and then the
other. The dual holder provided the capability of producing two films
during a single deposition run. Initially a DC motor with a reducing
gear drive was used to rotate the substrates with a linear speed. A
more sophisticated direct drive stepping motor was later added to the
system to permit variable exposure to the two plasma sources. The
stepping motor was interfaced with the Dascon I/O board in the Z-158
computer via an external digital control circuit.
One disadvantage of the COSAD configuration was that substrates
could not be heated or cooled and the temperature of the substrate
could not be monitored during a deposition. The utility of this
particular system configuration, however, is that single component
films could still be produced, simply by positioning the substrate
holders over the respective apertures, and exposing the substrate to
deposit a film.
RF Magnetron Sputtering
As described in the Bibliographic Review section, the only sputter
deposition of CdS was carried out by RF-diode sputtering. In this
configuration two parallel plates are used. The target is attached to
the cathode plate, and substrate is attached to the anode plate, which
can either be at ground or floating potential. A plasma is generated
between the two plates by applying RF power to the cathode plate.


122
Figure 37 SEM micrograph of thin film subject to rapid thermal
anneal (RTA) heat treatment.


CHAPTER IV
EXPERIMENTAL METHOD
Vacuum Deposition System
Thin films for this study were made with a specially designed
vacuum system. A schematic of the deposition chamber is shown in
Figure 10, and a picture of the complete system is shown in Figure 11.
The deposition chamber consists of a rectangularly shaped stainless
steel box which measures 12" X 18" X 18". This chamber was custom
built by MDC Vacuum Corp. (Haywood, CA.). Numerous ports and
feedthroughs on the chamber permit a wide variation of deposition
configurations. The vacuum pumping system utilizes a 330 1/sec
turbomolecular pump (Balzers, Hudson, NH), a molecular sieve trap (MDC
Vacuum Corp.), and a 300 1/min two-stage mechanical pump (Sargent
Welch, Skokie, IL). With the molecular sieve trap activated, pressures
in the high 10"^ torr range were possible using the mechanical pump
only. Pumpdown from atmosphere pressure to 1X10"^ torr could be
accomplished in 2 hours with use of the turbopump. Vacuum gauging was
performed with a Leybold Heraues (East Syracuse, NY) model CM 330
combined Penning discharge and thermocouple gauge controller.
Pressures for sputter deposition were controlled by using a
micrometer adjustable throttle valve (Sputtered Films Inc. Santa
Barbara, CA.) and a mass flow meter/controller (Matheson Gas Products
Norcross, GA). High purity argon gas (99.9995%) was used as the
52


108
silica substrate and its corresponding SAD pattern are shown in Figures
28a and 28b. The absence of any contrast in many of the grains
indicates a highly ordered crystal structure. The grains are again
polygonal in shape with an average size of 800 . The SAD pattern
shows that the preferred orientation is still maintained, but several
of the other hexagonal reflections are now more predominant. Although
fewer crystallographic defects are observed in these films, the "bulk"
structure was found to be very nonuniform. Variation in color of the
film across the substrate indicated a variation in composition and/or
thickness, which made these films unsuitable for optical studies. Films
deposited at temperatures greater than 200 C were found to be even
more nonuniform. The nonuniformities result from nonuniform thermal
contact because silica substrates are such poor conductors of heat.
Thermal contact paste was not used due to concerns about contamination
from the paste. The nonuniformities encountered at high deposition
temperatures will be detailed more thoroughly in a following section on
the scanning electron microscopy of thin films.
Heat treated thin films. Once the optical properties of as-
deposited thin films were determined, it became apparent that to obtain
optimum optical properties some type of thermal treatment had to be
conducted on the films to remove the defect structure produced by the
deposition process. However, due to difficulties with the increased
bonding strength of thin films to substrates as a result of even mild
heat treatments, very few TEH micrographs were obtained. The primary
technique used for observing the microstructure of heat treated thin
films was by SEM, and the results are presented in the following


99
Figure 23 TEM micrographs of thin films deposited onto alternate
substrates, a) silica glass slide; b) NaCl slab.


130
of the Table do, however, correlate closely to grain size observed by
TEM for as-deposited thin films. The shift of the higher rate film to
a smaller two-theta value indicates the presence of a tensile stress.
Higher stress levels in higher rate films may be due to growth related
defects and the inclusion of impurities. The higher stress level in
these films causes the film to exfoliate within several hours after
exposure to the atmosphere. Figure 41a shows an optical micrograph of
a film exhibiting the onset of this exfoliation, and Figure 41b
displays the appearance of a film after several days of exposure. The
appearance of the exfoliation shown in Figure 41a is characteristic of
a tensile stress. Exposure to atmosphere leads to the absorption of
water on the film surface, which apparently increases the stress to a
value large enough to cause the film to break away from the substrate.
Some type of stress related corrosion process may also contribute to
this result. Lower deposition rate films were found to be stable with
exposure to the atmosphere indefinitely, so X-ray line shift due to
uniform strain was expected to be less.
Precision lattice constant. A calculation of the precision
lattice constant of heat treated thin films was made from the positions
of diffraction peaks measured from thin films. Due to the strong
preferred orientation only the c-axis lattice constant could be
determined and because small two-theta values had to be used, the
calculated lattice parameter was plotted versus the Nelson-Riley
function. The plot shown in Figure 40b indicates an extrapolated
lattice parameter of co=6.707 , which is 0.09% less than the JCPDS
card file value of 6.713 .^3


78
where R^, R2, and R3 are the reflectivities of the air-film, film-
substrate, and substrate-air interfaces, respectively.16 The values of
the Rl, R2, and R3 are given by the same equation stated above with the
refractive indices of the three materials (air, film, and substrate)
substituted for n, respectively. Even if the value of the absorption
coefficient is known, the transmission must be measured to 0.5% to get
a 1% accuracy in the refractive index,which is experimentally very
difficult to do unless a specially designed spectrometer is used.
When the refractive index of the film is significantly different
from that of the substrate (as the case with CdS on silica) a very
strong wavelength dependent interference effect occurs. The effect can
be explained by examining Figure 16. When the incident light beam
passes through the thin film and encounters the interface it will be
reflected due to the differences in refractive indices. Depending on
the thickness and the refractive index of the film (the product of
these two values determines the optical path length) the reflected ray
will either constructively or destructively combine with the incident
beam. This is the same process that occurs in a Fabry-Perot
interferometer described previously. However, instead of changing the
physical distance of the cavity to change the optical path length,
resonance is achieved by scanning the wavelength of incident light. In
terms of the total transmission that is measured, a series of
interference fringes is observed, which vary with wavelength. These
fringes are known as fringes of equal chromatic order (FECO), and the
wavelengths at which maxima and minima in transmission occur is given
by the same equation used for resonance conditions in a Fabry-Perot


COUNTS
166
1 M UI 1 1 LU n
2.6 5.2 7.8 10.
ENERGY CKqV)
Figure 62 EDS X-ray spectrum of C0SAD1 film.


68
about 10 minutes with this holder, although most depositions were made
at 200 C. The unique feature of this holder is that it permitted
outgassing of the substrates prior to deposition. This was done by
heating to 200 C for 30 minutes in high vacuum.
Deposition rates
A variation in deposition rates was also studied to determine its
effect on thin film properties. For pure films of CdS, rates from 1
/sec to 5 /sec were used. These rates were monitored and controlled
by the IC 6000 deposition controller. Figure 15 shows a graph of the
sputtering rate versus power density for both CdS and BK-7 targets. As
shown, the glass target used for making COSAD films is much more
difficult to sputter, as an input of 275 watts (13.5 W/cm^) only
resulted in a deposition rate of 2 /sec. Power levels greater than
300 watts could only be used with the BK-7 glass target which had a
copper backing plate bounded to the back of the target. Even at 500
watts, however, a deposition rate of only 4 /sec could be obtained.
At this power level, considerable heating of the substrate was found to
occur, even when the substrate was rotated during COSAD runs.
Co-Sputter Alternating Deposition (COSAD)
By using the dual sputter gun configuration described above, a
very unique type of thin film could be produced. The acronym describes
the process by which these films were made; by rotating the substrate
above the two sputter guns, an alternating layer of glass and the CdS
was deposited, which was repeated continuously. Initially the two guns


Counts xlO
206
9. 0 K
Wavslength (nn)
Figure 81 Temperature dependence of blue edge photoluminescence.


TABLE OF CONTENTS
PAGE
ACKNOWLEDGMENTS iii
ABSTRACT vii
CHAPTERS
I INTRODUCTION 1
Optical Signal Processing 1
Nonlinear Optical Materials 2
Thin Films 5
II THEORY OF NONLINEAR OPTICS 8
Nonlinear Optical Susceptibility 8
Optical Bistability 19
III BACKGROUND BIBLIOGRAPHICAL REVIEW 28
Bulk Properties of Cadmium Sulfide 28
Photoluminescence of CdS 32
Resonant Raman Scattering in CdS 37
Nonlinear Susceptibility 38
CdS Thin Films 39
Vacuum Deposition 39
RF Sputtering 45
Semiconductor Doped Filter Glasses 50
IV EXPERIMENTAL METHOD 52
Vacuum Deposition System 52
RF Magnetron Sputtering 60
Thin Film Deposition 63
Substrate materials 64
Substrate temperatures 66
Deposition rates 68
Co-Sputter Alternating Deposition (COSAD) ... 68
Post Deposition Treatments 70
Thin Film Characterization 71
Microstructure 71
Chemical Analysis 73
v


70
were set up to sputter deposit at a static rate of 3 /sec. This is
the maximum rate that could be used with the glass target without
causing excessive heating. The circle over which the substrate was
rotated was eight inches in diameter, so with the three inch diameter
apertures used with the guns, the duty cycle was rather low, only 0.12
for each gun. With a rotation speed of 20 RPM, the resulting total
dynamic deposition rate is less than 1 /sec. One possible way to
increase this rate would have been to reduce the rotation speed. This,
however, would also mean that the substrate would be spending more time
over areas which were not depositing film and were actually depositing
impurities.
The use of the computer controlled stepping motor to rotate the
substrate holder alleviated the problem with the linear drive system.
With this setup is was possible to increase the amount of time the
substrate spent over a given source. At the same time, it reduced the
amount of impurities introduced into the film because the stepping
motor could be driven full speed between the two sources, which was
about 120 RPM. By simply altering the computer program, this system
permitted easy variation of deposited structure.
Post Deposition Treatments
To improve many of the physical properties of thin films a post
deposition heat treatment had to be instituted. Generally, heat
treatments on CdS thin films were made in a fused silica muffle tube
furnace under a flowing argon atmosphere. The argon used for heat
treatments was of the same grade as the gas used for sputtering. For


I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
Joseph H. Simmons, Chairman
Professor of Materials
Science and Engineering
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
Paul H. Holloway,
Professor of Materials
Science and Engineering
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
Robert T. Dehoff,
Professor of Materials
Science and Engineering


159
* * T a V .
* ***, at
§ A' * 3*?
*: \- ,*## J
l^^v VI *1
* * Ll ^ -
200 nm
Figure 57 TEM micrograph of heat treated C0SAD1 thin film, showing
large unidentified crystals.


a)
50 run
b)
Figure 52 TEH micrograph of C0SAD1 thin film, a) as-deposited on
carbon support film; b) corresponding SAD pattern.


37
exciton to be 0.5+/-0.1 nsec. The very fast decay time of this state
is what makes CdS so attractive for nonlinear optical application.
Resonant Raman Scattering in CdS
Further studies of exciton levels in CdS have involved the
measurement of multiple phonon scattering from exciton states. Leite,
Scott, and Damen found that the scattering of longitudinal optical (LO)
phonons was enhanced when the scattering-phonon frequency coincided
with that of excitons.^ They were able to show up to nine orders of
resonant Raman scattering occurring at frequencies shifted less than 1%
from multiples of the 305 cm'^ line (the first LO line). These authors
were not able to determine the identity of the exciton state (i.e. free
or bound) which was acting as the intermediate state for the resonant
scattering in this study, butin a latter paper by Leite, Scott, and
Damen free excitons were proven to be the primary intermediate state.^
They also showed that bound excitons participated as intermediates by
observing phonon sideband features on the photoluminescence of the
bound excitons. The 1^ exciton was found to be a stronger resonant
state compared to the I2 bound exciton.
In a more recent paper by Mashshenko,^ the temperature dependence
of the LO and 2L0 lines associated with the A exciton were
investigated. At 77 K the A-LO phonon predominates the emission, but
as the temperature is increased to 110 K, this line decreases and the
A-2L0 line increases. This increase in the 2L0 line indicates that an
increase in the probability of a two-phonon process occurs at high
temperatures.


43
properties. This is probably due to differences in thin film
microstructures, which in the above studies were only determined by X-
ray diffraction. No direct measurements (i.e. electron microscopy)
were used to examine thin films.
Although some of the optical properties of evaporated thin films
have been investigated, albeit there are differences in the results,
apparently very few studies have been made of the photoluminescence of
thin films produced by this technique. Christmann et al. c describe
evaporation deposited epitaxial CdS films which displayed both green
and blue edge luminescence. This is one of the first studies to relate
both the morphology and composition of thin films to the observed
photoluminescence. Thin films were deposited on cleaved surfaces of
SrF2 at temperatures from 210 to 310 C. Variation of the
supersaturation by controlling either the source temperature or the
substrate temperature resulted in three basic morphologies. At low
supersaturations, smooth CdS thin films were produced which only
displayed broad band green photoluminescence. By decreasing the
substrate temperature or increasing the source temperature, films were
produced which displayed a structure with many hexagonal flat tops.
These films showed green edge emissions and very low intensity blue
edge emissions. Finally, at very high supersaturations, films which
displayed a morphology with many hexagonal pyramids were found to show
very intense blue edge, or bound exciton photoluminescence.
Through cathodoluminescence and microprobe studies it was
determined that the hexagonal pyramids were cadmium rich, and they did
not contribute to the luminescence. Only the areas adjacent to the


165
typical EDS X-ray spectrum obtained while operating the JEOL 200 CX in
the STEM mode is shown in Figure 62. This spectrum was acquired from
the film displayed in Figure 61 and it shows that the particles are
CdS. Except for silicon, the other constituents of the glass matrix
(e.g. Ca and K) could not be determined with this technique because of
their low concentration and the thickness of the film. The copper peak
originates from the TEM support grid.
The compositions of thicker films made for optical analysis were
determined by XRF. One difficulty encountered in using this technique
was that the potassium Ka line overlapped the cadmium L3 line, so the
amount of potassium could only be roughly determined. For COSAD films
deposited on silica substrates the total amount of silicon also could
not be determined, due to fluorescence of the substrate. The relative
amounts of cadmium and sulfur, however, could be determined and it was
found that peak intensity ratios very close to those of as deposited
pure CdS films were obtained with COSAD films with both high and low
amounts of CdS. A few COSAD films were deposited on single crystal
sapphire to determine the composition separately from that of the
substrate.
Thin Film Optical Properties
Now that the physical properties of thin films have been fully
described, the results that are central to this dissertation will be
presented in this section. These results describe how the changes in
microstructure produced by various treatments affect the optical
properties. UV-visible absorption, photoluminescence, and resonant


APPENDIX A
BASIC PROGRAM USED FOR EXTERNAL CONTROL OF LAMBDA 9
1000 REM LAMBDA 9 COMMUNICATIONS PROGRAM
1010 REM VER. UWIS6.BAS
1015 REM WRITE COMMAND USED TO OUTPUT DATA TO FILE NAME *.PRN
1020 REM FOR ASYST PROGRAM INPUT AND LOTUS WORKSHEET
1030 CLEAR:CLOSE:KEY OFF:CLS
1040 DIM X(3000)
1045 REM ARRAY MUST BE LARGE ENOUGH TO ACCOMMODATE ENTIRE SCAN
1050 INPUT "TRANSMISSION OR ABSORBANCE SCAN T/A ";ANS$
1060 IF ANS$="T" THEN 1070 ELSE 1080
1070 SB=0:PRINT "TRANSMISSION SCAN"¡GOTO 1090
1080 SB=5:PRINT "ABSORBANCE SCAN"
1090 INPUT "SELECT ABSCISSA MAX ";AH
1100 INPUT "SELECT ABSCISSA MIN ";AL
1110 INPUT "SELECT SCAN SPEED ";SS
1120 INPUT "SELECT DATA INTERVAL ";DI
1130 INPUT "SELECT RESPONSE ";FR
1140 INPUT "SELECT SLIT WIDTH ";SL
1150 SL=SL*100:REM CONVERT SLIT SETTING
1160 REM CALCULATE NUMBER OF DATA POINTS
1170 XPTS=(AH-AL)/DI
1180 INPUT "SELECT NORMAL (1) OR EXTENDED RANGE (2) "jRA
1190 INPUT "SELECT CHART REGISTRATION: OFF(O).SERIAL DASH (1),
OVERLAY DASH (2)";CR
1200 IF CR=0 THEN 1260
1210 INPUT "SELECT PRINTER FUNCTION: OFF (0),GRID + SCALE (l) ";PR
1220 INPUT "SELECT PEN LINE MODE (1-4) ";PE
1230 INPUT "SELECT RECORDER FORMAT (nm/cra) ";RS
1240 INPUT "SELECT ORDIANTE MAXIMUM ";MX
1250 INPUT "SELECT ORDINATE MINIMUM ";MI
1260 IF CHG$="Y" THEN 1350
1270 REM OPEN COMMUNICATIONS PORT
1280 OPEN "COMI: 4800 E, 7,1" AS//1
1290 PRINT //I, "$RE 0"
1300 PRINT It 1, CHR$ (17)
1310 INPUT //I ,D$
1320 PRINT //1, "$PA 2"
1330 INPUT tfl,D$
1340 PRINT D$
1350 REM SET UP INSTRUMENT
1360 PRINT //I,"$SB" + STR$(SB)
1370 INPUT //I ,D$
1380 PRINT //I, "$AH" + STR$(AH)
1390 INPUT //1, D$
1400 PRINT //I, "$AL" + STR$ (AL)
227


6
investigative tools become available for following the underlying
processes.
The study of any thin film for this application should start with
examining the properties of the bulk material which contribute to the
specific origin for the nonlinear effect. In the case of CdS this
means looking at the electronic states of the material which lead to
the presence of excitons. These states have been thoroughly examined
for more than 30 years and they are probably the best understood in
this material. Exciton states have been shown to occur in thin
epitaxial films, but no one has investigated the presence of these
states in polycrystalline thin films, nor have the effects of
preparation conditions on the excitonic transitions of a thin films
been investigated. The objective of this study therefore is to
determine how exciton states would occur in polycrystalline thin films
of the material, how they are affected by structure and formation
conditions, and how size variations and grain boundary structures might
affect their energy level structure.
A supplementary part of this study will investigate the
possibility of producing thin film structures consisting of a glass
matrix with small isolated crystals whose sizes matched those needed to
develop quantum confinement effects. The investigation will use
spectroscopic techniques to determine if quantum confinement effects
can be induced on the exciton states in the material. The objective is
to produce a thin film of semiconductor doped filter glass.
The process of quantum confinement has been and still is under
investigation in bulk semiconductor doped filter glasses, which have


27
The value of nt is what actually determines the phase shift but as
indicated by equation 1.1, the figure of merit for nonlinear optical
applications also includes the switching times and the absorption
coefficient.


181
TABLE 6
Comparison of Exciton Reflection and Absorption
Peak positions
Peak Position
Energy
Peak Position
Energy
Exciton
CdS Platelet
Delta
CdS Thin Film
Delta
A
2.555
0.015
2.545
0.013
B
2.570
0.080
2.558
0.072
C
2.630
2.620
Note: All energy values given in electron volts, eV.


53
CRYSTAL
THICKNESS
MONITOR
cmo
O
LN2 COOLED
SUBSTRATE HOLDER
1
CAPACITANCE
MANOMETER
SHUTTER
mill ""'"'ll,,,
TURBOMOLECULAR PUMP
Figure 10 Schematic diagram of sputter deposition chamber. Computer
figure shown to indicate computer control of system.


41
M)
Figure 8 Variation of index of refraction reported by several
i ? Q
authors. *


66
were also required for TEM analysis of COSAD films, since the HE
solutions normally used to lift CdS off silica slides would strongly
attack these films.
Substrate temperatures
Several different substrate temperatures were used to investigate
the effect of deposition temperature on thin film grain size. With
the LN2 feedthrough assembly that was described above, substrates could
be cooled to LN2 temperatures in about 20 minutes. For some
depositions a low temperature thermal contact paste was use to mount
the substrate in the holder, although most frequently, standard hold
down clips were used. An Iron-Constantan thermocouple was initially
used to monitor the temperature of the substrate holder during a
deposition. To use a digital meter to measure the output from this
thermocouple during a plasma deposition, an RC circuit had to be used
to decouple the RF signal that was imposed on the DC thermocouple
signal. The RC circuit is essentially a low band pass filter. A
schematic diagram of the circuit is shown in Figure 14. If this filter
is not used, a very large RF signal can be induced in the lead wires,
which will destroy most D/A converters used in digital meters.
For depositing films at room temperature, the LN2 feedthrough
assembly (without LN2 in the reservoir) was used to hold substrates.
These depositions were very close to room temperature, since it was
found that even with the highest deposition rates, the holder would
only heat up to 35 C. When substrate temperatures greater than this
were required, the LN2 assembly was replaced with the resistively
heated holder. Temperatures as high as 300 C could be obtained in


INTENSITY INltNSIlY INTENSITY
124
a)
b)
c)
Figure 38 X-ray diffraction patterns of as-deposited thin films, a)
deposited at LN9 temperatures; b) deposited at room
temperature; c) deposited at 300 C.


121
diffusion, which is a low energy process. At higher temperatures,
enhanced diffusion of impurities at the grain boundaries results in a
large increase in grain growth. Grain growth is therefore controlled
by the activation energy for impurity diffusion. The curve appears to
flatten out at the highest temperatures, which is possibly due to two
effects. First, since the film is of a finite thickness, there must be
a limit on the maximum size of the grains. Generally the largest grain
size obtainable should correspond to the thickness of the film, but
when a film is in contact with a substrate, this constraint will depend
upon the interface energy between the film and substrate. The more
likely effect, however, is due to a loss of material at higher
temperatures. Thin films were found to totally evaporate at 800 C in
a very short time, even though the sublimation temperature of bulk CdS
is 925 C. At 650 C for long times a loss of material was also found
to occur which would disrupt the grain growth process.
Although some of the optical properties were optimized by heat
treatments to 650 C, the larger grains and cracking of these films
increased the amount of light scattering. Rapid thermal annealing
(RTA) permitted high temperature treatments for very short times, which
resulted in very little grain growth and no cracking of the film.
Figure 37 shows a high magnification micrograph of a film that received
an RTA treatment at 650 C for 30 sec. The film displays an average
grain size of only 900 , which is considerably smaller than the size
obtained by furnace heat treatments to this temperature. Since this
film was subjected to a very large thermal stress, it appears that the


10
e = (n + ) 2 (2.4)
2oj
where ca/2w is the extinction coefficient. By combining these two
equations a relation between the polarization and the index of
refraction can be written.
(n + ica )2= 1 + 4tt P( 2w
The nonlinear susceptibility can be defined by expansion of the
refractive index in terms of the intensity I, of the radiation inside
the sample:^
n = n-^ + n2l + + (2.6)
Next we assume that the extinction coefficient is very small compared
to n. Then by using equations 2.5 and 2.6 and expanding the terms,
n2 = n2 + 2nin2I + (n2l)2 = 1 + 4tt X(1) + 4tt X(3) [E]2. (2.7)
Finally we assume n2 is much smaller than n^ and by comparing
coefficients of [E]^ an j


Count*
194
Figure 74 Low resolution photoluminescence spectrum acquired with
optical multichannel analyzer (OMA).


239
58. D. Briggs and M.P. Seah, Practical Surface Analysis by Auger and X-
ray Photoelectron Spectroscopy, (John Wiley and Sons, New York,
1983), p.119.
59. S.W. Gaarenstroom and N. Winograd, "Initial and Final State Effects
in the ESCA Spectra of Cadmium and Silver Oxides," J. Chem. Phys.,
67, 3500 (1977).
60. K. Urbanek, "Magnetron Sputtering of SO2; An Alternative to
Chemical Vapor Deposition," Solid State Technol., April, 87 (1977).
61. V.E. Mashchenko, "Interaction of Excitons and LO Phonons in CdS
Under High Excitation and At High Temperature," Opt. Spectrosc.,
42, 116 (1977).
62. B.D. Cullity, Elements of X-ray Diffraction, (Addison-Wesley
Publishing Co., Reading, MA, 1978), p. 458.
63 B.D. Cullity, Elements of X-ray Diffraction, (Addison-Wesley Pub.
Co., Inc., Reading, MA, 1978), chap. 11.
64. E. Elizalde and F. Rueda, "On the Determination of the Optical
Constants n(A) and a(A) of Thin Supported Films," Elect. Opt.,
122, 45 (1984).
65. K.A. Simmons and J.H. Simmons "Graphical Method for Solution of
Ellipsometry Data" to be published in J. Appl. Phys.


140
b)
Figure 43 Auger electron spectroscopy survey scans. a) Target
material; b) heat treated thin film.


222
chalcogeni.de compounds is that this type of defect would be limited to
less than 1%. Calculations show that if all the excess sulfur were
present at the grain boundaries as a thin layer, then this amount of
sulfur would be six to seven monolayers, which corresponds to a grain
boundary layer 14 thick. For an average grain size of 3000 , this
is not an unreasonable thickness for grain boundaries. The uneven
contrast observed in X-ray maps of sulfur in heat treated films
indicates that there is an uneven distribution of sulfur and therefore
it may exist as small pockets, instead of a thin evenly distributed
second phase. The presents of this second phase does not effect the
optical properties significantly, but it may be responsible for the
large resistivity observed in high temperature heat treated thin films.
8. The limited study on COSAD films indicates that it is possible to
produce a structure where very fine particles of a semiconductor are
embedded in a glass matrix. By variation of the amount of CdS that is
co-deposited a wide variation in the structure of the film can be
accomplished. High amounts of CdS produce a film which readily phase
separates during examination in the TEM. When this type of film is
heat treated to high temperatures a very distinct two phase structure
results where the CdS is dispersed as a bimodal distribution of
spherical particles. The larger particles have an average size of 500
, and the smaller particles are sized at 200 . When very low
concentrations of CdS are co-deposited, then a structure which does not
phase separate results. After heat treatment, very small particles of
less than 70 result.


18
value ever reported. Additional advantages of bound exciton saturation
are that the state decays by a radiative transition and the lifetime is
on the order of 500 psec. This would make for a very fast, low power
switch. Also, since the excitons are bound to defect sites, there are
no carrier diffusion problems, which in the other three processes tend
to wash out the effect. The one primary disadvantage of utilizing this
process is that the strongest exciton resonance occurs at 2 K. As the
temperature is increased, the transition broadens, thereby requiring a
larger saturation intensity and hence a smaller x\2 is observed. In
addition, since the exciton binding energy is only a few millielectron
volts, the state is thermally annihilated at higher temperatures.
As previously described the process of quantum confinement could
be used to access exciton levels at higher temperatures if the physical
size of the material could be made small enough. Multiple quantum well
structures produce quantum confinement in one direction because the
structure is made up of alternating layers in which the layers act as
infinite potential wells and the layer thickness is smaller than the
exciton radius. Although the exciton is not confined in the other two
directions, the effect is strong enough that nonlinearity can be
observed at room temperature. The effect would be larger if
confinement was made in the other two directions.


84
manipulated later. The BASIC program used to remotely program, run,
and accumulate data from the Lambda 9 is listed in Appendix A.
For the determination of the refractive index a particular
software package available with the AT was found to be very useful.
This software called is Asyst (McMillain Software Co.) and it is a very
powerful mathematical analysis program. To determine the refractive
index, the numbers representing the FECO spectra were imported into
Asyst and stored in a two dimensional array. Using a very short
program, the array was plotted, and the equations for the curves
tangent to the maximum and minimum were fit. Next, arrays were
generated to represent each of the tangent curves. Asyst could then be
used to very easily determine the points where the three arrays
intersected, with an accuracy of 0.1%. The intersection points were
stored in a fourth array. This process was repeated for a 30
incidence spectra. The very long iterative BASIC program that was
originally used to calculate the dispersion equation could be shortened
considerably with utility functions available in Asyst. In fact, by
manipulating the numbers as arrays, the calculation was much simpler
and much faster.
Even though the above process produced an accurate result, a
second technique, ellipsometry, was used to verify these results.
Ellipsometry is based on the change in polarization of a light beam
when it is reflected from a surface. Generally a plane polarized light
beam is made incident on the surface of a material at an angle of 45.
The resulting reflected beam is elliptically polarized and from the


ABSORBANCE
182
Figure 70 Low temperature absorbance spectra of furnace heat treated
(FHT) and rapid thermal anneal (RTA) thin film.


80
m A = 2 n d (4.3)
where m is the order number of the fringe, d is the thickness, and A is
the wavelength. To take advantage of this effect to determine the
index of refraction two assumptions must be made. The first is that
the order number is not a discrete number but that it can vary
continuously. Why this is important will be described shortly. The
second assumption is that the dispersion of the refractive index can be
described by the following equation
n
2 =
+
B
(4.4)
where the square root of B gives the infinite wavelength refractive
index and A and x are constants.
To make use of this effect, a FECO spectrum is obtained at normal
incidence. A typical spectra from a 2 pm CdS film is shown in Figure
17a. Curves are drawn tangent to the maximum and minimum peaks and the
values of the wavelength of the tangent points are determined. The
sample is then rotated in the beam by 30, resulting in an increase in
the optical path length by a factor of l/(cos 30). The assumption
that the order number is a continuous mathematical variable means that
the normal incidence spectrum can be shifted to a shorter wavelength by
increasing each order number by an amount 6m. ^4 This increase produces
a shift in the FECO spectra shown in Figure 17b. Again, tangent curves
are drawn and the tangent points to the interference spectrum are
determined. The difference in the pairs of tangency points are then


148
nearly twice as wide as the peaks seen in Figure 45. Second, the peaks
are shifted to lower binding energy and third, a very definite doublet
peak is observed. The first two differences may be due to charging
effects. Charging was not observed in the Kratos spectrum (Figure 45)
because a low energy electron flood gun was used to compensate the
build-up of static charge. The clear resolution of a doublet that
was not resolved in Figure 45 is, however, an anomalous result. Also,
unexpectedly, the Cd peaks have shifted in the opposite direction after
the sputter etch. This may be due to preferential sputtering of the
sulfur, so that more metallic cadmium is present. It may also be due
to a change in grounding potential, which would effect the static
charge built-up on the sample.
Apparently XPS is not sensitive enough to determine if the small
amount of excess sulfur present in thin films exists as a distributed
second phase. The final technique that was used to determine this
distribution is X-ray mapping. Although this type of X-ray analysis by
EPMA would have been the most quantitative, the probe size used in the
EPMA is not small enough to spatially resolve the grain boundaries of
thin films, which is where the sulfur is most likely to exist. By
using EDS X-ray mapping with the SEM, high spatial resolution could be
obtained. Simultaneous maps of sulfur and cadmium were acquired at a
magnification of 20,000X, which represented about 150 grains. A low
energy electron beam of 10KV was used to reduce to volume of material
sampled and a high density 256 x 256 image, with 8 bit resolution, and
a 200 msec dwell time was used to generate the X-ray maps. Even at
this high magnification, if the cadmium and sulfur were evenly


25
Figure 3 Graphical solution to the Airy function showing the
critical phase shift required for the onset of
bistability.


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MC E )^E
147
XPS high resolution scan of cadmium 3d3 and 3d5 peaks
obtained from Perkin-Elmer instrument. Spectrum are of a
heat treated thin film, as-received and after a one minute
sputter etch.
Figure 49


Ul
igutre 9
fion of the
Variatio^g
authors-
absorption
coefficient
reported


180
listed in Table 6. The absolute values of the peak energies are
slightly different, but the energy difference between the A, B and C
levels is nearly the same in both spectra. The difference in absolute
values may be attributed to the same source which produces the
difference in the absorption band edge, displayed in Figure 68b. The
peaks are much sharper in the reflection spectra because the technique
is very sensitive to changes in reflectivity and also because the
reflection spectrum was taken at 4.2 K. The broader peaks observed
here may be due to temperature broadening, as well as orientational
broadening in the polycrystalline films.
The observation of exciton absorption peaks was made in all films
heat treated to above 500 C. With the large grain growth that occurs
with high temperature furnace heat treatments, it is not surprising
that exciton levels should occur in a polycrystalline thin film,
because even with a grain size of 3000 , the band gap should be well
established. An interesting result however, is indicated by Figure 70
which shows exciton levels to be present in a RTA film in which the
grain size is only 900 . The absorption peaks are less intense and
slightly shifted, but they are definitely present. This indicates that
large grain growth is not required to produce polycrystalline thin
films which display exciton levels. What is needed is only the
annealing of certain defects.
Another interesting observation is the variation of the exciton
peaks with temperature. Figure 71a shows that only a slight shift and
decrease in intensity occurs when the temperature is increased to 40
K, but at 100 K the A exciton peak has become a shoulder, the B and C


32
forming a bound exciton complex, which will have a lower energy than
the corresponding free exciton.
Of all the II-VI semiconductor materials, the exciton states in
bulk CdS have been studied the most and are perhaps the best
understood. Reflection, absorption, and luminescence studies dating
back to the mid-fifties have investigated the exciton states in this
material. Excitation of the states can be accomplished by either
electron bombardment or by photon absorption. When the emission is due
to the latter process it is known as photoluminescence and the results
reported for CdS are detailed below.
Photoluminescence of CdS
When CdS is excited by photons of energy greater than the band gap
the characteristic luminescence which results form the decay of excited
states is shown to consist of two primary emission bands. The first,
known as the "green-edge emission" is due to the edge emission of
various states in the band gap, i.e. shallow donor-acceptor
recombinations. Studies of the edge emission of CdS were made by
Kroger as early as 1940. At a slightly higher energy, a band known
as the "blue-edge emission" occurs and is due to emission from free and
bound exciton complexes.^ A typical low resolution spectrum
displaying these two bands is shown Figure 6, and a high resolution
spectrum of a portion of the blue band is shown in Figure 7. The
intensity of the bands is dependent upon the polarization of the
incident light with respect to the c-axis of the crystal.


50 run
Figure 61 High magnification TEH micrograph of heat treated C0SAD2
thin film showing fine dispersion of microcrystallites.