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

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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 -




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