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Enhanced luminescence from europium doped yttrium oxide thin films grown via pulsed laser deposition

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Enhanced luminescence from europium doped yttrium oxide thin films grown via pulsed laser deposition
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Jones, Sean Liam, 1967-
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Annealing ( jstor )
Cathodoluminescence ( jstor )
Electric potential ( jstor )
Electrons ( jstor )
Europium ( jstor )
Luminescence ( jstor )
Phosphors ( jstor )
Silicon ( jstor )
Thin films ( jstor )
Yttrium ( jstor )
Dissertations, Academic -- Materials Science and Engineering -- UF ( lcsh )
Materials Science and Engineering thesis, Ph.D ( lcsh )
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non-fiction ( marcgt )

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Thesis:
Thesis (Ph.D.)--University of Florida, 1997.
Bibliography:
Includes bibliographical references (leaves 173-181).
General Note:
Typescript.
General Note:
Vita.
Statement of Responsibility:
by Sean Liam Jones.

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ENHANCED LUMINESCENCE FROM EUROPIUM DOPED YTTRIUM OXIDE
THIN FILMS GROWN VIA PULSED LASER DEPOSITION













By

SEAN LIAM JONES


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


1997



























Copyright 1997

by

Sean L. Jones



























To my Grandmother Ethell and Grandfather Russell Hughes, Uncle

Eddie Jones, and best friend Edith Renee Hill who passed while I was on this

part of life's journey, this volume is respectfully dedicated.















ACKNOWLEDGMENTS


I first give honor and praise to God for allowing me to see this day.

Along with the Creator, my family and girlfriend are directly responsible for

this achievement. I especially thank my mother. I thank her for her sacrifices,

devotion, and little nudges to get me through the process. I would also like to

thank Dr. Holloway for giving me the opportunity to contribute. He has been

an untiring source of wisdom and inspiration. In that vain, I would also like to

acknowledge Dr. Singh, who has served as a second advisor. I appreciate all

of the individual time you have spent on this project and on my development. I

would also like to thank the balance of my committee for their service: Dr.

Hummel, Dr. Pearton, and Dr. Anderson.

I am particularly thankful for Dr. Jengyi Yu, the first graduate student I

was paired with upon arrival to the University of Florida. He was my first

mentor and colleague. For that, I will always be grateful to him. I would also

like to recognize Dr. John M. Anderson, Dr. William Edmondson, and Dr. Paul

Mason for their mentoring and friendship. Special recognition belongs to Dr.

Hendrick Swartz, Dr. Mark Davidson and Xiao-Ming Zhang for their support.

The three of them came into my life just as I was giving up hope and preparing









to leave school to begin a new life. Their interest in me and my work kept me

here far longer than I thought possible.

I would also like to thank Dr. Maggie Lambers for her assistance with

SIMS data, Dr. Eric Lambers for his assistance with AES data, Richard Crockett

for his help with SEM micrographs and EDS data, and Dr. Wayne O. Holland at

Oak Ridge National Labs for his assistance in collecting RBS data. In that

venue, I thank all those in Dr. Holloway's group and Dr. Singh's group for their

assistance. I thank Ludie, Paula, Dorothy, Nigel, Wishey, Balu, Brent, Ananth,

Karen, Mark, Steve, Jeff, TJ, Amondrea, Keith, and Michelle for their friendship

over the years. Special thanks go to Ludie for keeping the office running

smoothly and putting up with all of us.

Special acknowledgments go to the McKnight Doctoral family, members

of the Black Graduate Student Organization, National Society of Black

Engineers, Traveler's Rest Baptist Church, Mt. Moriah Baptist Church, and Mt.

Carmel Baptist Church. I am also grateful to the Brothers of Nu Eta Lambda,

Theta Sigma and Pi Alpha chapters of Alpha Phi Alpha Fraternity,

Incorporated.


















TABLE OF CONTENTS






ACKNOW LEDGMENTS ............................................. iv


ABSTRACT ....................................................... viii


CHAPTERS


1. INTRODUCTION .................


2. LITERATURE REVIEW ............


Field Emission Displays ........
Yttrium Oxide:Eu+3 (Y203:Eu) ....
Cathodoluminescence .........
Efficiency of Thin Film Phosphors
Elementary Scattering Theory ...
Luminescent Thin Film Research
Pulsed Laser Deposition (PLD) ...


3. EXPERIMENTAL PROCEDURE .....


Substrate and Target Preparation
Pulsed Laser Deposition(PLD) ...
PLD Film Characterization ......
X-Ray Diffraction(XRD)....


............ ......... .......... 1


. . . . 8


. . . . .. 8
. . . . .. 12
. . . .. .. 1 6
. . . . 19
......... .................. .. 23
. . . .. 2 8
. . . . 3 2


. . . .. .. .. 5 4


. .. . . . 54
. . . . .. 5 5
. .. .. . . 5 7
..... ..... ...... ............ 57


Scanning Electron Microscopy(SEM) ....................... 60
Rutherford Backscattering Spectroscopy(RBS) ............... 60
Atomic Force Microscopy(AFM) ......................... 62
Auger Electron Spectroscopy(AES) ........................ 65
Photoluminescence(PL) .................................. 66
Cathodoluminescence(CL) ............................... 67


4. RESULTS .......................................................76


Photolum inescence(PL) ....................................... 76









Cathodoluminescence(CL) ..................................... 78
Thin Film Grow th ............................................. 80
Surface Roughening .................... ......... .............. 82
Annealing ................................................. 85

5. DISCU SSIO N ................................................... 136

Enhanced Luminescence Through Scattering .................... 136
CIE Coordinates and Reduced Luminescence ................... 139
Thin Film Grow th ............................................ 141
A nnealing ......................................... ....... 144

6. SUMMARY AND CONCLUSIONS ................................ 164

7. FUTURE WORK ................................................ 168

APPENDICES

A CALCULATION FOR FRACTION OF LIGHT EMITTED ............... 171
B ALLOWED (hkl) VALUES FOR X-RAY DIFFRACTION ................ 172

REFERENCES .................................................. 173

BIOGRAPHICAL SKETCH .......................................... 182

























vii















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


ENHANCED LUMINESCENCE FROM EUROPIUM DOPED YTTRIUM OXIDE
THIN FILMS GROWN VIA PULSED LASER DEPOSITION

By

Sean Liam Jones

December 1997


Chairman: Professor Paul H. Holloway
Major Department: Materials Science and Engineering

Yttrium oxide thin films doped with europium were grown on silicon,

Coming 2947 glass, c-axis sapphire and quartz substrates. A pulsed laser

ablation process was used to deposit films onto substrates heated from 250 C

to 800 C and at oxygen pressures between 105 Torr and 600 mTorr. The

deposited films were characterized using x-ray diffraction, secondary electron

microscopy, Auger electron spectroscopy, atomic force microscopy, secondary

ion mass spectroscopy, photoluminescence and cathodoluminescence.

Crystalline as-deposited films were achieved at substrate temperatures

as low as 250 oC. Films grown in high vacuum were strongly (111) textured,

but became more random polycrystalline as the temperature was increased to

800 C and/or growth pressure was increased to 600 mTorr. The root mean









square roughness increased significantly as oxygen pressure increased. Films

grown at 10i5 Torr had RMS roughness values of 2-3 nm, but the roughness

increased to 71 nm for growth at 600 mTorr. Above 200-300 mTorr oxygen, the

film morphology changed from a smooth to a particulate surface. These

particles were 100 15 nm at the transition pressure and decreased to 80 +

15 nm at 600 mTorr. During annealing, the grain size remained well below 200

nm, even after 1400 C for 1 hour.

Photoluminescence and cathodoluminescence intensities increased as a

function of increasing growth temperature and pressure. PL was done using a

UV lamp with a wavelength of 254 nm as the excitation source. A PR 650

camera with an internal calibrated intensity standard was used as the

detector. The PL intensity increased from 0.093 fL for 1 pm thick films grown

at 10-5 Torr to 0.81 fL for films grown at 600 mTorr. The powder gave 11 fL.

The PL response from the film increased to 4 fL (36% that of the powder) for a

the thickness of 9 jm.

Cathodoluminescence was done using a Kimball Physics EFG7 electron

gun and an Ocean Optics S2000 fiber optic as the detector. Films were not

cathodoluminescent until growth and annealing at 800 oC or higher. The best

brightnesses were observed at temperatures > 1170 oC. This temperature

corresponds to 0.54 of the melting temperature for yttria. At 2 kV and 1

pA/cm2, 3 pm thick films grown at 100 mTorr (RMS of 3 nm) had CL efficiencies

of 0.85 Im/W, at 600 mTorr (RMS of 71 nm) had efficiencies of 2.63 lm/W, while









the powder had an efficiency of 9.5 Im/W. The CL efficiency for a 9 pm thick

rough film was 3.3 Im/W at 2 kV and 1 pA/cm2.

The CIE color chromaticity coordinates (x, y) of the films were different

from the powder {(x, y) of (0.62,0.35) for film instead of (0.644, 0.35) for

powders}, indicating that the concentration of active europium was lower in

the films, by as much as one mole percent. This contributed significantly to

lower CL efficiencies for the films. This was confirmed with RBS.

Improvements in luminescence with an increase in surface roughness

were attributed to forward light scattering caused by the rough surface. The

increase in light scattered at the surface was presumed to reduce the

percentage of light internally reflected within the film. The appropriate

scattering model was anomalous diffraction. Although the 3 pm thick rough

film was only 27.7% as efficient as the powder, anomalous diffraction theory

predicts that increasing the roughness to 200 nm would yield an efficiency of

12.7 Im/W (134% that of powder).















CHAPTER 1

INTRODUCTION


Much of the interest in field emission displays (FEDs) is due to the large

number of applications that would benefit from their realization. These flat

panel displays (FPD) are thinner, lighter, and consume less power than

conventional displays, such as the cathode-ray tube (CRT). Since the FED will

operate between 100 V and 5 kV, be significantly thinner and lighter, it is the

prime choice for notebook computers and 'thin TVs' over CRTs. See figure 1-1.

Field emission displays also provide significant improvements over the more

common of the FPD technologies, liquid crystal displays (LCDs).[Cas92] Table

1-1 shows a comparison of LCD versus FED technology.[Hol96] The improved

brightness, power efficiency, viewing angle and temperature range make an

FED the choice for laptop computers. For applications such as military and

medical, the reduced weight, lower power consumption, and wider operating

temperature range give FEDs a further advantage over LCDs.

Economic incentives have also fueled increased research interest into

developing this technology. It is estimated that the FPD industry will be a $20

billion market by the year 2000.[DeJ97] Table 1-2 illustrates the market share

for the various display technologies, where the price per display is given in the











parenthesis. As noted, the anticipated unit price for an FED is considerably

lower than that for LCDs or electroluminescent displays (ELD), the main

competing technologies. [DeJ97] The FED industry could be a half billion dollar

business by the year 2003 by filling niche markets alone.

The research presented in this thesis is primarily for advancing FED

technology. Current FED phosphor powder research is concerned with

increasing the efficiency and lifetime while reducing particle size. Reducing

the particle size allows for smaller pixels, yielding higher resolution. As the

optimized CRT phosphor powder particles are reduced in size, however, the

efficiency decreases significantly.

An alternative phosphor configuration, thin films, was studied in this

research. By overcoming some of the fundamental limitations of thin films,

several advantages would be realized. Table 1-3 lists the advantages and

disadvantages of powders versus thin films.[Fel57a] The intent of this

reserach was to show that modifing the surface could increase the efficiency of

thin film phosphors.

In this dissertation, the literature is reviewed in chapter 2 to provide a

background for this work. A brief overview of FED technology, an outline as to

why thin films are less efficient, a review of applicable scattering theory, and a

review of the pulsed laser ablation technique are presented. Past phosphor

thin film research is reviewed to place the gains achieved from this study into

proper perspective. The experimental procedures used to deposit the Y203:Eu









3

thin films are described in chapter 3, along with a variety of techniques for thin

film characterization. A brief outline of each technique is given with emphasis

placed on photoluminescence and cathodoluminescence. Results are

presented in chapter 4. A discussion of results follows in chapter 5 with an

emphasis on crystal quality and luminescence. Conclusions from this work are

presented in chapter 6, and future studies in chapter 7.











Table 1-1 Comparison of FED and LCD Technologies


CHARACTERISTIC FED (target) TFT-LCD

Thickness 6-10 mm 23 mm

Weight <0.2 kg 0.33 kg

Contrast ratio > 100:1 100:1

Viewing angle 160 V, 1600 H +600 V, +900 H

Max. brightness >200 cd/m2 60 cd/m2

Power @ 60 cd/m2 <1W 4 W

Max. Temperature -50 to + 80 C 0 to 50 C
[Hol96]












Table 1-2 Market Share information for Flat Panel Displays


Display 1997 1998 1999 2000 2001 2002 2003
LCD-TFT $8407 ($316)" $9917 ($306) $11743 ($298) $14317 ($296) $16227 ($282) $17905 ($264) $19165 ($242)
EL $115 ($442) $124 ($449) $135 ($464) $152 ($483) $168 ($494) $187 ($507) $207 ($517)
Plasma $325 ($1014) $416 ($1143) $665 ($1387) $964 ($1530) $1466 ($1640) $2085 ($1641) $2357 ($1548)
FED $1 ($169) $5 ($144) $35 ($165) $94 ($173) $219 ($167) $333 ($143) $444 ($132)
LEDb $542 ($3) $552 ($3) $567 ($3) $572 ($3) $586 ($3) $601 ($3) $617 ($3)
Vacuumb $752 ($5) $769 ($5) $791 ($5) $806 ($5) $815 ($5) $821 ($5) $826 ($5)
a Estimated sales revenue in millions of dollars. (Unit selling price in dollars.)
b These displays are alpha-numeric displays which are extremely small.
[DeJ971











Table 1-3 Advantages and Disadvantages of Thin Film
versus Powder Phosphor


THIN FILM PHOSPHOR
PHOSPHOR POWDER

Efficiency poor excellent

Resolution < < 1pm 5 prm to 10 Qpm

Screen Contrast excellent good

Lifetime(at high pA/cm2) good poor

Mechanical Stability excellent good

Thermal Stability excellent good
[Fel57a]








































Light
Emission
Glass Phosphor/Anode





Electrons-- :: i


Vacuum --- .







Base Plate /


Gate
1-isulalor


Figure 1-1. Spindt cathode type field emission display (FED). The Spindt type
is the most common FED structure.















CHAPTER 2

LITERATURE REVIEW




Field Emission Displays



Flat panel displays (FPDs) may be divided into two classes, emissive

and non-emissive.[Tan85] Non-emissive displays do not emit or radiate light,

but act as light valves for back lighting sources. In emissive displays,

however, luminescent materials are excited by an energy source and

subsequently radiate light. These luminescent materials are called phosphors.

Field emission displays operate on the same principles as conventional cathode

ray tubes (CRTs).

In a CRT, a triode e-gun cathode is used to generate and accelerate

electrons toward a phosphor screen.[Cas92] The cathode beam is scanned

across and rastered down the display illuminating one red, green, or blue

(RGB) phosphor pixel at a time. Scanning and rastering are accomplished by

deflecting the beam with magnetic coils. The scanning and rastering

requirement primarily accounts for the depth and bulk of CRTs.












In an FED, scanning and rastering are not used. Instead, an array of

field emission tips generate the electrons. Refer to figure 1-1. Upon applying a

very low voltage across the gate, electrons are emitted from the cone shaped

tips and are accelerated toward the phosphor screen. When the electrons

strike the phosphor screen, cathodoluminescence occurs.

Cathodoluminescence (CL) is the emission of light upon electron radiation.

These FED tips can be turned on one row of pixels at a time, eliminating

the need to deflect the electron beam. This eliminates the path needed for

electron deflection, therefore, the FED is a flat panel display technology.

Different geometries of field emission displays are shown in figures 1-1 and 2-

1, where the tips in figure 1-1 are referred to as a Spindt cathode, the more

common type. The second type is an edge emitter which offers another

configuration where the whole device is on one substrate. For both, the

spacing between the base and the face plate is on the order of a 1 to 3

millimeters. The whole display, including packaging, is thinner than 5

centimeters.

Charles A. Spindt developed field emitter tip technology in 1973 at SRI

International using VLSI processing.[Mat86] The emitter tips are metals such

as W and Mo, or semiconductors such as Si and diamond. When a voltage is

applied between the tip and the gate, Fowler-Nordheim tunneling [Hud92]

occurs and electrons are accelerated toward an anode. These tips emit

electrons above 20 V compared to etched wire emitters which require between











1 to 3 kV.[Adl91, Spi76] Low voltage operation yields stable, continuous

emission with long tip life. Field emission current density, J [A/cm2], due to

Fowler-Nordheim tunneling is described by Brodie and Spindt:


E2 (3/2
J = 1.54 x 10-6 exp(-6.87 x 107- v(y))
t 2(y) E



where E is the applied electric field [V/cm], (0 is the work function [eV], t2(y) is

approximately 1.1, and y is the Schottky image charge lowering contribution to

the work function.[Bro92] The variable t2(y) is explained in detail elsewhere.

[Adl91,Cut93]. The variables v(y) and y are equal to


v(y) = 0.95 y2


and


E 1/2
y = 3.79 x 10-




Figure 2-2 shows J as a function of electric field for varying work functions.

Even with low voltages acting over closely spaced electrodes, current

densities greater than 106 A/cm2 are possible.

Semiconductor field emission tips are degenerately doped (= 1019 cm-3)

so that the Fermi level is just above the bottom of the conduction band. In

such a case, the work function term in the Fowler-Nordheim equation is











replaced with the semiconductor's electron affinity, X.[Bro92] The electron

affinity is the energy needed to raise an electron from the bottom of the

conduction band to the vacuum level.

Utsumi [Uts91 showed that the tip shape is also critical in determining

the turn-on voltage and field emission current density. He concluded that

round whiskers were closest to ideal emitters. Cutler [Cut93] reported that

higher current densities were possible by reducing the tip radius, as seen in

figure 2-3.

The other half of the display is the phosphor screen. Phosphor powder

can be applied to glass plates using electrophoretic, dusting, or slurry

methods.[Has90] The slurry method is the most common where phosphor

powder is mixed with photosensitive chemicals and deposited using

photolithography techniques. For operating voltages greater than 2 kV, the

screen is back coated with a thin layer of aluminum as a reflector and charge

dissipater.[Lev68] At operating voltages below 2 kV, the electron penetration

depth is so small that the screens are left un-coated.[Bec96] In this study, the

starting phosphor powder was yttrium oxide powder doped with four and a

half weight percent of europium obtained from Osram Sylvania.











Yttrium Oxide: Eu3 (Y,2Q3:Eu



Yttrium oxide (Y203), commonly referred to as yttrium sesquioxide or

yttria, is a highly refractory oxide with a melting point of 2410 oC. At room

temperature yttria is a body centered cubic with the bixbyite or cubic-C

structure with a lattice parameter of 10.6 A.[Wyc64] There is an allotropic

phase transformation to hexagonal at 2367 oC.[Tro91] Yttria has very good

chemical stability, high resistivity and a high dielectric breakdown strength. It

has a measured room temperature bandgap of 5.3-5.5 eV [Jol90] and a

bandgap of 5.8 eV at 10 oK.[Tom86]

Yttria has a theoretical density of 5.033 g/cm3. There are 16 molecules of

80 atoms in its unit cell. The cation occupies two different crystallographic

sites, S6 and C2, as seen in figure 2-4. There is a full layer of C2 sites with a

layer of alternating S6 and C2 sites, resulting in 75% of the yttriums in C2

symmetry and 25% in S6 symmetry. In S6 symmetry, the yttrium is in contact

with body diagonal vacancies, but in contact with face diagonal vacancies in

C2 symmetry. In both symmetries, the cation is surrounded by 6

oxygens.[Mae92] These sites are, therefore, chemically equivalent but

crystallographically different. S, symmetry is also denoted in the literature as

C3i symmetry.

Yttria is a perfect host for the trivalent rare earth europium to make

highly efficient lamp and cathodoluminescent phosphors. Europium belongs to











the Lanthanide series which is characterized by an unfilled 4f electron shell

surrounded by filled 6s and 5p shells. Europium can have either a Eu+2 or a

Eu3 valence where the electron configurations are (Kr core) 4d1'4f65s25p65dl

and (Kr core) 4dl'4f65s25p65do, respectively. See figure 2-5. Note that due to

the electron shell filling rules, the 4f level is partially filled until the 5d shell has

been completely filled. In yttria, Eu substitutes for the Y in both the S6 and C2

sites.

For the Lanthanide series rare earths, the deep lying 4f electrons are

shielded from the effects of the host lattice by the 6s and 5p shells. This gives

rise to a number of discrete energy levels that, due to the shielding, resemble

the energy level diagram of a free ion.[Bla94] Since the excitation and emission

occur within the discrete levels of the 4f shell, a characteristic sharp line

emission 5D, to 7Fj results. See figures 2-6 and 2-7.

The allowed transitions (5Dj to 7'F) from the activator, Eu3, are a

function of the selection rules. [Hen89] Three types of transitions are possible:

electric-dipole (ED) transitions, magnetic dipole (MD), and electric quadrapole

(EQ) transitions. The strength of these transitions decreases from ED to MD to

EQ as the ratio 1:105:10-6. The weaker MD and EQ transitions are usually

allowed, so we are concerned with the selection rules governing ED

transitions. Laporte's rule states that electric dipole transitions within the 4f

level are forbidden due to unchanged parity. Parity refers to the state of the

orbital angular momentum quantum number, 1, which can be even or odd.











Since the 4f to 4f transitions occur within the same quantum number, the

parity does not change and are, therefore, not allowed.

Laporte's parity rule, however, can be lifted if there is mixing of parity

from the lattice.[Bla79] When the luminescent center is in a crystal site that

does not have inversion symmetry, there will be a mixing of parties and

LaPorte's rule is lifted. Lifting of the parity rule can result in a transition. This

is called a forced transition. If the site retains inversion symmetry, Laporte's

rule is upheld and no luminescence is observed from the ion. In yttria,

europium occupies both C2 and S6 sites. The S6 site has inversion symmetry

but the C2 site does not. This yields both an ED and MD transition where the

MD originates from the S6 and C2 sites and a forced ED transition originates

only from C2 sites. The primary emission peak at 611 nm for YO3:Eu+3

originates from forced ED transitions from Eu in C2 cation sites because of

inversion symmetry. [For69]

The specific levels from which these transitions occur are also controlled

by the selection rules. The Jodd-Ofelt's theory adds that the allowed ED (5Dj to

7Fj) transitions must adhere to the following:

1) AJ < 6,

and for an f shell filled with an even number of electrons:

2) when J = 0 then J'= 0 is forbidden,

3) if J = 0 then odd J' values are weak (MD),and

4) if J = 0 then J' = 2,4,6 are strong.[Hen89]











In this case, europium has 6 electrons in its 4f shell. According to the Jodd-

Ofelt theory, a strong 5Do-7F2 ED transition is expected to be accompanied by

weaker Do0-7F1 MD transitions. This explains the characteristic sharp line

emission peak at 611 nm (5D0-7F2) from Y203:Eu+3 seen in figure 2-7. Blasse

[Bla79] concludes that orange-red (D0o-7F,) emission results from europium ions

that occupy centers of inversion symmetry, and Eu+3 in sites that lack

symmetry emit in the red (5D0-7F2) and infra-red (5D0o-F4).

The site symmetry and site coordination will determine the number of

primary peaks seen for these transitions. For J values equal to 0, splitting of

levels is not allowed. For J> 0, triple degeneracy is possible and splitting can

occur. The field symmetry also determines the amount of allowed splitting.

Cubic symmetry does not cause splitting but tetragonal and trigonal fields

cause splitting into 2 levels. Lower symmetry sites cause splitting into three

levels yielding three peaks.[Bla79] Since the emission for Y203:Eu originates

from a 5Dj= level, only one peak is allowed. And since the Eu predominantly

sits in a cubic symmetry site, the 7F, level is not degenerate and no splitting

occurs. Thus only one primary 5D0o-F2 peak is seen.

The selection rules also help determine the speed of these transitions.

The Eu3 has an excited state lifetime of approximately 10-3 seconds which is =

105 times longer than the lifetime for allowed electric dipole transitions. This

difference in lifetimes illustrates the degree to which these forced ED (4f to 4f)











transitions are forbidden.[Bla79] Details of this transition via

cathodoluminescence are given below.



Cathodoluminescence



The term phosphor is reserved for materials that display

phosphorescence and/or fluorescence when excited with an energy source.

Phosphorescence is the non-thermal radiation that persists after the excitation

event has ceased. Fluorescence refers to non-thermal radiation given off while

the incident energy impinges the sample. Radiation that does not terminate

within 10 nanoseconds of energy excitation is classified as phosphorescence.

The incident energy can be in the form of X-rays, UV photons, energetic ions,

electrons, or result from mechanical disruption.[Mar88, Lev68] The types of

luminescence are chemiluminescence which is initiated by a chemical

reaction, photoluminescence obtained from photons, roentgenoluminescence

from X-rays, ion luminescence from energetic ions, electroluminescence

induced by an electric field or current, incandescence resulting from high

temperatures, and cathodoluminescence resulting from electrons.[Lev68]

Cathodoluminescence contains components from both phosphorescence and

fluorescence. Figure 2-8 and 2-9 depict qualitatively the interactions that

occur after an electron impinges on a surface. As seen in figure 2-8, there are a

number of physical processes that result from the primary beam interaction









17

with the solid. The depth that an energetic electron will penetrate into a solid

is dependent on the voltage. Figure 2-9 shows the excitation volume shape

and depth as a function of voltage. The number of luminescent centers excited

is controlled by the depth of the incident electrons and follows Terril's formula

for penetration depth:


x = 2.5 x 10 12p 1V2 [cm


where p is the material's density and Vo is the voltage.[Fel60] This equation is

valid for 1 to 10 kV. Figure 2-10 shows that for 10 kV electrons incident on

yttria the range is 500 nm, but is reduced to 125 nm for 5 kV electrons. The

observed luminescent area and intensity will be a function of the beam energy,

electron-hole pair generation and diffusion, subsequent emission processes,

competing absorption processes, and in the case of thin films, the amount of

internal reflection of photons.[Gol66]

The process by which the electrons travel to the rare earth activators to

excite them is different from the classical donor and acceptor model. Process 7

of figure 2-11 depicts the excitation and deexcitation from an impurity with

incomplete inner shells like rare earths and transition metals. [Yac92j

Processes 4,5, and 6 are transitions expected from classical donors and

acceptors impurities that establish localized states in the band gap.

The mechanism by which the electron and holes travel to the activator is

explained by Ozawa.[Oza71] Cathodoluminescence in phosphors is initiated by









18

1 of 3 mechanisms, (i) absorption from energy transferred from the host to the

activator, (ii) direct absorption by the activator ion from the incident energy,

and (iii) indirect excitation by the absorption of energy by recombination of

mobile electrons and holes, where processes (ii) and (iii) dominate. By using

concentration dependence (CD) curves (figure 2-12), the method of transfer

may be revealed. Phosphors whose CD curves contain only one inflection for

concentration quenching are excited directly by the electron beam (figure 2-

12b, Y203:Pr). CD curves that have two inflection points (figure 2-12a and b)

Y202S:Pr) are from phosphors that are excited indirectly by mobile electron-

hole pairs (EHs). In the case of indirectly excited phosphors, the first inflection

in the CD curve corresponds to the luminescence from electron-hole pairs that

have diffused beyond the primary beam range. Note that all of these phosphors

display direct excitation CD curves under photoluminescence.IOza90]

The process of cathodoluminesce by indirect excitation through

recombination of mobile carriers is as follows for Y203:Eu3.[Oza68, Die68]

When the cathode ray impinges the host lattice, electron-hole pairs are

created. The electrons and holes are free to migrate within the crystal. The

migration distance is given by


L = dC1


where L is the migration distance or the distance an electron or hole must

travel from the host ion to an activator, d is the distance between cations and











C is the average number of activator ions to cations. [Oza90] The model

assumes that the lattice ions do not act as recombination sites for the EHs and

that recombination only occurs at the activator ion. The incident electron

coverts the yttrium, y3+, to y2+. The divalent yttrium quickly releases the

electron to the next yttrium, converting back to y3. The model also assumes

that conversion of Y2+ to yl is not possible. The electron migrates along the

y3+s until it reaches an activator, Eu3+ in this case. The europium, having

trapped the electron, is converted to Eu2+, which results in a local field that is

negatively charged. The holes, which are transported on the 02- converting

them to 01', are attracted to this negative charge region. Once the hole is

captured by the Eu2+, the europium goes into the excited Eu3+* state. It relaxes

and returns to the ground state (Eu3+) emitting a photon with a characteristic

wavelength. This process can be repeated continuously, limited only by the

capture and decay time of the Eu3+* state.



Efficiency of Thin Film Phosphors



Before discussing previous research done on thin film phosphors, a

discussion as to why thin film brightness may be lower than powders is in

order. It has been generally accepted that thin films will always be less

efficient than powders, because it is estimated that 80 to 90 % of the light

generated within the film is lost in frustrated internal reflections.











The phosphor quantum efficiency, 7le, under electron beam

bombardment may be expressed according to Robertson and van Tol:


(1 -rb)hvem
leff = %t~alesc
gEg



where rb is the fraction of electrons backscattered, hvem is the average energy

of the emitted photons, and rt, r1a, and rsc correspond to the efficiency of

transferring the generated electron-hole pairs to the luminescent center, the

efficiency of radiative recombination at the luminescent center, and the

fraction of light escaping the sample, respectively. [Rob80] The minimum

energy needed to create an electron-hole pair is represented by the constant,

P, times the band gap,Eg. The constant P varies from 2 to 8 [Has90, Bec96], but

is usually set equal to 2.8 [Ait93]. For phosphor powders, it is assumed that all

of the luminescent centers are active, in the proper crystallographic sites, and

that once the light is generated within the powder, there are enough

reflections that all of the light escapes with minimum absorption. Thus T, and

r1,sc are equal to one.[Bec96]

For thin film phosphors, however, these assumptions may not be valid.

For poor crystalline quality films, the transfer of electron-hole pairs to

luminescent centers and the probability of radiative recombination at the

luminescent center will be lower than unity. The overall efficiency is further

reduced by a low value of .,nc due to internal reflections (figure 2-13). For thin












films with good crystallinity and optimized activator, internal reflection and

absorption is a major loss factor as explained by the following.

Light impinging on an interface between two different media with

different optical indices, will be reflected and refracted. Specifically, when

light traveling at some angle 08 reaches the interface, the wave is refracted

according to Snell's law


n-sin06 = n2sin02



where n1 and n2 are the indexes of refraction for each media and the 01 is the

angle of the incidence light and 02 is the angle at which the light refracts.

When medium 1 is more dense than medium 2 (n, > n2), an angle exist where

above its value, incident light is not refracted but is totally reflected back into

the film. This angle is called the critical angle for total internal reflection, 0c,

and is equal to


O6 = sin-( n
phosphor



where n, and rphosphor are the index of refraction for air and the film,

respectively. See figure 2-13, where ray c is incident at 6, and ray d is incident

at an angle > O0. Air has an n value of 1. Yttria has an index of refraction (at

611 nm) value of 1.93, yielding a 0, of 310. Figure 2-14 shows the change in the

critical angle as (l/nphosphor) is varied.









22

From the critical angle, a cone of emission forms where the volume of the

cone is a function of the critical angle. The fraction of light, f, that will escape

the thin film through this cone is approximated geometrically by


(1-cosoe) 1
2 2
4phosphor


as is detailed in Appendix A.[Car66] If emission is measured from only one

surface and if light propagating in the other direction is not reflected but is

lost, only 7% of the light would be transmitted out of yttria thin films. The light

impinging within this projection cone, is further subjected to the standard

rules for reflection and transmission across the interface. The percentage of

the light within the cone that will be transmitted, T, for a dielectric thin film is

given by


T phosphor
(nphosphor +1)2



and for yttria, approximately 10% will be reflected and not transmitted,

resulting in only 6.3% of the light leaving the thin film. Even if backside

reflections were included, the maximum output would be 13%.

This is analogous to waveguiding in fiber optic systems, where the

cladding layer has an index of refraction which keeps the light trapped within

the fiber. In thin film phosphors, this waveguiding effect is termed "light









23

piping". It can be seen from this discussion that perceived brightness can be

severely reduced in thin film phosphors, leading to lower efficiencies.



Elementary Scattering Theory



It will be shown in the following chapters that roughening the surface of a

dielectric thin film phosphor increases the photoluminescence (PL) and

cathodoluminescence (CL) brightness. Since this increase was attributed to

scattering at the film/air interface, an outline of scattering and how it pertains

to particles and films is presented. Although there is an abundance of

scattering theories, ranging from general to case specific solutions, the

discussion below is limited to solutions for nonconducting, non-absorbing,

relatively small particles with refractive indices close to 1. A brief overview of

applicable scattering theory is presented below.

It is generally accepted that scattering refers to the movement of a wave

due to the presence of an obstacle of a given size, form and composition.

[Van81] More specifically, the term is most applicable when the 'obstacle' has

a size smaller than the incident wavelength, A. The 'movement' due to an

obstacle having a size much greater than X. is referred to as diffraction,

reflection, or refraction.[Van81, Jen76j

For spherical particles, scattering occurs when the radius, a, is much

smaller than the wavelength, but geometric optics apply when a > X. Thus,












the first important parameter for scattering due to spherical particles is the

particle size. The size parameters that describe this scattering are x, and p,

given by the following equations:


2xa
X-


and


p = 2xIn- ,



where a and n are the radius and the index of refraction of the sphere,

respectively.[Van81] For the case of nonconducting, non-absorbing, relatively

small spheres (small x and p) with refractive indices close to 1, the following

scattering conditions apply; Rayleigh scattering, Rayleigh-Gans scattering,

intermediate scattering, anomalous diffraction or the limiting case of geometric

optics where a>A. The x and p requirements to meet each condition are

presented below with their effect on scattering.

The most well known of scattering theories is Rayleigh scattering,

developed by Lord Rayleigh in 1881.[Str64] Rayleigh scattering occurs when,

(a) the external field can be considered as a homogenous field across the

particle, mathematically expressed as


a ( ; xc( 1,
27T











and (b) the particle size and index of refraction are smaller than the

wavelength inside the particle, A/n, which is mathematically expressed as


In a (( ; nx 1 .
2t



A particle size of less than X/20 can be used as a guide.[Van81] Thus, when

the x and n*x values are much less than 1, Rayleigh scattering yields a

scattering efficiency, Q.ca, equal to:


8 x4m21 2
Osa XI 2'
3 m2+1



showing the well known (a/A)4 dependence, since x is equal to (27a/A).[Str64]

This is the Rayleigh scattering formula for nonconducting, non-absorbing

spheres as opposed to the more common formula expressed for n- 1. The term

Q.ca is the cross-section for scattering, Caa, normalized by the particle's area

projected perpendicular to the incident beam.[Boh95]

As the sphere size increases, the scattering condition moves from

Rayleigh to Rayleigh-Gans scattering [Van81], where the two main

assumptions are that (a) the index of refraction is close to 1 and (b) the

allowable phase shift is small such that


a <
4i ln-ll









26

The difference between Rayleigh and Rayleigh-Gans scattering is that a larger

phase shift is allowed. The scattering efficiency, OQa, is expressed as an

integral that is bound by two limiting cases of x < < 1 and when x > >1, where

the efficiency is expressed as


Osa = In-1 12(x).


The function *(x) is an integral equal to


5 sin4x 7( 1
r(x) + 2x (1-cos4x) +( 2)[0.577 + log4x Ci(4x)]
2 4x 16x2 2x2



where Ci(4x) is a cosine integral bound by x and c. When x < <1, the Rayleigh

scattering condition is obtained. As x becomes large and x(n-1) remains small,

the scattering changes from a (a/I)4 to a (a/A)2 dependence and is expressed as


sca = 2(n-1)2x2 = 0.5p2


This is the beginning of the intermediate scattering condition. Rayleigh-Gans

scattering can be thought of as being bound by the two extremes, Rayleigh

scattering and intermediate scattering. As seen by the formula, intermediate

scattering has a (a/A)2 dependence.

As x or n are increased further, the scattering condition changes again to

anomalous diffraction where the scattering efficiency is expressed in terms of a

sine integral:












4 4
sca = 2 sinp + -(1-cosp) .
P P2



Here, the assumptions are that x and p are large, where large is any value

close to or greater than 1.[Van81] Anomalous diffraction, just as Rayleigh

scattering, can also be used with large n values. As radius, a, becomes larger

than the wavelength of light, anomalous diffraction moves into geometric

optics, where the classical diffraction, reflection, and refraction rules are

appropriate. (Jen76]

In summary, scattering theory can be applied to particles, but the particle

size, wavelength of incident light and refractive index determine the type of

scattering which occurs. For extremely small particles, Rayleigh scattering

dominates, having a (a/I)4 dependence. As the particle size and index are

increased, Rayleigh-Gans scattering applies, having a dependence ranging

from (a/3)4 at low x to (a/A)2 at larger x. Once the x and x(n-1) values reach

unity, or larger, scattering is described by anomalous diffraction with an

integral that leads to between (a/A)2 and a (a/A)3 dependence for small and

large radius, respectively. For particle radii greater than the wavelength of

light, geometric optics, such as ray tracing, should be used.











Luminescent Thin Film Research



Williams [Wil47] was the first to publish data on the deposition of thin

films for cathodoluminescence phosphors. It was quickly realized that thin film

phosphors would provide advantages over phosphor powders, namely that

screens would be transparent and not opaque or white, improved resolution

would be easily obtained, the thickness could be optimized versus operating

voltages, and the thermal and electrical conductivities could be higher for

continuous thin films. Refer to table 1-3. Recently, additional research has

been reported on cathodoluminescent thin films whose efficiencies are shown

in table 2-1.

The majority of studies of thin film luminescence have focused on

electroluminescence (EL) and photoluminescence (PL) in thin films.

Cathodoluminescence data have predominantly been collected from powders,

due to the severe reduction of luminescence brightness observed with thin

films versus powders.[Gol66] Much of the early thin film research worked on

exploiting the advantages of thin film structures, such as fabricating penetron

devices.[Fel57a] Penetrons are layered phosphor films that change emission

color as a function of beam voltage due to change in penetration depth. (See

figures 2-8 & 2-9.)

Current research has focused on the growth of high quality thin films

and the quantification of thin film brightness. A variety of growth techniques









29

have been employed, including evaporation, spray pyrolysis, sputtering, metal

organic chemical vapor deposition and more recently pulsed laser deposition.

Williams [Wil47] employed an evaporation technique to deposit Mn doped zinc

fluoride phosphors. The thin films were grown at 100 C and were luminescent

as-deposited. However, these films degraded and decomposed readily under

electron beam bombardment. Studer et al. produced ZnS:Mn [Stu51,Cus52]

and ZnS:As,P [Stu55] using a chemical vapor deposition technique. They

achieved white brightnesses of 20% that of the powder with the ZnS:As,P

phosphor thin films. The films displayed an efficiency of 1.08 lm/W at 10 kV

and 1 mA/cm2. With the use of an integrating sphere, they also showed that

the lower brightness could not be attributed to internal reflections alone, but

lower crystallinity or the presence of defects reduced the efficiency. They

subsequently showed that thin film phosphors exhibited high spatial

resolution due to less light scattering.[Stu55,Stu56] Feldman et al. [Fel57b]

deposited several phosphors via vacuum evaporation followed by high

temperature vacuum anneals. They found low brightnesses for transparent

ZnS:Mn films and a 2.5x improvement using a frosted substrate. Their high

temperature, long time anneals turned the transparent films 'foggy' and

opaque. These 'fogged' films were 90% the brightness of the powder. They

deduced that the films were rough since they displayed the characteristic

opaque color resulting from severe diffuse reflectance. They also showed that

oxide phosphors like (Zn2PO4)3, CaWO4:W, and ZnSiO4:Mn could be formed









30
using vacuum evaporation. Koller et al [Ko160] evaporated ZnS:Mn in a H2S and

HC1 background gas. This helped to maintain stoichiometry and to incorporate

Cl into the film. They reported that while their films were crystalline as-

deposited at 100 C, they did not luminescence until heat treated at 650 C.

They also produced Zn2SiO4:Mn thin films by evaporating ZnF2:Mn onto heated

quartz (550 650 oC) substrates. Bateman [Bat60] also evaporated ZnS:Mn thin

films and a MgF2:Zn,Mn phosphor thin film. The MgF, phosphor performed as

well as the ZnS:Mn thin film.

Kirk et al. [Kir61] used spray pyrolysis to react ZnCl with a heated silica

substrate to make ZnSi2,4 doped with Mn and Ti thin films. They achieved

thin film efficiencies of 0.24 Im/W as-deposited and 0.32 Im/W for films

annealed at 1250 C for 1 hr. Their beam conditions were 7.5 kV and 10

mA/cm2. Spray pyrolysis was later used by Falcony [Fal92] to deposit ZnS:Mn

films from which PL data were reported. Luminescence was achieved with

substrate temperatures as low as 360 C. With PL, they showed the reduction

of a self-activated defects as the growth temperature was increased.

Hansen et al. [Han65] were the first to use electron beam evaporation to

deposit phosphor thin films. Yttrium oxide doped with various rare earths was

deposited onto heated molybdenum strips. They achieved 2.7% of the

brightness of the standard powder for europium doped films, but 41% of the

powder brightness for Tb doped. No explanation for the differences was given.











Dalacu et al [Dal92] later used electron beam evaporation to deposit

MgAl204:Mn phosphor thin films. No brightness data were given.

Sputtering has also been employed to deposit cathodoluminescent

phosphor thin films. Maple et al [Map73] deposited La2,OS doped with Eu+3

(red) and Tb+3 (green) and obtained the highest reported efficiencies of 7.37

Im/W at 1 W/cm2 after a 1000 C anneal in an H2-SO2 atmosphere. They also

showed that beveled edges on 20 pm x 20 pm segments improved the

efficiency by a factor of 3. Sella et al [Sel82] reported the deposition of

amorphous Y202S and La202S thin films doped with Eu+3 and Thb3. The films

were annealed to 600 to 850 C to get good luminescence. Like Feldman, they

reported an increase in brightness of 4x by using ground substrates over

smooth ones. They also employed laser annealing via a pulsed CO2 laser that

showed a 3x improvement over furnace annealing. Bondar [Bon95] grew

Y203:Eu and several oxysulphide:RE films using electron beam evaporation, RF

magnetron sputtering, and a RF ion-plasma diode sputter deposition

technique. The RF ion-plasma technique yielded superior results. No

luminescence was reported for the films grown using electron beam

evaporation. They achieved an efficiency of 1.4 Im/W and 3.06 Im/W at 8 kV

and 1 pA/cm2 for r.f. sputtered Y203:Eu and Y202S:Eu films, respectively. Hsieh

et al [Hsi94] employed rf magnetron sputtering to deposit ZnGa2O4:Ga. Ouyng

et al [Ouy95] also used rf sputtering to deposit Y203-SiO2:Eu thin films. Films









32

were grown at 300-450 C and annealed around 1000 C for 2 hours. Brightness

values of 10-20 % that of the powder were reported.

Liquid phase epitaxy and low pressure metal-organic chemical vapor

deposition were utilized by Robertson et al and West et al. Robertson et al

[Rob80] deposited Y3AlsO2:Ce thin films. West et al. [Wes90] did not report

efficiency values for their Y203:Eu films, but showed that there was an effect of

deposition parameters on the amount of active Eu in the S6 vs C, sites. This

was done by following the 611 nm peak (5Do 7F2 transition) versus the 533 nm

peak (DI 7F1 transition).

Rao [Rao96] utilized sol-gel processing to deposit Y203:Eu thin films.

Films were amorphous as-deposited at a substrate temperature of 400 C. The

films became crystalline and luminescent after a 600 C, 2 hour anneal. The

characteristic polycrystalline cubic x-ray diffraction pattern was not achieved

until annealing temperatures of 800 C. Like West et al., they showed that the

crystallographic occupation of europium was influenced significantly by

processing temperature.



Pulsed Laser Deposition (PLD)



More recently, pulsed laser deposition (PLD) has been used to deposit

phosphor films. McLaughlin et al. [McL93] deposited ZnS:Mn thin films by

PLD. Greer et al. [Gre94a] used PLD to deposit yttrium aluminum garnet











(YAG) doped with various rare earth dopants. They reported efficiencies for

films from 2 to 8 /pms thick. All films were annealed in oxygen between 1400

and 1650 C for varying times. Reported were efficiencies of 3.8 Im/W for a 8

pm thick film and 2.4 Im/W for a 2 pm film at 15 kV and 25 pA/cm2 were

reported. Hirata el al. [Hir96] also deposited ZnGa2O4:Tb, Y3A15012:Tb (YAG:Tb)

and Y203:Eu using PLD. Films deposited at 300 C were amorphous and

required anneals of 800 C or greater to establish good crystallinity and

luminescence. They reported similar results for the yttria doped with

europium thin films.[Hir97] No relative brightness or efficiency data were

given.

The growth technique used to deposit the phosphor films was pulsed

laser deposition (PLD). PLD has been used extensively as a growth technique

for superconductors and other complex oxide systems.[Nar87] It is the

deposition technique of choice to deposit superconducting films due to its

accuracy for stoichiometry over sputtering and other growth techniques. This

physical vapor deposition technique couples a focused pulsed laser onto a

target where absorption and subsequent evaporation take place. Excimer

lasers are normally used, varying in wavelengths from 1064 to 532, 355,or 248

nm. A main limitation of this growth technique are particulate evolution

during deposition and limitation in deposition area.

Stoichiometric deposition results from the laser-target interactions.

There are two basic laser-target interactions leading to ablation: surface









34

heating and volume heating.[Sin94] In the surface heating regime, the optical

absorption depth of the laser beam is much smaller than the thermal diffusion

distance. During ablation, either a planar vaporization interface propagates

through the bulk, or subsurface heating may occur leading to non-linear

ablation characteristics. The temperature rise in this regime is governed by


AT = (1-R)tp
C, p(2Dtp)1/2


where R is the reflectance, (1-R) is the amount of radiation absorbed, F is the

energy density, t, is the pulse duration, C, is specific heat, p is the density and

(2Dtp)1/2 is the thermal diffusion distance.[Sin90]

In volume heating, the optical absorption depth is much larger than the

thermal diffusion distance, and the optical absorption depth is inversely

proportional to the absorption coefficient, a. In this regime, sub-surface

heating effects dominate and the temperature rise is governed by


AT(z) (1-R)aexp(-az)
Cvp



where z is the absorption depth.[Sin94, Che88] For large band gap

semiconductors and dielectric materials, optical absorption can be quite high

at the shorter wavelengths. This leads to high surface temperatures since

high energy densities are confined to a small volume. For these materials,











temperatures greater than the melting temperature are reached at depths

greater than the thermal diffusion distance. Target components, therefore,

cannot segregate despite differences in vapor pressures. This yields

congruent evaporation from multicomponent targets.

Besides wavelength [Kor89], repetition rate [Cha90] and spot size

[Wu90] also influence the ablation characteristics and therefore thin film

growth. The spot size of the laser beam will affect the deposition rate and the

amount of particulates ejected by the beam. Larger beams are attenuated by

the plasma plume created when the laser strikes the target. As the spot size is

reduced, the deposition rate increases due to reduced beam-evaporation

plume interaction. The beam-plume interaction can reduce the number of

particulates by evaporating them in the vapor phase.[Eye87] The reduction of

laser wavelength has this same effect since there is a high absorption in the

vapor plume that further volatilizes the fragments.[Kor89] It has been shown

that changing the repetition rate affects the film crystallinity and morphology.

Reducing the repetition rate reduces the growth rate which increases the time

atoms can diffuse to equilibrium positions. This leads to a higher crystallinity

and film density. [Wu90] For every set of growth conditions there will be an

optimum repetition rate, or arrival rate, that yields smooth dense thin

films.[Cha90] Coupled with the ability to change the standard growth

parameters such as substrate temperature and growth pressure, PLD has









36

emerged as a technique capable of depositing a wide variety of

materials. [Che88]









Table 2-1 Efficiency Data for Phosphor Thin Films


Current Bright-
Volts Density ness Eff. Growth
Phosphor (kV) (pA/cm2) (Cd/m2) (Im/W) Tech. Ref. Comment
ZnS:Mn 10 1 34.3 1.08 a Cus52 also ZnS:Zn blue

ZnS:As,P 10 1 34.3 1.08 a Stu55 from plot
ZnS:As,P 20 1 119.9 1.88 a Stu55 from plot

ZnS:Mn 12 10 6.9 0.02 b Fel57b as-deposited
ZnS:Mn 12 10 68.5 0.18 b Fel57b annealed

ZnS:Mn 12 10 116.5 0.31 b Fel57b Al coated

ZnS:Mn 12 10 342.6 0.90 b Fel57b annealed, fogged film

CaF2:5%Mn 12 10 27.4 0.07 b Fel57b clear film

PZn2(PO4)3:3% Mn 12 10 411.1 1.09 b Fel57b
CaWO4:W 12 10 34.3 0.09 b Fel57b clear

CaWO4:W 12 10 68.5 0.18 b Fel57b fogged film

Zn2SiO4:8%Mn 12 10 205.6 0.54 b Fel57b clear

ZnrSiO4:8%Mn 12 10 1027.8 2.69 b Fel57b annealed, fogged film

CaWO4:W 10 10 54.1 0.14 b Fel57b annealed, fogged film

a-Zn2SiO4:Mn 7.5 10 58.2 0.24 c Kir61 vycor substrate











Current Bright-
Volts Density ness Eff. Growth
Phosphor (kV) (pA/cm2) (Cd/m2) (Im/W) Tech. Ref. Comment

a-ZrnSiO4:Mn 7.5 10 75.4 0.32 c Kir61 silica, anneal (1250 C, 1 hr.)
a-Zn2SiO4:Mn 7.5 10 13.7 0.06 c Kir61 pyrex substrate
a-Zn2SiO4:Mn 7.5 10 10.3 0.04 c Kir61 pyrex substrate
a-Zn2SiO4:Ti 7.5 10 3.4 0.01 c Kir61 pyrex substrate
a-Zn2SiO4:Ti 7.5 10 6.9 0.03 c Kir61 silica, anneal (1000 oC, 1 hr.)
CdSiO3:Mn 7.5 10 92.5 0.39 c Kir61 silica, anneal (1150 C, 1 hr.)

Y203:Eu 8 1 36.0 1.40 d Bon95 sapphire
La202S:Eu 8 1 5.2 0.20 d Bon95 sapphire

Y202S:Eu 8 1 78.0 3.10 d Bon95 sapphire

Y202S, Gd202S:Tb 8 1 26.0 1.02 d Bon95 sapphire
Y202S:Tb 8 1 24.0 0.94 d Bon95 sapphire
La20OS:Tb 8 1 40.0 1.60 d Bon95 sapphire
LaO2S:Nd 8 1 1.2 0.05 d Bon95 sapphire

La202S:Tb 685.2 7.37 e Map73 0.1 W/cm2
La2,OS:Tb 2000.0 2.15 e Map73 1.0 W/cm2











Current Bright-
Volts Density ness Eff. Growth
Phosphor (kV) (pA/cm2) (Cd/m2) (Im/W) Tech. Ref. Comment

Y3Ga2A13012:Tb 10 25 1.60 f Gre94 from graph, 2 pm film

Y3Ga2A13012:Tb 15 25 2.40 f Gre94 from graph, 2 pm film
Y3Ga2Al3012:Tb 10 25 2.80 f Gre94 from graph, 8 /m film

Y3Ga2A13012:Tb 15 25 3.80 f Gre94 from graph, 8 pm film
a Chemical Vapor deposition
b Thermal Vacuum Evaporation
c Spray Pyrolysis
d RF Ion-plasma Sputtering
e RF Magnetron Sputtering
'Pulsed Laser Deposition












































Faceplate


+-- Insulator





-- Baseplate


Phosphor/ Positive Electrodea


Figure 2-1. Alternative lifted edge emitter field emission display technology.
Display allows for thin film phosphor integration with edge
emitter onto same substrate.









41














109 "0-

I 3 eV / -5.0eV
losi_ -2*2c V
104 02v 4
0-4eV
ri
S107- 0- S. eV -

I- / /'






10




S 2 6 8 12
102

ELECTRIC FIELD E 107 V/cm



Figure 2-2. Plots of Fowler-Nordheim equation for current density, J, as a
function of electric field, E, for varying work functions, Q. [Bro92]





























































6 20


40 60
TIP RADIUS (nm)


Figure 2-3. Calculated log of current density, J, plotted as a function of field
emission tip radius. The applied external voltage was 30 V and
the tip to anode separation distance was fixed at 20 nm.[Cut93]


2-


1


0



-1


-2


O


-4



-5,



-6-








































Q- 0 ] -Vacancy

-Y




Figure 2-4. Representation of the Y20, crystal structure showing the two
different cation symmetries, Sg and C2. In the C2 site there is an
oxygen vacancy (squares) on the face diagonal, while the S6 site
has a body diagonal oxygen vacancy pair.[Jag94] This yields
chemically equivalent but crystallographically in-equivalent sites.








44











6do 0DDDL00

T-3 d []IE-Inn
T

t_ 00


T p 11n El
00DDDDDO


5. ,
5sD0
Tp ElF]E


4, 000

300J


I E


Figure 2-5. Fill order for electron sub-energy levels in solids.[Sie67]
























50 Y03a O BANO EDGE


3

2 5)


O0


6113 A



6
5
4


SHARP LINE
EMISSION


7,

BROAO BAND
ABSORPTION


Figure 2-6. Energy level diagram for europium, Eu+3, doped yttrium oxide.
The characteristic emission at 611 nm results from the inter-
atomic 5Do-7F2 transition within the europium. [Wic64]


30-


ta
0

z
4


-r~P~I~P





























Cathodoluminescence Spectrum of Y20,:Eu


560 580


5D- 7F2

























600 620 640 660 680 7


Wavelength, Inm]




Figure 2-7. Cathodoluminescence spectra for Y23O:Eu+3 showing sharp line
emission at 611 nm. Spectra taken with Oriel Multispec
Spectrometer.


~

























incident
electron
beam


Key
incident electron beam


y secondary electron production


Sback-scattered electrons


M primary X-ray excitation

W Bremsstrahlung (continuum radiation)


rC cathodotuminescence excitation


Figure 2-8. By-products produced by electron in the interaction volume,
where DI is the X-ray generation area and D2 is the full
cathodoluminescence width after penetration and diffusion of
electron-hole pairs. [Mar88]








































electron


beam


LA Lc
r LB L


Figure 2-9. Changes in interaction volume depth and beam diameter with
voltage. As the voltage is increased (A to C) the penetration
depth (LA to La), is increased while the cross-section of surface
interaction is decreased.[Has90]











49
























32 S ,. 1-------------------

16C

322C



23:c -C



DC 2 3 4
312-c




I Z



C 5 C15 20 25

Accelerating Voltage, [kV]




Figure 2-10. Plot of electron penetration depth versus accelerating voltage,
per Terril's equation for Y20O.[Fel60]








































Ec


EE -- ED- E Excitec
state

Eg 2 3 5 6 7

EA E EA -


Figure 2-11. Schematic diagram of luminescence transitions between the
conduction band (Ec) and the valence band (Eq). Processes 4-6 are
typical transitions occurring within semiconductors where
process 7 is typical for rare earth dopants.[Yac921


tv////////////////////




















Y,, : Eu (5 rm)


YOS : Eu(3 pm)


10 -


I I I I I


10-6 10-s 10-4 10- 10-2 10-'

Eu concentration (mole fraction)


Y,,S :Pr (lOpm)


Y20, : Pr


10-6


10- 10 -* 10-I 10-2
Pr concentration (mole fraction)


Figure 2-12. Cathodoluminesecent intensity versus activator concentration (a)
Eu in two host materials displaying indirect activator excitation
and (b) for Pr in two host materials displaying both direct (slope
=1.0) and indirect (slope= 0.66) behavior.[Oza90]


...... I


--


. I I I



































ELECTRON


(a) (b)


Figure 2-13. Illustration of light internally reflected in (a) a powder versus, (b)
a thin film. Light eventually escapes the powder, while only light
within the film incident the surface at angles less than Oc (rays a
& b) will escape. Light incident at angles greater (rays c & d) is
trapped within the film and will continue to reflect internally.







































Critical Angle vs Index of Refraction Ratio
90 .


80 ............ air/yttria ratio

70 -
60
60

50 -

H 40-
40

4J 30 ............... ... .

U
20

10

0
0.0 0.2 0.4 0.6 0.8 1.0


nair/nphosphor




Figure 2-14. Plot of critical angle versus the refractive index of air to phosphor
ratio. For Y20O at 611 nm, the index of refraction is equal to 1.93.
This yields a critical angle of 31.















CHAPTER 3

EXPERIMENTAL PROCEDURE




Substrate and Target Preparation



Yttrium oxide doped with europium thin films were deposited onto

Coming 2947 glass, c-axis sapphire (A1203), quartz, and (100) silicon (Si). The

glass, sapphire and the quartz substrates were cleaned using a modified RCA

cleaning procedure. First, the substrates were mechanically cleaned using

trichloroethylene (TCE) and a cotton swab. The substrates were then

ultrasonically degreased in heated (35 oC) TCE, acetone and methanol for 5

minutes each. They were then rinsed in de-ionized (DI) water and blown dry

with nitrogen gas. The same solvent cleaning procedure was employed on the

silicon wafers followed by a 5 second buffered hydrogen fluoride etch and a DI

rinse. The silicon substrates were then blown dry with nitrogen. Some silicon

substrates were roughened prior to cleaning to examine the effect on CL

brightness. Silicon carbide 600 grit paper was wetted with methanol and the

substrates were roughened by random hand motion for 10 seconds.









55

The phosphor PLD targets were manufactured from powder supplied by

Osram Sylvania (type 2342 Y203:Eu lamp phosphor). Ten weight percent of

various known fluxes, such as AgO and NaF, were sometimes added to assist

in the sintering process. The powders were pressed into 1 inch pellets and

sintered at 1400 C in air for 24 hours. A target was also manufactured from

the same powder via isostatic hot pressing by Cerac, Incorporated.



Pulsed Laser Deposition (PLD)



Films were created by PLD in a high vacuum chamber which used a

Leybold Trivac mechanical pump model # D4A for rough pumping and a

Pfeiffer Balzers TPU 450 H corrosive turbomolecular pump with a TCP305

controller and a MD4I diaphragm backing pump. The ultimate pressure of the

system was 4 x 1011 Torr. A KrF (X= 248 nm) 1 Watt Lambda Physik

Lasertechnik LPX300 excimer laser with a pulse width of 10 nanoseconds was

used. Pulse frequencies between 1 and 100 Hz were possible, but all films

were deposited at 10 Hz. A schematic of the deposition chamber is shown in

figure 3-1. By adjusting the lens-to-target distance, varying laser beam spots

with controlled sizes were achieved. The target was also rotated to reduce the

number of ablated target particles deposited on the thin film surface. [Che94b]

The incident laser beam struck the target at 450 away from the surface normal,

generating a plume perpendicular to the target and substrate. The substrate









56

holder was not rotated. The distance between the target and substrate could

be varied from 2.5 cm to 5 cm, but 3 cm was the most commonly used target-

to-substrate distance. Typically, deposition was accomplished over 15

minutes at 500 mJ/pulse with a focused spot of 0.2 cm2 and a pulse rate of 10

Hz. This yielded 600 pulses/minute with an energy density of 2.23 J/cm2 per

pulse or 22.3 W/pulse. A 12% loss due to absorption and reflections of the

mirror, lenses, and quartz vacuum window was factored into the energy

density calculation. The loss was measured using a laser power meter. The

combination of target-to-substrate distance and laser conditions yielded

deposition rates of 3.7 A/pulse (36.6 A/sec) at high vacuum conditions and 2.7

A/pulse (26.7 A/sec) at 600 mTorr.

Ultra high purity oxygen from compressed gas cylinders was used to

grow at higher pressures. The gas was introduced into the vacuum system

using a MKS model 247C mass flow controller at 80 sccm. The pressure was

regulated by throttling the turbo pump, using the gate valve to achieve the

desired pressure. Pressures of 10-6 Torr to 600 mTorr were obtained using this

method.

The substrates were heated via a quartz lamp to temperatures from 150

oC to 1000 oC. A stainless steel heater plate was used as the thermal contact

between the lamp and the substrate. Substrates were mounted onto the plate

with silver paste. Deposition temperatures were monitored using a

thermocouple in contact with the stainless steel plate via support clips. Some









57

of the as-deposited films were furnace annealed in air for varying temperatures

and times to a maximum temperature of 1450 C in a Lindberg Blue M model #

54433 high temperature furnace.



PLD Film Characterization



The PLD films were characterized using a variety of analytical

techniques including x-ray diffraction (XRD), secondary electron microscopy

(SEM), atomic force microscopy (AFM), scanning and static Auger electron

spectroscopy (AES), Rutherford back-scattering (RBS), secondary ion mass

spectroscopy (SIMS), photoluminescence (PL), and cathodoluminescence (CL).

Brief descriptions of these techniques follow.



X-ray Diffraction(XRD)


X-ray diffraction [Cul78] was the primary technique used for yttria

phase and crystallinity identification. A Phillips model APD 3720 x-ray

diffractometer was operated at 40 kV and 20 mA to generate Cu Ka radiation of

X= 1.5406 and 1.5444 A. The Cu Kp was removed using a Ni filter. Diffraction

spectra over the 20 range of 10 90 were measured. The goniometer was

scanned at 30 per minute in a continuous mode. XRD is primarily a structural

characterization technique but may be used for semi-quantitative composition

analysis, and determination of stress-strain, crystal size, film thickness and in









58

special cases, quantification of defect concentrations. [Smi861 These different

applications of XRD result from the ability to measure and interpret peak

intensity, position and width data. The Warren-Averbach method was used to

deduce the crystallite size from the FWHM data.ICul78] The (100) peak of a

silica standard obtained from the National Institute of Standards and

Technology was used to quantify the data from Y203:Eu.

Atoms in a solid are capable of scattering and diffracting electrons,

photons, other atoms and x-rays. Incident x-rays are either constructively or

destructively diffracted. Constructive diffraction results when the atoms are

aligned in a periodic fashion. The different rays reinforce rather than cancel

one another as in the destructive case and a diffracted signal, or peak, is

detected. This peak is also referred to as a line profile. The required atomic

alignment to yield constructive interference is governed by Bragg's law

nX = 2dhsineO



where X is the wavelength of incident radiation, n is a small integer (1,2,3...)

and is the order of the diffracted peak, typically first order with n= 1; dh is the

spacing between atomic (hkl) planes where hkl are the Miller indices of the

diffracting plane of atoms, and 0 is the angle between the normal of the (hkl)

plane and the incident x-ray beam. [Cul78] The interplanar spacings and

planes for low Miller indices are shown in figure 3-2.











Since the atomic arrangement varies for each of the seven crystal

systems, the allowed diffraction from various hkl atomic planes will also vary

per crystal system. Appendix B shows the allowed hkl combinations. The d

spacing is a function of the plane location and the lattice parameter. In cubic

systems, dh is given by


ao

h2 + k2 + 2



where a, is the lattice parameter of the crystal. The resulting peak contains

contributions from the sample, instrumentation error and spectral distribution.

By eliminating the latter two, the profile can be manipulated to quantify

crystalline defects.[Smi86] The amount of x-rays which constructively diffract

is dependent on the crystallinity, therefore, a direct comparison of % crystalline

can be made between samples by comparing the area under the peaks. Care

must be taken to measure the same physical area and thickness for all of the

samples to make meaningful comparisons of peak areas. If the sample

thickness is much greater than the characteristic absorption depth of x-rays,

thickness differences can be neglected. The x-ray absorption depth for yttria is

10 pm. Peak width can yield information about crystallinity and defects, and

position yields information about mismatch and strain.











Scanning Electron Microscope(SEM)


Scanning electron microscopy (SEM) is a powerful tool in the

characterization of surface topography and defects. [Bin92] It is capable of

providing micrographs with magnification as low as 10x and as high as

300,000x resulting in images with resolutions of a few nm. A JEOL 6400 SEM

with true secondary and backscatter electron detectors plus an energy

dispersive x-ray spectrometer (EDS) attachment was used similar to the one

pictured in figure 3-3. Primary electrons were generated from a tungsten

filament by thermionic emission. The operating voltage ranged from 5 kV to 30

kV, most commonly 15 kV. Since the samples were non-conductive, they were

coated with Au-Pd for imaging or carbon coated for EDS analysis. The high

magnification limit for these samples was 100,000x. A transmission electron

microscope (TEM) was employed for higher magnification. SEM was primarily

used to acquire surface morphology, cross section thickness and morphology,

and grain size data.



Rutherford Backscattering Spectroscopy(RBS)


Rutherford backscattering spectroscopy (RBS) was primarily used to

determine film composition and stoichiometry. [Bau92a] It is one of the few

quantitative surface analysis tools available. A high voltage van de Graaf

accelerator with an EDS detector at Oak Ridge National Laboratory was used.









61

Primary beam energies of 2.2 MeV and doses of 1019 atoms/cm2 were typically

used. RBS can detect a few parts per million (ppm) of heavy elements in a

light element matrix and is insensitive to chemical bonding and matrix effects,

unlike Auger electron spectroscopy (AES) and secondary ion mass

spectroscopy (SIMS). RBS is routinely used for quantitative compositional

analysis of thin films, layered structures and bulk specimens, surface dopant

analysis, defect depth distribution, quantification of percent crystallization,

and film thickness (or density).[Chu86]

Backscattering results from direct collisions between He ions and nuclei

of sample atoms. The energy of the back scattered particle is a function of

both the amount of energy lost due to momentum transferred during the

nuclear collision plus the energy loss during collisions incoming and scattered

He particle with electrons in the sample. Measuring the energy of the

scattered ion gives a direct measurement of the mass of the ion, and hence

mass identification. The projectile energy to incident ion energy after the

elastic collision ratio (kinematic factor) is given by the equation:


S= -((M/MsinO)2 + (M/M,)cosO)
K = E/E =
1 + (M1/M2)



where M1 is the mass of incident ion, M2 is the mass of target atom, 6 is the

angle between trajectory of He ion and the angle of detector.[Chu78] See

figure 3-4. Since K is a function of M1/M2, there is a larger energy separation











for lighter elements than heavier ones due to a significantly larger amount of

momentum transfer to lighter elements. This results in excellent mass

resolution between light elements and between light and heavy element

matrixes, but poor resolution between heavy elements. There is good mass

resolution for the Y203:Eu matrix.

The probability of backscattering into a given solid angle per incident

particle is referred to as the differential scattering cross section, do/d2, where


do (ZZ2e2)2 4 ( 1-((M/M2)sinO)2 + coso)2
dQ 4E sin4 1 -((M1/M2)sino)2


where Z, = atomic number of incident atom, Z2 is the atomic number of target

atom, E is the incident energy before scattering, and e is the charge. The

scattering cross section is usually simplified to (Z1Z2e2)2/4e. Since do/dQ is

proportional to Z22, larger atomic number elements are more efficient in

scatterering the incident ion. In the case of a Y203:Eu matrix, even though the

mass resolution is poor for heavier elements, RBS is very sensitive to the

yttrium and the europium. [Chu78]



Atomic Force Microscopy(AFM)


Atomic force microscopy (AFM) belongs to the family of scanning probe

microscopy (SPM). [Bin82, How93] AFM was used to quantify film roughness

and other morphological details about phosphor thin film surfaces. A Digital









63

Instrument Nanoscope III Multimode SPM was employed in both the contact or

tapping modes. The instrument was mounted on a pressurized air table to

minimize vibrational noise. AFM is based on attraction and repulsion forces

generated between an atomically sharp tip and the sample surface. These

forces can be measured to display topological data such as roughness,

grain size and step heights, as well as yield physical information such as

hardness, friction, and wear strength.[Bhu95]

AFM employs atomically sharp, etched Si tips attached to a cantilever

beam with resonant frequencies of 60 to 400 kHz. The beam is fabricated to

have a small spring constant to allow for beam deflection. As the tip is

brought close to the sample, it experiences a weak Van der Waal attractive

force. [How93] The tip is drawn closer causing the electron clouds of the tip

and sample to overlap which induces a repulsive electrostatic force. This force

goes to zero as the distance between the atoms reaches a few A. When the

Van der Waals force becomes repulsive, the tip is in contact with the sample,

[How93] as shown in figure 3-5. The repulsive Van der Waal force slope is very

steep in this regime and counteracts any force that attempts to push the

sample and tip atoms closer together.

In the contact mode, the AFM is operated in the repulsive force regime

shown in figure 3-5. Since the slope of the (+) force side is steep, changes in

the Z height causes the cantilever to deflect instead of propelling the tip

further into the sample. In the non-contact (tapping) mode, the AFM operates









64

in the (-) Van der Waals (attractive) force regime where the space between the

tip and the sample surface is on the order of tens to hundreds of A. The

cantilever beam is oscillated (or tapped up and down) relative to the surface,

and changes in the average Z height are detected to keep a constant

modulation force on the surface. The non-contact mode is more sensitive than

the contact mode.

With the Digital Nanoscope, both techniques utilize a laser to detect the

deflection. As pictured in figure 3-6, a laser beam is positioned on the tip at

the end of the cantilever. This beam is reflected off the cantilever and into a

position sensitive photo-diode detector where sub-angstrom movement can be

detected.[Bhu95] The x-y position is fed back to keep the cantilever level while

the deflection or height change is recorded.

To optimize the images or quantify data from AFM, the artifacts in data

from scanning characteristics and noise must be removed. The Digital

Nanoscope Instrument plane-fit-data-filter (3rd order) and a flatten-data-filter

(0h order) were used to eliminate scanner artifacts. Vibration induced noise

was erased using a scan-line-filter. This produced an image from data

averaging over the nearest points. Over-compensation from repeated use of

scan line filtering can introduce extra error,

Roughness, or the change in Z height, was the primary quantitative data

gathered. The roughness (Rq), also called root mean square roughness (RMS),

is given by













(Zz. Z )2
Rq = _N [unit of length]



where Zi is the surface height, Zav is the average height displacements and N

is the number of data points.



Auaer Electron Spectroscopy(AES)


Auger electron spectroscopy (AES) was done using a PHI model 660

scanning Auger microprobe to identify surface contaminants and changes in

the oxygen to yttrium ratio as a function of deposition and/or annealing

conditions.[Hol80] The Auger process results from the excitation a of core level

electron and compensation by the ejection and de-excitation of electrons

possessing characteristic energies.[Str92] AES utilizes energetic electrons to

initiate this process. Since Auger electrons are low energy electrons, only

those from a shallow depth from the surface will survive without inelastic

collisions to be detected, hence AES is a surface sensitive technique (5 30 A).

AES is widely used as a surface sensitive technique capable of providing

elemental composition and some information on surface chemical bonding.

AES survey spectra were taken on the samples as-received and after the

surface was cleaned for 1 minute by sputtering with an energetic 3keV Ar+

beam. This time was equivalent to the removal of ~ 100 A. Depth profiles











were also done by recording the AES spectrum, sputter for 2 minutes, and

record a new spectrum to detect changes in element concentrations.



Photoluminescence(PL)


Photoluminescence (PL) brightness was measured using a UVP UVG54

6 watt UV lamp and a Photo Research Pritchard 650 (PR650) camera.[Col92]

This camera was equipped with an internal light source and brightness could

be measured in footlamberts (fL). Instead of using peak height intensity as the

brightness value, the PR650 used the calculated area-under-the-curve between

380 and 780 nm. The PR650 also determined CIE coordinates from the

measured spectral distribution. The UV lamp consisted of a mercury vapor

tube with filters to pass primarily the short wavelength of 254 nm (figure 3-7).

The lamp intensity was 2250 j/W/cm2 at 254 nm at a distance of 3 inches. The

phosphor absorbed the radiation by promoting an electron from the ground

state to an excited state. The excited atom then relaxed radiatively emitting a

characteristic photon.[Col92]

The lamp and the spectrometer were 90 apart, therefore samples were

arranged at 450 from the lamp and the PR650. The spectrometer was focused

using focal lenses. The PR650 had a resolution of 5 nm, therefore it did not

resolve the true line width of the Eu3 but was sufficient for peak identification

and brightness comparisons. The samples were mounted on a black

background to reduce reflection. Since the primary data collected were from









67

films on silicon, PL was recorded in the reflectance mode at room temperature.

More complicated PL equipment may allow spectral analysis at cryogenic

temperatures, PL excitation spectroscopy, time-resolved PL, and PL

mapping.[Col92, Gui95]



Cathodoluminescence(CL)


Cathodoluminescence (CL) utilizes energetic electrons to excite

radiative relaxation.[Yac92] CL data was taken using two different systems.

The first system used the co-axial electron gun from a PHI model 545 scanning

Auger electron spectrometer with an Oriel Multispec optical spectrometer and

a CCD array detector. The AES gun was capable of providing a primary

electron beam with an energy from 100 eV to 5 keV and currents between 1

and 5 pA with a spot size from 0.5 to 3 mm. This resulted in DC current

densities from 10 to 510 pA/cm2.

The other CL system consisted of a EGPS-7H Kimball Physik electron

gun, and an optical fiber coupled to an Ocean Optics S2000 spectrometer with

a CCD array detector. The EGPS-7H electron gun was capable of energies from

50 to 5 keV and currents from 0.01 to 500 pA with spot sizes from 2 mm to 3 cm.

This yielded current densities ranging from 1.4 x 10-3 to 1.6 x 104 PA/cm2.

Both the Oriel and the Ocean Optics spectrometers were able to resolve

the characteristic Eu3 line emission since their resolution was less than 1 nm.

The sample current was determined by biasing the electrically isolated sample










68
carousel with a +90 volt battery with an ammeter in series to ground. The bias

collected the true secondary electrons to avoid error upon secondary electron

emission.

From the current density, incident power, and brightness, the efficiency

can be calculate. Efficiency is given by


Efficiency = 100* B Im/W
V*I
density



where B is the brightness [cd/m2], V is the voltage [VI, and Ienity is the current

density [pA/cm2] measured from the sample current and beam spot size. Since

neither spectrometers had an internal brightness standard, intensity was

measured in arbitrary units and calibrated using phosphor standards.

Brightness values from phosphor standards were used to calculate absolute

efficiencies. Efficiency versus accelerating voltage curves for standard

phosphors were provided by the Phosphor Technology Center of Excellence

(PTCOE). Data given for 1 pA/cm2 densities. The PR650 was used to measure

brightness values in cd/m2 and chromaticity values (x,y).




















































Figure 3-1. Schematic of pulsed laser deposition chamber.



















d210


o0- 1 s-6 0G--e-f-
\ -oo
a-- ---- -- --1-














(100) (110)







(111) (012)




Figure 3-2. Example of x-rays being diffracted by aligned atoms in a cubic
orientation. This shows the proper miller indices (hkl)for different
atom locations where ao represents the lattice parameter.[Ton92]






















































Figure 3-3. Schematic of secondary electron microscope.










































Target atom


Figure 3-4.


Schematic representation of elastic atom collision between
projectile atom with mass M1 and resting target atom with mass
M2. E represents the atom's energy, v is the velocity, and 0 and <
are the scatter angles for the projectile and the target atoms,
respectively. [Chu78]


Projectile


M2 MI
V2, M2 vo, EoM

- M- -0


vIEI































repulsive force







distance
(tip-to-sample separation)


attractive force


Figure 3-5. Interatomic force versus atomic distance curve illustrating the
two AFM regimes, contact and non-contact. How93]


intermittent-






















































Figure 3-6. Schematic diagram of AFM utilizing position sensitive photo-
diode detector to measure changes in laser position which
correlates to z height changes. [Bhu95]



















































wavelength, nm


Figure 3-7. Spectrum after filtering yielding short wave UV emission.















CHAPTER 4

RESULTS


In this chapter, microstructure and chemical data are presented for

Y203:Eu films grown by PLD. The objective of this study was to determine the

effects of roughness on the light out-coupling in europium doped yttria thin

films. It was found that increased surface roughness increased the out-

coupling. The increase in roughness was accomplished in-situ by increasing

the 02 growth pressure. Samples were categorized as smooth versus rough as

determined by Atomic Force Microscopy (AFM). Smooth films had RMS

roughnesses of 3 nm, while rough films had RMS roughnesses of 71 nm.

Luminescence data reported under PL and CL are presented first with the film

growth and annealing processes resulting in these improvements following.

Parameters such as deposition temperature, and annealing temperature also

had a significant effect PL and CL brightness.



Photoluminescence(PL)



The films yielded weak luminescence detectable only by eye at growth

temperatures of 250 C in vacuum. The PR650 camera was able to detect









77

emission consistently for films grown at 600 C or higher. Films grown at 800

C were much brighter than those deposited at 600 oC, showing that growth

temperature had a significant effect on luminescence properties (figure 4-1).

This effect of increasing the temperature to 800 C from 600 C was even

greater than the effect of mechanical roughening of the substrate prior to

growth, as illustrated in figure 4-2. This mechanical roughening step is

discussed in further detail below.

In addition to increasing temperature, the PL response also increased

as the oxygen growth pressure was increased as shown in figure 4-3.

Increasing 02 pressure above 200 mTorr increased the surface roughness from

3 nm to 71 nm, as discussed below. There was a 7 times improvement in PL

brightness, corresponding to an increase to 8% the brightness of the powder

standard.

Films were annealed to improve the crystallinity and further activate the

europium dopant. Smooth films (grown at 100 mTorr) and rough films (grown

at 600 mTorr) grown on (100) silicon at 460 oC were annealed from 600 C to

1200 C for 1 hour in air. Under PL, the films grown at 100 mTorr 02 pressure

did not yield detectable PL until annealed at 1000 C, where the response was

1.81 fL after a 1200 C anneal for 60 minutes versus 4.55 fL for an annealed film

grown at 600 mTorr (figure 4-4). As seen in figure 4-4, increasing annealing

temperature from 1000 C to 1200 C had a significant effect on PL intensity.











There was also an increase in PL intensity as the film thicknesses were

changed from 0.5 jm to 9 pm (figure 4-5) at 600 mTorr and 600 C.



Cathodoluminescence(CL)



Cathodoluminescence data were collected at a current density of 1

pA/cm2. Figure 4-6 shows the brightness versus voltage (BV) curves for the

Osram Sylvania Y203:Eu powder used as starting material for this study.

Brightness data was taken with both spectrometer systems described

previously. Since the power density was known, efficiency values were

obtained as shown in figure 4-7. The efficiency curves had the characteristic

linear and saturation regions at low and high power densities, respectively.

The change from linear to saturation behavior occurred between 1.5 and 2 kV.

The CL brightness from the thin films was not detectable until they had

been grown or annealed at temperatures greater than 800 oC, although the

films were photoluminescent at temperatures of 450 C (figures 4-1 to 4-3). For

800 C deposition, the CL brightness values were low but detectable as shown

in figure 4-8. The BV curves (figure 4-9) for a smooth film grown at 100 mTorr

and 460 C then annealed at 800 oC, 1000 oC, and 1200 oC in air for 1 hour show

that brightness and efficiency increased as the annealing temperature was

increased. There was a significant change from 1000 C to 1200 OC. Figure 4-

10 shows that annealing also increased the CL intensity from rough films











grown at 600 mTorr and 460 'C. Under cathodoluminescence, the rougher

films were significantly brighter than smooth films, similar to PL data (figure 4-

4). The significant improvement from annealing temperature 1000 oC to 1200

C was also similar (figures 4-4, 4-9, and 4-10)

A comparison of CL efficiency for films grown at 460 oC in either 100

mTorr or 600 mTorr and annealed at 1200 C for 1 hour in air is shown in figure

4-11. From AFM, the RMS roughness for films grown at 100 mTorr and 600

mTorr were 3 nm and 71 nm, respectively. The films grown at 600 mTorr were

- 3 times more efficient.

The effect of surface roughness versus CL efficiency was also

demonstrated by films deposited on c-axis sapphire. Films were grown on c-

axis sapphire at 800 C, 2 x10-5 Torr versus 600 C, 600 mTorr and annealed at

1200 oC in air for 60 minutes (figure 4-12). The smooth as-deposited films and

smooth annealed films (grown at 2 x 10-5 Torr) had similar CL efficiency values,

while the rough film (grown at 600 mTorr) had a significantly higher CL value.

The rougher films were- 3 times more efficient.

Films grown at a higher growth pressure than 600 mTorr (up to 6 Torr)

were also annealed at 1200 C in air for 1 hour. The efficiency was not

significantly higher than those grown at 600 mTorr as shown by the efficiency

data in figure 4-13.

Cathodoluminescence efficiency versus annealed film thickness was

measured as shown in figures 4-14 and 4-15. Just as for PL, increased film











thickness led to better CL efficiencies. The CL efficiency data for 2 kV

electrons are shown in figure 4-15 for thicknesses ranging from 0.6 to 9 pm.



Thin Film Growth



The results of thin fim growth are presented below starting with the

raw materials processing. The europium doped yttria powder had the

characteristic cubic Y203 structure and was a perfect match for the JCPDS 25-

1200 file. As-received powder pressed into targets and sintered at 1400 C for

24 hours also displayed the cubic structure (see figure 4-16).

Films were deposited at growth temperatures of 250 C to 800 C in 2 x

10-5 Torr. Yttria was successfully grown at temperatures as low as 250 C,

regardless of substrate material. As seen in figure 4-17, there was a small

(111) peak for films grown on glass substrates at 250 C. The (111) direction

remained the preferred orientation as the temperature was increased to 600 C.

There were additional peaks from films grown at 600 C resulting from grains

aligned in other directions. Similar results were seen for films grown on (100)

silicon and c-axis A1203 (sapphire), as shown in figures 4-18 and 4-19,

respectively. It is noted that for the yttrium oxide/silicon system, the (100)

silicon peak was at 690, which, for clarity, is not plotted on these graphs. There

were also small silicon K, and Ka peaks present at 610 and 32 in some of the

XRD plots.










81

As the growth temperature was increased, the FWHM for the (111) peak

changed with temperature as shown by figure 4-20. A poorly crystalline

polycrystalline pattern was observed for films grown at 250 C on all

substrates, yielding a high FWHM of 0.383. The crystallinity improved,

changing to a strong (111) texture at temperatures greater than 250 C,

corresponding to a decrease in FWHM. At 800 C, the films on silicon and

sapphire transformed back from highly (111) texture to a polycrystalline film

resulting in an increase in the FWHM.

There was also a corresponding change in film morphology with the

change in crystal structure and texture. The typical thin film morphology was

smooth (figure 4-21) for films grown at pressures less than 1 x 104 Torr on the

(100) silicon substrates. Particulates ranging from 0.125 to 0.25 pm seen in

figure 4-21 resulted from the ablation process. Particles from the target were

liberated as a result of the absorption and melting processes during ablation

and could be reduced by using denser targets, reducing the laser energy

density during ablation or by utilizing some of the other particulate reduction

schemes developed recently.[Che94b] As seen in the cross-section SEM

photograph (figure 4-22), the growth process yielded nice columnar grains. As

the growth temperature was increased for deposition on (100) silicon at 2 x 10"'

Torr, the films first became smoother from 250 oC to 400 C, then more granular

from 400 C to 800 oC, as seen by figures 4-23 to 4-25.









82

X-ray diffraction data were complemented with chemical analysis using

AES, RBS and SIMS. Figure 4-26 shows a typical Auger spectrum from a yttria

film deposited on (100) silicon at 600 C at 10'5 Torr, after cleaning the near

surface with Ar' sputtering for 1 minute. The surface of the as-received

samples was contaminated with carbon (272 eV) and chlorine (181 eV) peaks,

which is normally seen. Yttrium Auger peaks were detected at 75, 106,122,

1743 and 1817 eV. From the AES handbook, the low energy triplets were

expected to be at 77, 110, and 127 eV. The expected higher energy peaks are

at 1746 and 1821 eV. The oxygen peak was seen at 509 eV while 510 eV is

characteristic energy. The expected theoretical values are for the Y metal and

the differences in detected peak energies and theoretical values result from

chemical shifts in forming the Y203 solid. The small peak at 215 eV resulted

from argon implantation from the Ar+ sputtering. Europium was not detected

due to its low concentration in Y203. Europium's characteristic Auger peaks

are located at 109, 139 and 858 eV, but no signal was detected at 858 eV.



Surface Roughening



The effect of surface roughening was initially investigated by growing

europium doped yttria on smooth and mechanically roughened substrates.

The substrates were roughened prior to growth with 600 grit SiC paper as

described previously. As in figure 4-27, yttria was successfully deposited onto











the roughened substrate with a morphology indistinguishable from the

substrate.

The goal of changing surface roughness in-situ was accomplished by

increasing the 02 growth pressure. The crystalline texture changed as a

function of the growth pressure (figure 4-28), as it did with temperature (figure

4-18). The films transformed from preferred (111) orientation at low 02

pressure to more random polycrystalline at pressures greater than 400 mTorr.

As seen in figure 4-28, at 600 mTorr the (400) peak becomes very pronounced.

The (111) FWHM is plotted versus 02 pressure during growth in figure 4-29,

showing that the FWHM decreased as the pressure was increased up to 200

mTorr. Above this pressure, the reported FWHM increased again. As seen in

figure 4-28, pressures above 200 mTorr resulted in a change from preferential

(111) orientation to more random polycrystalline films. Just as with increasing

growth temperature (figure 4-20), as the film changed from a (111) preferred

orientation to a random polycrystalline film, the FWHM increased.

At 600 mTorr and 600 C, the peaks marked (*) in figure 4-28 were from

a yttrium silicate phase that formed during deposition. To avoid reactions

between Y20, and SiO2 to form a silicate, a nucleation layer of Y203 was

deposited at 200 mTorr for two minutes prior to continued deposition at 600

mTorr. As seen in figure 4-30, formation of a silicate phase was avoided and a

preferential (111) orientation was maintained by using this nucleation layer.









84

The change from a preferential (111) texture to a random polycrystalline

one was also evident from SEM photographs. Figures 4-31 to 4-36 show the

change in morphology as the oxygen pressure was increased. The morphology

became nodular in nature and rougher as the pressure was increased from 0.2

mTorr (figure 4-31) to 600 mTorr (figures 4-35 and 4-36). The nodule size, as

well as the grain size, were well below 1 pm. The nodule size varied from 100

15 nm for films grown at 400 mTorr and 80 15 nm for films grown at 600

mTorr (figure 4-36). This change in structure and nodule size is seen in figure

4-37, where the 02 pressure was increased from 0.05 mTorr to 80 mTorr, 600

mTorr, and then 1 Torr.

A change in film morphology was also apparent from AFM

measurements of the RMS roughness of surfaces versus oxygen pressure

during growth (figure 4-38). The RMS roughness, increased from 2 nm to 71

nm as the 02 pressure was increased from 0.02 to 600 mTorr.

Film thickness, at a fixed 02 pressure and temperature, was also varied

by controlling the deposition time. As expected, the growth was linear with

respect to deposition time as seen in figure 4-39, which shows film thickness

versus growth time for films grown at 600 C and 600 mTorr on (100) silicon.

The columnar growth was maintained even at these long growth times (figure

4-40).

The Y/O peak heights were compared using AES to determine if there

was any change in oxygen content with increasing oxygen pressure during











deposition. The change in Y/O peak height ratios is seen in figure 4-41. The

pressed target had a Y/O ratio of 14. According to figure 4-41, this would

correspond to a film grown in 02 pressures between 100 and 200 mTorr. RBS

data were collected from samples grown at 100 mTorr and 600 C to quantify

the Y, O and Eu composition (figure 4-42). The broad plateau from 0.7 to 1.275

MeV originated from scattering from the silicon substrate. Using a fitting

routine, the composition was determined to be YO.36700626Eu0.007. RBS data were

also collected from samples grown at 600 mTorr and 600 C. At 600 mTorr and

600 "C, films grown on (100) silicon were to rough to accurately analyze with

RBS.

Figure 4-43 shows a typical SIMS profile from films deposited at 100

mTorr and 600 C. Oxygen was detected at a mass/charge ratio of 16, while

yttrium was detected at 89 and europium at 151 and 153.



Annealing



Samples were annealed to improve the crystallinity and determine the

effects on the luminescence brightness. The effect of annealing on the crystal

structure is shown by the XRD patterns of figures 4-44 through 4-47. Figure 4-

44 shows the change in crystal structure for films grown at 600 mTorr and 460

"C, then annealed at 800 "C, 1000 "C, and 1200 C for 1 hour in air. The films

remained polycrystalline, but an interfacial reaction was detected at 1000 C











that converted the silicon/yttrium oxide to a yttrium silicate phase. The a-

Y2Si20O phase was the best fit. Figure 4-45 shows that annealing reduced the

FWHM from 0.51 to 0.140 as the annealing temperature was increased from

460 C growth temperatures up to 1200 C annealing temperatures. Films

grown at 100 mTorr at 460 C (figure 4-46) were (111) oriented and remained

textured for all annealing temperatures. The (111) FWHM (figure 4-47)

decreased from 0.180 to 0.130, a value similar to the film grown at 600 mTorr

and annealed at 1200 C. The (111) FWHM value for the powder was even

lower, ranging from 0.0960 to 0.11.

The nodule size increased with annealing temperature as shown by

figure 4-48. The sample was deposited at 600 mTorr and 600 oC on (100)

silicon (figure 4-48a) followed by furnace annealing at 1000 C for 30 minutes

in air (figure 4-48b). Grains increased from 80 15 nm to 100 20 nm in size.

This size of nodules was still an order of magnitude smaller than that of

phosphor powder particles at 5 jm.

RBS data from a film grown at 100 mTorr, 600 C and annealed at 1000

C for 30 minutes in air is shown in figure 4-49. The composition of the film

was the same prior to and after annealing. However, there was an interfacial

reaction with the silicon substrate resulting in a yttrium rich silicate phase.

This can be seen as a change in the low energy edges of the peaks for oxygen

and yttrium and for the high energy edge of Si. The fit routine yielded the

same composition for the annealed film as for the as-deposited condition.
























PL Intensity versus Growth Temperature
Y203:Eu films grown at 2 x 10'5 Torr, 600 and 800 C


520 540 560 580 600 620 640 660 680
wavelength, [nm]


Figure 4-1. Photoluminescence versus growth temperature showing an
increase in intensity as temperature is increased. Films grown
on (100) silicon at 600 and 800 oC at 2 x 10-5 Torr.

























PL Intensity for Various Temperatures


525 550


575 600
wavelength, [nm]


625 650 675


Figure 4-2.


Photoluminescence versus temperature for film grown at 600 and
800 C compared to a film deposited on a mechanically
roughened silicon substrate at 600 C. Substrate was roughened
with 600 grit SiC paper prior to deposition.




























PL Intensity versus Growth Pressure


1-




lO-S


I 0I I-3 1I 0- 10'
10-4 10- 10-2 10-1


Pressure, [mTorr]



Figure 4-3. Photoluminescence versus 02 growth pressure showing an
increase in PL intensity beginning at 200 mTorr. Films grown
on (100) silicon at 600 oC. Films were 1 pmn thick.



























PL Intensity versus Annealing Temperature
460 OC 100 mTorr and 600 mTorr on (100) silicon


5.0 -


4.5 -


4.0 -


3.5 -

S3.0
3.0 -


2.5-


2.0 -


1.5 -


1.0 -


0.5


I I 1 1
600 800 1000 1200


Temperature, [OC]




Figure 4-4. Photoluminescence versus annealing temperature showing
increase in PL intensity as the annealing temperature was
increased. Rough film grown at 600 mTorr, 460 C and annealed
at 1200 C is approximately 3 times brighter than smooth film
grown at 100 mTorr, 460 C and annealed at 1200 C. Films were
annealed for 60 minutes in air.


600 mTorr


100 mTorr -




Full Text
119
36E
1 5KU X25
1 H m
0 0 0
15mm
Figure 4-33. SEM photograph of film grown on (100) silicon at 200 mTorr and
600 C showing outgrowths and the on-set of surface roughness.
The estimated RMS roughness was 10 nm.


83
the roughened substrate with a morphology indistinguishable from the
substrate.
The goal of changing surface roughness in-situ was accomplished by
increasing the Oz growth pressure. The crystalline texture changed as a
function of the growth pressure (figure 4-28), as it did with temperature (figure
4-18). The films transformed from preferred (111) orientation at low Oz
pressure to more random polycrystalline at pressures greater than 400 mTorr.
As seen in figure 4-28, at 600 mTorr the (400) peak becomes very pronounced.
The (111) FWHM is plotted versus Oz pressure during growth in figure 4-29,
showing that the FWHM decreased as the pressure was increased up to 200
mTorr. Above this pressure, the reported FWHM increased again. As seen in
figure 4-28, pressures above 200 mTorr resulted in a change from preferential
(111) orientation to more random polycrystalline films. Just as with increasing
growth temperature (figure 4-20), as the film changed from a (111) preferred
orientation to a random polycrystalline film, the FWHM increased.
At 600 mTorr and 600 C, the peaks marked (*) in figure 4-28 were from
a yttrium silicate phase that formed during deposition. To avoid reactions
between Y203 and Si02 to form a silicate, a nucleation layer of Y203 was
deposited at 200 mTorr for two minutes prior to continued deposition at 600
mTorr. As seen in figure 4-30, formation of a silicate phase was avoided and a
preferential (111) orientation was maintained by using this nucleation layer.


YIELD
128
Figure 4-42. Rutherford Backscattering spectrum of film grown on (100)
silicon at 100 mTorr Oz pressure and 600 C for 1 minute.


176
Fel57b
Fol94
For69
Gav90
G0I66
Gre94a
Gre94b
Gro89
Gu91
Gui95
Han65
Has90
Hen89
Hir97
Feldman, C. & OHara, M., Journal of the Optical Society of
America 47, 300.
Foltyn, S. R., Pulsed Laser Deposition of Thin Films, edited by
Crisey, D. & Hubler, G.K., John Wiley and Sons, New York, NY,
chapter 4.
Forest, H. & Ban, G., Journal of the Electrochemical Society 116,
474.
Gavigan, J.,Givord, D.,Lienard, A., McGrath, O., Rebouillant, J., &
Souche, Y., MRS Symposium Proceedings volume 191.
Goldberg, P, Luminescence of Inorganic Solids, edited by
Goldberg, P., Academic Press, New York, NY, chapter 7.
Green, S., Pique, A., Harsjavardhan, K., & Bernstein, J., Pulsed
Laser Deposition of Thin Films, edited by Chrisey, D. & Hubler,
G.K., John Wiley and Sons, NY, New York, chapter 2.
Greer, J., Van Hook, V. H., Tabat, M., Nguyen, H., Gammie, G., &
Koufopoulos, P., MRS Symposium Proceedings volume 345, 281.
Grovemor, C. R. M., Microelectronic Materials, edited by Cantor,
B., Institute of Physics Publishing Ltd, Philadelphia, PA.
Gu, Z., Dummer, R. S., Maradudin, A. A., McGum, A. R., &
Mendez, E. R., Applied Optics 30, 4094.
Guillaume, C.B.A.L., Encyclopedia of Applied Physics vol 13.
edited by Trigg, G.L., VCH Publishers, Inc., NY, New York.
Hansen, W. W. & Myers, R. E., Applied Physics Letters 6, 58.
Hase, T., Kano, T., Nakazawa, E., & Yamamoto, E., Advances in
Electronics and Electron Physics vol. 79.
Henderson, B., & Imbusch, G.F., Optical Spectroscopy of
Inorganic Solids. Oxford University Press, New York, NY.
Hirata, G.A., McKittrick, J., Avalos-Borja, M., Siqueiros, J.M., &
Devlin, D., Applied Surface Science 113/114, 509.


99
CL Efficiency versus Voltage
films grown at 600 mTorr vs 6 Torr Oz pressure
Voltage, [kV]
Figure 4-13. Plot of cathodoluminescence efficiency versus voltage as Oz
pressure is increased from 600 mTorr to 6 Torr. Films were grown
on (100) Si at 460 C and annealed in air at 1200 C for 1 hour.
Efficiency data taken at 1 (lA/cm2.


93
Figure 4-7.
CL Efficiency for Osram Sylvania YgOgiEu
Voltage, [kV]
Cathodoluminescence efficiency of Osram Sylvania powder
standard at 1 [lA/cm2 versus applied voltage. Shows
characteristic linear low voltage region with saturation at higher
power density at around 2.5 kV.


CHAPTER 7
FUTURE WORK
This research suggest that light piping effects in thin films can be
significantly reduced or even eliminated through surface modifications. Even
though the efficiency of the films was improved by increasing the roughness,
the brightness was still much lower than the unprocessed, unscreened
powder. To utilize the benefits of the higher resolution possible from thin film
screens (50 times higher resolution), future work should be done in the
categories of optimizing the RMS roughness, optimizing the dopant
concentration, improving the as-deposited film crystallinity, and doing
experiments to understand and quantify the surface recombination effects in
Y203:Eu thin films. A few suggestions are offered below.
The optimum RMS roughness, according to the anomalous diffraction
equation, would be ~ 200 nm, suggesting that the roughness could be further
optimized. Roughnesses around this value should be tried, possibly using
template substrates manufactured utilizing photolithography techniques. An
increase in RMS from 71 nm to 200 nm, theoretically, would triple the efficiency
(Qsoa from 0.83 to 3.17). The CL efficiency at 2 kV and 1 jUA/cm2 for the 9 jum
thick film would have been 12.66 lm/W, whereas the powder was 9.5 lm/W.
168


CHAPTER 4
RESULTS
In this chapter, microstructure and chemical data are presented for
Y203:Eu films grown by PLD. The objective of this study was to determine the
effects of roughness on the light out-coupling in europium doped yttria thin
films. It was found that increased surface roughness increased the out-
coupling. The increase in roughness was accomplished in-situ by increasing
the Oz growth pressure. Samples were categorized as smooth versus rough as
determined by Atomic Force Microscopy (AFM). Smooth films had RMS
roughnesses of 3 nm, while rough films had RMS roughnesses of 71 nm.
Luminescence data reported under PL and CL are presented first with the film
growth and annealing processes resulting in these improvements following.
Parameters such as deposition temperature, and annealing temperature also
had a significant effect PL and CL brightness.
Photoluminescence(PL)
The films yielded weak luminescence detectable only by eye at growth
temperatures of 250 C in vacuum. The PR650 camera was able to detect
76


53
Critical Angle vs Index of Refraction Ratio
Figure 2-14. Plot of critical angle versus the refractive index of air to phosphor
ratio. For Y203 at 611 nm, the index of refraction is equal to 1.93.
This yields a critical angle of 31.


106
FWHM versus Growth Temperature
on (100) silicon
Figure 4-20. FWHM versus growth temperature for films grown on (100)
silicon at 2 x 10'5 Torr.


100
CL Efficiency versus Voltage
for varying film thicknesses
Voltage, [kV]
Figure 4-14. Cathodoluminescence versus voltage for increasing thin film
thicknesses taken at 1 (jA/cm2. Films grown on (100) Si at 600 C,
600 mTorr oxygen growth pressure and annealed in air at 1200 C
for 1 hour.


27
4 4
Qsca = 2 SnP + (1-COSp) .
P p2
Here, the assumptions are that x and p are large, where large is any value
close to or greater than l.[Van81] Anomalous diffraction, just as Rayleigh
scattering, can also be used with large n values. As radius, a, becomes larger
than the wavelength of light, anomalous diffraction moves into geometric
optics, where the classical diffraction, reflection, and refraction rules are
appropriate. [ J en7 6 ]
In summary, scattering theory can be applied to particles, but the particle
size, wavelength of incident light and refractive index determine the type of
scattering which occurs. For extremely small particles, Rayleigh scattering
dominates, having a (a/A)4 dependence. As the particle size and index are
increased, Rayleigh-Gans scattering applies, having a dependence ranging
from (a/A,)4 at low x to (a/A,)2 at larger x. Once the x and x(n-l) values reach
unity, or larger, scattering is described by anomalous diffraction with an
integral that leads to between (a/A)2 and a (a/A)3 dependence for small and
large radius, respectively. For particle radii greater than the wavelength of
light, geometric optics, such as ray tracing, should be used.


130
(431)
Figure 4-44. X-ray diffraction patterns showing the effect of annealling
temperature on yttria films deposited on (100) silicon in 600
mTorr Oz pressure at 460 C followed by annealing at (a)1200 C,
(b) 1000 C, (c) 800 C, and (d) as-deposited. Films were furnaced
annealed in air for 60 minutes.


7
Light
Emission
Gate
Insulator
Cathode
Base Plate
Emitter Tips
Figure 1-1. Spindt cathode type field emission display (FED). The Spindt type
is the most common FED structure.


35
temperatures greater than the melting temperature are reached at depths
greater than the thermal diffusion distance. Target components, therefore,
cannot segregate despite differences in vapor pressures. This yields
congruent evaporation from multicomponent targets.
Besides wavelength [Kor89], repetition rate [Cha90] and spot size
[Wu90] also influence the ablation characteristics and therefore thin film
growth. The spot size of the laser beam will affect the deposition rate and the
amount of particulates ejected by the beam. Larger beams are attenuated by
the plasma plume created when the laser strikes the target. As the spot size is
reduced, the deposition rate increases due to reduced beam-evaporation
plume interaction. The beam-plume interaction can reduce the number of
particulates by evaporating them in the vapor phase. [Eye87] The reduction of
laser wavelength has this same effect since there is a high absorption in the
vapor plume that further volatilizes the fragments. [Kor89] It has been shown
that changing the repetition rate affects the film crystallinity and morphology.
Reducing the repetition rate reduces the growth rate which increases the time
atoms can diffuse to equilibrium positions. This leads to a higher crystallinity
and film density. [Wu90] For every set of growth conditions there will be an
optimum repetition rate, or arrival rate, that yields smooth dense thin
films.[Cha90] Coupled with the ability to change the standard growth
parameters such as substrate temperature and growth pressure, PLD has


49
1C 15 20
Accelerating Voltage, [kV]
25
Figure 2-10. Plot of electron penetration depth versus accelerating voltage,
per Terrils equation for Y2O3.[Fel60]


61
Primary beam energies of 2.2 MeV and doses of 1019 atoms/cm"2 were typically
used. RBS can detect a few parts per million (ppm) of heavy elements in a
light element matrix and is insensitive to chemical bonding and matrix effects,
unlike Auger electron spectroscopy (AES) and secondary ion mass
spectroscopy (SIMS). RBS is routinely used for quantitative compositional
analysis of thin films, layered structures and bulk specimens, surface dopant
analysis, defect depth distribution quantification of percent crystallization,
and film thickness (or density).[Chu86]
Backscattering results from direct collisions between He ions and nuclei
of sample atoms. The energy of the back scattered particle is a function of
both the amount of energy lost due to momentum transferred during the
nuclear collision plus the energy loss during collisions incoming and scattered
He particle with electrons in the sample. Measuring the energy of the
scattered ion gives a direct measurement of the mass of the ion, and hence
mass identification. The projectile energy to incident ion energy after the
elastic collision ratio (kinematic factor) is given by the equation:
K = EJE,
O
1 + (M/M2)
where Mj is the mass of incident ion, M2 is the mass of target atom, 0 is the
angle between trajectory of He ion and the angle of detector. [Chu78] See
figure 3-4. Since K is a function of Mx/M2, there is a larger energy separation


the powder had an efficiency of 9.5 lm/W. The CL efficiency for a 9 /im thick
rough film was 3.3 lm/W at 2 kV and 1 /jA/cm2.
The CIE color chromaticity coordinates (x, y) of the films were different
from the powder {(x, y) of (0.62,0.35) for film instead of (0.644, 0.35) for
powders}, indicating that the concentration of active europium was lower in
the films, by as much as one mole percent. This contributed significantly to
lower CL efficiencies for the films. This was confirmed with RBS.
Improvements in luminescence with an increase in surface roughness
were attributed to forward light scattering caused by the rough surface. The
increase in light scattered at the surface was presumed to reduce the
percentage of light internally reflected within the film. The appropriate
scattering model was anomalous diffraction. Although the 3 jUm thick rough
film was only 27.7% as efficient as the powder, anomalous diffraction theory
predicts that increasing the roughness to 200 nm would yield an efficiency of
12.7 lm/W (134% that of powder).
x


116
Diffraction Angle, [26]
Figure 4-30. X-ray diffraction patterns showing the elimination of yttrium
silicate phase for a film grown at (a) 200 mTorr for 2 minutes and
11 minutes at 6 Torr versus (b) a film grown at 600 mTorr for 15
minutes. Silicon substrate temperature was 600 C.


Table 1-2 Market Share information for Flat Panel Displays
Display
1997
1998
1999
2000
2001
2002
2003
LCD-TFT
$8407 ($316)a
$9917 ($306)
$11743 ($298)
$14317 ($296)
$16227 ($282)
$17905 ($264)
$19165 ($242)
EL
$115 ($442)
$124 ($449)
$135 ($464)
$152 ($483)
$168 ($494)
$187 ($507)
$207 ($517)
Plasma
$325 ($1014)
$416 ($1143)
$665 ($1387)
$964 ($1530)
$1466 ($1640)
$2085 ($1641)
$2357 ($1548)
FED
$1 ($169)
$5 ($144)
$35 ($165)
$94 ($173)
$219 ($167)
$333 ($143)
$444 ($132)
LEDb
$542 ($3)
$552 ($3)
$567 ($3)
$572 ($3)
$586 ($3)
$601 ($3)
$617 ($3)
Vacuumb
$752 ($5)
$769 ($5)
$791 ($5)
$806 ($5)
$815 ($5)
$821 ($5)
$826 ($5)
a Estimated sales revenue in millions of dollars. (Unit selling price in dollars.)
b These displays are alpha-numeric displays which are extremely small.
[DeJ97]
cn


122
-*
PLD 32
Hh jflk
^ vt '
% *
^ jffm
|L #
wv
1 0 0 n m
15KU
X 1 0 0,0 0 0 15mm
Figure 4-36. SEM (100,000x) photograph showing nanoparticle features of film
grown at 600 mTorr and 600 C on (100) silicon.


144
found at 600 mTorr. As seen in figure 4-38, the root mean square roughness
(RMS) was approximately 2 nm for low 02 pressures, but increased to 71 nm at
600 mTorr. Previous work has shown that pulsed laser ablation can be used
for film growth (UHV to 300 mTorr) or to generate particulates (1 Torr to 760
Torr).[Mat86]
The ultimate goal of increasing the brightness by surface modification
was achieved. Here, it has been shown that films can be fabricated in-situ
with a particle like texture,which improves the efficiency. Surface modification
has previously been accomplished by changing the repetition rate [Cha90] or
by post deposition laser processing [Fol94]. This is, however, the first report of
intentionally utilizing the growth pressure to effect the surface morphology.
As noted by Chen [Che94b], the use of reactive or inert gases during growth
has not been fully explored beyond the extremes of achieving stoichiometry or
producing small sized powder particles. Only recently has it been explored as
a technique to fabricate composites and nanophases.[Che94b]
Annealing
Annealing improved the crystallinity of the films. However, the (111)
FWHM from annealed films (minimum of 0.14) were not as low as the value of
the powder (0.09 to 0.11). Annealing at high temperatures should have
improved the FWHM. However, the melting temperature of yttria is 2683 K, so


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edited by Brundle, C.R., Evans, C. A., & Wilson, S., Manning
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173


50
Figure 2-11. Schematic diagram of luminescence transitions between the
conduction band (Ec) and the valence band (Ev). Processes 4-6 are
typical transitions occurring within semiconductors where
process 7 is typical for rare earth dopants.[Yac92]


120
Figure 4-34. SEM photograph of film grown on (100) silicon at 400 mTorr and
600 C showing a nodular surface with a RMS roughness of 20 nm.


97
Figure 4-11.
CL Efficiency versus Voltage
for films grown at 100 mTorr and 600 mTorr annealed at 1200 C
Voltage, [kV]
Cathodoluminescence efficiency of films grown on (100) Si at 460
C and 100 mTorr versus 600 mTorr. Efficiency taken at 1 |iA/cm2.
Typical RMS values of 3 nm and 71 nm were obtained at 100
mTorr and 600 mTorr, respectively.


21
films with good crystallinity and optimized activator, internal reflection and
absorption is a major loss factor as explained by the following.
Light impinging on an interface between two different media with
different optical indices, will be reflected and refracted. Specifically, when
light traveling at some angle 01 reaches the interface, the wave is refracted
according to Snell's law
r^sinOj = n2sin02
where n, and n2 are the indexes of refraction for each media and the 0t is the
angle of the incidence light and 02 is the angle at which the light refracts.
When medium 1 is more dense than medium 2 (n, > n2), an angle exist where
above its value, incident light is not refracted but is totally reflected back into
the film. This angle is called the critical angle for total internal reflection, 0C,
and is equal to
c = sin '( )
^phosphor
where naiI and nphosphor are the index of refraction for air and the film,
respectively. See figure 2-13, where ray c is incident at 0C and ray d is incident
at an angle > 0C. Air has an n value of 1. Yttria has an index of refraction (at
611 nm) value of 1.93, yielding a 0C of 31. Figure 2-14 shows the change in the
critical angle as (l/nphosphor) is varied.


67
films on silicon, PL was recorded in the reflectance mode at room temperature.
More complicated PL equipment may allow spectral analysis at cryogenic
temperatures, PL excitation spectroscopy, time-resolved PL, and PL
mapping. [Col92, Gui95]
Cathodoluminescence(CL)
Cathodoluminescence (CL) utilizes energetic electrons to excite
radiative relaxation.[Yac92] CL data was taken using two different systems.
The first system used the co-axial electron gun from a PHI model 545 scanning
Auger electron spectrometer with an Oriel Multispec optical spectrometer and
a CCD array detector. The AES gun was capable of providing a primary
electron beam with an energy from 100 eV to 5 keV and currents between 1
and 5 /jA with a spot size from 0.5 to 3 mm. This resulted in DC current
densities from 10 to 510 /jA/cm2.
The other CL system consisted of a EGPS-7H Kimball Physik electron
gun, and an optical fiber coupled to an Ocean Optics S2000 spectrometer with
a CCD array detector. The EGPS-7H electron gun was capable of energies from
50 to 5 keV and curents from 0.01 to 500 /JA with spot sizes from 2 mm to 3 cm.
This yielded current densities ranging from 1.4 x 103 to 1.6 x 104 jUA/cm2.
Both the Oriel and the Ocean Optics spectrometers were able to resolve
the characteristic Eu 3 line emission since their resolution was less than 1 nm.
The sample current was determined by biasing the electrically isolated sample


181
Tro95
Uts91
Van81
Wes90
Wic64
W147
Wu90
Wu94
Wyc64
Wys78
Yac92
Tropf, W. J., Thomas, M.E., & Harris, T. J., Handbook of Optics
Volume II. Academic Press, Boston, MA, chapter 33..
Utsumi, T., IEEE Transactions on Electron Devices 38, 2276.
VanDe Hulst, H. C., Light Scattering bv Small Particles. Dover
Publications, Incorporated, New York, NY.
West, G. A & Beeson, K. W., Journal of Materials Research 5,
1573.
Wickersheim, K. A. & Lefever, R.A., Journal of the Electrochemical
Society 111, 47.
William, F. E., Journal of the Optical Society of America 37, 302.
Wu, X., Muenchavsen, R.E., & Foltyn, S., Applied Physics Letters
56, 1483.
Wu, X. D., Muenchavsen, R. E., Foltyn, S., Estler, R.C., Dye, R.C.,
Flamme, C., Nogar, N.S., Garcia, A.R., Martin J., & Tesmer, J.,
Applied Physics Letters 56, 1481.
Wycoff, R.W.G., Crystal Structures Volume 2. Interscience
Publishers, New York, NY.
Wyszecki, G., Handbook of Optics, edited by Drscoll, W.G.,
McGraw-Hill, Inc., New York, NY, chapter 9.
Yacobi, B.G., Encyclopedia of Materials Characterization, edited
by Brundle, C.R., Evans, C.A., & Wilson, S., Manning Publications
Co., chapter 3.3.


66
were also done by recording the AES spectrum, sputter for 2 minutes, and
record a new spectrum to detect changes in element concentrations.
Photoluminescence(PL)
Photoluminescence (PL) brightness was measured using a UVP UVG54
6 watt UV lamp and a Photo Research Pritchard 650 (PR650) camera. [Col92]
This camera was equipped with an internal light source and brightness could
be measured in footlamberts (fL). Instead of using peak height intensity as the
brightness value, the PR650 used the calculated area-under-the-curve between
380 and 780 nm. The PR650 also determined CIE coordinates from the
measured spectral distribution. The UV lamp consisted of a mercury vapor
tube with filters to pass primarily the short wavelength of 254 nm (figure 3-7).
The lamp intensity was 2250 ¡jW/cm2 at 254 nm at a distance of 3 inches. The
phosphor absorbed the radiation by promoting an electron from the ground
state to an excited state. The excited atom then relaxed radiatively emitting a
characteristic photon. [Col92]
The lamp and the spectrometer were 90 apart, therefore samples were
arranged at 45 from the lamp and the PR650. The spectrometer was focused
using focal lenses. The PR650 had a resolution of 5 nm, therefore it did not
resolve the true line width of the Eu+3 but was sufficient for peak identification
and brightness comparisons. The samples were mounted on a black
background to reduce reflection. Since the primary data collected were from


62
for lighter elements than heavier ones due to a significantly larger amount of
momentum transfer to lighter elements. This results in excellent mass
resolution between light elements and between light and heavy element
matrixes, but poor resolution between heavy elements. There is good mass
resolution for the Y203:Eu matrix.
The probability of backscattering into a given solid angle per incident
particle is referred to as the differential scattering cross section, do/d, where
do
dQ
(Z,Z2e2)2 4
4£ sin40
(^-((M/MJsin)2 + cos0)2
y/l-((M1/M2)sin0)2
where = atomic number of incident atom, Z2 is the atomic number of target
atom, E is the incident energy before scattering, and e is the charge. The
scattering cross section is usually simplified to (Z1Z2e2)2/4e. Since do/dQ is
proportional to Z22, larger atomic number elements are more efficient in
scatterering the incident ion. In the case of a Y203:Eu matrix, even though the
mass resolution is poor for heavier elements, RBS is very sensitive to the
yttrium and the europium. [Chu78]
Atomic Force Microscopv(AFM)
Atomic force microscopy (AFM) belongs to the family of scanning probe
microscopy (SPM).[Bin82, How93] AFM was used to quantify film roughness
and other morphological details about phosphor thin film surfaces. A Digital


30
using vacuum evaporation. Koller et al [K0I6O] evaporated ZnS:Mn in a H2S and
HC1 background gas. This helped to maintain stoichiometry and to incorporate
Cl into the film. They reported that while their films were crystalline as-
deposited at 100 C, they did not luminescence until heat treated at 650 C.
They also produced Zn2Si04:Mn thin films by evaporating ZnF2:Mn onto heated
quartz (550 650 C) substrates. Bateman [Bat60] also evaporated ZnS:Mn thin
films and a MgF2:Zn,Mn phosphor thin film. The MgF2 phosphor performed as
well as the ZnS:Mn thin film.
Kirk et al. [Kir61] used spray pyrolysis to react ZnCl with a heated silica
substrate to make ZnSi204 doped with Mn and Ti thin films. They achieved
thin film efficiencies of 0.24 lm/W as-deposited and 0.32 lm/W for films
annealed at 1250 C for 1 hr. Their beam conditions were 7.5 kV and 10
mA/cm2. Spray pyrolysis was later used by Falcony [Fal92] to deposit ZnS:Mn
films from which PL data were reported. Luminescence was achieved with
substrate temperatures as low as 360 C. With PL, they showed the reduction
of a self-activated defects as the growth temperature was increased.
Hansen et al. [Han65] were the first to use electron beam evaporation to
deposit phosphor thin films. Yttrium oxide doped with various rare earths was
deposited onto heated molybdenum strips. They achieved 2.7% of the
brightness of the standard powder for europium doped films, but 41% of the
powder brightness for Tb doped. No explanation for the differences was given.


Normalized
156
2%, 3% 5%
Eu concentration (mole %)
Figure 5-10. Plot of luminescence intensity versus europium concentration for
Y202S, Y203 and YV04 phosphors showing the optimum europium
concentration for each phosphor system. Concentration
quenching occurs at mole percents greater than this critical
value. For Y203, this value is 3 mole percent. [Oza90]


74
4
Figure 3-6. Schematic diagram of AFM utilizing position sensitive photo
diode detector to measure changes in laser position which
correlates to z height changes.[Bhu95]


162
Temperature, [C]
1500 1400 1300 1200 1100
1000/T [K'1]
Figure 5-16. Plot of activation energy for change in CL efficiency versus
annealing temperature showing similar activation energies for
films grown on (100) silicon at 460 C and annealed at various
temperatures for 1 hour in air. The smooth film (100 mTorr) has
an activation energy, Ea, equal to 26 cal/mole while the rough film
(600 mTorr) has an Ea equal to 28 cal/mole.


73
Figure 3-5.
Interatomic force versus atomic distance curve illustrating the
two AFM regimes, contact and non-contact. [How93]


59
Since the atomic arrangement varies for each of the seven crystal
systems, the allowed diffraction from various hkl atomic planes will also vary
per crystal system. Appendix B shows the allowed hkl combinations. The d
spacing is a function of the plane location and the lattice parameter. In cubic
systems, dhkl is given by
H ao
^hkl ,
\¡h2 + k2 + l2
where aD is the lattice parameter of the crystal. The resulting peak contains
contributions from the sample, instrumentation error and spectral distribution.
By eliminating the latter two, the profile can be manipulated to quantify
crystalline defects.[Smi86] The amount of x-rays which constructively diffract
is dependent on the crystallinity, therefore, a direct comparison of % crystalline
can be made between samples by comparing the area under the peaks. Care
must be taken to measure the same physical area and thickness for all of the
samples to make meaningful comparisons of peak areas. If the sample
thickness is much greater than the characteristic absorption depth of x-rays,
thickness differences can be neglected. The x-ray absorption depth for yttria is
10m. Peak width can yield information about crystallinity and defects, and
position yields information about mismatch and strain.


APPENDIX B. ALLOWED (hkl) VALUES FOR X-RAY DIFFRACTION
Bravais lattice
Reflections possibly present
Reflections necessarily absent
Simple
all
none
Base-centered
h and k unmixed*
h and k mixed*
Body-centered
(h + k + /) even
(h + k + 1) odd
Face-centered
h. k. and / unmixed
h, k, and / mixed
Cul78]
172


26
The difference between Rayleigh and Rayleigh-Gans scattering is that a larger
phase shift is allowed. The scattering efficiency, Qsca, is expressed as an
integral that is bound by two limiting cases of x < < 1 and when x > > 1, where
the efficiency is expressed as
Q. = |n-l|2i|r(x).
The function ijf(x) is an integral equal to
i|/(x) = + 2x2 (1 -cos4x) +( 2)[0.577 + log4x Ci(4x)]
2 4x i6x2 2x2
where Ci(4x) is a cosine integral bound by x and . When x < < 1, the Rayleigh
scattering condition is obtained. As x becomes large and x(n-l) remains small,
the scattering changes from a (aA)4 to a (aA)2 dependence and is expressed as
Qsca = 2(n-l)2x2 = 0.5p2 .
This is the beginning of the intermediate scattering condition. Rayleigh-Gans
scattering can be thought of as being bound by the two extremes, Rayleigh
scattering and intermediate scattering. As seen by the formula, intermediate
scattering has a (a/A)2 dependence.
As x or n are increased further, the scattering condition changes again to
anomalous diffraction where the scattering efficiency is expressed in terms of a
sine integral:


20
The phosphor quantum efficiency, r)eff, under electron beam
bombardment may be expressed according to Robertson and van Tol:
'eff
(l-rjhv
t 'a 'esc
where rb is the fraction of electrons backscattered, hvem is the average energy
of the emitted photons, and r|t, r|a, and r|esc correspond to the efficiency of
transferring the generated electron-hole pairs to the luminescent center, the
efficiency of radiative recombination at the luminescent center, and the
fraction of light escaping the sample, respectively. [Rob80] The minimum
energy needed to create an electron-hole pair is represented by the constant,
P, times the band gap,Eg. The constant P varies from 2 to 8 [Has90, Bec96], but
is usually set equal to 2.8 [Ait93], For phosphor powders, it is assumed that all
of the luminescent centers are active, in the proper crystallographic sites, and
that once the light is generated within the powder, there are enough
reflections that all of the light escapes with minimum absorption. Thus r|a and
r|esc are equal to one.[Bec96]
For thin film phosphors, however, these assumptions may not be valid.
For poor crystalline quality films, the transfer of electron-hole pairs to
luminescent centers and the probability of radiative recombination at the
luminescent center will be lower than unity. The overall efficiency is further
reduced by a low value of r)esc due to internal reflections (figure 2-13). For thin


13
the Lanthanide series which is characterized by an unfilled 4f electron shell
surrounded by filled 6s and 5p shells. Europium can have either a Eu+2 or a
Eu+3 valence where the electron configurations are (Kr core) 4d104f65s25p65d1
and (Kr core) 4d104f65s25p65d, respectively. See figure 2-5. Note that due to
the electron shell filling rules, the 4f level is partially filled until the 5d shell has
been completely filled. In yttria, Eu substitutes for the Y in both the S6 and C2
sites.
For the Lanthanide series rare earths, the deep lying 4f electrons are
shielded from the effects of the host lattice by the 6s and 5p shells. This gives
rise to a number of discrete energy levels that, due to the shielding, resemble
the energy level diagram of a free ion.[Bla94] Since the excitation and emission
occur within the discrete levels of the 4f shell, a characteristic sharp line
emission 5Dj to 7Fj results. See figures 2-6 and 2-7.
The allowed transitions (5Dj to 7Fj) from the activator, Eu+3, are a
function of the selection rules. [Hen891 Three types of transitions are possible:
electric-dipole (ED) transitions, magnetic dipole (MD), and electric quadrapole
(EQ) transitions. The strength of these transitions decreases from ED to MD to
EQ as the ratio 1:105:10'6. The weaker MD and EQ transitions are usually
allowed, so we are concerned with the selection rules governing ED
transitions. Laportes rule states that electric dipole transitions within the 4f
level are forbidden due to unchanged parity. Parity refers to the state of the
orbital angular momentum quantum number, 1, which can be even or odd.


Cathodoluminescence(CL) 78
Thin Film Growth 80
Surface Roughening 82
Annealing 85
5. DISCUSSION 136
Enhanced Luminescence Through Scattering 136
CIE Coordinates and Reduced Luminescence 139
Thin Film Growth 141
Annealing 144
6. SUMMARY AND CONCLUSIONS 164
7. FUTURE WORK 168
APPENDICES
A CALCULATION FOR FRACTION OF LIGHT EMITTED 171
B ALLOWED (hkl) VALUES FOR X-RAY DIFFRACTION 172
REFERENCES 173
BIOGRAPHICAL SKETCH 182
Vll


(I/Io Io), [arb. units]
151
Figure 5-5.
PL Intensity versus RMS Roughness
film response and theoretical
Photoluminescence intensity versus RMS roughness of
experimental values and theoretical according to anomalous
diffraction, as seen in figures 5-2 and 5-3. Plot shows that
reported brightness increases due to scattering from surface
roughness fits anomalous diffraction theory well.


79
grown at 600 mTorr and 460 C. Under cathodoluminescence, the rougher
films were significantly brighter than smooth films, similar to PL data (figure 4-
4). The significant improvement from annealing temperature 1000 C to 1200
C was also similar (figures 4-4, 4-9, and 4-10)
A comparison of CL efficiency for films grown at 460 C in either 100
mTorr or 600 mTorr and annealed at 1200 C for 1 hour in air is shown in figure
4-11. From AFM, the RMS roughness for films grown at 100 mTorr and 600
mTorr were 3 nm and 71 nm, respectively. The films grown at 600 mTorr were
~ 3 times more efficient.
The effect of surface roughness versus CL efficiency was also
demonstrated by films deposited on c-axis sapphire. Films were grown on c-
axis sapphire at 800 C, 2 xlO'5 Torr versus 600 C, 600 mTorr and annealed at
1200 C in air for 60 minutes (figure 4-12). The smooth as-deposited films and
smooth annealed films (grown at 2 x 10'5 Torr) had similar CL efficiency values,
while the rough film (grown at 600 mTorr) had a significantly higher CL value.
The rougher films were- 3 times more efficient.
Films grown at a higher growth pressure than 600 mTorr (up to 6 Torr)
were also annealed at 1200 C in air for 1 hour. The efficiency was not
significantly higher than those grown at 600 mTorr as shown by the efficiency
data in figure 4-13.
Cathodoluminescence efficiency versus annealed film thickness was
measured as shown in figures 4-14 and 4-15. Just as for PL, increased film


150
Figure 5-4.
PL Intensity versus Roughness
films grown on (100) Si at 600 C
RMS, [nm]
Photoluminescence intensity versus root mean square roughness
(RMS) for films deposited at various growth pressures on (100)
silicon at 600 C.


6
Table 1-3 Advantages and Disadvantages of Thin Film
versus Powder Phosphor
THIN FILM
PHOSPHOR
PHOSPHOR
POWDER
Efficiency
poor
excellent
Resolution
<< ljL/m
5 jUm to 10 j(im
Screen Contrast
excellent
good
Lifetime(at high /jA/cm2)
good
poor
Mechanical Stability
excellent
good
Thermal Stability
excellent
good
[Fel57a]


167
(28% of powder) for smooth and rough films, respectively. The CL efficiency for
a 9 /jm thick film at 2 kV and 1 jL/A/cm2 was 3.3 lm/W (35 % of powder).
All of the films had CIE x and y coordinates which were lower and
higher, respectively, as compared to the powder. The powder had (x, y)
values equal to (0.644, 0.352) at 4 kV and 1 jUA/cm2. The films varied with x:
0.601 to 0.625 and y: 0.359 to 0.362. This indicated that the amount of active
europium was at least a half of a mole percent less than that of the powder,
resulting in much lower efficiency. RBS confirmed that the europium
concentration was lower than 3 mole percent.
From the data presented, it is seen that increasing the europium
concentration and increasing the scattering efficiency by increasing RMS
roughness, the thin film phosphor efficiency could be advanced significantly.
These advances could lead to an efficient thin film phosphor system, even at
lower processing temperatures. This would provide the FED industry with a
high resolution display that is physically and thermally rugged.


LD
1780
1997
.31X5
UNIVERSITY OF FLORIDA
I ill ill mu mu '1" 0
3 1262 08555 0472


Table 2-1 Efficiency Data for Phosphor Thin Films
Phosphor
Volts
(kV)
Current
Density
OuA/cm2)
Bright
ness
(Cd/m2)
Eff.
(lm/W)
Growth
Tech.
Ref.
Comment
ZnS:Mn
10
1
34.3
1.08
a
Cus 52
also ZnS:Zn blue
ZnS:As,P
10
1
34.3
1.08
a
Stu55
from plot
ZnS:As,P
20
1
119.9
1.88
a
Stu55
from plot
ZnS:Mn
12
10
6.9
0.02
b
Fel57b
as-deposited
ZnS:Mn
12
10
68.5
0.18
b
Fel57b
annealed
ZnS:Mn
12
10
116.5
0.31
b
Fel57b
A1 coated
ZnS:Mn
12
10
342.6
0.90
b
Fel57b
annealed, fogged film
CaF2:5%Mn
12
10
27.4
0.07
b
Fel57b
clear film
pZn2(P04)3:3% Mn
12
10
411.1
1.09
b
Fel57b
CaW04:W
12
10
34.3
0.09
b
Fel57b
clear
CaW04:W
12
10
68.5
0.18
b
Fel57b
fogged film
Zn2Si04:8%Mn
12
10
205.6
0.54
b
Fel57b
clear
Zn2Si04:8%Mn
12
10
1027.8
2.69
b
Fel57b
annealed, fogged film
CaW04:W
10
10
54.1
0.14
b
Fel57b
annealed, fogged film
a-Zn2Si04:Mn
7.5
10
58.2
0.24
c
Kir61
vycor substrate
"CJ


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, 1997
p -i.
i ^ Winfred M. Phillips^
Dean, College of Engineering
Karen A. Holbrook
Dean, Graduate School


46
Cathodoluminescence Spectrum of Y203:Eu
Wavelength, [nm]
Figure 2-7. Cathodoluminescence spectra for Y203:Eu+3 showing sharp line
emission at 611 nm. Spectra taken with Oriel Multispec
Spectrometer.



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Log (Counts / Second)
129
Figure 4-43. SIMS surface scan of film grown on (100) silicon at 600 C and 600
mTorr, showing surface contaminants, oxygen at a mass/charge
ratio of 16, yttrium at 89 and europium at 151 and 153.


24
the first important parameter for scattering due to spherical particles is the
particle size. The size parameters that describe this scattering are x, and p,
given by the following equations:
2na
x =
p = 2x|n-l| ,
where a and n are the radius and the index of refraction of the sphere,
respectively. [Van81] For the case of nonconducting, non-absorbing, relatively
small spheres (small x and p) with refractive indices close to 1, the following
scattering conditions apply; Rayleigh scattering, Rayleigh-Gans scattering,
intermediate scattering, anomalous diffraction or the limiting case of geometric
optics where a>A. The x and p requirements to meet each condition are
presented below with their effect on scattering.
The most well known of scattering theories is Rayleigh scattering,
developed by Lord Rayleigh in 1881.[Str64] Rayleigh scattering occurs when,
(a) the external field can be considered as a homogenous field across the
particle, mathematically expressed as
X
a
2n
x 1,


INTENSITY [ARB. UNITS]
104
Figure 4-18. X-ray diffraction patterns of yttria films grown on (100) silicon at
(a) 800 C, (b) 600 C, (c) 400 C, and (d) 250 C at 2 x 10 5 Torr.


56
holder was not rotated. The distance between the target and substrate could
be varied from 2.5 cm to 5 cm, but 3 cm was the most commonly used target-
to-substrate distance. Typically, deposition was accomplished over 15
minutes at 500 mJ/pulse with a focused spot of 0.2 cm2 and a pulse rate of 10
Hz. This yielded 600 pulses/minute with an energy density of 2.23 J/cm2 per
pulse or 22.3 W/pulse. A 12% loss due to absorption and reflections of the
mirror, lenses, and quartz vacuum window was factored into the energy
density calculation. The loss was measured using a laser power meter. The
combination of target-to-substrate distance and laser conditions yielded
deposition rates of 3.7 /pulse (36.6 /sec) at high vacuum conditions and 2.7
/pulse (26.7 /sec) at 600 mTorr.
Ultra high purity oxygen from compressed gas cylinders was used to
grow at higher pressures. The gas was introduced into the vacuum system
using a MKS model 247C mass flow controller at 80 seem. The pressure was
regulated by throttling the turbo pump, using the gate valve to achieve the
desired pressure. Pressures of 106 Torr to 600 mTorr were obtained using this
method.
The substrates were heated via a quartz lamp to temperatures from 150
C to 1000 C. A stainless steel heater plate was used as the thermal contact
between the lamp and the substrate. Substrates were mounted onto the plate
with silver paste. Deposition temperatures were monitored using a
thermocouple in contact with the stainless steel plate via support clips. Some


40
Figure 2-1.
Alternative lifted edge emitter field emission display technology.
Display allows for thin film phosphor integration with edge
emitter onto same substrate.


CHAPTER 3
EXPERIMENTAL PROCEDURE
Substrate and Target Preparation
Yttrium oxide doped with europium thin films were deposited onto
Coming 2947 glass, c-axis sapphire (A1203), quartz, and (100) silicon (Si). The
glass, sapphire and the quartz substrates were cleaned using a modified RCA
cleaning procedure. First, the substrates were mechanically cleaned using
trichloroethylene (TCE) and a cotton swab. The substrates were then
ultrasonically degreased in heated (35 C) TCE, acetone and methanol for 5
minutes each. They were then rinsed in de-ionized (DI) water and blown dry
with nitrogen gas. The same solvent cleaning procedure was employed on the
silicon wafers followed by a 5 second buffered hydrogen fluoride etch and a DI
rinse. The silicon substrates were then blown dry with nitrogen. Some silicon
substrates were roughened prior to cleaning to examine the effect on CL
brightness. Silicon carbide 600 grit paper was wetted with methanol and the
substrates were roughened by random hand motion for 10 seconds.
54


Scattering Efficiency, Q
149
Figure 5-3.
Scattering Efficiency, Qsca, versus Size Factor, p
for RMS roughness ranging from 0 250 nm
RMS roughness, |nm]
0 40 80 120 160 200 240
Magnification of figure 5-2 for RMS roughness values between 0
and 250 nm showing the scattering efficiency factor, Qsca, versus
the size factor, p, and RMS roughness. Plot shows the increase in
scattering as the critical dimension, RMS roughness, is increased.
[Van81] Dotted line represents the maximum RMS roughness
obtained in this study in relation to the maximum attainable
brightness.


15
In this case, europium has 6 electrons in its 4f shell. According to the Jodd-
Ofelt theory, a strong 5D0-7F2 ED transition is expected to be accompanied by
weaker 5D0-7F1 MD transitions. This explains the characteristic sharp line
emission peak at 611 nm (5D0-7F2) from Y203:Eu+3 seen in figure 2-7. Blasse
[Bla79] concludes that orange-red (5D0-7F1) emission results from europium ions
that occupy centers of inversion symmetry, and Eu+3 in sites that lack
symmetry emit in the red (5D0-7F2) and infra-red (5D0-7F4).
The site symmetry and site coordination will determine the number of
primary peaks seen for these transitions. For J values equal to 0, splitting of
levels is not allowed. For J> 0, triple degeneracy is possible and splitting can
occur. The field symmetry also determines the amount of allowed splitting.
Cubic symmetry does not cause splitting but tetragonal and trigonal fields
cause splitting into 2 levels. Lower symmetry sites cause splitting into three
levels yielding three peaks.[Bla79J Since the emission for Y203:Eu originates
from a 5DJ=0 level, only one peak is allowed. And since the Eu predominantly
sits in a cubic symmetry site, the 7F2 level is not degenerate and no splitting
occurs. Thus only one primary 5D0-7F2 peak is seen.
The selection rules also help determine the speed of these transitions.
The Eu+3 has an excited state lifetime of approximately 10'3 seconds which is ~
105 times longer than the lifetime for allowed electric dipole transitions. This
difference in lifetimes illustrates the degree to which these forced ED (4f to 4f)


159
ZONE I¡
ZONE T | ZONE I
ZONE nr
1
GRAIN ¡
l
' ONSET OF
! EXTENSIVE
RENUCLE ATION i
1 GRANULAR
j GRAIN
1
1
1
1
1
1
| EPITAXY
i
i
i
i GROWTH
i
i
i
s/t
M
0.7
Figure 5-13. Zone model of grain size changes for metals deposited at
substrate temperature, Ts versus their melting temperature,
Tmelt. Temperature is in Kelvin. [Ohr92]


41
Figure 2-2.
Plots of Fowler-Nordheim equation for current density, J, as a
function of electric field, E, for varying work functions,

85
deposition. The change in Y/O peak height ratios is seen in figure 4-41. The
pressed target had a Y/O ratio of 14. According to figure 4-41, this would
correspond to a film grown in 02 pressures between 100 and 200 mTorr. RBS
data were collected from samples grown at 100 mTorr and 600 C to quantify
the Y, O and Eu composition (figure 4-42). The broad plateau from 0.7 to 1.275
MeV originated from scattering from the silicon substrate. Using a fitting
routine, the composition was determined to be Y0 367O0 626Eu0 007. RBS data were
also collected from samples grown at 600 mTorr and 600 C. At 600 mTorr and
600 C, films grown on (100) silicon were to rough to accurately analyze with
RBS.
Figure 4-43 shows a typical SIMS profile from films deposited at 100
mTorr and 600 C. Oxygen was detected at a mass/charge ratio of 16, while
yttrium was detected at 89 and europium at 151 and 153.
Annealing
Samples were annealed to improve the crystallinity and determine the
effects on the luminescence brightness. The effect of annealing on the crystal
structure is shown by the XRD patterns of figures 4-44 through 4-47. Figure 4-
44 shows the change in crystal structure for films grown at 600 mTorr and 460
C, then annealed at 800 C, 1000 C, and 1200 C for 1 hour in air. The films
remained polycrystalline, but an interfacial reaction was detected at 1000 C


Brightness, [Arb. units]
96
CL Brightness versus Voltage
for film grown at 460 C and 600 mTorr
Voltage, [kV]
Figure 4-10. Brightness of film grown at 460 C, 600 mTorr of 02 pressure and
annealed at 800, 1000, and 1170 C for 1 hour in air. The films
grown at 460 and 600 C did not have detectable
cathodoluminescence.


CHAPTER 5
DISCUSSION
The primary intent of this work was to determine the effects of changing
surface morphology on the PL and CL brightness of Y203:Eu thin films grown
on silicon. The data show that in-situ changes in surface morphology at high
Oz pressure during PLD resulted in enhanced luminescence. This was
attributed to light scattering effects that reduced the light piping. This is
further discussed below in terms of optical scattering theory. The effects of
film growth temperature, pressure and annealing on PL and CL brightness are
also discussed.
Enhanced Luminescence Through Scattering
Increasing the RMS roughness through increased growth pressures
above 200 mTorr resulted in increased luminescent brightness detailed by the
optical spectrometer. This was attributed to forward scattering of light at the
film/air interface being increased while internal reflectance was decreased at
higher roughness. The reported values for RMS roughness were between 3 to
71 nm and the grain sizes were approximately 80 nm.
136


Intensity, [arb. units]
87
Figure 4-1.
PL Intensity versus Growth Temperature
Y203:Eu films grown at 2 x 10 5 Torr, 600 and 800 C
Photoluminescence versus growth temperature showing an
increase in intensity as temperature is increased. Films grown
on (100) silicon at 600 and 800 C at 2 x 10'5 Torr.


134
Figure 4-48. SEM photograph comparing (a) as-deposited film (600 mTorr, 600
C) versus (b) a film annealed at 1000 C for 30 minutes.


179
Met94
Mey34
Nar87
Neu94
Ohr92
Ort92
Ouy95
Oza90
Oza71
Oza68
Pea97
Pin92
Rao96
Rob80
Sel82
Sie67
Sim95
Metev, S., Pulsed Laser Deposition of Thin Films, edited by
Chrisey, D. & Hubler, G.K., John Wiley and Sons, New York, NY,
chapter 9.
Meyer-Arendt, J., Introduction to Classical and Modem Optics
2nd edition. Prentice-Hall Inc, NJ.
Narayan, J., Biunno, N., Singh, R., Holland, O. W., & Auchiello,
O., Applied Physics Letters 51, 1845.
Neumann-Spallart, M., Levy-Clement, C., & Grabner, G., Journal
of Applied Physics D 27, 407.
Ohring, M., The Materials Science of Thin Films. Academic Press,
Inc., San Diego, CA.
Ortiz, C., & Blatter, A., Thin Solid Films 218, 209.
Ouyang, X., Kitai, A. H., & Siegele, R., Thin Solid Films 254, 268.
Ozawa, L., Cathodoluminescence: Theory and Applications, VCH
Publishers, New York, NY.
Ozawa, L., Forest, H., Jaffe, P. M., & Ban, G., Journal of the
Electrochemical Society 118, 482.
Ozawa, L., & Jaffe, P. M., Journal of Electrochemical Society 117,
1297.
Peach, L. A., Laser Focus World 33, 18.
Pinto, R., Pai, S., P., DSouza, C. P., Gupta, R. C., Vijayaraghavan,
R., Kumar, D., & Sharon, M., Physica C 196, 264.
Rao, R., Solid State Comm. 99, 439.
Robertson, J. M., & van Tol, M. W., Applied Physics Letters 37,
471.
Sella, C., Martin, J., & Charreire, Y., Thin Solid Films 90, 181.
Siebring, B. Chemistry. Macmillan Company, New York, NY.
Simile, M., Information Display 11, 18.


16
transitions are forbidden. [Bla79] Details of this transition via
cathodoluminescence are given below.
C athodoluminescence
The term phosphor is reserved for materials that display
phosphorescence and/or fluorescence when excited with an energy source.
Phosphorescence is the non-thermal radiation that persists after the excitation
event has ceased. Fluorescence refers to non-thermal radiation given off while
the incident energy impinges the sample. Radiation that does not terminate
within 10 nanoseconds of energy excitation is classified as phosphorescence.
The incident energy can be in the form of X-rays, UV photons, energetic ions,
electrons, or result from mechanical disruption.[Mar88, Lev68] The types of
luminescence are chemiluminescence which is initiated by a chemical
reaction, photoluminescence obtained from photons, roentgenoluminescence
from X-rays, ion luminescence from energetic ions, electroluminescence
induced by an electric field or current, incandescence resulting from high
temperatures, and cathodoluminescence resulting from electrons.[Lev68]
Cathodoluminescence contains components from both phosphorescence and
fluorescence. Figure 2-8 and 2-9 depict qualitatively the interactions that
occur after an electron impinges on a surface. As seen in figure 2-8, there are a
number of physical processes that result from the primary beam interaction


138
Since the RMS roughness was not greater than X, geometric optic ray
tracing is not valid. Anomalous diffraction is appropriate, however, since x is
larger than A/20 and p is greater than 1. The scattering efficiency, Qsca, is
therefore equal to
4 4
Qsca = 2 SinP + (l-cosp)
p p2
The anomalous diffraction scattering efficiency for the yttria system
versus p and RMS roughness is shown in figure 5-2. It can be seen that as the
RMS roughness is increased, the scattering efficiency increases and passes
through a maximum of 3.17 at 214 nm. Figure 5-3 shows the same curve
magnified for RMS roughness values up to 250 nm. As seen from figure 5-4,
the PL response from films grown on (100) silicon at 600 C show the same sine
integral increase as seen in figure 5-3. Using the scattering efficiency, the
predicted PL brightness versus RMS roughnesses is shown in figure 5-5. The
figure shows that anomalous diffraction theory gives a good fit for surface
scattering. So, according to figures 5-2 and 5-3, the PL brightness can be
improved significantly with further increases in RMS roughness to values of
214 nm.
This anomalous diffraction also increased the CL efficiency as seen n
figures 5-6 and 4-12. Although anneals at temperatures greater than 800 C
were required, the rougher films had brightnesses 3x the smooth films just as


72
Figure 3-4.
Schematic representation of elastic atom collision between
projectile atom with mass Ml and resting target atom with mass
M2. E represents the atoms energy, v is the velocity, and 0 and 0
are the scatter angles for the projectile and the target atoms,
respectively. [Chu78]


CHAPTER 1
INTRODUCTION
Much of the interest in field emission displays (FEDs) is due to the large
number of applications that would benefit from their realization. These flat
panel displays (FPD) are thinner, lighter, and consume less power than
conventional displays, such as the cathode-ray tube (CRT). Since the FED will
operate between 100 V and 5 kV, be significantly thinner and lighter, it is the
prime choice for notebook computers and thin TVs over CRTs. See figure 1-1.
Field emission displays also provide significant improvements over the more
common of the FPD technologies, liquid crystal displays (LCDs).[Cas92) Table
1-1 shows a comparison of LCD versus FED technology. [Hol96] The improved
brightness, power efficiency, viewing angle and temperture range make an
FED the choice for laptop computers. For applications such as military and
medical, the reduced weight, lower power consumption, and wider operating
temperature range give FEDs a further advantage over LCDs.
Economic incentives have also fueled increased research interest into
developing this technology. It is estimated that the FPD industry will be a $20
billion market by the year 2000.[DeJ97] Table 1-2 illustrates the market share
for the various display technologies, where the price per display is given in the
1


145
annealing at 1443 K only yields 0.54*Tmelt. The temperature ratio (Ts/Tmelt) of
0.54 is low in the zone for grain growth (figure 5-13). As seen in the SEM
photos of figure 4-48, an annealed film at 1273 K (0.47*Tmelt) did not show
significant grain growth or increase in particle size.
Throughout the annealing process, the smooth films FWHM remained
the lowest. This suggest that crystallinity was better for smooth versus rough
polycrystalline films. The brightness and efficiency, however, of smooth films
were significantly lower than rough films grown at 600 mTorr (figures 5-6, 5-14
and 5-15).
The effect of improved crystallinity versus increased dopant activation
was shown with CL efficiency versus temperature and (111) FWHM (figures 5-
14 and 5-15). As seen in the figures, there was a significant increase in
efficiency with increasing temperature and a corresponding decrease in
FWHM. However, from the activation energy plots (figures 5-16 and 5-17) the
reverse was possibly true. The activation energies as a function of annealing
temperature were approximately equal, 26 cal/mole for a smooth versus 28
cal/mole for rough films. The activation energies as a function of FWHM,
however, show no correlation at all, where Ea is equal to 107 cal/mole for
smooth versus 33 cal/mole for rough films. This implies that although there
was a significant reduction in FWHM, it was the activation of the europium
that was responsible for such an increase in efficiency with annealing
temperature. This was also seen in a change in CIE values as the temperature


148
Figure 5-2.
Scattering Efficiency, Qsca, versus Size Factor, p
RMS roughness, [nm]
0 200 400 600
p = 2x|m-l |
Scattering efficiency factor, Qsca, versus the size factor, p showing
the increase in scattering as the critical dimension, RMS
roughness, is increased.[Van81]


135
0.8 1.0 1.2 1.4 1.6 1.8 2.0
BACKSCATTERED ENERGY (MeV)
Figure 4-49. RBS spectrum of film annealed at 1000 C for 30 minutes
showing Si, O, Y, and Eu peaks. Also shows a reaction at the Si
interface with O, Si, and Y. Solid line represents fit of data to
determine composition.


175
Che88
Chu86
Chu78
Chu95
Col92
Cul78
Cus52
Cut93
Dal92
DeJ97
Die68
Eye87
Fal92
Fel60
Fel57a
Cheung, J. T., & Sankur, H., CRC Critical Reviews in Solid State
and Materials Sciences 15, 63.
Chu, W., Metals Handbook 9th Edition: Vol 10 Materials
Characterization, edited by Davis, J., American Society for Metals,
Metals Park, Ohio.
Chu, W., Mayer, J. W., Nicolet, M., Backscattering Spectrometry.
Academic Press, New York, NY.
Church, E. L., & Takacs, P. Z., Handbook of Optics volume I.
edited by Bass, M., McGraw-Hill, Inc., New York, NY, chapter 7.
Colvard, C., Encyclopedia of Materials Characterization, edited by
Brundle, C.R., Evans, C.A., & Wilson, S., Manning Publications
Co., chapter 7.1.
Cullity, B.D., Elements of X-rav Diffraction. Addison-Wesley Pub.
Co., Reading, MA.
Cusano, D.A., & Studer, F. J., Journal of the Optical Society of
America 42, 878.
Cutler, P. H., He, J., Mislovsky, N. M., Sullivan, T. E., & Weiss, B.,
Journal of Vacuum Science and Technology B 11, 387.
Dalacu, N., Kitai, A. H., Sanders, B. W., & Huang, R., Thin Solid
Films 209, 207.
DeJule, R., Semiconductor International 20, 59.
Dieke, G. H., Spectra and Energy Levels of Rare Earth Ions in
Crystals. International Publishers, New York.
Eyett, M. & Bauerle, D., Applied Physics Letters 51, 2054.
Falcony, C., Garcia, M., Ortiz, A., & Alonso, J., C., Journal of
Applied Physics 72, 1525.
Feldman, C., Physical Review 117, 455.
Feldman, C., M., Journal of the Optical Society of America 47, 790.


11
replaced with the semiconductors electron affinity, x-[Bro92] The electron
affinity is the energy needed to raise an electron from the bottom of the
conduction band to the vacuum level.
Utsumi [Uts91] showed that the tip shape is also critical in determining
the turn-on voltage and field emission current density. He concluded that
round whiskers were closest to ideal emitters. Cutler [Cut93] reported that
higher current densities were possible by reducing the tip radius, as seen in
figure 2-3.
The other half of the display is the phosphor screen. Phosphor powder
can be applied to glass plates using electrophoretic, dusting, or slurry
methods.[Has90] The slurry method is the most common where phosphor
powder is mixed with photosensitive chemicals and deposited using
photolithography techniques. For operating voltages greater than 2 kV, the
screen is back coated with a thin layer of aluminum as a reflector and charge
dissipater.[Lev68] At operating voltages below 2 kV, the electron penetration
depth is so small that the screens are left un-coated.[Bec96] In this study, the
starting phosphor powder was yttrium oxide powder doped with four and a
half weight percent of europium obtained from Osram Sylvania.


166
form
Qsca = 2 sinP
P
+
(1-cosp).
p2
Using this theory, it was predicted that the maximum gain in efficiency from
this type of scattering would be at RMS roughness values close to 200 nm.
This roughness was well beyond the in-situ capabilities of PLD.
Annealing samples grown at 460 C in various oxygen pressures to
temperatures between 600 C and 1200 C increased the CL brightness and
efficiency. As the annealing temperature increased, the brightness and
efficiency increased and the (111) FWHM decreased. For smooth films
deposited at 100 mTorr and 400 C, the (111) FWHM decreased from 0.18 to
0.13 degrees after annealing at 1170 C in air for 1 hour. Rough films grown at
600 mTorr and 400 C decreased in (111) FWHM from 0.51 to 0.14 degrees upon
the same annealing schedule. The smooth films retained the (111) texture,
while rough films changed to more random polycrystalline orientations. Again,
a-yttrium silicate phase formed at the Y203/Si interface. After the 1170 C
anneal, the smooth films had a PL brightness of 1.8 fL while rough films had
4.6 fL. Although the FWHMs were equal for smooth and rough films, the 70 nm
RMS roughness film was 2.5 times more efficient in CL. The CL efficiency at 2
kV and 1 ¡jA/cm2 was 0.85 lm/W (9% of the powder efficiency) and 2.63 lm/W


Brightness, [arb. units]
92
Figure 4-6.
CL Brightness versus Voltage
for Osram Sylvania Y203:Eu powder
Cathodoluminescence brightness versus voltage for Osram
Sylvania standard powder at 1.0 (iA/cm2 current density.


139
in PL. The rough film CL efficiency was still lower than the target and the
powder (figure 5-7). Possible reasons for this deviation are given below.
CIE and Reduced Luminescence
The Commission Internationale dEclairage (CIE) chromaticity
coordinates (x,y) were recorded during CL measurements to characterize the
color from the phosphors. The 1931 CIE diagram shown in figure 5-8 shows
the (x,y) chromaticity values that correspond to the different primary
wavelengths of blue, green and red at 400, 520, and 700 nm,
respectively. (Wys78] The (x,y) chromaticity values for a phosphor are
functions of the spectral distribution and define the color a phosphor emits.
Phosphors doped with europium may emit in the red (611 to 626 nm),
but can have chromaticity or color from whitish- orange to a deep red
dependent on the europium concentration. [Bla74, Oza90] As shown in figure 5-
9, as the europium mole percent is increased, there is an increase in the x-
chromaticity value and a decrease in the y-chromaticity value which
corresponds to a shift to deep red, which is also seen in figure 5-8.[Oza90] This
shift results from changes in the ratio of from 5D, 7Fj and 5DQ 7F2 emission
peaks at 538 and 611 nm, respectively (figure 5-9). As the concentration of
europium active in C2 sites increases, the ratio decreases and there is a shift
towards higher x-chromaticity and lower y-chromaticity values.[Oza71,Oza90]


169
Roughnesses greater than 2\ should also be tried where geometric optics
apply. Recent advances in GaAs (n=3.5) LEDs have been achieved by utilizing
ray optics to increase the efficiency. [Pea97] The size should be kept close to
1.1 jUm so that the particle size advantage over powder phosphors is retained.
Since the CIE versus voltage experiments showed that the europium
concentration was much lower than the concentration of the starting powder,
a true comparison of efficiency was not possible. Therefore, the europium
concentration should be increased to bring the (x,y) coordinates to their proper
value and improve the efficiency of the Y203:Eu films. This could be
accomplished by adding EuC13, EuF3, or Eu203 to the starting targets at various
weight percents. The chlorine or fluorine addition would also act as fluxing
agents that would aid in sintering.
More dopant could also be added by depositing a europium thin film on
top of the existing films followed by drive-in anneals to diffuse, homogenize
and activate the europium. It would also be worth trying ion implanting
europium to the desired concentration followed by annealling to activate the
europium and repair the lattice damage done during implantation.
It was also evident that temperature, especially annealing
temperature, played a significant role in improving the CL efficiency. The
maximum annealing temperature used in this study was 1443 K, which
corresponds to only 0.54*Tmelt of yttria. Such high temperature anneals are not
an acceptable option for most flat panel display manufacturers. The


89
Figure 4-3.
PL Intensity versus Growth Pressure
Pressure, [mTorr]
Photoluminescence versus 02 growth pressure showing an
increase in PL intensity beginning at 200 mTorr. Films grown
on (100) silicon at 600 C. Films were 1 |im thick.


90
PL Intensity versus Annealing Temperature
460 C 100 mTorr and 600 mTorr on (100) silicon
Figure 4-4. Photoluminescence versus annealing temperature showing
increase in PL intensity as the annealing temperature was
increased. Rough film grown at 600 mTorr, 460 C and annealed
at 1200 C is approximately 3 times brighter than smooth film
grown at 100 mTorr, 460 C and annealed at 1200 C. Films were
annealed for 60 minutes in air.


133
(111) FWHM vs Annealling Temperature
Tgr = 460 C, 100 mTorr
Temperature, [C]
Figure 4-47. FWHM versus annealing temperature for films grown on (100)
silicon at 460 C and 100 mTorr and annealed in air for 1 hour at
various temperatures.


CHAPTER 2
LITERATURE REVIEW
Field Emission Displays
Flat panel displays (FPDs) may be divided into two classes, emissive
and non-emissive.[Tan85] Non-emissive displays do not emit or radiate light,
but act as light valves for back lighting sources. In emissive displays,
however, luminescent materials are excited by an energy source and
subsequently radiate light. These luminescent materials are called phosphors.
Field emission displays operate on the same principles as coventional cathode
ray tubes (CRTs).
In a CRT, a triode e-gun cathode is used to generate and accelerate
electrons toward a phosphor screen. [Cas92] The cathode beam is scanned
across and rastered down the display illuminating one red, green, or blue
(RGB) phosphor pixel at a time. Scanning and rastering are accomplished by
deflecting the beam with magnetic coils. The scanning and rastering
requirement primarily accounts for the depth and bulk of CRTs.
8


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. Holioway, Chair
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.
iA
jiv K. Singn
Rajiv K. Singf
Professor of Materials
Engineering
Science and
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.
P-C
Rolf E. Hummel
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 Philos
Pearton
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.
Timothy J. vtnclerson
Timothy,
Professor of Chemical Engineering


177
Hir96
Hir89
H0I8O
Hol96
Hol95
Hor94
How93
How92
Hsi94
Hud92
Huo86
Ino95
Jae93
Jag94
Hirata, G. A., McKittrick, J., Shea L. E., Avalos-Borja, M., & Lopez,
O. A., Applied Surface Science 113/114, 509.
Hirofumi, F., Imura, T., & Osaka, Y., Applied Physics Letters 55,
360.
Holloway, P. H., Advances in Electronics and Electron Physics
volume 54. Academic Press, New York, NY.
Holloway, P. H., Jones, S.L., Rack, P., Sebastian, J., & Trottier, T.,
Proc. Of Tenth Inti. Sym. Applications of Ferroelectrics. edited by
B. Kulwicki, East Brunswick, NJ, (IEEE, NY).
Holloway, P. H., Sebastian, J., Trottier, T., Swart, H. & Peterson,
H., Solid State Technology 38, 47.
Horwitz, J. S., & Sprague, J. A., Pulsed Laser Deposition of Thin
Films, edited by Chrisey, D. & Hubler, G.K., John Wiley and Sons,
New York, NY, chapter 8.
Howland, R., & Benatar, L., A Pratical Guide to Scanning Probe
Microscopy. Park Scientific Instruments.
Howland, R. S., & Kirk, M.D., Encyclopedia of Materials
Characterization, edited by Brundle, C.R., Evans, C.A., & Wilson,
S., Manning Publications Co., chapter 2.3.
Hsieh, I., J., Feng, M., S., Kuo, K. T., & Lin, P., Journal of the
Electrochemical Society 141, 1617.
Hudson, J., Surface Science: An Introduction. Butterworth-
Heinmann, Boston, MA.
Huo, D., & Huo, T., Journal of the Electrochemical Society 133,
1492.
Inoue, Y., Tanaka, K., Okamoto, S., Kobayashi, K., & Fujimato, K.,
Japanese Journal of Applied Physics 34, 180.
Jaeger, R. C., Introduction to Microelectronic Fabrication: Volume
5, Addison-Wesley Publishing Company, Reading, MA.
Jagannathan, R., Kutty, T.R.N., Kottaisamy, M. & Jeyagopal, M.,
Japanese Journal Applied Physics 33, 6207.


22
From the critical angle, a cone of emission forms where the volume of the
cone is a function of the critical angle. The fraction of light, f, that will escape
the thin film through this cone is approximated geometrically by
f C1-005^) = l
2 4n2
phosphor
as is detailed in Appendix A.[Car66] If emission is measured from only one
surface and if light propagating in the other direction is not reflected but is
lost, only 7% of the light would be transmitted out of yttria thin films. The light
impinging within this projection cone, is further subjected to the standard
rules for reflection and transmission across the interface. The percentage of
the light within the cone that will be transmitted, T, for a dielectric thin film is
given by
T =
4n
phosphor
^phosphor*1)
and for yttria, approximately 10% will be reflected and not transmitted,
resulting in only 6.3% of the light leaving the thin film. Even if backside
reflections were included, the maximum output would be 13%.
This is analogous to waveguiding in fiber optic systems, where the
cladding layer has an index of refraction which keeps the light trapped within
the fiber. In thin film phosphors, this waveguiding effect is termed light


113
Figure 4-27.
SEM photograph of a film grown on (100) silicon at 600 C and 2 x
10 Torr on a mechanically roughened substrate with 600 grit SiC
paper.


SUBSTRATES
Figure 3-1.
PUMP
Schematic of pulsed laser deposition chamber.


INTENSITY (ARB. UNITS1
103
Figure 4-17. X-ray diffraction patterns of yttria grown on glass substrates for
(a) 600 C, (b) 400 C, and (c) 250 C at 2 x 10 5 Torr.


143
processes like PLD and sputtering frequently produce this type of
microstructure. [ Smi9 5 ]
As the oxygen pressure was increased from 0.02 mTorr, the film quality
improved. The decrease in (111) FWHM peak (figure 4-29) showed an
improvent in crystallinty, with a minimum of 0.15 degrees at 200 mTorr.
However, further increases in growth pressure increased the FWHM. This also
corresponded to the end of (111) preferential orientation and the onset of
polycrystalline behavior (figure 4-28). The AES Y/O peak-to-peak height ratios
show that the stoichiometry approached the best ratio between 100 to 200
mTorr, which correlates with the minimum (111) FWHM XRD data quite nicely.
This was expected since PLD is usually carried out in the presence of either a
reactive gas (02, N2, H2) or an inert gas (Ar) in the range of a few hundred
mTorr to influence stoichiometry. [Hor94] The background gas influences film
growth by reducing the energy of the vapor flux impinging the substrate and
providing a high flux of gas particles bombarding the surface during
deposition. For PLD of oxides in particular, it has been shown that growth
pressures between 100 to 300 mTorr are standard in achieving proper
stoichiometry and the best bulk film properties.[Pin92]
There was also a change in the surface as the background gas was
increased which was expected. In a background pressure of 200 mTorr,
outgrowths resulted in a rough surface. Above 200 mTorr, modest
improvements in roughness were obtained, and the largest roughness was


63
Instrument Nanoscope III Multimode SPM was employed in both the contact or
tapping modes. The instrument was mounted on a pressurized air table to
minimize vibrational noise. AFM is based on attraction and repulsion forces
generated between an atomically sharp tip and the sample surface. These
forces can be measured to display topological data such as roughness,
grain size and step heights, as well as yield physical information such as
hardness, friction, and wear strength. [Bhu95]
AFM employs atomically sharp, etched Si tips attached to a cantilever
beam with resonant frequencies of 60 to 400 kHz. The beam is fabricated to
have a small spring constant to allow for beam deflection. As the tip is
brought close to the sample, it experiences a weak Van der Waal attractive
force. [How93] The tip is drawn closer causing the electron clouds of the tip
and sample to overlap which induces a repulsive electrostatic force. This force
goes to zero as the distance between the atoms reaches a few . When the
Van der Waals force becomes repulsive, the tip is in contact with the sample,
[How93] as shown in figure 3-5. The repulsive Van der Waal force slope is very
steep in this regime and counteracts any force that attempts to push the
sample and tip atoms closer together.
In the contact mode, the AFM is operated in the repulsive force regime
shown in figure 3-5. Since the slope of the ( +) force side is steep, changes in
the Z height causes the cantilever to deflect instead of propelling the tip
further into the sample. In the non-contact (tapping) mode, the AFM operates


147
Figure 5-1. Illustration of critical dimensions for roughness of thin film scat
terer, where a is the root mean square (RMS) roughness, b is the
grain size, and c is the spacing between features.


4
Table 1-1 Comparison of FED and LCD Technologies
CHARACTERISTIC
FED (target)
TFT-LCD
Thickness
6-10 mm
23 mm
Weight
<0.2 kg
0.33 kg
Contrast ratio
>100:1
100:1
Viewing angle
160 V, 160 H
60 V, 90 H
Max. brightness
>200 cd/m2
60 cd/m2
Power @ 60 cd/m2
<1 W
4 W
Max. Temperature
-50 to + 80 C
0 to 50 C
[Hol96]


178
Jen76
Jol90
Kir61
K0I6O
Kor89
Koy92
Lev68
Lid97
Mae92
Map73
Mar88
Mat86
McC93
McL93
Jenkins, F. A., & White, H. E., Fundamentals of Optics, edited by
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Physical Review B 42, 7587.
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Koren, G., Gupta, A., Baseman, R. J., Lutwyche, M.I., 8c Laibowitz,
R. B., Applied Physics Letters 55, 2450.
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Crystal Growth 117, 156.
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CRC 78th Edition Handbook of Chemistry and Physics. Academic
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Maestro, P., Huguenin, D., Seigneurin, A., 8c Deneuve, F., Journal
of the Electrochemical Society 139, 1470.
Maple, T., 8c Buchanan, R., Journal of Vacuum Science and
Technology 10, 616.
Marshall, D., Cathodoluminescence of Geological Materials.
Unwin Hyman Ltd, Boston, MA.
Matsunawa, A., Katayama, S., Susuki, A., 8c Ariyasu, T., JWRI15,
205.
McCamy, J., Lowndes, D., Budai, J., Zuhr, R., 8c Zhang, X., Journal
of Applied Physics 73, 7818.
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63, 1865.


55
The phosphor PLD targets were manufactured from powder supplied by
Osram Sylvania (type 2342 Y203:Eu lamp phosphor). Ten weight percent of
various known fluxes, such as Ag20 and NaF, were sometimes added to assist
in the sintering process. The powders were pressed into 1 inch pellets and
sintered at 1400 C in air for 24 hours. A target was also manufactured from
the same powder via isostatic hot pressing by Cerac, Incorporated.
Pulsed Laser Deposition (PLD1
Films were created by PLD in a high vacuum chamber which used a
Leybold Trivac mechanical pump model # D4A for rough pumping and a
Pfeiffer Balzers TPU 450 H corrosive turbomolecular pump with a TCP305
controller and a MD4I diaphragm backing pump. The ultimate pressure of the
system was 4 x 10 11 Torr. A KrF (A=248 nm) 1 Watt Lambda Physik
Lasertechnik LPX300 excimer laser with a pulse width of 10 nanoseconds was
used. Pulse frequencies between 1 and 100 Hz were possible, but all films
were deposited at 10 Hz. A schematic of the deposition chamber is shown in
figure 3-1. By adjusting the lens-to-target distance, varying laser beam spots
with controlled sizes were achieved. The target was also rotated to reduce the
number of ablated target particles deposited on the thin film surface. [Che94b]
The incident laser beam struck the target at 45 away from the surface normal,
generating a plume perpendicular to the target and substrate. The substrate


25
and (b) the particle size and index of refraction are smaller than the
wavelength inside the particle, A/n, which is mathematically expressed as
i A
|n|a ; nx 1 .
2tx
A particle size of less than A/20 can be used as a guide.[Van81] Thus, when
the x and n*x values are much less than 1, Rayleigh scattering yields a
scattering efficiency, Qsca, equal to:
Q
sea
8 4lm2-l
- I
3 m2+l
2
showing the well known (a/A)4 dependence, since x is equal to (27ia/A).[Str64]
This is the Rayleigh scattering formula for nonconducting, non-absorbing
spheres as opposed to the more common formula expressed for n-l. The term
Qsca is the cross-section for scattering, Csca, normalized by the particles area
projected perpendicular to the incident beam.[Boh95]
As the sphere size increases, the scattering condition moves from
Rayleigh to Rayleigh-Gans scattering [Van81], where the two main
assumptions are that (a) the index of refraction is close to 1 and (b) the
allowable phase shift is small such that
4rr 173 -11


29
have been employed, including evaporation, spray pyrolysis, sputtering, metal
organic chemical vapor deposition and more recently pulsed laser deposition.
Williams [Wil47] employed an evaporation technique to deposit Mn doped zinc
fluoride phosphors. The thin films were grown at 100 C and were luminescent
as-deposited. However, these films degraded and decomposed readily under
electron beam bombardment. Studer et al. produced ZnS:Mn [Stu51,Cus52]
and ZnS:As,P [Stu55] using a chemical vapor deposition technique. They
achieved white brightnesses of 20% that of the powder with the ZnS:As,P
phosphor thin films. The films displayed an efficiency of 1.08 lm/W at 10 kV
and 1 mA/cm2. With the use of an integrating sphere, they also showed that
the lower brightness could not be attributed to internal reflections alone, but
lower crystallinity or the presence of defects reduced the efficiency. They
subsequently showed that thin film phosphors exhibited high spatial
resolution due to less light scattering.[Stu55,Stu56] Feldman et al. [Fel57b]
deposited several phosphors via vacuum evaporation followed by high
temperature vacuum anneals. They found low brightnesses for transparent
ZnS:Mn films and a 2.5x improvement using a frosted substrate. Their high
temperature, long time anneals turned the transparent films foggy and
opaque. These fogged films were 90% the brightness of the powder. They
deduced that the films were rough since they displayed the characteristic
opaque color resulting from severe diffuse reflectance. They also showed that
oxide phosphors like (Zn2P04)3, CaW04:W, and Zn2Si04:Mn could be formed


124
Root Mean Square Film Roughness versus Growth Pressure
films grown at 600 C on (100) silicon
Figure 4-38. Plot of RMS versus increasing 02 growth pressures. Silicon
substrate temperature was 600 C for all depositions.


71
Figure 3-3. Schematic of secondary electron microscope.


110
PLD23

1 P m
-
1 5KU
X25.-000 15mm
SEM photograph of Y203:Eu film grown on (100) silicon at 400 C at
2 x 105 Torr.
Figure 4-24.


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
ENHANCED LUMINESCENCE FROM EUROPIUM DOPED YTTRIUM OXIDE
THIN FILMS GROWN VIA PULSED LASER DEPOSITION
By
Sean Liam Jones
December 1997
Chairman: Professor Paul H. Holloway
Major Department: Materials Science and Engineering
Yttrium oxide thin films doped with europium were grown on silicon,
Coming 2947 glass, c-axis sapphire and quartz substrates. A pulsed laser
ablation process was used to deposit films onto substrates heated from 250 C
to 800 C and at oxygen pressures between 105 Torr and 600 mTorr. The
deposited films were characterized using x-ray diffraction, secondary electron
microscopy, Auger electron spectroscopy, atomic force microscopy, secondary
ion mass spectroscopy, photoluminescence and cathodoluminescence.
Crystalline as-deposited films were achieved at substrate temperatures
as low as 250 C. Films grown in high vacuum were strongly (111) textured,
but became more random polycrystalline as the temperature was increased to
800 C and/or growth pressure was increased to 600 mTorr. The root mean
viii


121
Figure 4-35. SEM photograph of film grown on (100) silicon at 600 mTorr and
600 C showing a nodular surface with a RMS roughness of 71 nm.


19
C is the average number of activator ions to cations.[Oza90] The model
assumes that the lattice ions do not act as recombination sites for the EHs and
that recombination only occurs at the activator ion. The incident electron
coverts the yttrium, Y3+, to Y2+. The divalent yttrium quickly releases the
electron to the next yttrium, converting back to Y3+. The model also assumes
that conversion of Y2+ to Y1+ is not possible. The electron migrates along the
Y3+s until it reaches an activator, Eu3+ in this case. The europium, having
trapped the electron, is converted to Eu2+, which results in a local field that is
negatively charged. The holes, which are transported on the O2' converting
them to O1', are attracted to this negative charge region. Once the hole is
captured by the Eu2+, the europium goes into the excited Eu3+* state. It relaxes
and returns to the ground state (Eu3+) emitting a photon with a characteristic
wavelength. This process can be repeated continuously, limited only by the
capture and decay time of the Eu3+* state.
Efficiency of Thin Film Phosphors
Before discussing previous research done on thin film phosphors, a
discussion as to why thin film brightness may be lower than powders is in
order. It has been generally accepted that thin films will always be less
efficient than powders, because it is estimated that 80 to 90 % of the light
generated within the film is lost in frustrated internal reflections.


131
(111) FWHM vs Annealling Temperature
films grown at 460 C, 600 mTorr 02
Temperature, [C]
Figure 4-45. FWHM versus annealing temperature for films grown on (100)
silicon at 460 C and 600 mTorr and annealed in air for 1 hour at
varous temperatures showing a decrease in FWHM as the
temperature is increased.


125
Film Thickness as a Function of Growth Time
600 mTorr, 600 C
Time, [minutes]
Figure 4-39. Film thickness versus growth time for 600 C substrate
temperature and 600 mTorr of Oz pressure. Plot shows linear
increase in film thickness as growth time is increased.


77
emission consistently for films grown at 600 C or higher. Films grown at 800
C were much brighter than those deposited at 600 C, showing that growth
temperature had a significant effect on luminescence properties (figure 4-1).
This effect of increasing the temperature to 800 C from 600 C was even
greater than the effect of mechanical roughening of the substrate prior to
growth, as illustrated in figure 4-2. This mechanical roughening step is
discussed in further detail below.
In addition to increasing temperature, the PL response also increased
as the oxygen growth pressure was increased as shown in figure 4-3.
Increasing 02 pressure above 200 mTorr increased the surface roughness from
3 nm to 71 nm, as discussed below. There was a 7 times improvement in PL
brightness, corresponding to an increase to 8% the brightness of the powder
standard.
Films were annealed to improve the crystallinity and further activate the
europium dopant. Smooth films (grown at 100 mTorr) and rough films (grown
at 600 mTorr) grown on (100) silicon at 460 C were annealed from 600 C to
1200 C for 1 hour in air. Under PL, the films grown at 100 mTorr Oz pressure
did not yield detectable PL until annealed at 1000 C, where the response was
1.81 fL after a 1200 C anneal for 60 minutes versus 4.55 fL for an annealed film
grown at 600 mTorr (figure 4-4). As seen in figure 4-4, increasing annealing
temperature from 1000 C to 1200 C had a significant effect on PL intensity.


48
Figure 2-9.
Changes in interaction volume depth and beam diameter with
voltage. As the voltage is increased (A to C) the penetration
depth (La to Lc), is increased while the cross-section of surface
interaction is decreased. [Has90]


68
carousel with a +90 volt battery with an ammeter in series to ground. The bias
collected the true secondary electrons to avoid error upon secondary electron
emission.
From the current density, incident power, and brightness, the efficiency
can be calculate. Efficiency is given by
Efficiency = 100 *ti [lm/W]
V*E .
density
where B is the brightness [cd/m2], V is the voltage [V], and Idensity is the current
density [¡JA/cm2] measured from the sample current and beam spot size. Since
neither spectrometers had an internal brightness standard, intensity was
measured in arbitrary units and calibrated using phosphor standards.
Brightness values from phosphor standards were used to calculate absolute
efficiencies. Efficiency versus accelerating voltage curves for standard
phosphors were provided by the Phosphor Technology Center of Excellence
(PTCOE). Data given for 1 /jA/cm2 densities. The PR650 was used to measure
brightness values in cd/m2 and chromaticity values (x,y).


x chromaticity values
158
Chromaticity Value versus Voltage
Voltage, [kV]
Figure 5-12. Change in x chromaticity value for powder standard versus film
grown at 460 C, 600 mTorr and annealed 1 hour in air at 1200 C
showing that the films x-value is 0.625 instead of 0.644 (powder)
or 0.639 as in the case of the target.


Phosphor
Volts
(kV)
Current
Density
(jL/A/cm2)
Bright
ness
(Cd/m2)
Eff.
(lm/W)
Growth
Tech.
Ref.
Comment
a-Zn2Si04:Mn
7.5
10
75.4
0.32
c
Kir61
silica, anneal (1250 C, 1 hr.)
a-Zn2Si04:Mn
7.5
10
13.7
0.06
c
Kir61
pyrex substrate
a-Zn2Si04:Mn
7.5
10
10.3
0.04
c
Kir61
pyrex substrate
a-Zn2Si04:Ti
7.5
10
3.4
0.01
c
Kir61
pyrex substrate
a-Zn2Si04:Ti
7.5
10
6.9
0.03
c
Kir61
silica, anneal (1000 C, 1 hr.)
CdSi03:Mn
7.5
10
92.5
0.39
c
Kir61
silica, anneal (1150 C, 1 hr.)
Y203:Eu
8
1
36.0
1.40
d
Bon95
sapphire
La202S:Eu
8
1
5.2
0.20
d
Bon95
sapphire
Y202S:Eu
8
1
78.0
3.10
d
Bon95
sapphire
Y202S, Gd202S:Tb
8
1
26.0
1.02
d
Bon95
sapphire
Y202S:Tb
8
1
24.0
0.94
d
Bon95
sapphire
La202S:Tb
8
1
40.0
1.60
d
Bon95
sapphire
La202S:Nd
8
1
1.2
0.05
d
Bon95
sapphire
La202S:Tb
685.2
7.37
e
Map73
0.1 W/cm2
La202S:Tb
2000.0
2.15
e
Map 7 3
1.0 W/cm2
CO
00


47
incident
electron
beam
L
D,
Key
incident electron beam
secondary electron production
back-scattered electrons
primary X-ray excitation
Bremsstrahlung (continuum radiation)
cathodolummescer.ee excitation
Figure 2-8. By-products produced by electron in the interaction volume,
where D1 is the X-ray generation area and D2 is the full
cathodoluminescence width after penetration and diffusion of
electron-hole pairs.[Mar88]


17
with the solid. The depth that an energetic electron will penetrate into a solid
is dependent on the voltage. Figure 2-9 shows the excitation volume shape
and depth as a function of voltage. The number of luminescent centers excited
is controlled by the depth of the incident electrons and follows Terrils formula
for penetration depth:
x = 2.5 x 10 12p V02 [cm]
where p is the materials density and VQ is the voltage.[Fel60] This equation is
valid for 1 to 10 kV. Figure 2-10 shows that for 10 kV electrons incident on
yttria the range is 500 nm, but is reduced to 125 nm for 5 kV electrons. The
observed luminescent area and intensity will be a function of the beam energy,
electron-hole pair generation and diffusion, subsequent emission processes,
competing absorption processes, and in the case of thin films, the amount of
internal reflection of photons.[Gol66]
The process by which the electrons travel to the rare earth activators to
excite them is different from the classical donor and acceptor model. Process 7
of figure 2-11 depicts the excitation and deexcitation from an impurity with
incomplete inner shells like rare earths and transition metals.[Yac92]
Processes 4,5, and 6 are transitions expected from classical donors and
acceptors impurities that establish localized states in the band gap.
The mechanism by which the electron and holes travel to the activator is
explained by Ozawa.[Oza71] Cathodoluminescence in phosphors is initiated by


57
of the as-deposited films were furnace annealed in air for varying temperatures
and times to a maximum temperature of 1450 C in a Lindberg Blue M model #
54433 high temperature furnace.
PLD Film Characterization
The PLD films were characterized using a variety of analytical
techniques including x-ray diffraction (XRD), secondary electron microscopy
(SEM), atomic force microscopy (AFM), scanning and static Auger electron
spectroscopy (AES), Rutherford back-scattering (RBS), secondary ion mass
spectroscopy (SIMS), photoluminescence (PL), and cathodoluminescence (CL).
Brief descriptions of these techniques follow.
X-rav DiffractionfXRDl
X-ray diffraction [Cul78] was the primary technique used for yttria
phase and crystallinity identification. A Phillips model APD 3720 x-ray
diffractometer was operated at 40 kV and 20 mA to generate Cu Ka radiation of
A= 1.5406 and 1.5444 . The Cu Kp was removed using a Ni filter. Diffraction
spectra over the 20 range of 10 90 were measured. The goniometer was
scanned at 3 per minute in a continuous mode. XRD is primarily a structural
characterization technique but may be used for semi-quantitative composition
analysis, and determination of stress-strain, crystal size, film thickness and in


APPENDIX A. CALCULATION FOR FRACTION OF LIGHT EMITTED
h
/ \ (r'h
> A / (
/
/ \ \
\ / \
\
/ 0 \ / \
Phosphor film
L-L _v_ )
dA/r2 = solid angle subtended by 20c [Boa83]
20c = total solid angle
0c = half angle = critical angle
A = 27trh
20c = dA/r2 = 27trh/r2 = 2n(h/r)
from the half angle triangle of the emission cone
cos(0c) = (r-h)/r = 1 (h/r)
h/r = 1 cos0
c
20c = 27t(l cos 0c)
For a sphere: the total area = 4k, therefore
The percentage emitted over the total area, f = 20c/47t
20c/47t = f = 1/2 (1- cos0c)
f= O.5(l-cos0c)
171


115
(111) FWHM versus Oxygen Growth Pressure
Pressure, [mTorr]
Figure 4-29. Change in FWHM of (111) peak for films grown on (100) silicon at
600 C at 0.02 mTorr, 0.2 mTorr, 20 mTorr, 200 mTorr, 400 mTorr and
600 mTorr of oxygen growth pressure.


to leave school to begin a new life. Their interest in me and my work kept me
here far longer than I thought possible.
I would also like to thank Dr. Maggie Lambers for her assistance with
SIMS data, Dr. Eric Lambers for his assistance with AES data, Richard Crockett
for his help with SEM micrographs and EDS data, and Dr. Wayne O. Holland at
Oak Ridge National Labs for his assistance in collecting RBS data. In that
venue, I thank all those in Dr. Holloway's group and Dr. Singhs group for their
assistance. I thank Ludie, Paula, Dorothy, Nigel, Wishey, Balu, Brent, Ananth,
Karen, Mark, Steve, Jeff, TJ, Amondrea, Keith, and Michelle for their friendship
over the years. Special thanks go to Ludie for keeping the office running
smoothly and putting up with all of us.
Special acknowledgments go to the McKnight Doctoral family, members
of the Black Graduate Student Organization, National Society of Black
Engineers, Travelers Rest Baptist Church, Mt. Moriah Baptist Church, and Mt.
Carmel Baptist Church. I am also grateful to the Brothers of Nu Eta Lambda,
Theta Sigma and Pi Alpha chapters of Alpha Phi Alpha Fraternity,
Incorporated.
v


78
There was also an increase in PL intensity as the film thicknesses were
changed from 0.5 [dm to 9 ¡dm (figure 4-5) at 600 mTorr and 600 C.
CathodoluminescencefCLl
Cathodoluminescence data were collected at a current density of 1
/dA/cm2. Figure 4-6 shows the brightness versus voltage (BV) curves for the
Osram Sylvania Y203:Eu powder used as starting material for this study.
Brightness data was taken with both spectrometer systems described
previously. Since the power density was known, efficiency values were
obtained as shown in figure 4-7. The efficiency curves had the characteristic
linear and saturation regions at low and high power densities, respectively.
The change from linear to saturation behavior occurred between 1.5 and 2 kV.
The CL brightness from the thin films was not detectable until they had
been grown or annealed at temperatures greater than 800 C, although the
films were photoluminescent at temperatures of 450 C (figures 4-1 to 4-3). For
800 C deposition, the CL brightness values were low but detectable as shown
in figure 4-8. The BV curves (figure 4-9) for a smooth film grown at 100 mTorr
and 460 C then annealed at 800 C, 1000 C, and 1200 C in air for 1 hour show
that brightness and efficiency increased as the annealing temperature was
increased. There was a significant change from 1000 C to 1200 C. Figure 4-
10 shows that annealing also increased the CL intensity from rough films


160
CL Efficiency at 2 kV versus Annealing Temperature
smooth and rough film grown on silicon
Figure 5-14. Cathodoluminescen.ee efficiency versus annealing temperature
for a smooth film (100 mTorr) and rough film (600 mTorr) showing
the increase in the efficiency with annealing temperature. Films
were deposited on (100) silicon at 460 C and annealed at various
temperatures for 60 minutes in air.


31
Dalacu et al [Dal92] later used electron beam evaporation to deposit
MgAl204:Mn phosphor thin films. No brightness data were given.
Sputtering has also been employed to deposit cathodoluminescent
phosphor thin films. Maple et al [Map73] deposited La202S doped with Eu+3
(red) and Tb+3 (green) and obtained the highest reported efficiencies of 7.37
lm/W at 1 W/cm2 after a 1000 C anneal in an H2-SOz atmosphere. They also
showed that beveled edges on 20 )Jm x 20 ¡Jm segments improved the
efficiency by a factor of 3. Sella et al [Sel82] reported the deposition of
amorphous Y2OzS and La202S thin films doped with Eu+3 and Tb+3. The films
were annealed to 600 to 850 C to get good luminescence. Like Feldman, they
reported an increase in brightness of 4x by using ground substrates over
smooth ones. They also employed laser annealing via a pulsed C02 laser that
showed a 3x improvement over furnace annealing. Bondar [Bon95] grew
Y203:Eu and several oxysulphide:RE films using electron beam evaporation, RF
magnetron sputtering, and a RF ion-plasma diode sputter deposition
technique. The RF ion-plasma technique yielded superior results. No
luminescence was reported for the films grown using electron beam
evaporation. They achieved an efficiency of 1.4 lm/W and 3.06 lm/W at 8 kV
and 1 /L/A/cm2 for r.f. sputtered Y203:Eu and Y202S:Eu films, respectively. Hsieh
et al [Hsi94] employed rf magnetron sputtering to deposit ZnGa204:Ga. Ouyng
et al [Ouy95] also used rf sputtering to deposit Y203-Si02:Eu thin films. Films


108
Figure 4-22. SEM cross-section photograph showing columnar grain structure
for Y203:Eu deposited on (100) silicon at 2 x 10'5 Torr at 600 C.


14
Since the 4f to 4f transitions occur within the same quantum number, the
parity does not change and are, therefore, not allowed.
Laportes parity rule, however, can be lifted if there is mixing of parity
from the lattice. [Bla79] When the luminescent center is in a crystal site that
does not have inversion symmetry, there will be a mixing of parities and
LaPortes rule is lifted. Lifting of the parity rule can result in a transition. This
is called a forced transition. If the site retains inversion symmetry, Laportes
rule is upheld and no luminescence is observed from the ion. In yttria,
europium occupies both C2 and S6 sites. The S6 site has inversion symmetry
but the C2 site does not. This yields both an ED and MD transition where the
MD originates from the S6 and C2 sites and a forced ED transition originates
only from C2 sites. The primary emission peak at 611 nm for Y203:Eu+3
originates from forced ED transitions from Eu in C2 cation sites because of
inversion symmetry.[For69]
The specific levels from which these transitions occur are also controlled
by the selection rules. The Jodd-Ofelt's theory adds that the allowed ED (5Dj to
7Fj) transitions must adhere to the following:
1) AJ < 6,
and for an f shell filled with an even number of electrons:
2) when J = 0 then J= 0 is forbidden,
3) if J = 0 then odd J values are weak (MD),and
4) if J = 0 then J = 2,4,6 are strong.[Hen89]


65
R
E(Z|-
z Y
ave
N
[unit of length]
where Z, is the surface height, Zave is the average height displacements and N
is the number of data points.
Auger Electron Spectroscopy(AES)
Auger electron spectroscopy (AES) was done using a PHI model 660
scanning Auger microprobe to identify surface contaminants and changes in
the oxygen to yttrium ratio as a function of deposition and/or annealing
conditions.[H0I8O] The Auger process results from the excitation a of core level
electron and compensation by the ejection and de-excitation of electrons
possessing characteristic energies.[Str92] AES utilizes energetic electrons to
initiate this process. Since Auger electrons are low energy electrons, only
those from a shallow depth from the surface will survive without inelastic
collisions to be detected, hence AES is a surface sensitive technique (5 30 ).
AES is widely used as a surface sensitive technique capable of providing
elemental composition and some information on surface chemical bonding.
AES survey spectra were taken on the samples as-received and after the
surface was cleaned for 1 minute by sputtering with an energetic 3keV Ar+
beam. This time was equivalent to the removal of ~ 100 . Depth profiles


2
parenthesis. As noted, the anticipated unit price for an FED is considerably
lower than that for LCDs or electroluminescent displays (ELD), the main
competing technologies.[DeJ97] The FED industry could be a half billion dollar
business by the year 2003 by filling niche markets alone.
The research presented in this thesis is primarily for advancing FED
technology. Current FED phosphor powder research is concerned with
increasing the efficiency and lifetime while reducing particle size. Reducing
the particle size allows for smaller pixels, yielding higher resolution. As the
optimized CRT phosphor powder particles are reduced in size, however, the
efficiency decreases significantly.
An alternative phosphor configuration, thin films, was studied in this
research. By overcoming some of the fundamental limitations of thin films,
several advantages would be realized. Table 1-3 lists the advantages and
disadvantages of powders versus thin films. [Fel57a] The intent of this
reserach was to show that modifing the surface could increase the efficiency of
thin film phosphors.
In this dissertation, the literature is reviewed in chapter 2 to provide a
background for this work. A brief overview of FED technology, an outline as to
why thin films are less efficient, a review of applicable scattering theory, and a
review of the pulsed laser ablation technique are presented. Past phosphor
thin film research is reviewed to place the gains achieved from this study into
proper perspective. The experimental procedures used to deposit the Y203:Eu


INTENSITY [ARB. UNITS]
114
(a)
(b)
(c)
(d)
Figure 4-28. X-ray diffraction patterns of Y203:Eu grown on (100) silicon at 600
C at oxygen partial pressures of (a) 600 mTorr, (b) 400 mTorr, (c)
200 mTorr, and (d) 0.02 mTorr. Increasing O pressure increased
the RMS roughness of the films. Yttrium silicate phase denoted
by (*)


161
CL Efficiency at 2 kV versus (111) FWHM
Figure 5-15. Cathodoluminescence efficiency versus FWHM for a smotth film
(100 mTorr) and a rough film (600 mTorr) showing the increase in
the efficiency for increased crystallinity. Films were deposited on
(100) silicon at 460 C and annealed at various temperatures for
60 minutes in air.


INTENSITY [ARB. UNITS)
105
(211) (ill)
(110)
(a)
(b)
Figure 4-19. X-ray diffraction patterns for yttria films grown on c-axis sapphire
for (a) 800 C, (b) 600 C, and ( c) 250 C at 2 x 105 Torn


18
1 of 3 mechanisms, (i) absorption from energy transferred from the host to the
activator, (ii) direct absorption by the activator ion from the incident energy,
and (iii) indirect excitation by the absorption of energy by recombination of
mobile electrons and holes, where processes (ii) and (iii) dominate. By using
concentration dependence (CD) curves (figure 2-12), the method of transfer
may be revealed. Phosphors whose CD curves contain only one inflection for
concentration quenching are excited directly by the electron beam (figure 2-
12b, Y203:Pr). CD curves that have two inflection points (figure 2-12a and b)
Y202S:Pr) are from phosphors that are excited indirectly by mobile electron-
hole pairs (EHs). In the case of indirectly excited phosphors, the first inflection
in the CD curve corresponds to the luminescence from electron-hole pairs that
have diffused beyond the primary beam range. Note that all of these phosphors
display direct excitation CD curves under photoluminescence. (Oza90]
The process of cathodoluminesce by indirect excitation through
recombination of mobile carriers is as follows for Y203:Eu3+.[0za68, Die68]
When the cathode ray impinges the host lattice, electron-hole pairs are
created. The electrons and holes are free to migrate within the crystal. The
migration distance is given by
L = dC1
where L is the migration distance or the distance an electron or hole must
travel from the host ion to an activator, d is the distance between cations and


170
development of lower melting temperature phosphors, the incorporation of
fluxes to lower the growth temperature, growth schemes utilizing glass
formers, or utilizing rapid thermal processing (lasers annealing, rapid
thermally annealing) to anneal the film without effecting the substrate should
be investigated.
Finally, it is curious that the CL efficiency kept increasing with film
thicknesses (9 jL/m) well beyond the primary electron penetration distance (0.5
¡jm at 2 kV) and the theoretical electron hole pair generation and diffusion
distances. Could there be a reduction of the surface recombination velocity as
the distance between the substrate and the film surface increases? Is this
velocity the limiting factor in determining the electron-to-activator transfer
efficiency, r|t? This could be checked by doing PL lifetime measurements
on a smooth film, a rough film, the powder, and the target. If the
surface recombination is a significant factor, the decay time will be lower for
the films versus the powder and target. Comparison between the smooth
versus rough film will also show if there is an effect of surface roughness on
the SR. Understanding the surface recombination velocity and what it is a
function of may be the key to increasing the efficiency not just for thin films but
for powder phosphors as well.


Ill
Figure 4-25.
PLD 24
*

1 K* m
1 5KU
X 2 5 j10 0 0 15mm
SEM photograph of Y203:Eu film deposited on (100) silicon at 600
C at 2 x 105 Torr.


10
1 to 3 kV.[Adl91, Spi76] Low voltage operation yields stable, continuous
emission with long tip life. Field emission current density, J [A/cm2], due to
Fowler-Nordheim tunneling is described by Brodie and Spindt:
n* 2 (f)3/2
J = 1.54 x 1CT6 exp(-6.87 x 107^ v(y))
Ot 2(y) E
where E is the applied electric field [V/cm], $ is the work function [eV], t2(y) is
approximately 1.1, and y is the Schottky image charge lowering contribution to
the work function. [Bro92] The variable t2(y) is explained in detail elsewhere.
[Adl91,Cut93]. The variables v(y) and y are equal to
v(y) = 0.95 y2
and
y
E> 1/2
= 3.79 x 10 4 -
Figure 2-2 shows J as a function of electric field for varying work functions.
Even with low voltages acting over closely spaced electrodes, current
densities greater than 10s A/cm2 are possible.
Semiconductor field emission tips are degenerately doped (~ 1019 cm'3)
so that the Fermi level is just above the bottom of the conduction band. In
such a case, the work function term in the Fowler-Nordheim equation is


square roughness increased significantly as oxygen pressure increased. Films
grown at 10'5 Torr had RMS roughness values of 2-3 nm, but the roughness
increased to 71 nm for growth at 600 mTorr. Above 200-300 mTorr oxygen, the
film morphology changed from a smooth to a particulate surface. These
particles were 100 15 nm at the transition pressure and decreased to 80
15 nm at 600 mTorr. During annealing, the grain size remained well below 200
nm, even after 1400 C for 1 hour.
Photoluminescence and cathodoluminescence intensities increased as a
function of increasing growth temperature and pressure. PL was done using a
UV lamp with a wavelength of 254 nm as the excitation source. A PR 650
camera with an internal calibrated intensity standard was used as the
detector. The PL intensity increased from 0.093 fL for 1 fjm thick films grown
at 10'5 Torr to 0.81 fL for films grown at 600 mTorr. The powder gave 11 fL.
The PL response from the film increased to 4 fL (36% that of the powder) for a
the thickness of 9 ¡Jm.
Cathodoluminescence was done using a Kimball Physics EFG7 electron
gun and an Ocean Optics S2000 fiber optic as the detector. Films were not
cathodoluminescent until growth and annealing at 800 C or higher. The best
brightnesses were observed at temperatures > 1170 C. This temperature
corresponds to 0.54 of the melting temperature for yttria. At 2 kV and 1
jUA/cm2, 3 ¡Jm thick films grown at 100 mTorr (RMS of 3 nm) had CL efficiencies
of 0.85 lm/W, at 600 mTorr (RMS of 71 nm) had efficiencies of 2.63 lm/W, while
IX


3
thin films are described in chapter 3, along with a variety of techniques for thin
film characterization. A brief outline of each technique is given with emphasis
placed on photoluminescence and cathodoluminescence. Results are
presented in chapter 4. A discussion of results follows in chapter 5 with an
emphasis on crystal quality and luminescence. Conclusions from this work are
presented in chapter 6, and future studies in chapter 7.


98
CL Efficiency versus Voltage
smooth and rough film grown on c-axis sapphire
Figure 4-12. Cathodoluminescence efficiency versus voltage for Y203:Eu
deposited onto c-axis sapphire. Rough annealed film grown at
600 C and 600 mTorr is approximately 3 times more efficient than
smooth annealed films grown at 800 C at 2 x 105 Torr. Films
were annealed at 1200 C in air for 60 minutes. Data taken at 1
|iA/cm2.


44
Figure 2-5.
"
>. 1 -
1
1
1 !
1 *
j T =<,
55 Q
' *

I
{=
35 Q
!=
Fill order for electron sub-energy levels in solids.[Sie67j


To my Grandmother Ethell and Grandfather Russell Hughes, Uncle
Eddie Jones, and best friend Edith Renee Hill who passed while I was on this
part of lifes journey, this volume is respectfully dedicated.


N(E),diff9
112
Figure 4-26.
Auger electron spectrum from Y203:Eu thin film deposited on
(100) silicon at 600 C and 200 mTorr, showing the characteristic
low energy yttrium triplet (75, 106, 122 eV), oxygen (509 eV), and
the high energy yttrium peaks (1743, 1817 eV). The small peak at
215 eV was present from implantation of argon during
sputtering.


123
Figure 4-37. SEM photograph illustrating in-situ change in morphology and
microstructure with increasing growth pressure. Film grown on
(100) silicon at 600 C at 0.05 mTorr to 80 mTorr, 600 mTorr, and
then 1 Torr.


BIOGRAPHICAL SKETCH
The author was bom September 9, 1967, in Willimantic, Connecticut.
His mother moved to South Carolina in 1969 where he graduated from
Orangeburg Wilkinson High School, Orangeburg, SC, with honors in 1985. He
received a Bachelor of Science degree in ceramic engineering from Clemson
University in 1990 in Clemson, South Carolina, and a Master of Science in
materials science and engineering from the University of Florida in 1995. He
enjoys membership in several honor societies and professional organizations
such as Beta Eta Sigma Honor Society, Alpha Sigma Mu Honor Society, Tiger
Brotherhood Honorary Society, American Vacuum Society, Society for
Information Display, American Ceramic Society, Materials Research Society,
National Society of Black Engineers, Black Graduate Student Organization,
and Alpha Phi Alpha Fraternity, Incorporated.
182


x chromaticity value
157
Chromaticity Values versus Applied Voltage
voltage, [kV]
Figure 5-11. Change in x and y chromaticity values for powder standard as a
function of applied voltage, where the x value increases to 0.644
and the y value decreases to 0.350 as the voltage is increased.
y chromaticity value


34
heating and volume heating. [Sin94] In the surface heating regime, the optical
absorption depth of the laser beam is much smaller than the thermal diffusion
distance. During ablation, either a planar vaporization interface propagates
through the bulk, or subsurface heating may occur leading to non-linear
ablation characteristics. The temperature rise in this regime is governed by
AT
(i -mtp
Cv p(2Dtp)V2
where R is the reflectance, (1-R) is the amount of radiation absorbed, F is the
energy density, tp is the pulse duration, Cv is specific heat, p is the density and
(2Dtp)1/2 is the thermal diffusion distance.[Sin90]
In volume heating, the optical absorption depth is much larger than the
thermal diffusion distance, and the optical absorption depth is inversely
proportional to the absorption coefficient, a. In this regime, sub-surface
heating effects dominate and the temperature rise is governed by
Ar^ = (l-.R)$exp(-az)
C p
where z is the absorption depth.|Sin94, Che88] For large band gap
semiconductors and dielectric materials, optical absorption can be quite high
at the shorter wavelengths. This leads to high surface temperatures since
high energy densities are confined to a small volume. For these materials,


117
1 ^ m
36B 15KU X25.000 16mm
Figure 4-31. SEM photograph of film grown at 0.2 mTorr on (100) silicon and
600 C showing a smooth surface.


153
Figure 5-7.
CL Efficiency versus Voltage
for powder, ablation target, and thin film
Cathodoluminescence efficiency versus voltage taken at 1 |JA/
cm2 of standard, ablation target, and 9 Jim thick rough film.
Target was sintered at 1400 C for 24 hours. Thin film was
deposited on (100) silicon at 600 C, 600 mTorr and annealed at
1200 C in air for 60 minutes.


70
Figure 3-2.
Example of x-rays being diffracted by aligned atoms in a cubic
orientation. This shows the proper miller indices (hkl)for different
atom locations where ao represents the lattice parameter. [Ton92]


64
in the (-) Van der Waals (attractive) force regime where the space between the
o
tip and the sample surface is on the order of tens to hundreds of A. The
cantilever beam is oscillated (or tapped up and down) relative to the surface,
and changes in the average Z height are detected to keep a constant
modulation force on the surface. The non-contact mode is more sensitive than
the contact mode.
With the Digital Nanoscope, both techniques utilize a laser to detect the
deflection. As pictured in figure 3-6, a laser beam is positioned on the tip at
the end of the cantilever. This beam is reflected off the cantilever and into a
position sensitive photo-diode detector where sub-angstrom movement can be
detected. [Bhu95] The x-y position is fed back to keep the cantilever level while
the deflection or height change is recorded.
To optimize the images or quantify data from AFM, the artifacts in data
from scanning characteristics and noise must be removed. The Digital
Nanoscope Instrument plane-fit-data-filter (3rd order) and a flatten-data-filter
(0th order) were used to eliminate scanner artifacts. Vibration induced noise
was erased using a scan-line-filter. This produced an image from data
averaging over the nearest points. Over-compensation from repeated use of
scan line filtering can introduce extra error,
Roughness, or the change in Z height, was the primary quantitative data
gathered. The roughness (Rq), also called root mean square roughness (RMS),
is given by


132
Diffraction angle, [26]
Figure 4-46. X-ray diffraction patterns showing the effect of annealing
temperature on yttria films grown on (100) silicon at 100 mTorr
and 450C fumaced annealed in air at (a) 1200 C, (b) 1000 C, and
(c) 800 C for 1 hour.


28
Luminescent Thin Film Research
Williams [W147] was the first to publish data on the deposition of thin
films for cathodoluminescence phosphors. It was quickly realized that thin film
phosphors would provide advantages over phosphor powders, namely that
screens would be transparent and not opaque or white, improved resolution
would be easily obtained, the thickness could be optimized versus operating
voltages, and the thermal and electrical conductivities could be higher for
continuous thin films. Refer to table 1-3. Recently, additional research has
been reported on cathodoluminescent thin films whose efficiencies are shown
in table 2-1.
The majority of studies of thin film luminescence have focused on
electroluminescence (EL) and photoluminescence (PL) in thin films.
Cathodoluminescence data have predominantly been collected from powders,
due to the severe reduction of luminescence brightness observed with thin
films versus powders. [Gol66] Much of the early thin film research worked on
exploiting the advantages of thin film structures, such as fabricating penetron
devices.[Fel57a] Penetrons are layered phosphor films that change emission
color as a function of beam voltage due to change in penetration depth. (See
figures 2-8 & 2-9.)
Current research has focused on the growth of high quality thin films
and the quantification of thin film brightness. A variety of growth techniques


163
Activation Energy versus FWHM
Figure 5-17. Plot of activation energy for change in FWHM showing dissimilar
activation energies for films grown on (100) silicon at 460 C and
annealed at various temperatures for 60 minutes in air. The
smooth film (100 mTorr) has an activation energy, Ea, equal to 107
cal/mole while the rough film (600 mTorr) has an Ea equal to 33
cal/mole.


140
The increase in europium concentration also has a direct effect on the
CL efficiency. [Oza68,Oza90] As the europium concentration is increased, the
efficiency increases until there are so many activators that concentration
quenching is observed. As shown in figure 5-10, the optimum concentration
for Y203:Eu is 3 mole percent and according to figure 5-9, the (x,y) chromaticity
coordinates for this concentration are (0.64,0.35).
The Osram Sylvania powder had CIE chromaticity (x,y) values of (0.644,
0.35) at 2 kV. As seen in figure 5-11, the (x,y) coordinates for the powder
changed as a function of voltage below 2 kV. The most probable explanation
for this behavior is that above 2 kV, the electron penetration depth was large
enough to reach the average 3 mole % europium concentration below the
surface, resulting in the x,y coordinates of 0.64, 0.35. Below 2 kV, penetration
was shallow and the low x, high y suggests that the europium is depleted from
the surface. Figure 5-12 shows the change in x for the powder standard,
ablation target and a typical annealed film versus the electron voltage. Note
that the thin film phosphors x-coordinate value is significantly lower, leading
to a lower efficiency since the (x,y) coordinate and the efficiency are directly
proportional to the activator concentration.[Oza68, Oza90] The films grown
from 0.02 mTorr to 600 mTorr had x values that ranged from 0.609 to 0.62 and
had an orange-red color associated with them under CL again suggesting Eu
depletion. According to figure 5-9, the active Eu concentration in the thin films
was about between 2.25 to 2.5 mole percent. This percentage should be


127
Auger Y/O Peak Height Ratios vs Growth Pressure
Figure 4-41. Auger Y/O peak height ratios for different oxygen growth
pressures. The target (dotted line) had a Y/O ratio of 14.


107
PLD 1 7




v-

, %





lHm
15KU X5,000
14mm
Figure 4-21. SEM photograph of film grown at 2 x 10'5 Torr at 600 C on (100)
silicon.


32
were grown at 300-450 C and annealed around 1000 C for 2 hours. Brightness
values of 10-20 % that of the powder were reported.
Liquid phase epitaxy and low pressure metal-organic chemical vapor
deposition were utilized by Robertson et al and West et al. Robertson et al
[Rob80] deposited Y3Al5012:Ce thin films. West et al. [Wes90] did not report
efficiency values for their Y203:Eu films, but showed that there was an effect of
deposition parameters on the amount of active Eu in the S6 vs C2 sites. This
was done by following the 611 nm peak (5D0 7F2 transition) versus the 533 nm
peak (5Dt 7Fa transition).
Rao [Rao96] utilized sol-gel processing to deposit Y203:Eu thin films.
Films were amorphous as-deposited at a substrate temperature of 400 C. The
films became crystalline and luminescent after a 600 C, 2 hour anneal. The
characteristic polycrystalline cubic x-ray diffraction pattern was not achieved
until annealing temperatures of 800 C. Like West et al., they showed that the
crystallographic occupation of europium was influenced significantly by
processing temperature.
Pulsed Laser Deposition (PLD)
More recently, pulsed laser deposition (PLD) has been used to deposit
phosphor films. McLaughlin et al. [McL93] deposited ZnS:Mn thin films by
PLD. Greer et al. [Gre94a] used PLD to deposit yttrium aluminum garnet


165
pressures between 2 xlO'5 Torr and 200 mTorr, the (111) FWHM decreased
from 0.47 to 0.15, then increased to 0.25 degrees at 600 mTorr. However, the
PL and CL intensities increased dramatically up to 600 mTorr of 02 with little
improvement at higher oxygen pressures. The grain and particulate size was
also reduced as the pressure was increased. Since the average particle size of
the Y203:Eu PLD thin films was less than 200 nm, there is hope that films with
sub-micron or even nanosized grains can be efficient emitters. Phosphor
powders less than 1 ¡dm are significantly less efficient than powders optimized
for CRT applications having particle sizes ranging from 3 to 10 /Jm. At
processing conditions of T> 600 C and PQ2 > 600 mTorr, a-Y2Si207 was formed
at the Y203/silicon interface.
The reported increased PL brightness with increased oxygen pressure
resulted from an altered surface morphology. The RMS surface roughness
increased from 2 nm to 71 nm upon increasing the growth pressure from 200
mTorr to 600 mTorr, with a concomitant increase in PL brightness from 0.162 fL
to 0.833 fL for 1 jL/m thick films, respectively. However, brightness was only
1.5% and 7.9% that of a powder standard for a smooth film and rough film,
respectively. Increasing film thickness to 3 /im and 9 jUm increased the
brightness to 2.5 fL (22.7%) and 3.9 fL (35.6%), respectively for rough films
deposited at 600 mTorr and 600 C.
The increase in RMS lead to increased forward diffuse scattering which
was fitted to anomalous diffraction scattering equation of the sine integral


33
(YAG) doped with various rare earth dopants. They reported efficiencies for
films from 2 to 8 fJms thick. All films were annealed in oxygen between 1400
and 1650 C for varying times. Reported were efficiencies of 3.8 lm/W for a 8
jUm thick film and 2.4 lm/W for a 2 fJm film at 15 kV and 25 juA/cm2 were
reported. Hirata el al. [Hir96] also deposited ZnGa204:Tb, Y3Al5012:Tb (YAG:Tb)
and Y203:Eu using PLD. Films deposited at 300 C were amorphous and
required anneals of 800 C or greater to establish good crystallinity and
luminescence. They reported similar results for the yttria doped with
europium thin films. [Hir97] No relative brightness or efficiency data were
given.
The growth technique used to deposit the phosphor films was pulsed
laser deposition (PLD). PLD has been used extensively as a growth technique
for superconductors and other complex oxide systems.[Nar87] It is the
deposition technique of choice to deposit superconducting films due to its
accuracy for stoichiometry over sputtering and other growth techniques. This
physical vapor deposition technique couples a focused pulsed laser onto a
target where absorption and subsequent evaporation take place. Excimer
lasers are normally used, varying in wavelengths from 1064 to 532, 355,or 248
nm. A main limitation of this growth technique are particulate evolution
during deposition and limitation in deposition area.
Stoichiometric deposition results from the laser-target interactions.
There are two basic laser-target interactions leading to ablation: surface


emerged as a technique capable of depositing a wide variety of
materials. [Che88 ]


ENHANCED LUMINESCENCE FROM EUROPIUM DOPED YTTRIUM OXIDE
THIN FILMS GROWN VIA PULSED LASER DEPOSITION
By
SEAN LIAM JONES
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
1997


Copyright 1997
by
Sean L. Jones


94
CL Brightness versus Voltage
for film grown at 800 C and 1 x 10 5 Torr on (100) Silicon
Figure 4-8. Brightness versus voltage for film grown at 800 C on (100) silicon
at 2 x 105 Torr. Data taken at 1 ¡iA/cm2.


9
In an FED, scanning and rastering are not used. Instead, an array of
field emission tips generate the electrons. Refer to figure 1-1. Upon applying a
very low voltage across the gate, electrons are emitted from the cone shaped
tips and are accelerated toward the phosphor screen. When the electrons
strike the phosphor screen, cathodoluminescence occurs.
Cathodoluminescence (CL) is the emission of light upon electron radiation.
These FED tips can be turned on one row of pixels at a time, eliminating
the need to deflect the electron beam. This eliminates the path needed for
electron deflection, therefore, the FED is a flat panel display technology.
Different geometries of field emission displays are shown in figures 1-1 and 2-
1, where the tips in figure 1-1 are referred to as a Spindt cathode, the more
common type. The second type is an edge emitter which offers another
configuration where the whole device is on one substrate. For both, the
spacing between the base and the face plate is on the order of a 1 to 3
millimeters. The whole display, including packaging, is thinner than 5
centimeters.
Charles A. Spindt developed field emitter tip technology in 1973 at SRI
International using VLSI processing. [Mat86] The emitter tips are metals such
as W and Mo, or semiconductors such as Si and diamond. When a voltage is
applied between the tip and the gate, Fowler-Nordheim tunneling [Hud92]
occurs and electrons are accelerated toward an anode. These tips emit
electrons above 20 V compared to etched wire emitters which require between


52
ELECTRON
a
/.
/ ^
"air
\\
\ \ \
\ \ w
\ b
3
(a)
(b)
Figure 2-13. Illustration of light internally reflected in (a) a powder versus, (b)
a thin film. Light eventually escapes the powder, while only light
within the film incident the surface at angles less than 0c (rays a
& b) will escape. Light incident at angles greater (rays c & d) is
trapped within the film and will continue to reflect internally.


152
Figure 5-6.
Efficiency versus Voltage
for film grown at 100 mTorr vs 600 mTorr annealed at 1200 C
Efficiency of 3 |im thick smooth (100 mTorr) versus rough film (600
mTorr) taken at 1 |iA/cm2 at varying voltages. Films grown on
(100) silicon at 460 C and annealed at 1200 C in air for 60 minutes.


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142
full width at half maximum (FWHM) for the (111) XRD peak decreased as the
substrate temperature was increased, indicating an improvement in
crystallinity (figure 4-17). Similar results for FWHM were seen for films
deposited onto glass and c-axis A1203.
Based on data in figures 4-18 and 4-19, films deposited at 250 C, < 104
Torr, exhibited slight polycrystalline behavior on crystalline substrates.
Between growth temperatures of 250 C and 800 C, the films exhibited strong
preferential (111) orientation. Above 800 C, strong polycrystalline texture was
developed. This tendency toward strong (111) texturing at temperatures of
800 C and below has also been reported with PLD of Y203 by Hirofumi et
al.[Hir89] Textured films have also been reported with other growth
techniques such as sputtering [Ait93], evaporation [Hud95] and by sol gel
[Rao96]. Rao et al. also reported the change to polycrystalline texture at
growth temperatures of 800 C or more.
Films grown at low pressures (2 x 10'5 Torr) were fairly smooth (figure 4-
24) while cracked films were observed at growth temperatures of 600 C or
greater (figure 4-25). This most likely resulted from cooling the substrate too
rapidly after growth, where the thermal expansion mismatch induced
cracking. Silicon has a linear coefficient of thermal expansion(CTE) of 2.49 x
10'6 K1 at 300 K [Lid97] while yttria has a CTE value of 8.9 x 106 K1 [Tro95],
which is 3.6 times that of silicon. Columnar growth was also seen (figure 4-22,
4-37 and 4-40) as expected. Growth from high energetic thin film growth


60
Scanning Electron MicrosconefSEM)
Scanning electron microscopy (SEM) is a powerful tool in the
characterization of surface topography and defects.[Bin92] It is capable of
providing micrographs with magnification as low as lOx and as high as
300,000x resulting in images with resolutions of a few nm. A JEOL 6400 SEM
with true secondary and backscatter electron detectors plus an energy
dispersive x-ray spectrometer (EDS) attachment was used similar to the one
pictured in figure 3-3. Primary electrons were generated from a tungsten
filament by thermionic emission. The operating voltage ranged from 5 kV to 30
kV, most commonly 15 kV. Since the samples were non-conductive, they were
coated with Au-Pd for imaging or carbon coated for EDS analysis. The high
magnification limit for these samples was ~ 100,000x. A transmission electron
microscope (TEM) was employed for higher magnification. SEM was primarily
used to acquire surface morphology, cross section thickness and morphology,
and grain size data.
Rutherford Backscatterina SoectroscopvfRBS)
Rutherford backscattering spectroscopy (RBS) was primarily used to
determine film composition and stoichiometry. [Bau92a] It is one of the few
quantitative surface analysis tools available. A high voltage van de Graaf
accelerator with an EDS detector at Oak Ridge National Laboratory was used.


180
Sin94
Sin90
Smi86
Smi95
Spi76
Str92
Str64
Stu56
Stu55
Stu51
Tan85
Tom86
Ton92
Tro91
Singh, R. K., Journal of Non-Crystalline Solids 178, 199.
Singh, R. K., & Narayan, J., Physical Review B 41, 8843.
Smith, D., Metals Handbook 9th Edition: Vol 10 Materials
Characterization, edited by Davis, J., American Society for Metals,
Metals Park, Ohio.
Smith, D. L., Thin-Film Deposition. Principles and Practices.
McGraw-Hill, Inc., New York, NY.
Spindt.C. A., Brodie, I., Humphrey, L., & Westerberg, E. R.,
Journal of Applied Physics 47, 5248.
Strausser, Y.E., Encyclopedia of Materials Characterization,
edited by Brundle, C.R., Evans, C.A., & Wilson, S., Manning
Publications Co., chapter 5.3.
Strutt, J. W., Scientific Papers bv Lord Ravleiah Volumes I & 2.
Dover Publications, New York, NY.
Studer, F.J., Journal of the SMPTE 65, 197.
Studer, F.J., & Cusano, D.A., Journal of the Optical Society of
America 45, 493.
Studer, F.J., Cusano, D.A., & Young, A., H., Journal of the Optical
Society of America 41, 559.
Tannas, L., Flat-panel displays and CRTs, van Nostrand Reinhold,
New York, NY.
Tomiki, T., Tamashiro, J., Tanahara, Y., Yamda, A., Fukutani, H.,
Miyahara, T., Kato, H., Shin, S., & Ishigame, M., Journal of the
Physical Society of Japan 55, 4543.
Toney, M., Encyclopedia of Materials Characterization, edited by
Brundle, C.R., Evans, C.A., & Wilson, S., Manning Publications
Co., chapter 4.1.
Tropf, W. J. & Thomas, M.E., Handbook of Optical Constants of
Solids II. edited by Palik, E., Academic Press, Boston, MA.


80
thickness led to better CL efficiencies. The CL efficiency data for 2 kV
electrons are shown in figure 4-15 for thicknesses ranging from 0.6 to 9 /Jm.
Thin Film Growth
The results of thin fim growth are presented below starting with the
raw materials processing. The europium doped yttria powder had the
characteristic cubic Y203 structure and was a perfect match for the JCPDS 25-
1200 file. As-received powder pressed into targets and sintered at 1400 C for
24 hours also displayed the cubic structure (see figure 4-16).
Films were deposited at growth temperatures of 250 C to 800 C in 2 x
10'5 Torr. Yttria was successfully grown at temperatures as low as 250 C,
regardless of substrate material. As seen in figure 4-17, there was a small
(111) peak for films grown on glass substrates at 250 C. The (111) direction
remained the preferred orientation as the temperature was increased to 600 C.
There were additional peaks from films grown at 600 C resulting from grains
aligned in other directions. Similar results were seen for films grown on (100)
silicon and c-axis A1203 (sapphire), as shown in figures 4-18 and 4-19,
respectively. It is noted that for the yttrium oxide/silicon system, the (100)
silicon peak was at 69, which, for clarity, is not plotted on these graphs. There
were also small silicon Kp and Ka peaks present at 61 and 32 in some of the
XRD plots.


Figure 3-7. Spectrum after filtering yielding short wave UV emission.


51
YjOj : Eu (5 fim)
Pr concentration (mole fraction)
Figure 2-12. Cathodoluminesecent intensity versus activator concentration (a)
Eu in two host materials displaying indirect activator excitation
and (b) for Pr in two host materials displaying both direct (slope
= 1.0) and indirect (slope=0.66) behavior. [Oza90]


23
piping. It can be seen from this discussion that perceived brightness can be
severely reduced in thin film phosphors, leading to lower efficiencies.
Elementary Scattering Theory
It will be shown in the following chapters that roughening the surface of a
dielectric thin film phosphor increases the photoluminescence (PL) and
cathodoluminescence (CL) brightness. Since this increase was attributed to
scattering at the film/air interface, an outline of scattering and how it pertains
to particles and films is presented. Although there is an abundance of
scattering theories, ranging from general to case specific solutions, the
discussion below is limited to solutions for nonconducting, non-absorbing,
relatively small particles with refractive indices close to 1. A brief overview of
applicable scattering theory is presented below.
It is generally accepted that scattering refers to the movement of a wave
due to the presence of an obstacle of a given size, form and composition.
[Van81] More specifically, the term is most applicable when the obstacle has
a size smaller than the incident wavelength, k. The movement due to an
obstacle having a size much greater than k is referred to as diffraction,
reflection, or refraction. [Van81, Jen76]
For spherical particles, scattering occurs when the radius, a, is much
smaller than the wavelength, but geometric optics apply when a > k. Thus,


Phosphor
Volts
(kV)
Current
Density
(jUA/cm2)
Bright
ness
(Cd/m2)
Eff.
(lm/W)
Growth
Tech.
Ref.
Comment
Y3Ga2Al3012:Tb
10
25
1.60
f
Gre94
from graph, 2 /Jm film
Y3Ga2Al3012:Tb
15
25
2.40
f
Gre94
from graph, 2 /Jm film
Y3G a2Al3012:Tb
10
25
2.80
f
Gre94
from graph, 8 /Jm film
Y3Ga2Al3012:Tb
15
25
3.80
f
Gre94
from graph, 8 /Jm film
a Chemical Vapor deposition
b Thermal Vacuum Evaporation
c Spray Pyrolysis
d RF Ion-plasma Sputtering
e RF Magnetron Sputtering
f Pulsed Laser Deposition
CO
CO


INTENSITY [ARB. UNITS]
102
DIFFRACTION ANGLE [20]
Figure 4-16. X-ray diffraction patterns of (a) yttria ablation target pressed
from (b) Osram Sylvania ytrria powder doped with 4.5 weight
percent Eu and (c) the cubic yttria JCPDS file 25-1200.


CHAPTER 6
SUMMARY AND CONCLUSIONS
Yttrium oxide doped with 4.5 weight percent europium films were
deposited onto (100) silicon, glass, and c-axis A1203 substrates using pulsed
laser ablation (PLD). The luminescence versus deposition conditions and
microstructure were characterized and correlated. The PLD target
demonstrated cathodoluminescence (CL) and photoluminescence (PL)
efficiencies and brightnesses a factor of 2 lower than the Y203:Eu starting
powder phosphor while the thin films were a factor 3.3 lower. This correlated
to an increase in theoretical and experimental values of 7% outcoupling for
smooth films to 30% from rough films.
As-deposited films were crystalline at substrate temperatures as low as
250 C, even though the melting temperature of Y203 is 2410 C. The PLD films
had particules varying in size from 100 15 nm to 80 15 nm for pressures of
2 x 105 Torr to 02 pressures of 600 mTorr, respectively. PLD at temperatures
less than 600 C and oxygen pressures below 200 mTorr resulted in
preferentially oriented films with a (111) texture. An increase in the 02
pressure above 200 mTorr and growth temperature above 600 C changed the
film from (111) texture to a random polycrystalline orientation. For 02
164


84
The change from a preferential (111) texture to a random polycrystalline
one was also evident from SEM photographs. Figures 4-31 to 4-36 show the
change in morphology as the oxygen pressure was increased. The morphology
became nodular in nature and rougher as the pressure was increased from 0.2
mTorr (figure 4-31) to 600 mTorr (figures 4-35 and 4-36). The nodule size, as
well as the grain size, were well below 1 /Jm. The nodule size varied from 100
15 nm for films grown at 400 mTorr and 80 15 nm for films grown at 600
mTorr (figure 4-36). This change in structure and nodule size is seen in figure
4-37, where the Oz pressure was increased from 0.05 mTorr to 80 mTorr, 600
mTorr, and then 1 Torn
A change in film morphology was also apparent from AFM
measurements of the RMS roughness of surfaces versus oxygen pressure
during growth (figure 4-38). The RMS roughness, increased from 2 nm to 71
nm as the Oz pressure was increased from 0.02 to 600 mTorr.
Film thickness, at a fixed Oz pressure and temperature, was also varied
by controlling the deposition time. As expected, the growth was linear with
respect to deposition time as seen in figure 4-39, which shows film thickness
versus growth time for films grown at 600 C and 600 mTorr on (100) silicon.
The columnar growth was maintained even at these long growth times (figure
4-40).
The Y/O peak heights were compared using AES to determine if there
was any change in oxygen content with increasing oxygen pressure during


81
As the growth temperature was increased, the FWHM for the (111) peak
changed with temperature as shown by figure 4-20. A poorly crystalline
polycrystalline pattern was observed for films grown at 250 C on all
substrates, yielding a high FWHM of 0.383. The crystallinity improved,
changing to a strong (111) texture at temperatures greater than 250 C,
corresponding to a decrease in FWHM. At 800 C, the films on silicon and
sapphire transformed back from highly (111) texture to a polycrystalline film
resulting in an increase in the FWHM.
There was also a corresponding change in film morphology with the
change in crystal structure and texture. The typical thin film morphology was
smooth (figure 4-21) for films grown at pressures less than 1 x 10'4 Torr on the
(100) silicon substrates. Particulates ranging from 0.125 to 0.25 ¡Jm seen in
figure 4-21 resulted from the ablation process. Particles from the target were
liberated as a result of the absorption and melting processes during ablation
and could be reduced by using denser targets, reducing the laser energy
density during ablation or by utilizing some of the other particulate reduction
schemes developed recently.[Che94b] As seen in the cross-section SEM
photograph (figure 4-22), the growth process yielded nice columnar grains. As
the growth temperature was increased for deposition on (100) silicon at 2 x 10'5
Torr, the films first became smoother from 250 C to 400 C, then more granular
from 400 C to 800 C, as seen by figures 4-23 to 4-25.


101
CL Efficiency versus Thickness
for annealed films grown at 600 C and 600 mTorr
Figure 4-15. Cathodoluminescence efficiency as a function of film thickness for
films grown on (100) Si at 600 C, 600 mTorr and annealed in air at
1200 C for 1 hour. Electron beam conditions were 2 kV and 1 (iA/
cm2.


174
Bla79
Bla94
Boa83
Boh96
Bon95
Bor64
Bro92
Bui87
Car66
Cas92
Cha90
Che94a
Che94b
Blasse, Chemistry and Physics of R-Activated Phosphors, ed.
Gscheindner, K. A. & Eyring, L., Handbook on the Physics and
Chemistry of Rare Earths: volume 4. North-Holland Publishing
Comp., New York, NY.
Blasse, G., & Grabmaier, B.C., Luminescent Materials. Springer-
Verlag, Berlin, Germany.
Boas, M. L., Mathematical Methods in the Physical Sciences. 2nd
Edition. John Wiley and Sons, New York, NY.
Bohren, C. F., Handbook of Optics volume I. edited by Bass, M.,
McGraw-Hill, Inc., New York, NY, chapter 6.
Bondar, V., Grytsiv, M., Groodzinsky, A., & Vasyliv, M., SPIE 2648,
338.
Bom, M., & Wolf, E., Principles of Optics: Electromagnetic Theory
of Propagation. Interference and Diffraction of Light. Pergamon
Press, New York, NY.
Brodie, I, & Spindt, C. A., Advances in Electronics and Electron
Physics volume 83. Academic Press, New York, NY.
Buijs, M & Meyerink, A. & Blasse, G., Journal of Luminescence 37,
9.
Carr, W. N., Infrared Physics 6, 1.
Castellano, J., Handbook of Display Technology. Academic Press,
Inc., San Diego, CA.
Chang, C. C. & Wu, X. D. & Ramesh, R. & Xi, X. X. & Ravi, T. S. &
Venkatesan, T. & Hwang, D.M. & Muenchausen, R. & Foltyn, S. &
Nogar, N. S., Applied Physics Letters 57, 1814.
Chen, L., Pulsed Laser Deposition of Thin Films, edited by Crisey,
D. & Hubler, G.K., John Wiley and Sons, New York, NY, chapter 6.
Cheung, J. T., Pulsed Laser Deposition of Thin Films, edited by
Crisey, D. & Hubler, G.K., John Wiley and Sons, New York, NY,
chapter 1.


45
Figure 2-6.
en
CE
UJ
CD
2
3
Z
UJ
5

SHARP LINE broao bao
EMISSION ABSORPTION
Energy level diagram for europium, Eu+3, doped yttrium oxide.
The characteristic emission at 611 nm results from the inter
atomic 5Dq-7F2 transition within the europium. [Wic64]


12
Yttrium Oxide: Eu+3 (Y2Q3:Eu')
Yttrium oxide (Y203), commonly referred to as yttrium sesquioxide or
yttria, is a highly refractory oxide with a melting point of 2410 C. At room
temperature yttria is a body centered cubic with the bixbyite or cubic-C
structure with a lattice parameter of 10.6 .[Wyc64] There is an allotropic
phase transformation to hexagonal at 2367 C.[Tro91] Yttria has very good
chemical stability, high resistivity and a high dielectric breakdown strength. It
has a measured room temperature bandgap of 5.3-5.5 eV [Jol90] and a
bandgap of 5.8 eV at 10 K.[Tom86]
Yttria has a theoretical density of 5.033 g/cm3. There are 16 molecules of
80 atoms in its unit cell. The cation occupies two different crystallographic
sites, S6 and C2, as seen in figure 2-4. There is a full layer of C2 sites with a
layer of alternating S6 and C2 sites, resulting in 75% of the yttriums in C2
symmetry and 25% in S6 symmetry. In S6 symmetry, the yttrium is in contact
with body diagonal vacancies, but in contact with face diagonal vacancies in
C2 symmetry. In both symmetries, the cation is surrounded by 6
oxygens. [Mae92] These sites are, therefore, chemically equivalent but
crystallographically different. S6 symmetry is also denoted in the literature as
C3i symmetry.
Yttria is a perfect host for the trivalent rare earth europium to make
highly efficient lamp and cathodoluminescent phosphors. Europium belongs to


95
CL Brightness versus Voltage
for annealed film grown at 460 C and 100 mTorr on (100) Silicon
Voltage, [kV]
Figure 4-9. Brightness of film grown at 460 C, 100 mTorr of 02 pressure and
annealed at 800, 1000, and 1170 C for 1 hour in air. The films
grown at 460 and 600 C did not have detectable
cathodoluminescence.


118
Figure 4-32.
1 !- m
36D 15KU X25.000 16mm
SEM photograph of film grown on (100) silicon at 20 mTorr and
600 C showing a smooth surface.


58
special cases, quantification of defect concentrations.[Smi86] These different
applications of XRD result from the ability to measure and interpret peak
intensity, position and width data. The Warren-Averbach method was used to
deduce the crystallite size from the FWHM data.[Cul78j The (100) peak of a
silica standard obtained from the National Institute of Standards and
Technology was used to quantify the data from Y203:Eu.
Atoms in a solid are capable of scattering and diffracting electrons,
photons, other atoms and x-rays. Incident x-rays are either constructively or
destructively diffracted. Constructive diffraction results when the atoms are
aligned in a periodic fashion. The different rays reinforce rather than cancel
one another as in the destructive case and a diffracted signal, or peak, is
detected. This peak is also referred to as a line profile. The required atomic
alignment to yield constructive interference is governed by Bragg's law
nl = 2dmsmm
where A is the wavelength of incident radiation, n is a small integer (1,2,3...)
and is the order of the diffracted peak, typically first order with n=l; d^ is the
spacing between atomic (hkl) planes where hkl are the Miller indices of the
diffracting plane of atoms, and 0 is the angle between the normal of the (hkl)
plane and the incident x-ray beam. [Cul78] The interplanar spacings and
planes for low Miller indices are shown in figure 3-2.


Intensity, [arb. units]
88
Figure 4-2.
PL Intensity for Various Temperatures
Photoluminescence versus temperature for film grown at 600 and
800 C compared to a film deposited on a mechanically
roughened silicon substrate at 600 C. Substrate was roughened
with 600 grit SiC paper prior to deposition.


43
Figure 2-4.
(^) O |~| Vacancy
Y
Representation of the Y203 crystal structure showing the two
different cation symmetries, S6 and C2. In the C2 site there is an
oxygen vacancy (squares) on the face diagonal, while the S6 site
has a body diagonal oxygen vacancy pair. [ Jag94] This yields
chemically equivalent but crystallographically in-equivalent sites.


TABLE OF CONTENTS
ACKNOWLEDGMENTS iv
ABSTRACT viii
CHAPTERS
1. INTRODUCTION 1
2. LITERATURE REVIEW 8
Field Emission Displays 8
Yttrium Oxide:Eu+3 (Y203:Eu) 12
Cathodoluminescence 16
Efficiency of Thin Film Phosphors 19
Elementary Scattering Theory 23
Luminescent Thin Film Research 28
Pulsed Laser Deposition (PLD) 32
3. EXPERIMENTAL PROCEDURE 54
Substrate and Target Preparation 54
Pulsed Laser Deposition(PLD) 55
PLD Film Characterization 57
X-Ray Diffraction(XRD) 57
Scanning Electron Microscopy(SEM) 60
Rutherford Backscattering Spectroscopy(RBS) 60
Atomic Force Microscopy(AFM) 62
Auger Electron Spectroscopy(AES) 65
Photoluminescence(PL) 66
Cathodoluminescence(CL) 67
4. RESULTS 76
Photoluminescence(PL) 76
vi


91
Figure 4-5.
PL Intensity vs Film Thickness
600 C, 600 mTorr Oz on (100) Silicon
Film Thickness, [pm]
Photoluminescence versus Y203:Eu film thickness showing an
increase in PL intensity as film thickness was increased. Films
grown on (100) silicon at 600 mTorr and 600 C.


ACKNOWLEDGMENTS
I first give honor and praise to God for allowing me to see this day.
Along with the Creator, my family and girlfriend are directly responsible for
this achievement. I especially thank my mother. I thank her for her sacrifices,
devotion, and little nudges to get me through the process. I would also like to
thank Dr. Holloway for giving me the opportunity to contribute. He has been
an untiring source of wisdom and inspiration. In that vain, I would also like to
acknowledge Dr. Singh, who has served as a second advisor. I appreciate all
of the individual time you have spent on this project and on my development. I
would also like to thank the balance of my committee for their service: Dr.
Hummel, Dr. Pearton, and Dr. Anderson.
I am particularly thankful for Dr. Jengyi Yu, the first graduate student I
was paired with upon arrival to the University of Florida. He was my first
mentor and colleague. For that, I will always be grateful to him. I would also
like to recognize Dr. John M. Anderson, Dr. William Edmondson, and Dr. Paul
Mason for their mentoring and friendship. Special recognition belongs to Dr.
Hendrick Swartz, Dr. Mark Davidson and Xiao-Ming Zhang for their support.
The three of them came into my life just as I was giving up hope and preparing
IV


154
Figure 5-8.
The 1931 Commission Internationale de lEclairage (CIE) (x,y)
chromaticity diagram showing the three primary color regions
blue, green and red as a function of chromaticity values x and y.
[Wys78] The arrow shows the change in (x,y) as europium
concentration is increased.


82
X-ray diffraction data were complemented with chemical analysis using
AES, RBS and SIMS. Figure 4-26 shows a typical Auger spectrum from a yttria
film deposited on (100) silicon at 600 C at 10'5 Torr, after cleaning the near
surface with Ar+ sputtering for 1 minute. The surface of the as-received
samples was contaminated with carbon (272 eV) and chlorine (181 eV) peaks,
which is normally seen. Yttrium Auger peaks were detected at 75, 106,122,
1743 and 1817 eV. From the AES handbook, the low energy triplets were
expected to be at 77, 110, and 127 eV. The expected higher energy peaks are
at 1746 and 1821 eV. The oxygen peak was seen at 509 eV while 510 eV is
characteristic energy. The expected theoretical values are for the Y metal and
the differences in detected peak energies and theoretical values result from
chemical shifts in forming the Y203 solid. The small peak at 215 eV resulted
from argon implantation from the Ar+ sputtering. Europium was not detected
due to its low concentration in Y203. Europiums characteristic Auger peaks
are located at 109, 139 and 858 eV, but no signal was detected at 858 eV.
Surface Roughening
The effect of surface roughening was initially investigated by growing
europium doped yttria on smooth and mechanically roughened substrates.
The substrates were roughened prior to growth with 600 grit SiC paper as
described previously. As in figure 4-27, yttria was successfully deposited onto


155
Figure 5-9.
Eu (mole %)
2 3 4 5 6
Plot of x and y chromaticity values versus europium mole percent
and europium S6/C2 site emission for Y2OzS under cathode ray
excitation. [Oza90]
Color coordinate


109
PLD 22
1 5KU
1 P m
X25.000 15mm
Figure 4-23. SEM photograph of Y203:Eu film deposited on (100) silicon at 250
C at 2x10 Torr.


141
increased so an accurate comparison of efficiency between the powder and
thin films can be made.[Oza90] This lower europium concentration, evident
from lower x-chromaticity values, contributes to a lower CL efficiency from
these PLD thin films.
Since the target x,y values closely matched the powder, the europium
depletion in the film was a result of the PLD growth process. The RBS data
showed that the atomic formula was Y0 367Eu0 007O0 626 which corresponds to
Yj 835Eu0 035O313. Using 3 mole percent of europium as the target amount, the
molecular formula should have been Yj 85Eu01503. The europium concentration
was half that, suggesting that the percentage is as low as 1 to 1.5 mole
percent. It is possible that the plume distance from the target was large
enough that heavy atoms were not incorporated ideally but were resident in
the fringes of the plume. Europium has atomic mass of 153, while yttrium is
89. It is known from spectroscopy experiments on laser ablation plumes that
heavier atoms and clusters fall out" from the plume where the concentration
decreases from the center to the outer edge of the plume by a cosine law
dependence.
Thin Film Growth
Deposition of Y2Q3:Eu thin films as reported above yielded crystalline as-
deposited thin films at temperatures of 250 C and higher. As expected, the


126
Figure 4-40. Cross-section SEM photograph of film grown on (100) silicon at
600 mTorr and 600 C for 60 minutes.


137
A model of surface scattering assumes that the grains and surface
particles are uniform and that the surface looks similar to figure 5-1, where a is
the RMS roughness, b is the surface hillock (grain) size and c is the spacing
between particles. It is assumed that the RMS roughness is the proper
roughness term to model the parameter a and that c is large enough that the
surface particles do not act as closely packed particles but as individual
scatters. It was also assumed that the film had the bulk index of refraction
value for yttria at 611 nm (n=1.93) and that due to the columnar structure of
the films, the light impinges as a plane wave incident parallel to the scattering
surface.
From the previous discussion on scattering (chapter 2), it was shown
that scattering theory, as opposed to geomtric optics, was appropriate for
small particles with index of refraction close to 1. From the size parameters, x,
and p, the critical dimensions at the largest RMS are equal to
OtTP
x = £51 = 0.73 ; p = 2x|m-l | = 1.36
X
where the sample RMS roughness substitutes for a. These values are too high
to satisfy the conditions for Rayleigh scattering (a < X/20 and 1.93x Although Rayleigh-Gans scattering allows for a larger x value, both x and p are
still too small ((m-l)x < < 1). Even the limiting case of Rayleigh-Gans,
intermediate scattering, is excluded since the p value is greater than 1.


146
was increased from 1000 C to 1200 C, where the x changed from 0.611 to
0.625 and y from 0.364 to 0.359, respectively at 4 kV and 1 jL/A/cm2. This shows
an increase in acitve europium.


86
that converted the silicon/yttrium oxide to a yttrium silicate phase. The a-
Y2Si207 phase was the best fit. Figure 4-45 shows that annealing reduced the
FWHM from 0.51 to 0.14 as the annealing temperature was increased from
460 C growth temperatures up to 1200 C annealing temperatures. Films
grown at 100 mTorr at 460 C (figure 4-46) were (111) oriented and remained
textured for all annealing temperatures. The (111) FWHM (figure 4-47)
decreased from 0.18 to 0.13, a value similar to the film grown at 600 mTorr
and annealed at 1200 C. The (111) FWHM value for the powder was even
lower, ranging from 0.096 to 0.11.
The nodule size increased with annealing temperature as shown by
figure 4-48. The sample was deposited at 600 mTorr and 600 C on (100)
silicon (figure 4-48a) followed by furnace annealing at 1000 C for 30 minutes
in air (figure 4-48b). Grains increased from 80 15 nm to 100 20 nm in size.
This size of nodules was still an order of magnitude smaller than that of
phosphor powder particles at 5 um.
RBS data from a film grown at 100 mTorr, 600 C and annealed at 1000
C for 30 minutes in air is shown in figure 4-49. The composition of the film
was the same prior to and after annealing. However, there was an interfacial
reaction with the silicon substrate resulting in a yttrium rich silicate phase.
This can be seen as a change in the low energy edges of the peaks for oxygen
and yttrium and for the high energy edge of Si. The fit routine yielded the
same composition for the annealed film as for the as-deposited condition.


42
7-1 i i i i i i i i i | i i i ri-n-i i | i i i n i i i i | i i i i i i i i i | i i i i i i i i t~|
0 20 40 60 80 100
TIP RADIUS (nm)
Figure 2-3. Calculated log of current density, J, plotted as a function of field
emission tip radius. The applied external voltage was 30 V and
the tip to anode separation distance was fixed at 20 nm. [Cut93]