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Measurement and modeling of the effects of pulsed laser deposited coatings on cathodoluminescent phosphors

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Title:
Measurement and modeling of the effects of pulsed laser deposited coatings on cathodoluminescent phosphors
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Thomes, William Joseph, 1974-
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English
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x, 322 leaves : ill. ; 29 cm.

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Subjects / Keywords:
Beam currents ( jstor )
Cathodoluminescence ( jstor )
Electric potential ( jstor )
Electron beams ( jstor )
Electrons ( jstor )
Energy ( jstor )
Luminescence ( jstor )
Phosphors ( jstor )
Stopping power ( jstor )
Wavelengths ( jstor )
Dissertations, Academic -- Materials Science and Engineering -- UF ( lcsh )
Materials Science and Engineering thesis, Ph.D ( lcsh )
City of Gainesville ( local )
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bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

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Thesis:
Thesis (Ph.D.)--University of Florida, 2000.
Bibliography:
Includes bibliographical references (leaves 310-321).
General Note:
Printout.
General Note:
Vita.
Statement of Responsibility:
by William Joseph Thomes.

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University of Florida
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MEASUREMENT AND MODELING OF THE EFFECTS OF
PULSED LASER DEPOSITED COATINGS ON
CATHODOLUMINESCENT PHOSPHORS
















By

William Joseph Thomes, Jr.


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

UNIVERSITY OF FLORIDA


2000
































This work is dedicated to the memory of my grandfather,
Gilbert Walter Thorpe,
who passed away in the course of my graduate studies.















ACKNOWLEDGMENTS


There are so many people whom I wish to thank that it is hard to decide where to

begin. First, I would like to thank my fiance, Cynthia, for her loving support all these

years. She has seen me through the ups and downs of graduate school and has always

been there to pick me up when I was down or to share in my joy. She also provided

invaluable editorial advice and revisions for this dissertation. I would also like to thank

my family for their love throughout my life. Even when I was not sure that it was

possible to make it all the way to the end, they believed in me and provided the

encouragement to stick it out no matter what happened.

Dr. Paul Holloway deserves a special word of thanks for providing guidance

during my graduate education. His door was always open, and he never hesitated to

discuss my work with me and give me some direction when needed. He is truly one of

the greatest teachers that I have been fortunate enough to come across in my educational

pursuits.

Special thanks are also due to Dr. Carl Seager and Dr. Dave Tallant for providing

me with the opportunity and the funding to come to Sandia National Laboratories to

conduct many of my experiments with them. I am especially grateful to Carl, who, in

addition to providing me access to all of the equipment in his labs so that I could measure

my samples, was a mentor and a friend during my stay in New Mexico. Without his help








and guidance, much of the work contained in this dissertation would not have been

possible.

There are many others who made my stay in New Mexico a memorable one.

Although I cannot mention them all in the space allotted to me, I would like to point out a

few. John Hunter was everything I could have asked for in a friend, and never once did

he hesitate to allow me to stay at his house a little longer when I said, "I think they want

me to stay one more month." I would also like to thank my many other friends out in

New Mexico, including John, Shasta, Jerry, and Shawna. Thanks are due to Jonathan

Campbell for sedimenting all of my phosphor screens, helping me learn all the quirks of

the lab equipment, introducing me to everyone at the lab, and teaching me about all of the

fascinating things that New Mexico has to offer (including green chiles). I'd also like to

thank Guild, who always seemed to be in a good mood, for his help with the SEM work.

Many thanks go to those in Florida who have made these past four years

memorable ones. I wish to thank the entire Holloway group past and present, especially

Billie, Jay, Bo, Chris, Sean, Troy, Loren, Caroline, Bill, Mark, and Ludie (without whom

we might all be lost). I am grateful to the following for not only providing me with their

friendship, but also for performing measurements on my samples: Eric Lambers for AES

measurements, Wish Krishnamoorthy for TEM analysis, and Dr. Kumar for help with the

PLD of the coatings used in this work.

I'd also like to thank all of my friends in Cuong Nhu, especially Chi-Wah, who

suffered with me for many years under some of the best senseis in the martial arts. I will

never find another set of instructors who provide the type of training that I found in

Sensei Mark's and Sensei Joyce's classes.









Although I have made every attempt to include all those who deserve

acknowledgement in these pages, I am sure that there are some who have not been

mentioned. I regret the omission, but after many long days of putting this dissertation

into its final form, it's surprising that more of you are not missing. Please know that each

of you is appreciated.















TABLE OF CONTENTS


ACKN OW LED GM ENTS.................................................................................................. iii

ABSTRA CT ............................................................................................................................ix

CHAPTERS

1 INTROD UCTION ........................................................................................................... 1

2 LITERATURE REVIEW ................................................................................................ 5

Introduction.................................................................................................................. 5
Field Em mission D display (FED )..................................................................................... 6
Cathodolum inescence (CL).......................................................................................... 8
Phosphors ................................................................................................................... 14
Y203:Eu................................................................................................................. 15
Y2SiOS:Tb ............................................................................................................. 17
Efficiency ................................................................................................................... 18
Charging..................................................................................................................... 19
Cathodolum inescent Degradation.............................................................................. 25
Coatings for Phosphors .............................................................................................. 31
Pulsed Laser Deposition (PLD ) ................................................................................. 35
M otivation for this W ork ........................................................................................... 38

3 EXPERIM ENTAL PROCEDURES ............................................................................. 61

Introduction............................................................................................................... 61
Sedim entation of Phosphor Screens........................................................................... 61
Pulsed Laser D position (PLD ) of Coatings.............................................................. 63
Characterization Techniques...................................................................................... 66
Steady-State Cathodolum inescence ...................................................................... 67
Pulsed Cathodolum inescence................................................................................ 69
Ellipsom etry .......................................................................................................... 71
Transm mission Electron M icroscopy (TEM )........................................................... 74
A uger Electron Spectroscopy (AES)..................................................................... 76
Scanning Cathodoluminescence (CL) in a Scanning Electron
M icroscope (SEM ) ............................................................................................. 77








4 EXPERIM ENTAL RESULTS...................................................................................... 88

Introduction........................................................................................................ ........ 88
Cathodoluminescent Spectra...................................................................................... 88
Beam Energy Effects on Luminescence.................................................................... 92
Pulsed Cathodoluminescence..................................................................................... 93
Coating Thickness and Uniform ity............................................................................ 97

5 M ODELING OF RESULTS....................................................................................... 174

Introduction.............................................................................................................. 174
Energy Loss in Dead Layer...................................................................................... 174
Incident Angle Contributions and Coating Uniform ity........................................... 177
Path Length of Electrons in the Coating.................................................................. 184
Calculation of the Cathodoluminescence from Coated Phosphors.......................... 185
New Energy Loss Equation...................................................................................... 188
Backscattering Coefficients ..................................................................................... 190
Index of Refraction .................................................................................................. 192

6 DISCUSSION ............................................................................................................. 233

Introduction.............................................................................................................. 233
Initial Uniform Coating M odel ................................................................................ 233
Validity of the Energy Loss M odel.......................................................................... 236
Backscattering Coefficient....................................................................................... 241
Scattering ................................................................................................................. 242
Surface Recom bination and Charging..................................................................... 243
Surface Segregation ................................................................................................. 244
Surface Roughness................................................................................................... 245

7 SUM M ARY AND CONCLUSIONS.......................................................................... 247

APPENDICES

A MATHCAD DATA AVERAGING PROGRAM...................................................... 252

B LUMINESCENCE DUE TO THE INCIDENT ANGLE OF THE ELECTRONS.... 256

C NONUNIFORM COATING SHAPE ........................................................................ 260

D STOPPING POW ER .................................................................................................. 263

E MATHCAD PROGRAM FOR MgO (4 min) / Y203:Eu........................................... 265

F MATHCAD PROGRAM FOR MgO (8 min) / Y203:Eu........................................... 270








G MATHCAD PROGRAM FOR A1203 (1.2 min) / Y203:Eu....................................... 275

H MATHCAD PROGRAM FOR A1203 (2.4 min) / Y203:Eu....................................... 280

I MATHCAD PROGRAM FOR MgO (4 min) / Y2SiO5:Tb ......................................... 285

J MATHCAD PROGRAM FOR MgO (8 min) / Y2SiO5:Tb......................................... 290

K MATHCAD PROGRAM FOR A1203 (1.2 min) / Y2SiOs5:Tb ................................... 295

L MATHCAD PROGRAM FOR A1203 (2.4 min equiv.) / Y2SiO5:Tb ......................... 300

M MATHCAD PROGRAM FOR A1203 (5 min) / Y2SiO5:Tb...................................... 305

B IB LIO G R A PH Y ........................................................................................................... 310

BIOGRAPHICAL SKETCH ........................................................................................... 322















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

MEASUREMENT AND MODELING OF THE EFFECTS OF PULSED LASER
DEPOSITED COATINGS ON CATHODOLUMINESCENT PHOSPHORS

By

William Joseph Thomes, Jr.

August 2000


Chairman: Dr. Paul Holloway
Major Department: Materials Science and Engineering


Coatings have been shown to reduce the degradation of field emission display

phosphors and, in certain cases, improve the low voltage luminescence. To study the

energy loss mechanisms in coatings and to predict their impact on cathodoluminescence,

coatings of MgO and A1203 were pulsed laser deposited onto sedimented screens of

Y203:Eu and Y2SiO5:Tb. The thickness of the pulsed laser deposited coatings (which

were characterized by ellipsometry, transmission electron microscopy, and Auger

electron spectroscopy sputter profiles) were varied from 7.5 to 50 nm by changing the

deposition time and oxygen background pressure. A Si shadow mask was used to cover

half of the sedimented powder during deposition. This allowed for comparison of coated

with uncoated powder that experienced the same processing. Cathodoluminescence data

were collected for beam energies from 0.8 to 4 keV at a constant current density of

0.16 1A/cm2.








The coating thickness on the particles was modeled by assuming a uniform

deposition flux over a spherical powder surface. Spatially resolved electron energy loss

was calculated to predict the cathodoluminescence intensity versus beam energy and

incident angle relative to the local surface normal. A modified Bethe stopping power

equation was initially used to predict the luminescence from the coated phosphors. This

was found to overestimate the attenuation of the luminescence at beam energies below

about 3 keV. To provide a more accurate fit to the low energy region, a new energy loss

equation based on a form similar to a Makhov power loss equation was introduced. By

using the new energy loss equation, the cathodoluminescence intensity of the coated

phosphors could accurately be predicted for all energies up to 4 keV.

The model developed in this work was used to fit luminescence losses from as

low as 4.71% (at 4 keV) for a 10 nm coating of A1203 on Y203:Eu to as large as 27.4%

(at 4 keV) for a 43 nm MgO coating on Y2SiO5:Tb. No change in the surface

recombination rate was observed. The coatings were also found to have no effect on the

decay states under pulsed excitation, except those attributed to beam energy loss.















CHAPTER 1
INTRODUCTION


Cathode ray tubes (CRTs) have dominated the visual display market since their

invention in the 1920s.1 However, due to their large size, they are not suitable for use in

many modem applications that require compact screens. To fill this role, a class of

devices known as flat panel displays (FPDs) has been invented. These displays are very

thin in comparison to the CRT, which makes them the ideal display media for portable

electronic devices. So far, liquid crystal displays (LCDs) have led the market in the field

of FPDs.2 However, new types of FPDs are being developed to improve upon the LCD.

At the forefront of the new FPDs is the field emission display (FED). It offers

advantages in terms of its wide operating temperature range, wide viewing angle, fast

response time, low power consumption, high brightness, high durability, low weight, and

scalability.2-12 For these reasons, the FED is likely to challenge the LCD for dominance

in the FPD market.

The FED excites luminescence from phosphors deposited on a glass faceplate,

similar to a CRT. However, unlike a conventional CRT that relies on just three electron

guns that create beams that are rastered across the phosphor screen, the FED uses

thousands of tiny emitters behind each phosphor pixel. The depth of a CRT is

determined by the distance necessary for deflection of the electron beam over the entire

screen surface. The FED removes this limitation due to the fact that the emitters are








located directly behind the pixels and can be turned on and off, unlike scanning in the

CRT, in which the electron gun is continuously on. Therefore, the device can be made

very thin (the typical spacing between the emitter array and phosphor screen is around

10 mm).

The proximity of the emitters to the pixels also allows much lower voltage to be

used in the FED (4 or 5 keV, compared to 25 keV in a CRT). The problem with using a

lower voltage, however, is a reduction in the luminescence intensity from the phosphors.

To counteract this loss in intensity, the current from the emitters is increased. The

increased current helps regain the lost luminescence, but it also leads to more rapid

degradation. 13

Due to the operating conditions of FEDs (low voltages and high currents),

phosphor degradation is one of the limiting factors in the lifetime of the device. Coatings

have been shown to greatly improve the degradation resistance of the phosphors.14 One

of the drawbacks to using coatings is that they also cause a reduction in the

cathodoluminescence from the phosphor. Under certain circumstances, however,

coatings have been shown to improve the low-voltage efficiency of phosphors.15 If the

efficiency of the phosphors is increased at low voltages, then the power consumption of

the device and the rate of degradation can be reduced.

The focus of this work has been on examining the effects on cathodoluminescence

of wide-bandgap oxide coatings, specifically MgO and A1203, deposited by pulsed laser

deposition onto two commercially available phosphors, Y203:Eu and Y2SiOs:Tb, used in

FEDs. A review of the literature available on selected aspects of the cathodoluminescent

process is given in Chapter 2. Also included in the chapter is a discussion of the








mechanisms behind the degradation process and how the use of coatings can slow or

eliminate this loss. Near the end of the chapter, the characteristics and merits of pulsed

laser deposition are presented, along with the reason for choosing this technique to

deposit the coatings.

The experimental procedures used to deposit the coatings and collect the

cathodoluminescence from the phosphors are discussed in Chapter 3. This chapter also

includes a description of ellipsometry, transmission electron microscopy, Auger electron

spectroscopy, and scanning electron microscopy, all of which were used to characterize

the coatings.

Results of the cathodoluminescent measurements on the coated samples showed

that the coatings attenuated the luminescence from the phosphors. The effect was larger

at lower voltages due to the interaction volume of the electron beam. Thicker coatings

were found to produce a larger reduction in the cathodoluminescent intensity. These

results are discussed in Chapter 4.

A model was developed to explain the loss in luminescence based on a dead layer

approximation. The model relied on calculation of the energy loss of the electron as it

travels through the coating. Resultant energy after passing through the coating was then

used to find the luminescence. This model allows for calculation of the

cathodoluminescence over the entire energy range up to 4 keV. Spatially resolved energy

loss was found using a cosine dependent coating based on a uniform deposition flux onto

spherical phosphor powders. An improved energy loss equation was incorporated to

account for the extended range of the low energy electrons. Calculated results from this

model were found to accurately predict the cathodoluminescence from the coated







4

phosphors for all measured thicknesses. Derivation of the model is given in Chapter 5.

A discussion of the applicability of the model is presented in Chapter 6.

The conclusions from this work are given in Chapter 7. Appendices A through M

contain the various Mathcad programs used during the calculations.















CHAPTER 2
LITERATURE REVIEW


Introduction

Electronics have become an ever-increasing part of everyday human life. The

ability to process information is continually advancing as new and faster ways of

computing are developed. One important aspect of this trend is the ability of the machine

to transmit data to a human operator. Shown in Figure 2-1 are the many different means

of interfacing between a computer and a person. It can clearly be seen that visual

communication is currently one of the fastest means of data transfer, at around 300

Mb/sec. Many different types of visual signal media can be used to accomplish this goal.

Historically, cathode ray tubes (CRTs) have dominated the visual display market.

However, new types of displays have recently evolved. Most of these new technologies

have been designed to serve a more specific purpose that allows them to compete with

the CRT. For example, the development of smaller, more portable electronic devices

created a need for compact screens. Liquid crystal displays (LCD) have filled this role

thus far, with a 90% market share in 1995.2 However, in demanding applications such as

military uses, medical instruments, vehicles, and dusty environments, to name a few, the

weaknesses of the LCD (one of which is its inability to operate over wide temperature

ranges) can have severe consequences. It is in these areas that new devices are being

developed to challenge the dominance of the LCD. All of these new devices can be









lumped under the general heading of flat panel displays (FPDs) because of their slim

profile when viewed from the side. Each type of display has certain advantages and

disadvantages when used for different applications. Table 1 lists the various FPDs and

some of the attributes of each. Portable applications are not the only market for FPDs;

their strengths over other visual media will allow them to compete in all arenas of display

technology, from tiny head-mounted displays to huge billboard-size screens.

Among the various types of FPDs, field emission displays (FEDs) offer many

advantages, such as wide viewing angle, fast response time for video refresh rates and

"instant-on" capability, low power consumption, high brightness, durability, wide

operating temperature range, low weight, and scalability to a larger size. Each of these

attributes makes FEDs desirable for a wide variety of applications.2-12



Field Emission Display (FED)

Cathode ray tubes (CRTs), the displays found in all conventional televisions, rely

on cathodoluminescence for their operation. Figure 2-2 shows the basic design of a CRT.

An electron gun accelerates electrons toward a phosphor screen. These electrons are

deflected by means of plates or magnetic fields and are then raster scanned across the

screen. Red, green, and blue phosphors are deposited on the screen in small holes known

as pixels. (Actually, each hole that contains a single color phosphor is called a subpixel.

Three subpixels together, one of each color, make up a pixel.) In a full color display,

three separate guns are used, each specific to an individual color.

One of the biggest problems with CRTs is their depth, which is needed in order to

have sufficient distance of travel for deflection of the 20 keV electron beams across the






7

entire screen. FEDs remove this size restriction by using thousands of individual electron

sources behind each subpixel. Figures 2-3 and 2-4 show typical FED setups. Electrons

are extracted from very sharp tips by tunneling when high fields are present. These

electrons are then accelerated toward the faceplate, which contains the phosphors. The

emitter tips are usually located only about 10mm away from the phosphor screen, thus

allowing the display to be extremely thin.

Emitter arrays can consist of up to thousands of tips per pixel. They are usually

made from molybdenum, tungsten, platinum, or silicon2, although carbon has recently

been found to be an effective cathode material as well. Carbon emitters are not grown

like traditional tips and are either diamond thin films or are in the form of

nanotubes.4' 12, 16-18 Regardless of the type of material chosen for the emitters, electron

extraction from the tip is carried out in the same fashion. By applying an appropriate

bias, dependent upon the type of material and the specific device setup, electric fields can

be generated to cause tunneling of electrons out of the tip. This process is accurately

described by the Fowler-Nordheim tunneling theory for electrons.6'7,9, 19-21 Figure 2-5

depicts the field with and without applied bias. As can be seen from the picture, with an

applied bias, the potential energy barrier on the surface has been reduced in height and

has a finite width. The applied field is magnified at the tip because of its extreme

sharpness (its radius is approximately 50 nm). This results in more tunneling current at

lower voltages. Due to the finite barrier and large effective fields near the surface, it is

possible for the electrons to quantum mechanically tunnel out of the tip and enter the

vacuum as field-emitted electrons.22 For a more detailed discussion of the use of the

Fowler-Nordheim equation and suggestions for improving the description of electron








emission from sharp tips, the reader is referred to the previously-cited paper by Cutler

and associates.19

One of the challenges faced in applying this technology in a commercial flat panel

display is achieving uniform emission from the thousands of tips. To combat the

problem of nonuniform emission, resistors have been fabricated in series with the tips to

help control tip-to-tip nonuniformity in current.23 This has provided a partial solution to

the problem, but it is not a perfect solution. The best method would be to fabricate a

constant-current source to the tip to regulate current more effectively. A group from

Japan has reported doing this using silicon tips grown as part of a metal-oxide-

semiconductor field-effect-transistor (MOSFET structure).23 Using such a device

structure allows for uniform and precise control of electron emission. With these

advances in cathode technology, research attention is shifting to the improvement of

other aspects of the FED.

The phosphor screen is an integral part of the FED. In terms of the phosphors,

some of the hurdles that still need to be overcome in order to realize a commercially

viable full color display include: better low voltage efficiency, enhanced chromaticity,

reduced saturation, and lower degradation.3



Cathodoluminescence (CL)

When some materials are impacted with certain forms of energy, they will emit

photons in excess of thermal radiation. This process is known as luminescence, and it

can be categorized based on the type of excitation source. These categories include

photoluminescence, cathodoluminescence, chemiluminescence, triboluminescence, X-ray







9

luminescence, and electroluminescence, among others.24, 25 Photoluminescence refers to

excitation by photons, usually from a UV lamp. Cathodoluminescence is excitation from

a beam of energetic electrons, also known as cathode rays. Chemiluminescence results

from energy released during a chemical reaction. Triboluminescence is a result of

mechanical energy, such as friction or fracture, and can also be seen during processes

such as grinding. X-ray luminescence comes from X-ray excitation, as the name

suggests. Finally, electroluminescence is produced by an electric field resulting from an

applied voltage across the material. It should be noted however, that

thermoluminescence does not refer to thermal excitation from the ground state. Instead,

this term is used to describe the thermal stimulation of electrons from excited state traps

that can then recombine and produce luminescence. The applied heat does not actually

excite the electrons; this occurred during a prior excitation event. Rather, the heat only

gives the electrons the energy needed to surmount the energy barrier holding them in the

trapped state.25

In addition to the characterization of luminescence based on the different types of

excitation, there is a distinction made according to the length of the delay between the

excitation and emission of photons. Materials can exhibit either fluorescence or

phosphorescence. "Fluorescence" is the term used when the emission of photons occurs

within about 10-8 seconds after excitation. "Phosphorescence" pertains to any material

that displays luminescence for longer than this. These phosphorescent materials are

called phosphors, and they can display luminescence lifetime ranging from 10-7 seconds

to hours after the excitation source is removed.24









Phosphors have many technologically important roles, especially in the display

industry. They are responsible for converting incident electrons into light in devices such

as CRTs and FEDs. However, their luminescent decay rate must be considered in

determining their suitability in a particular application. In a display, the phosphor must

continue to luminesce long enough to display an image, but also extinguish quickly

enough to allow for fast video refresh rates. This is accomplished by choosing suitable

phosphor materials and processing, which allows the decay time of the phosphor to be

tailored.26 This is not usually a concern because most phosphors display a suitable

response time without any special processing. However, in applications such as the new

high-definition TVs, in which very fast video refresh rates are needed, the decay times of

various phosphors under different types of processing will need to be taken into

consideration.

When an electron beam enters a solid, it will undergo collisions with the host

atoms. These collisions are either elastic or inelastic. During elastic collisions, the

incident electron interacts with a nucleus of the atom and is deflected with little or no

energy loss. If the electron is deflected back toward the surface, it can be lost from the

solid as a backscattered electron. Inelastic collisions involve interaction between the

incident electron and the electrons of the atom. In these collisions, the incident electron

loses part of its energy to the atom. When the electrons in an atom return from the

excited state to their ground state, a host of signals can be produced. Among them are

Auger electrons, X-rays, secondary electrons, photons, and phonons (thermal effects such

as electroacoustic signals).24 For this study, the generated photons

(cathodoluminescence) are of primary interest.






11

The incremental energy lost (dE) over a distance of travel (ds) due to an inelastic

collision can be found from the Bethe equation14,24' 27-30:

dE _-785pZ l(1166EE [2.1]
ds A*E [ J

where A is the atomic weight in g/mol, Z is the atomic number, E is the electron energy,

p is the density in g/cm3, and J is the mean ionization potential in eV. The latter is the

average energy loss per interaction (for all possible energy loss processes), and for Z > 13

it can be expressed as:

J = 9.76Z + 58.5 [2.2]
Z0.19

These equations have been shown to be accurate for E > 6.34 multiplied by the mean

ionization potential.14'24, 28-30 Below this energy, the modification by Rao-Sahib and

Wittry needs to be considered30:

dE = -785*p*Z [2.3]
ds 1.2588*A*V*K V

This equation corrects for the low energy region while mimicking the behavior of the

original Bethe equation at high energy. This new equation can be used for energies as

low as approximately 500 V. Attempting to reduce this limit even further, Joy and Luo

suggested a new expression for the mean ionization potential using an energy dependent

term14, 29, 31:

J='- [2.4]
1+k -
[E








where J' is the new mean ionization potential, J is still the average energy loss per

interaction, and k is a fitting parameter. The constant k varies from 0.7 to 0.9, but it is

usually around 0.85.

The stopping power equations cited above provide a measure of how the electrons

lose energy in the solid, but they do not give an accurate indication of the interaction

volume. During the electron's trip through the lattice, it will also undergo elastic

collisions. These will not result in an energy loss, but rather will lead to a change in the

direction of the electron's travel. Equation 2.1 can be integrated to find the length of an

individual electron "random walk" trajectory. This is known as the Bethe range.

However, the interaction volume will be much smaller due to scattering. Typically, the

interaction volume is known as the Gruen, electron beam, or penetration range. It can be

expressed as:


Re = [2.5]


where p is the density, E0 is the electron beam energy, k' depends on the atomic number

and is a function of energy, and a depends on the atomic number and E. Various

equations have been given for the solution to this problem, but the one most widely

accepted was proposed by Kanaya and Okayama:

rf0.0276A [,667
\pZ08 )
'Re Jo [2.6]


measured in gm, where Eo is in keV, A is in g/mol, p is in g/cm3, and Z is the atomic

number.24 Figure 2-6 demonstrates how this interaction volume moves deeper into the

material as the beam energy is increased.









Every incident electron gives off energy to the lattice in multiple steps.

Therefore, it is possible for one electron to generate a multitude of secondary electrons.

The number of electron-hole pairs generated per incident electron is known as the

generation factor; it can be expressed as:

=E(1- [2.7]
E,

where y represents the fractional electron beam energy lost due to backscattered

electrons. E, is the ionization energy, the energy needed to form a single electron-hole

pair. The ionization energy is related to the bandgap of the material by:

E1 =2.8E9 +M [2.8]

where M is between 0 and 1 eV, depending on the material in question. For a more

detailed discussion, the reader is referred to Yacobi and Holt's work on the subject.24

Once the incident electron has transferred its energy to the solid, this energy can

be used to produce visible light. Luminescence from inorganic phosphors usually takes

place at an impurity, which is referred to as an activator.24' 32, 33 However, not all

impurities are activators. Those that do not lead to luminescence are referred to as

quenchers or killers.32 Since the activators are an imperfection in the crystal lattice, they

will usually exhibit energy levels between those of the conduction and valence bands of

the host lattice. These states can become populated either through direct excitation or

trapping of secondary electrons generated in the host lattice. When the activator returns

to its ground state through recombination of an electron and hole, the energy can be

released as luminescence.24,25, 32-36 This is known as a radiative transition. When the

atom relaxes without the emission of a photon, it is called a nonradiative transition.









Competition between these two types of transitions determines the intensity of the

emitted light.25

When an electron and hole radiatively recombine, the emitted photon will be

characteristic of the energy levels involved, which is determined by the electronic states

of the activator. The photon energy (hv) is given by:

hv = Ef -E, [2.9]

where Ef and Ei are the energy of the final and initial states, respectively. In the literature

on the subject, this energy is often referred to instead by its wavelength equivalent.

There is a simple relation relating the two:

1.2398 [2.101
E

where E is the photon energy in eV and X is in Rm.24, 36 For most materials, the

recombination of the electron and hole will be the rate- limiting step to the luminescent

process.37



Phosphors

Phosphors are responsible for converting incident energy into photons (light).

Without them, visual display media would not be possible. There are a plethora of

different materials that exhibit this behavior. The following is only a partial list of

phosphors that can be used to generate the three primary colors (red, blue, and green) of a

common display. For red light, there are Y202S:Eu, Y203:Eu, CaS:Eu2, SrS:Eu2, and

YVO4:Eu. For green light, there are ZnS:Cu:Au:Al, Zn2SiO4:Mn, Y2SiO5:Tb,

Gd2O2S:Tb, La2O2S:Tb, ZnO:Zn, Y3(AI,Ga)5012, Y2GeO5:Pr, Y202S:Pr, ZnGa204:Mn,

Y3AI5012:Ce, and SrGa2S4:Eu. For blue light, there are ZnS:Ag:CI, Zn2SiO4:Ti,






15

Y2SiO5:Ce, ZnGa204, and SrGa2S4:Ce.3,38 For this work, Y203:Eu and Y2SiO5:Tb were

chosen to be representative of the red and green groups, respectively. This allowed for

examination of not only the coating effects, but also the consequences of the underlying

host material. The attributes of each will be discussed below.



Y203:Eu

Y203:Eu shows a main emission peak in the visible spectrum at 611 nm (red)

under cathodoluminescent excitation. This results from a 5Do 7F2 transition of the Eu+3

site.25'39-43 Figure 2-7 shows such a transition. Eu is a rare earth ion and is

characterized by an incompletely filled 4f shell. The 4f 6 shell is shielded by the filled 5s2

and 5p6 shells.25'41 Shielding allows the Eu ion to retain its atomic character and have

energy levels close to that of a free ion.25, 44 As a result, the emission is very sharp at the

transmission wavelength.44 This is also the reason that Eu is usually able to keep its

characteristic luminescence, regardless of the host material. There are two possible

crystallographic configurations for the Eu atoms, either C2 or C3i.39-41 Figure 2-8 shows

these. The main emission from the phosphor is due to transitions in the C2 sites. Energy

absorbed into the C3i sites is effectively transferred into the C2 site for recombination.40

In the literature, C3i symmetry is often referred to as S6 symmetry.39

Of importance in the Y203 host is the charge transfer to the luminescent center.

The highest occupied levels of the ground state in the host lattice are the 2p orbitals of

oxygen, while the lowest unoccupied levels are a mixture of the 3s orbitals of oxygen and

the 4d orbitals of yttrium. When electrons are promoted to an excited state under electron

bombardment, they can be transferred into the 5D levels of Eu.25 The configurational








coordinate diagram can be used to represent such a transition. For a fixed central atom,

the diagram represents the potential energy (E) curves of the atom as a function of the

distance between it and its neighbors (R). Figure 2-9 shows such a diagram. The solid

lines in the figure represent the ground states and excited states of the activator, while the

dashed line represents another excited state of the activator known as the charge-transfer

state. The parabolic shape results from the assumption that the restoring force on the

neighboring atoms is proportional to their displacement from equilibrium. The charge-

transfer state is one which excited electrons can easily be fed into because of allowed

transitions from the ground state of the activator or from the host lattice. The

luminescent state is not directly accessible because the transitions are forbidden. In

Figure 2-9 (a), the minimum in the charge-transfer band causes excited electrons to be

effectively transferred into the luminescent states. This is the case for Y203:Eu. If,

however, the charge-transfer band had a minimum located closer to the ground state

bands, as shown in Figure 2-9 (b), then excited electrons would be effectively transferred

into the ground state through nonradiative relaxation. Due to the location of the charge-

transfer band in Y203:Eu, excited electrons populate the luminescent states, giving this

phosphor its high quantum efficiency.45

The efficiency of different FED phosphors are shown in Table 2.2 Y202S:Eu has

a larger efficiency than Y203:Eu, but it has other disadvantages in terms of degradation

that make it a less desirable material for FED applications. 14









Y2SiOs:Tb

Y2SiO5:Tb is a green phosphor with a main emission at 538 nm.46 This is a result

of a 5D4-7F5 electron transition on the Tb3 site, as shown in Figure 2-7. The other three

peaks seen in a typical Tb-doped spectrum are the result of transitions to the other 7Fn

states.47 As with the Eu atom in yittria, the Tb in Y2SiO5 is well shielded by electrons in

the outer shells. In a similar fashion, this usually makes the resulting spectrum invariant

to host lattice composition.44,48 It also makes the transitions exhibit little spread in the

emitted wavelength.44 Also similar to Y203:Eu, Y2SiOs:Tb has a charge-transfer band

that efficiently deposits excited electrons into the luminescent states, as shown in

Figure 2-9 (a). In the Y2SiOs:Tb system, this is the 4f75d band of Tb, and it results in a

relatively high quantum efficiency.25

One of the properties that makes Y2SiO5:Tb favorable over other green phosphors

is its saturation behavior. In a FED application, as compared to a CRT, lower voltages

are used (hundreds to a few thousand volts versus 10 to 20 kV). To regain brightness,

higher currents are employed (up to 1000 A/cm2 versus 0.5 A/cm2).11.43 Most phosphors

will begin to show saturation effects as the current is increased, especially at these low

voltages.3 This is speculated to result from ground state depletion.49'50 As the ground

state of the activators becomes depleted, the probability of exciting an electron into a

luminescent state is reduced. The result is an increase in the nonradiative recombination

rate, which leads to a reduction in the efficiency of the phosphor (see section on

efficiency in this chapter). However, this is not as prevalent in Y2SiOs:Tb because of the

fast decay rate of the excited state. Because the excited electrons recombine so quickly,

they refill the ground state and are thus available for excitation again.49' 50 Because









Y2SiO5:Tb shows little or no saturation effects, it is gaining considerable interest in the

display market.



Efficiency

A phosphor is "a solid which converts certain types of energy into

electromagnetic radiation over and above thermal radiation."25 In other words, when an

electron beam strikes a phosphor, it can impart some of its energy to exciting electrons

into higher energy levels. The rest of the energy is lost in collisions with the host

material. The excited electrons can then recombine with a hole in a radiative or

nonradiative transition. Radiative transitions give off electromagnetic radiation, whereas

nonradiative transitions transfer energy to Auger electrons, ionizations, or phonons.25

Efficiency is defined as the brightness per unit of input power and is measured in

lumens per watt.3,25,39,51 In a phosphor, charge transfer to a luminescent center is

critical to achieving high efficiency. If the electron and hole are not effectively

transferred to the luminescent impurities, then they will have a high probability of

recombining nonradiatively. This is one of the major problems in FEDs, where lower

voltages are used, as compared to CRTs. As the accelerating voltage is decreased, the

efficiency also decreases. 11, 35, 51-53 Table 3 and Figure 2-10 depict this situation. From

the table, it is apparent that the reduced efficiency at low voltage is one of the primary

disadvantages to FED operation. In order to regain high brightness, larger currents need

to be used. These larger currents, in turn, lead to saturation effects and enhanced

phosphor degradation and thus short device lifetime.3









Many researchers have attempted to devise ways of improving efficiency in this

low voltage (under 4 kV) range. One method involved placing the phosphor material

directly on top of the gate electrode, as seen in Figure 2-11. Other researchers have tried

using quantum confinement to enhance luminescence by spatially confining electrons and

holes. This method usually requires phosphor particles smaller than 10 nm in diameter,

as compared with the 5 nm phosphors currently in production.54-59

The problem with most of the aforementioned approaches is that they tend to be

very difficult to implement. The fundamental problem to solve in order to improve

efficiency is that of how to transport charge to luminescent centers while decreasing the

probability of the charge reaching nonradiative sites. In order to realize this goal,

charging in the phosphor during electron bombardment needs to be controlled.



Charging

Surface recombination refers to the return of excited electrons to the ground state

by recombination with an available hole at the phosphor surface. Due to the disruption in

the phosphor lattice at the surface, there will be a large number of defects in this region.

These defects provide efficient nonradiative pathways for electron-hole recombination.

In order to alleviate this problem, surface passivation is often used to regain

luminescence. Surface passivation involves coating the surface with another material so

that charge is effectively radiated and not allowed to reach the nonradiative surface

states.60-63 These surface states become critical when considering charging of the

phosphor. This is especially important in a FED environment, where low voltages are

used. As the accelerating voltage of the incident electrons is decreased, the generation of








electron-hole pairs will occur closer to the surface due to the lower penetration of the

incident electrons.50 Figure 2-6 shows a schematic of the electron beam penetration into

the phosphor at various beam energies. From the figure, it can be seen that at lower

voltages the electrons are localized closer to the surface. Therefore, surface states will

have a larger influence on the generated secondary electrons at these lower electron beam

voltages.

While surface recombination is possible, other effects can occur upon electron

bombardment of phosphors. Yoo and Lee attributed decreased luminescence on the basis

of trapped electrons on the surface. They postulate that low energy incident electrons are

trapped on the surface. These trapped electrons create an electrostatic potential barrier

for subsequent primary electron bombardment and therefore decrease luminescence.63

Ozawa further contributes to this explanation with what he calls secondary bound

electrons. When an incident electron enters a material, it creates secondary electrons in

the near surface region during interaction with the host. If these electrons are within the

mean free path of secondary electrons, then they can be ejected from the material. The

secondary electron coefficient (5) is the number of ejected secondary electrons per

incident electron. Typical values range from 1.5 to 3 for a primary beam energy of about

1 keV, meaning that more secondaries escape the surface than enter it. These leave

behind holes, which give the surface a net positive charge. If the secondaries do not have

sufficient energy to escape or reenter the crystal, then they can become trapped a short

distance above the surface. An electron cloud is thus produced above the surface, known

as a space charge, and it acts as a barrier for subsequent incident electrons.35






21

Bennewitz and associates studied the surface potential by looking at desorption of

F' ions from a CaF2 sample under 1 keV electron bombardment. In their work, they

point out that the positive surface potential is a result of secondary electron loss from the

near surface region. Therefore, it does not depend strongly on the primary beam current,

but only on the secondary electron distribution. The primary beam current will influence

this potential when there are current pathways to the surface from other pathways, such

as desorption of positive ions from the surface, or a leakage current between the

irradiated spot and the sample holder.64

Seager et al. have postulated that instead of trapping electrons outside the surface,

internal fields lead to charging during electron beam bombardment.52 Utilizing

secondary electron emission energies and carbon Auger peak shifts during irradiation

with electron beams of energies from 0.5 to 5 keV, they were able to provide evidence of

surface charging. Four phosphors were studied: ZnS:Ag, SrGa2S4:Eu, ZnO:Zn, and

Y203:Eu. All were powders sedimented onto -1 cm2 Au foil. Particle diameters varied

from 1 to 10 imn, and the layers were about 5 to 10 particles thick. When the researchers

changed the potential of the plate that the samples were mounted on, surface potential on

the front of the sample (closest to the beam) varied. Figure 2-12 shows results for

Y203:Eu, which is representative of the other phosphors. The figure shows that the

energy of the C Auger electrons changed linearly with application of the applied bias.

This suggests that the backing plate bias led to a change in the surface potential that

altered the escape energy of the Auger electrons. Similar shifts were seen for the

secondary electrons. Thus, it is possible to change the potential of the phosphor with

application of a backing plate bias.









Seager et al. also used dual electron beams to irradiate the sample during

excitation. The second electron flood beam was used to control surface potential of the

phosphor. Their results led them to believe that secondary electrons are not recaptured

and used to build an electron cloud, as postulated by Ozawa, Yoo, Lee, and others; rather,

an internal mechanism is responsible for bringing the phosphor into a steady state

condition. They postulated that this internal mechanism is a buildup of positive charge

near the surface, with a corresponding negative charge near the end of range of the

incident electrons.

One problem with the experiments of Seager et al. is the fact that the potential of

the front surface of the phosphor was unpinned. This made application of a known field

within the phosphor difficult. To circumvent this problem, Seager applied a metallic

mesh to the front surface of the phosphor to pin the surface potential at electrical ground.

Electric fields were then applied across the phosphor layer to either enhance or retard the

cathodoluminescence, depending on polarity.65 Two phosphors -- Y203:Eu and

Y2SiO5:Tb -- were sedimented onto brass plates. Total thickness of the phosphor layer

was around 25 pm. Metal mesh with square holes (7.5 gm) was attached to the top

surface of each sample. These grids allowed electric fields to be induced within the

phosphor layer while still allowing incident electrons to reach the phosphor surface.

Figure 2-13 shows this sample configuration. It was found that the maximum voltage

that could be applied before breakdown was between 400 and 450 V. This voltage is

important because it gives a measure of the dielectric strength of the material.

Initial results suggested that internal fields could be used to sweep generated

charges away from or toward the surface. This would, in turn, lead to an increase or








decrease in cathodoluminescence because of losses to surface recombination.65

However, further experimentation led to the conclusion that the results seen could be

caused by surface potential fluctuations. Instead of changing the internal fields of the

material, these fluctuations caused the energy of the incident electrons to be altered. The

most likely explanation for the unpinned potential of the surface is that there was an air

gap between the phosphor and grid as a result of the roughness of the powder.66 Thus,

intimate contact is needed if the internal fields of a material are to be altered in a

controlled fashion. Coatings applied directly to the phosphors could provide such a

situation.

Recently, Pantano et al. published an explanation of charging that takes into

account the internal fields, with the addition of allowing for a negative surface potential.

They rely on a model proposed by Cazaux67 which depends on the stored charges per unit

area, q+ and q-, and the charge densities, p+ and p.:


q =J08t P -
ds

qt J0(1-rnt)
q_ = Jo(1-Tlt) P_= -
dp

where Jo is the incident primary-beam current, 8 is the secondary-electron emission

coefficient, T" is the backscatter coefficient, t is the time, ds is the thickness of the

positively charged surface layer, and dp is the penetration depth of the incident

electrons.68 If 1 ri ) 8, the total negative charge q- will exceed q+ in time and the surface

will charge up negatively. The negative surface charge will then act as a potential barrier

for incoming electrons. This can only occur for 8 < 1, such as for beam energies below








about 1 kV. The reduced energy of the incoming electrons will limit the luminescence

from the material. If 8 > 1 il, the positive surface charge q+ will exceed the negative

end of range charge q.. This process will be self-regulating because as the positive

surface potential increases, the low energy secondary electrons will not have enough

energy to escape. The result is a limited, but stable, positive surface.68 Under such

conditions, the excited electrons generated within the phosphor are swept toward the

surface, where there is a high probability of nonradiative recombination.

One possible method from the presented models above for affecting the

luminescence is to change the velocity of the incoming electrons. Yoo and Ozawa claim

that charge buildup on the surface produces such a velocity change.35,63 Internal fields

could also build up similar surface fields if 8 < 1, as shown in the models by Seager and

by Pantano et al. As a consequence, incident electrons are repelled by an electrostatic

potential at the surface, which lowers their velocity. Due to their lower incident energy,

the electrons that make it into the material have less energy with which to excite

luminescence. Based on calculations of the dielectric strength of the phosphors, as

discussed below, however, this does not seem to be a reasonable model.

Dielectric strength is a measure of the maximum electric field that can be applied

across a dielectric before breakdown. The equation is as follows:

(V/l

where represents the electric field, V represents the voltage, and d represents the

distance. Most materials have a dielectric strength of 105 to 107 V/cm.69, 70 For

Y203:Eu, it has been reported that breakdown occurred at 450 V over a 25 gim film.65

This would correspond to a dielectric strength of 1.8* 105 V/cm, which is within the









expected range. The value calculated is lower than the bulk dielectric strength because

the numbers were taken from a powder sample. Therefore, the number has a larger

contribution from the lower surface dielectric strength than from the higher bulk

dielectric strength. Given the above dielectric strength and that the penetration depth of

the electrons within the phosphor surface is on the order of 1000 A (probably an

overestimate), breakdown would be expected at around 1.8 V. Even if one assumed a

dielectric strength one order of magnitude larger, breakdown would be expected at 18 V.

When this is compared to the energy of the incident electrons (500 4000 V), it does not

seem possible that these small voltages could be affecting the velocity of the incident

electrons enough to cause a large decrease in luminescence.

Considering the above argument, the most plausible explanation of charging is

that internal fields are causing excited secondary electrons to be swept toward the

surface, where they have a high probability of nonradiative recombination or emission.

These internal fields are a result of a positive surface charge due to secondary electron

emission and a negative charge region near the end of range of the primary electrons. In

order to reduce charging in phosphors, the internal fields need to be minimized or

removed so that generated secondary electrons remain in the bulk of the phosphor, where

they have a higher probability of radiative recombination. This, in turn, will lead to a

higher overall efficiency.



Cathodoluminescent Degradation

One of the main problems that needs to be overcome in order to make an FED

device for mass production is that of increasing FED lifetime. This is especially









important for a low voltage device. As the accelerating voltage of the electrons is

reduced, the brightness of the device decreases rapidly, due to a loss in efficiency. To

attempt to regain the original luminescence, higher currents are used.43 In turn, these

higher currents cause operational problems in the device. Besides saturation, as

discussed earlier, degradation is greatly enhanced due to the high current densities

employed. When the lifetime of the device is considered, the degradation of both the

phosphor and field emitters needs to be taken into account. Both can lead to a reduction

in the cathodoluminescence seen from the FED.

In a CRT, it is relatively easy to maintain a good vacuum over the lifetime of the

device. This is due to two reasons. First, the relatively small surface-to-volume ratio of

CRTs makes it easy to initially pump the device. Second, there is a large area over which

to apply a getter material, giving a large ratio of active getter area to system volume. The

FED does not have either of these benefits. Due to its closely spaced anode and cathode,

it has a large surface-to-volume ratio. This makes it difficult to initially pump the device

due to the conductance between the plates. There is also little area over which to apply

the getter material.71 Some improvements to the pumping could be realized by

increasing the gap between the plates, but this creates problems with the focusing of the

electrons and would require the insertion of a focusing grid into the device. Placing the

getter between the anode and cathode could help with the pumping, but this too would

cause problems with the actual operation of the display.71 New techniques are being

explored for gettering, such as placing a non-evaporable getter around the sides of the

package or attaching a getter to the back with small holes to allow pumping. Although








these might help in the maintenance of the vacuum within the FED, the role of gases

needs to be considered when examining the device operation.

There are two main sources of background gas in a FED device, those resulting

from desorption from the device structure and those released from the phosphor due to

electron beam impingement. Gases released from the phosphors depend on the specific

material in question and will be discussed later in this section. All other gases are mainly

a result of outgassing from the structural components of the device, such as the spacers,

cathode, and black matrix. Other sources of background gas are present -- for example,

permeation -- but these usually contribute a negligible amount to the overall composition

of gases in the device. The main outgassing products inside a FED are H2, H20, CO,

CO2, and hydrocarbons such as methane, ethane, and propane.72'73 Depending on the

relative amount of these gases and the specific material, various processes can lead to

decreased cathodoluminescence. If the environment is dominated by carbon-containing

gas molecules, then carbon deposition on the phosphor surface will most likely

result.74-76 Under steady-state conditions, gas molecules are constantly physisorbing on

and desorbing off the phosphor surface. When an electron beam is present, it can impart

energy to these molecules and cause them to crack (i.e., break the bonds holding them

together) to atomic species. The carbon is then free to form bonds with the surface, while

the other components combine with each other and are released back to the gas phase.

The rate at which this will occur depends on the residual vacuum pressures of the specific

atoms and molecules. As the carbon layer grows, it will substantially reduce the

luminescence from the phosphor particle in the electron beam exposed area.74-76 After






28

exposure in such an environment, a dark spot is usually present that can be seen with the

naked eye.

If hydrogen and water, rather than carbon compounds, dominate the FED

environment, then degradation is still present, but it often will be a result of a change in

the internal efficiency of the phosphor.77 Although different materials degrade in

different ways and at different rates, certain generalizations can be made for various types

of phosphors. Some of the most efficient high voltage phosphors currently known

contain sulfur. Examples include Y202S:Eu for red, ZnS:Ag:Cl for blue, and

ZnS:Cu:Al:Au for green. Under the electron beam, dissociation of residual gas

molecules can occur. These atoms can then form bonds with the atoms in the outer layer

of the phosphor. Due to the volatile nature of the sulfur compounds such as SOx, H2S,

etc., they desorb back into the vacuum. As a result, sulfur is leached from the

surface.13, 78-83 It would be expected that this phenomenon would be limited to the near

surface region, however, the lack of sulfur causes a diffusion gradient that brings more

sulfur to the surface region.68 As the sulfur is removed, the particle surface is converted

into its oxide equivalent. Swartz et al. showed that when ZnS is subjected to an electron

beam, its surface is converted into the non-luminescent ZnO.13,78'80,82 Trottier

demonstrated that Y202S:Eu is transformed into the less efficient Y203:Eu on the surface

under an electron beam. In addition to the decrease in luminescence, a peak shift was

observed for Y202S:Eu that was due to the emission from the Y203:Eu layer.13, 14 These

types of degradation reactions have been termed electron-stimulated surface chemical

reactions (ESSCR) because of their mechanism.13' 14








It would seem, based on the mechanism discussed above, that oxide-based

phosphors would not be susceptible to electron beam degradation. However, this is not

the case. Oxides usually degrade because of electron-stimulated desorption on the

surface. This causes the introduction of lattice defects in the material.68, 84 Because they

are a disruption in the lattice symmetry, defects tend to increase the nonradiative

recombination rates. As the rate of nonradiative transitions increases, the efficiency of

the phosphor decreases and, thus, so does the cathodoluminescent brightness. Although

this is fundamentally a surface phenomenon, the electron interaction proceeds deep into

the material. Once the outermost surface is changed, an activity gradient exists to drive

the diffusion of species in subsurface regions. Thus, the damage continues into the

phosphor. Compounds most affected by this are ones in which the cation and anion have

Pauling electronegativity differences greater than 1.7, a category that includes many

oxides.68 In a way, this is similar to the sulfide case described above. Even though

ESSCR causes the majority of the degradation in sulfide-based materials, defect

introduction is occurring simultaneously to further decrease the luminescence.

In all of the scenarios discussed, the electron beam caused the degradation to take

place. It seems natural, therefore, that the length of exposure determines the extent of

degradation. By convention, this is measured as the coulomb load to which the phosphor

has been exposed. It can be expressed as:

Coulomb load (Coulombs/cm2) = (I t) / A

where I is the sample current (Coulombs/sec), t is the time of exposure (sec), and A is the

area of the beam spot (cm2). For this reason, this type of degradation is often referred to

as Coulombic aging.85-87 There have been different formulas presented to represent the






30

loss in luminescence with Coulomb load, but the one most widely referenced was derived

by Pfanhl at Bell Labs. It simply states that the intensity as a function of dose can be

expressed as:
I(N)=- O

(I+ c'N)

where I is the cathodoluminescent intensity at any given dose N (number of

electrons/cm2), Io is the initial cathodoluminescent intensity, and C' is the burn parameter

(cm2).14,71 The bum parameter C' will often be replaced by a quantity l/Qso% in which

Q50% is equal to the charge dose, for which the initial intensity of the phosphor is

halved.88 Not all phosphors behave in such an ideal way. Sometimes a phosphor will

show an initial rapid decrease in luminescence that is followed by a slight rise and

eventual inflection and further decrease in intensity. To account for this rise, Cappels and

associates have derived an equation that uses two decay curves similar to the equation

above.89 In essence, their equation is just the superposition of two simple inverse decay

curves.

Holloway, et al. modeled the degradation they observed in sulfide-based

phosphors based on an ESSCR sulfur removal rate. Their model predicted that the

concentration of sulfur on the surface is exponentially dependent on the dose. It is a

more in-depth study of the exact mechanisms behind the degradation process. Their

equation is as follows:

1 e(K'PN)
e
10






31

where K' is a constant, Prg is residual gas pressure, and N is the electron dose. However,

they pointed out that within experimental error, the data could be made to fit their

equation or Pfahnl's.13

At the beginning of this section, it was mentioned that the electron beam can not

only degrade the phosphor, but can lead to degradation of the emitter tips as well.

Usually, this is an indirect contamination due to volatile species being ejected from the

phosphors. It is well established that sulfide-containing phosphors "poison" the emitter

tips during excitation with an electron beam.11, 90-93 This is suspected to be a result of

the ejected volatile species traveling through the vacuum and reacting with the emitters.

Upon reaction, these species can change the work function of the tip material. Due to the

change in work function, the emission characteristics will be altered.3,94 Therefore, not

only a sulfide, but any phosphor has the potential to degrade the cathode emitter tips.

Consequently, the more volatile species have a much larger effect on field emission from

the cathode than their more stable counterparts. For this reason, oxides have received

considerable attention. However, due to efficiencies lower than sulfides or oxysulfides,

they represent a trade-off in performance.



Coatings for Phosphors

Key issues in designing phosphors for low voltage FED applications are improved

efficiency and reduced degradation. Many researchers have attempted to use coatings to

accomplish these goals.

In CRT manufacturing, an aluminum backing layer is applied to the phosphor

screen. Usually, a lacquer film is first deposited to provide a planer surface on which to








deposit the aluminum film. Then the lacquer is baked out. The aluminum reflects

generated light back toward the front of the device to improve brightness and contrast.

Even though the aluminum is an effective energy barrier for the incoming electrons, it is

not a problem due to the high voltages used in CRTs.71,95

Many groups have looked at coating for protecting phosphor particles from

electron degradation. Some of the earlier work in this area was performed by Kingsley

and Prener. They used sol-gel processes to coat ZnS:Cu particles with non-luminescent

ZnS layers. Thickness of the layers was determined based on weight gain and size

distribution of the powder after processing. Figures 2-14 and 2-15 show results of their

work. As can be seen from the figures, the coatings did not change the slope of the

luminescence versus voltage curves; they only changed the turn-on voltage. The turn-on

voltage is found by taking an extrapolation from the linear portion of the luminescence

versus voltage curve. The voltage at which the linear extrapolation crosses the axis (i.e.,

when the intensity is zero) is the turn-on voltage. The fact that the coatings only changed

the turn-on voltage suggests that the coating did not alter the luminescent states in the

phosphor, but only caused a decrease in the incoming electron energy. (A layer that

exhibits this type of behavior is often referred to as a "dead layer" because the energy lost

in the layer is no longer available to excite luminescence.) Using this assumption,

Kingsley and Prener were able to model the decrease in luminescence with a Makhov

power loss equation with corrections for the spherical surface and surface

recombination.96 The Makhov power loss equation is given by:

P(x, jo,Vo)= joVo exp(-X2 XO9)






33

where P is the beam power per unit area, jo is the beam current density at the surface, Vo

is the accelerating potential, and X is a normalized depth in the material given by:


X(X,VO) = X-
X
p *C*Vo


where p is the density, C and n are materials constants, and x is the depth in the

material.96

As noted above, sulfide-based coatings may be degraded under electron beam

exposure due to ESSCR at the surface. For this reason, other materials have been studied

as potential capping layers. Bechtel reports on the use of a phosphate coating to improve

the degradation characteristics of ZnS-based phosphors.97 Dmitrienko suggests the use

of stable oxides such as SiO2, MgO, or A1203, however, he notes that the optimal

thickness and deposition techniques still need to be determined.98

One of the first studies to show the protection offered by applying coatings to

phosphors was done by Trottier and Fitz-Gerald. They demonstrated that coatings of

TaSi2 and Ag deposited by pulsed laser ablation onto Y202S:Eu, a highly efficient red

phosphor, could be effective at reducing the degradation of this phosphor. Figure 2-16

shows the results from their work. The pulsed laser deposited coatings of TaSi2 and Ag

were compared against wet-chemistry coatings of SiO2 and phosphate. The pulsed laser

coating was better at slowing degradation and exhibited far less loss in brightness after

aging to 20 C/cm2. Of the two laser ablated coatings, TaSi2 protected the underlying

phosphor better than Ag. This is believed to be due to Ag migration or volatilization

under the electron beam. 14, 99

Recent interest in phosphor coatings has concentrated not only on their protective

properties, but also on their use to increase cathodoluminescent efficiency at low






34

voltages. The group of Yang and Yokoyama showed that coating CuxS on ZnS:Ag:Cl led

to an increase in the efficiency of the phosphor.100 However, sulfides are susceptible to

ESSCR in a FED environment. It seems that wide-band oxides are the preferred choice

for increasing the efficiency of the phosphors. 101 The group of Kominami et al. showed

that coating ZnS:Ag:Cl with In203 led to improved efficiency at voltages below 500 eV.

They attributed this to an increase in the conductivity of the phosphor, which allowed

charge to be dissipated. An increase in the phosphor lifetime was also reported.102

Villalobos and associates coated SiO2 on ZnS:Ag particles using a newly designed spray

coating process, in which the host particle is suspended in a liquid mixture and then

sprayed into a hot zone where the coating gels on the surface. The particles are then heat

treated as a final step. Using this technique, increases in both the brightness and

degradation resistance were reported. 103

Attempts to model these phenomena have been made by groups at the Georgia

Institute of Technology.15,93' 104 They used a 1-D discrete computer model to calculate

the efficiency of the phosphor with varying thicknesses of coating applied. A correction

term was added to account for surface recombination velocity and the band offset of the

coating compared to the host material. Experimental results were collected for SiO2

coatings on ZnS:Cu:AI deposited by sol-gel methods. If the pH was kept around 6.5 and

the silica concentration at 1 wt%, then enhancement of the cathodoluminescent efficiency

was possible below around 2 keV. However, the enhancement was only seen for coatings

less than 40 nm thick. Beyond this value, electron stopping in the coating was reported to

overshadow the surface recombination gains. 15,93, 104









It should be noted that processing of the phosphors will have an impact on their

cathodoluminescent response. Many commercially available phosphors have undergone

a series of growth, grinding, and annealing steps. Many researchers have shown that

improved cathodoluminescence is possible by tailoring these processes to produce a more

uniform and less defective particle surface.105' 106 The reader should be aware of this

when examining some reports on the effects of coatings. Some authors will apply

coatings to phosphors that have been grown in their own laboratory. The coated

phosphors will then be compared against commercially available materials of the same

composition. This is an unfair comparison because the commercial powders may have a

lower CL response than the laboratory-grown phosphors. Thus, it is difficult to separate

the effects of the coating from those of the phosphor processing.

Another issue to consider when coating phosphors is the consequences of the

coating on the ability to apply the phosphor to the anode faceplate. Certain materials will

cause aggregation of the phosphor particles in the slurry before screening, while others

will help them stay dispersed. In order to create screens with the highest possible

brightness, dispersion of the particles needs to be maintained, and thus the coatings must

help maintain this condition.107



Pulsed Laser Deposition (PLD)

Advances in PLD have made it an increasingly popular method for applying

coatings to materials. 108, 109 In this technique, short laser pulses are used to evaporate

material from a target. Several different things happen during the laser pulse. Lowndes

describes these as "rapid heating and vaporization of the target; increasing absorption by








the vapor until breakdown occurs to form a dense plasma; and absorption of the

remainder of the laser pulse to heat and accelerate the plasma."110 The particles in this

plasma undergo collisions, which in turn produce a highly directional expansion away

from the target. All of this is occurring in the Knudsen layer above the target."10 These

evaporated materials undergo gas phase collisions and are subsequently deposited onto a

substrate. By controlling the ambient gas mixture, background pressure, substrate

temperature, and laser energy and duration, films can be grown to desired specifications.

Due to the method of ablation, the flux of material leaving the target is strongly peaked in

the direction perpendicular to the target surface.110. 11 However, recent advances in the

use of target rotation and apertures have made uniform deposition over larger areas

possible. 112

Of particular interest are the advances being made in the application of PLD to

optical materials. An example includes the deposition of ultrathin (<10 nm) indium tin

oxide (ITO) films. ITO is one of the most widely used transparent conductive coatings in

electro-optic applications.113 PLD is particularly good for growing oxides due to the fact

that the material transfer from target to substrate is normally stoichiometric, meaning that

the deposited film has the same composition as the target. 110, 114 This is a result of the

high initial rate of heating and highly nonthermal target erosion. 110 Large bandgap

semiconductors and dielectric materials usually have large optical absorption at short

wavelengths. Therefore, large amounts of the laser energy are deposited in a small

volume close to the surface. As a result, the sublimation temperature of the material is

attained at depths greater than the thermal diffusion distance of the constituents. Target






37

components are not able to segregate, therefore they leave the target in proportion to their

bulk concentrations. This results in deposits with the same composition as the target.39

By varying the conditions under which thin film phosphors are deposited, their

optical properties can be improved. Y203:Eu is a good example of these effects. It has

been shown that by increasing the oxygen partial pressure during deposition, rougher

films are produced. This leads to an increase in brightness due to forward scattering by

anomalous diffraction.39' 115 Microcrystallites of f-IV compounds such as CdTe and

CdS have been grown in a similar fashion. 116 Temperature of the substrate can also

affect film properties. Models suggest that higher substrate temperature will lead to

rougher films. 117 The type of substrate will also determine whether amorphous,

polycrystalline, or epitaxial single-crystal films are grown.110 In one experiment,

Y203:Eu was grown on bare (100) Si wafers and on diamond-coated Si wafers. The

diamond coating was prepared using a hot filament chemical vapor deposition. The

phosphor films were grown under identical conditions on the two substrates. There was a

substantial improvement in luminescent brightness from the diamond-coated substrate.

This was attributed to the higher roughness of the surface; see Figures 2-17 and

2-18.41,118

Sapphire substrates have also been used to improve the brightness of Y203:Eu

thin films. The increase in brightness is believed to be due to the low absorption and low

refractive index of red light in sapphire and to the improved growth of grains with

unidirectional orientation on the (0001) sapphire substrates.119

Finally, Y203:Eu films have been epitaxed onto LaA103 substrates. These are

good substrates because the lattice mismatch is only 0.8% with an orientation of









[110]Y20311[100]LaA103 and [-110]Y203||[010]LaAlO3. Z-contrast scanning

transmission electron microscopy (STEM) was used to demonstrate the absence of

precipitates of Eu in the deposited films.120 This is advantageous in a luminescent film

because the activators are thus spread out.

Besides its use to deposit phosphor thin films, PLD has also been used to coat

phosphor powders to improve luminescence and reduce degradation. Fitz-Gerald

deposited Y203:Eu onto powders of SiO2 and A1203. The powders were agitated in a

fluidized bed setup so that the deposited coating would be uniformly distributed over the

powder surface.121-123 The photoluminescent spectra showed the 612 nm peak from the

5Do-7F2 transition in Y203:Eu; so the coatings were effectively deposited onto the core

particles.122 Transmission electron microscopy (TEM) was used to confirm that the

coatings were continuous.123 Next, coatings of TaSi and Ag were deposited on

Y202S:Eu, a highly efficient red phosphor, to help reduce degradation under electron

beam excitation. As discussed in the previous section, degradation curves clearly showed

the advantage of these two coatings for protecting the phosphor.14, 99



Motivation for this Work

Coatings have been shown to be effective in slowing the degradation of

phosphors, especially at the low voltages and high currents of a FED environment.

Improvements in the efficiency of cathodoluminescence at low voltages have also been

demonstrated. Thus far, most of the work on coatings has focused on putting a coating

around the entire phosphor particle, using such techniques as sol-gel processing or spray

pyrolysis. These steps add considerably to the cost of making the powder and must be








monitored closely to ensure proper results. In this work, the commercially available

phosphors Y203:Eu and Y2SiO5:Tb were sedimented onto Mo substrates and coated using

pulsed laser deposition. This is a much simpler process that can be applied to large area

screens using existing coating techniques.

To ensure that all results observed were due to the coating and not the processing

of the screen (such as deposition, CL geometry, etc.), half of the screen was masked

during deposition. This allowed side-by-side comparison of coated and uncoated

phosphor material. Coatings of MgO and A1203 were deposited at various thicknesses to

examine their effects on cathodoluminescence. These materials were chosen because

they both have large work functions compared to the host materials. This should lead to

enhanced efficiency due to repulsion of electrons from the particle-coating interface and

thus a lower surface recombination velocity.

Phosphor powders were chosen for this study instead of thin films because they

represent real world devices. Thin films provide an excellent surface on which to study

the basics of electron interactions, but they can introduce other problems. Lattice

mismatch between the luminescent film and substrate can lead to a drastic reduction in

cathodoluminescence. This can be alleviated in some instances by the careful choice of

substrate. (For example, LaAIO3 for growth of Y203:Eu.124) However, this is often a

difficult challenge. There is also the problem of a much lower intensity due to greater

internal reflection in the thin film.39 Because the initial intensity from the thin films is

low, detecting changes in the brightness becomes more difficult. Because phosphor

powders are used in FED devices and offer high initial brightness, they were chosen for

this study.
























Direct
Connection
to Brain


I ........Sens -- Visual (300 Mb/oec)

Audio (20 kb/ec)

Touch (10 b/sec)


Smell (4 b/sec)

Heat (0.1 b/sec)






Figure 2-1 The different means of interfacing between a human and a computer. Also
shown are the speeds of each type of transfer.6



















Table 1

The different types of flat panel displays and the advantages of each.


Teehnnlnov


Tvne


Size


Power


Advantages


Barriers


lat----. e.L _Rugged,___high-_
AMEL emissive small moderate Rugged, high- Pixel size limit, lack
resolution, high- of suitable phosphor
brightness for full-color.
OEL emissive small low High luminous Lithography
efficiency, low incompatibilities,
drive voltage instability, short
lifetime, limited
temperature range
Plasma emissive large high Mature High-power, slow
technology, high refresh rates, limited
______ ___brightness military market
Laser light large, moderate High brightness, Still in early stages
Projection valve scaleable display size is of development, no
scaleable full-scale prototype
MEMS light moderate Some Development of new
Projection valve large, commercially technologies costly,
scaleable available (TI- digital artifacts may
____________ ________ DMD), scaleablebe a problem
Reflective light small to very low Very low power, Limited temperature
LCD valve medium bistable range.
manufacturability,
durability


FED


emissive


small to
medium


low


Low power, mign
brightness, wide
viewing angle


Manuracturaoility,
lifetime


Reproduced from reference 8.
The acronyms in the table stand for the following:
AMEL Active Matrix Electroluminescent
OEL Organic Electroluminescent
LCD Liquid Crystal Display
FED Field Emission Display






42

















/^I"


c/t Electron
_Beam





Electron Gun Vertical
Deflection Horizontal
Plates Deflection
Plates

Phosphor
Screen


Figure 2-2 Typical cathode ray tube (CRT), consisting of an electron gun and deflection
plates to raster the electron beam across the phosphor screen.6

























Anode --
Assembly

'"I^i rr "J 'I~ .I


Spacer







Cathode
Assembly

Baseplate Row Electrodes Column Electrodes



Figure 2-3 A typical FED setup, consisting of the cathode assembly, which houses the
field emitter tips, and the anode assembly, a glass plate on which the
phosphors are deposited. Spacers are used to separate the two halves of the
device.























light
B i I l T T Transparent Conductor (-1000V),



EmttF a plate+
... .... ........ ....... .................tte... ....

j \;/ \ / \ .\;/ Electrons i j \;
I, t /^ xtracionti u/ u/ If I II



IEmitter Baseplate Emitter & Elerodes


Figure 2-4 Schematic cross section of a typical FED. Not drawn to scale.9


Ph




















Enery


-Fex


Total Potential


Figure 2-5 Potential energy curves of an electron near a metal surface. The "Image
Potential" is with no external field. An externally applied field that is
electrically negative to the surroundings can be represented by the "Applied
Potential" curve. The resultant "Total Potential" will then be the
superposition of the two curves. Due to the lower energy barrier at the
surface, electrons in the metal have a finite probability of quantum
22
mechanically tunneling into the vacuum.
























ary electron


beam



phosphor




Electron penetration in a phosphor at various beam energies. Incident beam
energy is increasing from A to B to C. This also demonstrates how the
interaction volume moves closer to the surface as the beam energy is
decreased. When surface recombination is considered, the interaction
volume in A will clearly have a stronger influence from the surface than that
in C. As a result, surface effects become more pronounced at lower beam
voltages .35


prim


Figure 2-6
























Erouc2 --
35 -



Esmxr+




25 -
-.-'O3

20 SD



CP)



10 C

o 3 2 H, --'

_H4


Fa7

W* -7F. -7F

Ce03 Gd3 Tm3 Tbh3 Eu3'




Figure 2-7 Expected transitions in various rare-earth dopants used for FED phosphors.47










, a a a 0 0 000*l


0,

Y


~ IN WW m


C2


S6


Figure 2-8 The two different crystallographic configurations of Eu in a Y203 host lattice.
The S6 symmetry is often referred to as C31 symmetry.4


















CT E
c, E c!
%, I % %I


a_



Figure 2-9


\,TH h T R
R b i R



The dashed curves represent the charge-transfer (CT) state. In (a), the CT
state helps feed the emissive 5D levels because of its position relative to the
7F ground state. This is the situation found in Y203:Eu and Y2SiO5:Tb. In
(b), the offset of the CT state causes the electrons to be returned to the ground
state, thus reducing the luminescence of the material.25















Table 2


Composition, color and efficiency
FED phosphors


at low-voltage operation of selected


Composition Color Efficiency (Im W )

500 V
ZnO:Zn Green 10.7
Gd202S:Tb Green 7.9
YAI O2:Tb Green 2.0
Y202S:Eu Red 3.5
Y20:Eu Red 2.2
YVO4:Eu Red 0.4

300 V, 131 iA
CaS:Ce Green 3.10
SrGa2S4:Eu Green 3.00
La2O2S:Tb Green 5.20
Gd2O2S:Tb Green 3.52
Y202S:Eu Red 2.20
Y203:Eu Red 1.57
Y2SiO.:Ce Blue 0.25
Y2SiO5:Tb Green 1.05
LaOBr:Tb Green 1.95
LaOCI:Tb Green 0.36
LaOBr:Tb Blue 0.54
LaOBr:Tm Blue 0.17
Reproduced from reference 2.























Table 3

Classification of FED structures
Anode Anode-cathode
Type voltage separation Advantages Disadvantages
(kV) (Mnm)

High CRT phosphors 8 *Focussing electrodes required
-voltage 4-10 2-3 processes applicable *Spacers with a high aspect ratio
-High efficiency *Breakdown

Medium 1-4 0.2-0.8 *Simple structure -Spacers with a relatively higk
-voltage 'Fair efficiency aspect ratio
Low -Simple structure -'Limited efficiency
-voltage 0.4-1 0.1-0.2 'High reliability
*Low cost

Reproduced from reference 11.












































500 1000 1500 2000 2500 3000 3500 4000 4500
acceleration voltage [volts]


Figure 2-10 Efficiency versus acceleration voltage for Y203:Eu.38

























faceplate


Electrons


Phosphor


Figure 2-11 FED with phosphor on the gate electrode to double the light output from the
device.6
device.






























2 3 0 1iiiiiiiiiiiiiiiiiiiiiiiiiiii
-40-30-20-10


0 10


20 30 40


-COLLECTION BIAS ( V )


Figure 2-12 Carbon Auger peak shifts after changes in the bias of the sample mounting
plate.52


310


0

ei

w
z
w
01


w
z

w
a

U
,=x:


300


290


280


270


260


250


240
















- m m


- 0 0 a U m


a an .a


PHOSPHOR
LAYER


II


BRASS BACK
PLATE


m


Figure 2-13 Setup for metal mesh placed on the phosphor surface for taking internal field
measurements under electron excitation.65


MESH


v


E.I




















S02 0.127p

>- 14 t T
a: .:^ /
I.-








0 4 8 12 16 200.254









ACCELERATING POTENTIAL (kV)



Figure 2-14 Luminescence versus accelerating potential for ZnS-coated (0.127 lHtm,
0.254 Inm, or 0.389 r thick) ZnS:Cu. The similar slope of the lines
suggests that the coating did not alter the luminescent states of the
phosphor, but only decreased the incoming electron energy.z
LU
rO O-389F
LU 6
Q

4-

2-


00 4 8 12 16 20
ACCELERATING POTENTIAL Mk)



Figure 2-14 Luminescence versus accelerating potential for ZnS-coated (0.127 g.m,
0.254 g~m, or 0.389 g~m thick) ZnS:Cu. The similar slope of the lines
suggests that the coating did not alter the luminescent states of the
phosphor, but only decreased the incoming electron energy. 9






57




20 -|1 i-|-| --- |-1 -\-1 -1

18 0.0508AF(

16- 0
S0.0847 0

| 14- o84
49

12-
S0. 188,

8- o



C.,
co
S4- -


0.50opt
2-
A/
0 A,, / I I I A I ~ -



0 4 8 12 16 20
ACCELERATING POTENTIAL (kV)



Figure 2-15 Luminescence versus accelerating potential for ZnS-coated (0.0508 pnm,
0.0847 gm, or 0.188 Lim thick) ZnS:Cu. The similar slope of the lines
suggests that the coating did not alter the luminescent states of the
phosphor, but only decreased the incoming electron energy.96















1

0.9

. 0.8
*
S0.7

I o.6

d 0.5


0.4

0.3


0 5 10 15 20 25
Coulomb Load (C/cm2)


Figure 2-16


Cathodoluminescence degradation of Y202S:Eu with various coatings. The
SiO2 and phosphate coatings were applied by wet chemistry techniques,
while the Ag and TaSi coatings were deposited by pulsed laser ablation in a
fluidized bed setup. "Original" refers to the uncoated powder.14
















































Figure 2-17 Atomic force microscopy (AFM) images of Y203:Eu film grown on bare Si
substrates and on diamond-coated Si substrates. The images clearly show
the higher roughness of the films on the diamond-coated material.41











100 ..
p Eu:Y203 Film on Diamond
90 "-i- Eu:Y203 Film on Silicon

o? 80

Z 70
Co
S 60

~g 50

"0 40
N
-1 30
20
z
10

0 I ..I I.....II
300 400 500 600 700

DepositionTemperature (C)


Figure 2-18 Photoluminescence from Y203:Eu films grown on bare Si and diamond-
coated Si substrates. The curves show the higher luminescence attained
with the diamond-coated substrate. This is attributed to the higher
roughness of these films.41














CHAPTER 3
EXPERIMENTAL PROCEDURES


Introduction

The effects of coatings on field emission display (FED) phosphors were studied in

this work. Coatings of A1203 and MgO were deposited by pulsed laser deposition (PLD)

onto screens of Y203:Eu and Y2SiOS:Tb. These screens were prepared by sedimentation

from a phosphor slurry. During deposition of the coatings, a Si shadow mask was used to

provide coating on only half of the sample. This allowed for direct comparison of coated

and uncoated phosphor material.

Many different types of characterization techniques were used to measure the

samples. Cathodoluminescence measurements were taken under steady-state and pulsed

conditions to examine the response of the phosphors to an electron beam. Ellipsometry,

transmission electron microscopy (TEM), and Auger electron spectroscopy (AES) were

used to determine the thickness of the coatings. Finally, AES and a scanning electron

microscope (SEM) with a fiber optic attachment used for measuring CL maps were used

to examine coating uniformity.



Sedimentation of Phosphor Screens

The phosphor screens of Y203:Eu and Y2SiO5:Tb were prepared by sedimentation

of the powder from a slurry. The apparatus consists of plastic beakers with a hole drilled








in the bottom. Attached through this hole is a connector that allows for a tube to be

inserted onto the bottom of the beaker. A vise-style clamp on this tube controls the flow

of the liquid out of the container. Samples are held down on the bottom of the vessel by

spring clips that are attached along the bottom edge.

The first step in the sedimentation process is preparing the slurry mixture. The

phosphor is weighed and added to a magnesium nitrate hexahydrate and isopropyl

alcohol mixture. This is then placed in a sonicator for 30 minutes to ensure complete

dissociation of the phosphor throughout the slurry. Natural heating also takes place

during the agitation, which helps hold the powder in suspension. At this time, the

substrates are cleaned by rubbing them with a methanol-soaked swab and then immersing

them in methanol and placing them in the sonicator for about five minutes. After

cleaning, the substrates are attached to the bottom of the beaker. The hose is then

attached and clamped off. With all of this in place, the slurry is poured into the beakers.

It takes many hours for the phosphor to settle out of solution and coat the samples.

Usually, the beakers are covered and left overnight. In the morning, the liquid is slowly

drained from the beaker by loosening the clamp on the hose. The samples are then air

dried in the container for several hours.

Both of the phosphors used for this work, Y203:Eu and Y2SiOs:Tb, were standard

Nichia powders. These were chosen because they were readily available and would be

typical for use in a FED. Molybdenum was chosen as a substrate material for all of the

samples because of its good electrical conduction and high melting temperature.

(Originally, gold was used as a substrate material because it is inert to most materials and








has a very high electrical conductivity. Unfortunately, the gold would not endure the

750 C temperature needed during the pulsed laser deposition of the coating materials.)



Pulsed Laser Deposition (PLD) of Coatings

Since PLD has been shown to be particularly good for growing oxides due to the

fact that the material transfer from target to substrate is generally stoichiometric110' 114, it

was chosen as the means of depositing the A1203 and MgO coatings for this work. The

apparatus used consists of a stainless steel chamber pumped by a Pfeiffer Balzers TPU

450 H corrosive turbomolecular pump with a TCP305 controller and backed by a MD41

diaphragm pump. A Leybold Trivac mechanical pump model # D4A was used for

roughing the chamber. To control background pressure, a gate valve was used between

the pump and chamber to adjust the pumping speed. Precision leak valves provide

further control of pressure through backfilling of the chamber with a variety of gasses.

Ablation energy comes from a KrF (X=248 nm) 1 Watt Lambda Physik Lasertechnik

LPX300 excimer laser. Pulse width was set at 10 nanoseconds and laser energy kept at

about 350 mJ for all depositions. Pulse frequency was variable between 1 and 100 Hz,

but 10 Hz was chosen for all experiments. Laser light enters through a viewport on the

side of the chamber. Control of the laser dimensions and position is accomplished with

collimating and focusing lenses located on an optics bench positioned between the laser

and vacuum chamber.

Directly across from the laser entry viewport inside the chamber is the target

holder. The holder is positioned so that the laser strikes at 45 from the surface normal,

which in turn generates a plume perpendicular to the target surface.110, 111 The target








holder is connected to a rotary motion feedthrough to allow target rotation during

deposition, thus allowing better utilization of the targets. Target rotation also helps

minimize deposition of large particulates from the target onto the growing film.125

Substrates are mounted on a sample holder located directly across from the target.

Target-to-substrate distance was set such that the samples were located near the end of

the plasma plume, which was around 3 centimeters in the chamber used for this work.

Plasma expansion models suggest that this is the optimal position for the substrate.114

The sample was not rotated during deposition. Heating of the samples was accomplished

using a quartz lamp located within the sample holder. This allowed for a range of 150C

to 1000C, although 750C was used throughout these experiments. A stainless steel

heater plate was used to transfer heat to the substrates, which were mounted to this plate.

A thermocouple attached to the plate provided accurate control of the deposition

temperature throughout the run.

The A1203 and MgO targets were made from their powders. First, approximately

five grams of the powder was placed in a hardened stainless steel die. The die used was

designed to produce targets an inch and a half in diameter. A hydraulic press was used to

compress the powder in the die to around 120 psi for 4 minutes. The green powder

compacts of these two materials were found to hold together very well. Both materials

came out of the die without excessive force, so no lubrication of the die was necessary.

Had such lubrication been used, it could have been a source of contamination for the

target. For similar reasons, no flux was used to help hold the compacts together, although

a flux is occasionally added to increase the sintering of the powder in the compact.

During firing of the compacts, the flux is supposed to evaporate, but if it does not, it can








also be a source of contamination. The compacts were then placed in a tube furnace at

1200C for 12 hours. This anneal is used to greatly increase the density of the target.

Finished targets were then silver pasted onto holders used to attach them to the rotary arm

of the target holder.

Phosphor screens were attached to the stainless steel heating plate of the sample

holder by silver paint. A piece of Si was placed over half of the screen to act as a shadow

mask. A spring clip attached to the side of the sample holder was used to hold the mask

in place. The target and substrate were then inserted into the vacuum chamber. Once

sealed off, the chamber was evacuated to a base pressure of around 1 105 Torr.

Temperature of the substrate was then raised to 750C. At this point, the gate valve was

partially closed and the precision leak valve opened to adjust the background pressure of

oxygen in the system. Oxygen was fed to the leak valve from ultra-high purity

compressed gas cylinders. A MKS model 247C mass flow controller set at 80 sccm was

used between the gas cylinder and leak valve. Background pressure of 50 mTorr was

used for the majority of the experiments, with 200 mTorr used for the remainder (as

noted later in the experimental results section).

Contrary to most other deposition techniques, such as electron beam, thermal

evaporation, and sputtering, PLD does not show a strong decrease in deposition rate with

increasing pressure above a few mTorr. This is due to the inherent quasi-free jet

expansion of the plume during pulsing. The particles in the plume impart momentum to

the background gas molecules. Therefore, the background gas and plume travel together

toward the substrate. This greatly reduces the scattering of deposition species due to






66
collisions with gas molecules. The result is a very long mean-free path for the particles,

which allows deposition at higher pressures."1

Control of the coating thickness was achieved by closely monitoring the

deposition time. Materials ablate at different rates, so the coating time had to be adjusted

for each of the material systems used. For the system used for this work, it was found

that the deposition rate at 50 mTorr was around 65 A/min for A1203 and 18.2 A/min for

MgO.



Characterization Techniques

Many different characterization techniques were used to determine the coatings'

thickness and effect on the luminescence of the phosphor. The techniques include

steady-state and pulsed cathodoluminescence (CL), ellipsometry, transmission electron

microscopy (TEM), Auger electron spectroscopy (AES), and scanning CL in a scanning

electron microscope (SEM).

Figure 3-1 shows the energy distribution of electrons emitted from a sample

surface under electron bombardment. Electrons in various energy regions on this curve

are measured in some of the aforementioned characterization techniques. The secondary

electrons are used for secondary electron (SE) imaging in the SEM. The Auger electrons

are measured in AES. The elastically backscattered electrons are used for diffraction

analysis in the TEM.








Steady-State Cathodoluminescence

Cathodoluminescence (CL) is the process of generating light by electron beam

excitation of a phosphor material. Incident electrons impart some of their energy to the

phosphor material through electron excitation. These excited electrons can then

recombine with an available hole (the absence of an electron) to produce visible

light.25, 35

Cathodoluminescence measurements were carried out in a stainless steel vacuum

chamber pumped with a CTI-Cryogenics cryopump and a Varian ion pump. Roughing

was accomplished using a rotary-vane mechanical pump. Base pressure for this

combination of pumps was between 5* 10-9 and 1 108 Torr with no bake-out. Although

baking the chamber would have allowed for a lower pressure by removing some of the

desorbed gas on the walls, it was not an option due to a fiber optic feedthrough.

However, these pressures are more than adequate for taking CL measurements. Electron

excitation was from a Kimball Physics EFG-7F electron gun with a EGPS-7H power

supply and RGDU-3C raster generation unit. The electron gun was capable of energies

up to 5 keV and currents from 0.01 to 500 giA. Focus was set to maintain a spot size of

approximately 2 mm.

Phosphor luminescence was collected using a fiber optic connected to an Ocean

Optics S2000 spectrometer. This spectrometer uses a diffraction grating and a charge

couple device (CCD) array detector, which allows luminescent spectra to be collected

over the entire visible wavelength range. To further improve wavelength resolution, a

homemade slit was fashioned on the end of the fiber before it entered the spectrometer.

This greatly reduced the broadening of the luminescent peaks. This is especially helpful








in materials with closely spaced peaks that can become indistinguishable due to peak

overlap.

To improve signal-to-noise in the collected spectra, the raster unit was utilized to

redirect the electron beam off-axis. Due to the electron gun design, the filament used to

generate electrons within the gun is visible through the exit aperture. This causes a bright

white spot of light on the sample directly in front of the gun. Using a mechanical

manipulation system, this filament light can be redirected away from the area of interest

on the sample. This greatly reduces the background signal detected by the spectrometer.

By setting the appropriate bias on the raster unit, voltage was applied to deflection plates

on the electron gun to redirect the electron beam to the center of interest on the sample.

Accurate current measurement requires the collection of secondary as well as

primary electrons. For this reason, a +100 V bias from a Fluke 343A DC Voltage

Calibrator power supply was applied to the electrically isolated sample carousel. A

Keithley 619 Electrometer/Multimeter was connected in series between the power supply

and the carousel to measure the current. Background subtraction was used to account for

any stray currents present in the system. A switch was installed between the meter and

carousel to allow the carousel to be shorted to ground. This is necessary during

measurement of the luminescence because the sample bias will impart extra energy to the

incoming electrons.

The sample carousel held samples perpendicular to the incident electron beam.

Samples were inserted such that the uncoated half of the powder screen was above the

coated half, instead of beside it. In this arrangement, the electron beam could be moved

from an uncoated to a coated area by adjusting the vertical position of the carousel.








Using only vertical repositioning assured that the electron beam-to-sample and sample-

to-fiber alignment would not be changed. It is critical to keep these alignments constant

because changing them will result in a change in the measured luminescence.



Pulsed Cathodoluminescence

Pulsed cathodoluminescence measurements are used to study the decay of the

luminescence from a phosphor under a pulsed electron beam. The same chamber was

utilized as in the steady-state CL measurements above. The Kimball Physics electron

gun has a fast pulsing option. A grid located inside the gun is used to blank the beam.

By applying a large negative bias to the grid, a potential barrier for the electrons is

established. The magnitude of this potential barrier, adjusted through the grid voltage,

determines how many electrons can make it past the grid. If an electron has enough

energy to surmount this barrier, then it will proceed through the gun as normal. Pulsing

is accomplished by sending a positive voltage pulse along the grid supply line to reduce

the grid retarding voltage. By selecting the appropriate grid bias, the positive pulse can

be used to turn the electron beam on and off. Thus, the characteristics of the positive

pulse determine the electron beam characteristics. Kimball Physics supplies a box that

can be installed in the grid voltage line for such an operation. This box allows the pulse

generator to be connected to the grid line and prevents the grid voltage from being sent

back through the line to the pulse generator. Due to the design of the electron gun, the

grid voltage (typically 100 to 250 V) is added to the accelerating voltage (typically 0.5 to

4 keV) before being transmitted down the grid line. During operation, it was noticed that








the pulsing box would not give consistent pulsing. To correct this, the circuit was

redesigned and a new box built to provide a stable, low noise operation.

A Hewlett Packard (HP) 8112A Pulse Generator was used to provide the positive

voltage pulses to turn the electron gun on. Pulse width was set to 10 is with a delay of

65 ns and a period of 100 ms. Amplitude was adjusted to produce a 16 volt turn-on

pulse.

Luminescence was collected using a fiber optic mounted inside the vacuum

system. The other end of the fiber was connected to a Hamamatsu Photomultiplier Tube

(PMT). A 1150 V power supply was used to power the PMT. A box was used to house

the PMT with a fitting for the fiber on the outside. During operation, a thick black piece

of fabric was wrapped around the box to reduce the amount of stray light. A holder was

designed for the inside of the box to allow filters to be placed between the fiber and PMT.

Filters were used to select specific wavelength regions. This gave the ability to study the

decay of single peaks (different decay states) of the phosphors. Filters used included a

535 nm bandpass and Coming glass 2-73, 3-70, and 5-58 highpass filters. Figures 3-2 to

3-4 show the transmission of these Coming filters. The output of the PMT was

connected through a "T" junction to an oscilloscope and a HP 3478A Multimeter. The

multimeter was used to look at the voltage from the PMT due to background light in the

chamber, most of which was due to filament light from the electron gun. During actual

measurement, the multimeter was disconnected to reduce the amount of noise in the

signal. The oscilloscope used was a 500 MHz 2 GS/s Hewlett Packard Infinium

Oscilloscope Model 54825A. A built-in disk drive was used to save files and transfer

them to another computer.








Sweep rate on the oscilloscope was chosen to ensure that the entire decay pulse

was measured. This meant making sure that the after-pulse luminescence returned to

background level. Once the sweep rate was set, the sample averaging was adjusted to

give 25044 data points per scan. To further improve signal-to-noise, 3000 scans were

collected for every set of conditions. The oscilloscope automatically averaged these as

they were collected. Therefore, every data file consisted of a 25044x2 matrix of time and

intensity. Comparison of multiple files of this size is very computer processor intensive,

so further averaging was done. Files were loaded into Mathcad, where a sliding time

average was applied. The program takes a preset number of data points and finds an

average, which it assigns to that interval. It then moves on to the next interval and

repeats the process. In this fashion, the entire data set is averaged to produce a more

manageable size. See Appendix A for a more detailed description of the program.

Typically, the data interval was set for 25 points and the last 44 points discarded, which

produced a 1000 point file.



Ellipsometry

Ellipsometry, also known as polarimetry and polarization spectroscopy, can be

used to obtain the thickness and optical constants of thin films.126 Some of the

advantages of this technique over others is that it is simple to operate, nondestructive, and

requires no vacuum system. Measurements can be taken in a vacuum system, in air, or in

a liquid.36 Figure 3-5 shows the typical experimental arrangement. Collimated

monochromatic light is passed through a polarizer (Glan-Thomson calcite prism) and a

quarter-wave compensator (mica plate with 45 retardation) to give elliptically polarized








light. This is reflected off the sample surface into a second polarizer (Glan-Thomson

calcite prism) that acts as an analyzer. The polarizer and analyzer are then rotated to find

the maximum extinction of the reflected light. 127

It is important to know the angle of incidence of the incoming light and its

wavelength and to keep these constant throughout the measurement. Once the initial set

of polarizer and analyzer settings is found, the two are rotated to find a second set of

extinction conditions. The polarizer is adjusted to 90 plus the original polarizer angle.

The analyzer is adjusted to 180 minus the original analyzer angle. Both are then rotated

to find maximum light extinction. In order to ensure accurate measurements, the new

angles should not differ from the calculated positions by more than four degrees.128 With

these data, the values of psi (T) and delta (A) can be determined from the relationships:

S_180 -(A2 -Al)
2

A= 3600 -(P + P)

where A2, A&, P2, and P, are the second and first set of analyzer and polarizer angles,

respectively. 127, 128

Light is an electromagnetic wave and therefore must obey Maxwell's equations.

As a consequence, there are certain relations that must be obeyed when light encounters a

boundary between two media. First, the angle of incidence must equal the angle of

reflectance. Second, in the case of one material on top of another, Snell's Law must

hold:


n, sin 01 = no sin 00









where niand no are the complex indices of refraction in material 1 and material 0 and 01

and 02 are the angles from the surface normal in material 1 and material 0.36 Third, the

Fresnel reflection coefficients are given by:

no Cos 00 n, Cos 01
no cos 0o + n, cos 01

n, cos 00 no cos 0

no cos0o + n, cos01

where s refers to the light vector component perpendicular to the plane of incidence and p

the parallel component.127 The plane of incidence is defined by the incident and reflected

beams and the surface normal.36 It is these last two relations that are important for the

ellipsometry measurement. They are related to the T and A values obtained earlier by the

following equation:

rp = (tan )T .
r,

These equations can also be related to the reflection coefficients through the following:

R rol,, + rlsub^,se-2i
gs +
I + rol, rl,, e~-2i

R rol.p + rlsubPe-2
1 + ro.p'rsub,pe -2i

where the subscripts 0, 1, and sub represent the measurement medium, film, and

substrate, respectively. The film thickness, di, can be found from the equation for the

phase angle, 13:


P=27j i cos0.









It is this equation that relates the index of refraction to the thickness, as well as to the

phase changes due to reflection at the interface.36' 126 There are many computer

programs available that use these relations to determine film thickness or index of

refraction from the I and A values found from the measured analyzer and polarizer

settings. Due to time and space constraints, a more detailed discussion of the equations

will not be attempted here. However, it is possible to analyze multiple films using this

technique, although such analysis is very complex.127, 129

For the current work, two different types of ellipsometry equipment were

utilized. The first was a Gaertner Scientific Corp. ellipsometer with a HeNe laser

(632.8 nm) and manual polarizer and analyzer. For this apparatus, the angle was set at

70, and the sample positioned to give reflection into the detector. Once this was set,

polarizer and analyzer readings could be collected. The second piece of ellipsometry

equipment used was a fully automated rotating analyzer ellipsometer. In this

arrangement, a fixed polarizer is used and the analyzer is rotated to determine extinction

values. Advantages of the latter include multiple wavelength capabilities and more

accurate measurement of the analyzer and polarizer relative positions. The computer

program also allows for multiple angles to be used to further improve analysis. Built-in

data libraries for most elements and compounds make data analysis much simpler and

more precise.



Transmission Electron Microscopy (TEM)

TEM is a powerful technique for obtaining information about the atomic structure

of a material. The main requirement is that the sample is thin enough to transmit






75

electrons.36, 126 Figure 3-6 shows a typical TEM setup. For the current work, TEM was

used to determine coating thickness from the Si shadow masks used during deposition.

One of the most difficult aspects of using TEM is sample preparation. Since coating

thickness was desired in the present study, cross-sectioned samples were prepared. This

is done by first thinly slicing the sample using a diamond saw. Two of these thin slices

are then laid flat with the coated sides touching. Wax is used to hold the sample together.

In order to obtain a region thin enough for electrons to propagate through, the sample is

placed in an ion-mill. This machine uses energetic ions to sputter thin the sample. Once

a hole appears, the sample is ready for analysis. By looking at regions on the periphery

of the hole, films sufficiently thin for analysis yield information about the sample.

In the microscopy, electrons are scattered as they pass through the samples. It is

the nature of the scattering that determines the type of information that is obtained. There

are two types of scattering events: elastic and inelastic scattering. Elastic scattering is a

result of Coulombic interactions of the incoming electron and the potential field of the

ion cores, and it results in no loss in energy to the electron. This type of process is

known as Rutherford scattering and gives rise to diffraction patterns. The magnitude of

the interaction scales with the charge on the nucleus, and thus with the atomic number.

Inelastic scattering is the interaction between the primary electron beam and electrons in

quantum states around the nuclei or in the solid. Energy is transferred during the

scattering, giving rise to spatial variation in the intensity of the transmitted beam

dependent upon defects and heterogeneities.126 By examining the intensity of the

transmitted electrons, different layers and their interfaces are visible. From this, the

thickness of the coating layer can be determined.










Auger Electron Spectroscopy (AES)

AES is a very useful tool for looking at surface compositions. By utilizing ion

beam sputtering, composition versus depth is attainable. Figure 3-7 shows a typical AES

setup. Ultra-high vacuum (UHV) conditions are needed to reduce surface contamination

during analysis. Sample characterization takes place as follows. Energetic primary

electrons (-5 keV) are focused by an electron gun onto the sample surface. These

primary electrons lose energy as they traverse the sample. Similar to the process

described for the TEM, some of this lost energy goes into exciting ground state electrons

into empty quantum states of the atoms or into continuum energy states. The atoms have

two options for recombining an electron with an available hole and returning to the

ground state. These are to produce an (1) X-ray or (2) Auger electron; see Figure 3-8.

Both of these processes happen simultaneously, but this analysis is concerned only with

the Auger electrons. In addition, for de-excitation energies less than approximately

2000 eV, Auger electron emission dominates over X-ray emission.

Auger electrons get their energy from the atom when it relaxes back to its ground

state. Therefore, they are characteristic of the energy levels of that specific atom. Once

Auger electrons are ejected from the atom, they must make it through the material and be

ejected without energy loss. Figure 3-9 shows the mean free path of various atomic

species. Immediately obvious from this graph is the fact that none of the elements listed

have an Auger electron escape depth over 30 A. This is the reason that AES

characterizes the near surface region. With the use of computers, data can now be

collected directly in N(E) versus E mode and then manipulated to obtain the differential








dN(E)/dE versus E. Historically, the data was collected in differential form due to the

use of an ac modulation on the signal and detection with lock-in amplifiers, as shown in

Figure 3-7, but with modem detectors and computers, this is no longer necessary. Saving

the data in its non-differentiated form presents advantages in noise and processing.36

However, the differential form is best for viewing the data since it highlights the Auger

peaks.22, 36, 126

For the current work, a Perkin-Elmer PHI660 Scanning Auger Multiprobe system

was utilized. Electron beam energy was kept at 5.0 keV with a 25 uLA current. Pressure

in the system was around 3*10-8 Torr during sputter profiling. A 3x3 raster was used on

the 3 keV Ar ion gun during sputter analysis. Initial surface scans were collected from

coated and uncoated parts of the sample to find peaks for analysis during depth profiling.

The surface scans of the coated materials can give information about the uniformity of

the coating within a thickness range equal to small multiples of the escape depth of the

Auger electrons. Depth profiling was used to determine thickness of the coating on

powders as compared to complementary coatings on Si masks.



Scanning Cathodoluminescence (CL) in a Scanning Electron Microscope (SEM)

Uniformity is also thought to be important to the success of the coatings. A SEM

with a fiber optic attached to a spectrometer was used for producing CL maps of the

sample surface. The SEM is a useful tool for magnifying the sample surface (about O10X-

300,OOOX).36 To produce an image, a focused electron beam is rastered across the

sample surface. When the electrons enter the sample, they lose energy through inelastic

collisions, as discussed above. This inelastic energy loss is transferred to the host lattice






78

and gives rise to a multitude of different electron energies that leave the surface.

Figure 3-1 shows a plot of these different electrons. In addition, other signals can be

produced, such as X-rays, light, and heat. Due to the inherent roughness of a sedimented

powder screen, there are intensity variations in the CL map of the surface. In order to

provide a complementary image of the surface being studied, a secondary electron (SE)

image was collected. Uniformity of the coating could then be checked by comparison of

the two images. The SE image provided a picture of the different phosphor surfaces.

The corresponding surfaces on the CL image could then be located and studied.





















ELECTRON
YIELD


SECONDARY ELASTIC BACK
ELECTRONS SCATTERED
ELECTRONS
AUGER
\ ELE CTRNS


5 50
ELECTRON ENERGY (eV)


2000
2000


Energy distribution of electrons emitted from a sample surface under electron
bombardment. The secondary electrons are used in secondary electron (SE)
imaging in a scanning electron microscope (SEM). The Auger electrons are
used in Auger electron spectroscopy (AES). The elastic energy is used for
diffraction analysis in the transmission electron microscope (TEM).126


Figure 3-1






























: t I. I I I 1 4 1 I I I I I I I I i I
S- --- -- -f



| ^ ^ ~ ~Z~-I.ZZ~LZ 'Z.Z'r rF' l'i I il I I /I;S/ i -,'~", "
-- ... i Ii 1 i i I fmw i C i I
"5' --'" f


I w I L 1 I1 I W1"
...2:I l 0 l, i
10l l! lIl l


400 420 440 460 480 500 520 640 S0 W80 00 020 640 600 W
WAVE LENGTH MILUMICRONS


Figure 3-2 Transmission of light through a Coming glass 2-73 filter. Taken from data
sheet supplied with filter.


70T 720 740


- 240 260 "D 300 SM 040 SOO no





























101 -- -- -- T- -- ---.-- .- -- -- -- ,- -_ _ _
o -: -4-- -4. ,_ ~~^ ~_ ^ ^ ^ ^^._
-c----------z-_--.:-I- ---I- --I-- I-__



I I 4 \ I "I
TO---------------- -2- 5 S _
40 i <; t \ t
S\ -\ -
20!= = =/ = =s == E E
so /E... ,
1. 11"=^=\^^ == \5=====....=I==
+o / I#1 \ \ 5c=^= = I,== /
I-/L/ x \ ;\l // == ::::


2, 2w 3 3 20 U40 a 30 000 400 420 440 <60 490 500 520 540 560 80 600 620 40
WAVE LENGTH MILLIMICRONS


60 6O 700 720 740


Figure 3-3 Transmission of light through a Coming glass 5-58 filter. Taken from data
sheet supplied with filter.


*w wO










































Figure 3-4 Transmission of light through a Coming glass 3-70 filter. Taken from data
sheet supplied with filter.


















SUBSTRATE


LYZER

TELESCOPE


,. LIGHT SOURCE


DETECTOR V
(EYE OR MICROPHOTOMETER)


Figure 3-5 Schematic of a typical ellipsometer. Monochromatic light is passed through a
polarizer and a quarter-wave compensator to give elliptically polarized light.
This is reflected off the sample into the analyzer (a second polarizer). The
polarizer and analyzer are rotated to determine maximum extinction of the
reflected light. 126





















Electron gun -
Anode ---- L ^
Gun alignment coils
Gun airlock --f

1 st Condenser lens -
2nd Condenser lena -s
Beam tilt 0oils sl
Condenser 2 aperture --- ION
Objective lens GETTER
Specimen block PUMP

Diffraction aperture
Diffraction lens -
Intermediate lens __ |

1 ast Projector lens --
2nd Projector lens -- -


Column vacuum block
35 mm Roll film camera --
Focussing screen
Plate camera /
16cm Main screen I








Figure 3-6 A typical transmission electron microscope. The top portion of the
instrument above the specimen block is used to generate a focused high-
energy electron beam. The apertures and lenses below the specimen block
are used to select specific regions of the diffracted electron beam for imaging.
Most instruments allow for collection of generated images on a photographic
film.36


















































Figure 3-7 A typical Auger electron spectroscopy (AES) setup, based on a cylindrical
mirror electron energy analyzer (CMA).22











C harvwcstc X-ray
O -
huE .EK-E

0


C Ev
0
@amC

c

C
I0..


e~

UI !L E Edwao


EwE, ELI


n(E)


Figure 3-8 Schematic representation of the processes of X-ray fluorescence and Auger
electron production. A KLL transition is shown as an example. Initial
excitation comes from ejection of a core shell electron by an incident
electron. Both processes occur in a material under electron beam
bombardment.22


Auge ElKctro
Emisio
E -E -E-























Au


I I
t0 20


-I I I


I I -- I I I I I
s0 100 200 300 500 1000 2000
Electron Energy, eV


Electron escape depth as a function of initial kinetic energy. This is a
measure of the average distance the electron will travel before undergoing an
inelastic collision with the host lattice. It is also commonly referred to as the
electron's inelastic mean free path.22


0.



0


w
LU


Figure 3-9















CHAPTER 4
EXPERIMENTAL RESULTS


Introduction

Results are separated into four main categories. These include the steady-state

cathodoluminescence (CL) spectra, which show the luminescence over the visible

wavelength range and at multiple beam energies. Next, the effects of beam energy on

steady-state CL intensities at specific wavelengths are reported. Third, pulsed CL decay

curves at various beam energies and wavelengths are reported. Fourth, the coating

thickness and uniformity data are presented. A list of all samples is given in Table 4.



Cathodoluminescent Spectra

Figure 4-1 shows the cathodoluminescent spectra of Y203:Eu as reported in the

Phosphor Technology Center of Excellence: Low Voltage Phosphor Data Sheets.38 This

figure shows the relative radiant intensity (i.e., cathodoluminescence intensity) as a

function of wavelength at 1 keV and 1 gA/cm2. The intensity has been normalized to the

value of the 611 nm peak. Figures 4-2 through 4-5 depict the cathodoluminescent spectra

for Y203:Eu with coatings of MgO deposited for 4 or 8 minutes, and A1203 deposited for

1.2 or 2.4 minutes, respectively. These curves show the intensity as a function of

wavelength from 520 to 720 nm. Spectra are displayed for uncoated and coated samples









at beam energies of 0.8 keV, 1.4 keV, 2.5 keV, and 4 keV. Current density was kept

constant at 0.16 IA/cm2 for each measurement. From the data, it can be seen that the

coating reduces the cathodoluminescent intensity over the entire wavelength range.

However, no measurable change occurred in the wavelength dependence of the

luminescent peaks, as will be discussed later in this section. Another feature of these

graphs is that coatings of the same material deposited for longer times (which were

therefore thicker) led to a larger attenuation of the intensity at all beam energies from 0.8

to 4 keV. This can be seen by comparing the intensity ratio of coated to uncoated

phosphors at any given beam energy for the same coating material and time.

Figure 4-6 shows a typical cathodoluminescent spectra for Y2SiOs:Tb from 400 to

720 nm measured at 1 keV, as reported in the literature.49 The intensity has been

normalized to the main emission peak. Figures 4-7 through 4-10 are spectra of uncoated

and coated Y2SiOs5:Tb. Coatings of MgO, deposited for 4 and 8 minutes, and A1203,

deposited for 2.4 and 5 minutes, are presented. No spectra are reported for the 1.2 minute

A1203 coating on Y2SiO5:Tb because the data files were corrupted. The intensity as a

function of wavelength from 460 nm to 680 nm at beam energies of 0.8 keV, 1.4 keV,

2.5 keV, and 4.0 keV are shown. Similar to the Y203:Eu samples, the MgO and A1203

coatings caused a reduction in the cathodoluminescent intensity over the entire

wavelength range. Also, coatings of the same material deposited for longer times (which

were therefore thicker) attenuated the luminescence more strongly. Wavelength

dependence of the luminescent peaks was unaffected by the coatings.

No calibrated light source was available inside the chamber, so luminescence in

Figures 4-2 to 4-5 and 4-7 to 4-10 was plotted in arbitrary units. During measurement,








the integration time of the spectrometer was adjusted to provide a main peak intensity

signal that was between 80 and 90 percent of the maximum allowable intensity. This

helps increase the signal-to-noise ratio. To further improve this ratio, 64 scans were

collected and averaged. Sixty-four background spectra scans were also collected and

averaged before each run and were subtracted from the luminescent spectra in real time.

Averaging of the background spectra is needed in order to keep from introducing noise

during subtraction.

All spectra in a given figure have been corrected for any differences in integration

time and can therefore be directly compared against one another. However, care must be

taken when comparing curves from different figures because not all figures have been

normalized to the same overall integration time. This was necessitated by small changes

in sample positioning and light collection between experiments. In order to compare

curves from different figures, the ratio of the coated to uncoated spectra needs to be used.

Since all powders of each type of phosphor are the same, the luminescence of the

uncoated side should be the same for all samples of that phosphor. This is one of the

main reasons for masking half of the phosphor screen during deposition. Therefore, by

comparing the ratio of intensities at different beam energies, the effects of the coating can

be obtained. Using this method, it can be concluded from the figures that coatings of a

given material deposited for longer times show larger attenuation of the

cathodoluminescence intensity, as stated in the previous paragraph.

In order to better understand the beam energy and coating effects on the CL peak

positions and relative heights, spectra for beam energies from 0.8 to 4 keV were

normalized to the intensity at the main emission peak. These are shown in Figures 4-11




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O' 5e ORFR 7 81,9(56,7< 2)


MEASUREMENT AND MODELING OF THE EFFECTS OF
PULSED LASER DEPOSITED COATINGS ON
CATHODOLUMINESCENT PHOSPHORS
By
William Joseph Thornes, Jr.
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
2000

This work is dedicated to the memory of my grandfather,
Gilbert Walter Thorpe,
who passed away in the course of my graduate studies.

ACKNOWLEDGMENTS
There are so many people whom I wish to thank that it is hard to decide where to
begin. First, I would like to thank my fiancée, Cynthia, for her loving support all these
years. She has seen me through the ups and downs of graduate school and has always
been there to pick me up when I was down or to share in my joy. She also provided
invaluable editorial advice and revisions for this dissertation. I would also like to thank
my family for their love throughout my life. Even when I was not sure that it was
possible to make it all the way to the end, they believed in me and provided the
encouragement to stick it out no matter what happened.
Dr. Paul Holloway deserves a special word of thanks for providing guidance
during my graduate education. His door was always open, and he never hesitated to
discuss my work with me and give me some direction when needed. He is truly one of
the greatest teachers that I have been fortunate enough to come across in my educational
pursuits.
Special thanks are also due to Dr. Carl Seager and Dr. Dave Tallant for providing
me with the opportunity and the funding to come to Sandia National Laboratories to
conduct many of my experiments with them. I am especially grateful to Carl, who, in
addition to providing me access to all of the equipment in his labs so that I could measure
my samples, was a mentor and a friend during my stay in New Mexico. Without his help
in

and guidance, much of the work contained in this dissertation would not have been
possible.
There are many others who made my stay in New Mexico a memorable one.
Although I cannot mention them all in the space allotted to me, I would like to point out a
few. John Hunter was everything I could have asked for in a friend, and never once did
he hesitate to allow me to stay at his house a little longer when I said, “I think they want
me to stay one more month.” I would also like to thank my many other friends out in
New Mexico, including John, Shasta, Jerry, and Shawna. Thanks are due to Jonathan
Campbell for sedimenting all of my phosphor screens, helping me learn all the quirks of
the lab equipment, introducing me to everyone at the lab, and teaching me about all of the
fascinating things that New Mexico has to offer (including green chiles). I’d also like to
thank Guild, who always seemed to be in a good mood, for his help with the SEM work.
Many thanks go to those in Florida who have made these past four years
memorable ones. I wish to thank the entire Holloway group past and present, especially
Billie, Jay, Bo, Chris, Sean, Troy, Loren, Caroline, Bill, Mark, and Ludie (without whom
we might all be lost). I am grateful to the following for not only providing me with their
friendship, but also for performing measurements on my samples: Eric Lambers for AES
measurements, Wish Krishnamoorthy for TEM analysis, and Dr. Kumar for help with the
PLD of the coatings used in this work.
I’d also like to thank all of my friends in Cuong Nhu, especially Chi-Wah, who
suffered with me for many years under some of the best senseis in the martial arts. I will
never find another set of instructors who provide the type of training that I found in
Sensei Mark’s and Sensei Joyce’s classes.
IV

Although I have made every attempt to include all those who deserve
acknowledgement in these pages, I am sure that there are some who have not been
mentioned. I regret the omission, but after many long days of putting this dissertation
into its final form, it’s surprising that more of you are not missing. Please know that each
of you is appreciated.
v

TABLE OF CONTENTS
ACKNOWLEDGMENTS iii
ABSTRACT ix
CHAPTERS
1 INTRODUCTION 1
2 LITERATURE REVIEW 5
Introduction 5
Field Emission Display (FED) 6
Cathodoluminescence (CL) 8
Phosphors 14
Y203:Eu 15
Y2Si05:Tb 17
Efficiency 18
Charging 19
Cathodoluminescent Degradation 25
Coatings for Phosphors 31
Pulsed Laser Deposition (PLD) 35
Motivation for this Work 38
3 EXPERIMENTAL PROCEDURES 61
Introduction 61
Sedimentation of Phosphor Screens 61
Pulsed Laser Deposition (PLD) of Coatings 63
Characterization Techniques 66
Steady-State Cathodoluminescence 67
Pulsed Cathodoluminescence 69
Ellipsometry 71
Transmission Electron Microscopy (TEM) 74
Auger Electron Spectroscopy (AES) 76
Scanning Cathodoluminescence (CL) in a Scanning Electron
Microscope (SEM) 77
vi

4 EXPERIMENTAL RESULTS 88
Introduction 88
Cathodoluminescent Spectra 88
Beam Energy Effects on Luminescence 92
Pulsed Cathodoluminescence 93
Coating Thickness and Uniformity 97
5 MODELING OF RESULTS 174
Introduction 174
Energy Loss in Dead Layer 174
Incident Angle Contributions and Coating Uniformity 177
Path Length of Electrons in the Coating 184
Calculation of the Cathodoluminescence from Coated Phosphors 185
New Energy Loss Equation 188
Backscattering Coefficients 190
Index of Refraction 192
6 DISCUSSION 233
Introduction 233
Initial Uniform Coating Model 233
Validity of the Energy Loss Model 236
Backscattering Coefficient 241
Scattering 242
Surface Recombination and Charging 243
Surface Segregation 244
Surface Roughness 245
7 SUMMARY AND CONCLUSIONS 247
APPENDICES
A MATHCAD DATA AVERAGING PROGRAM 252
B LUMINESCENCE DUE TO THE INCIDENT ANGLE OF THE ELECTRONS.... 256
C NONUNIFORM COATING SHAPE 260
D STOPPING POWER 263
E MATHCAD PROGRAM FOR MgO (4 min) / Y203:Eu 265
F MATHCAD PROGRAM FOR MgO (8 min) / Y203:Eu 270
vii

G MATHCAD PROGRAM FOR A1203 (1.2 min) / Y203:Eu 275
H MATHCAD PROGRAM FOR A1203 (2.4 min) / Y203:Eu 280
I MATHCAD PROGRAM FOR MgO (4 min) / Y2Si05:Tb 285
J MATHCAD PROGRAM FOR MgO (8 min) / Y2Si05:Tb 290
K MATHCAD PROGRAM FOR AI203 (1.2 min) / Y2Si05:Tb 295
L MATHCAD PROGRAM FOR A1203 (2.4 min equiv.) / Y2Si05:Tb 300
M MATHCAD PROGRAM FOR A1203 (5 min) / Y2Si05:Tb 305
BIBLIOGRAPHY 310
BIOGRAPHICAL SKETCH 322

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
MEASUREMENT AND MODELING OF THE EFFECTS OF PULSED LASER
DEPOSITED COATINGS ON CATHODOLUMINESCENT PHOSPHORS
By
William Joseph Thornes, Jr.
August 2000
Chairman: Dr. Paul Holloway
Major Department: Materials Science and Engineering
Coatings have been shown to reduce the degradation of field emission display
phosphors and, in certain cases, improve the low voltage luminescence. To study the
energy loss mechanisms in coatings and to predict their impact on cathodoluminescence,
coatings of MgO and AI2O3 were pulsed laser deposited onto sedimented screens of
Y203:Eu and Y2Si05:Tb. The thickness of the pulsed laser deposited coatings (which
were characterized by ellipsometry, transmission electron microscopy, and Auger
electron spectroscopy sputter profiles) were varied from 7.5 to 50 nm by changing the
deposition time and oxygen background pressure. A Si shadow mask was used to cover
half of the sedimented powder during deposition. This allowed for comparison of coated
with uncoated powder that experienced the same processing. Cathodoluminescence data
were collected for beam energies from 0.8 to 4 keV at a constant current density of
0.16 /¿A/cm2.
IX

The coating thickness on the particles was modeled by assuming a uniform
deposition flux over a spherical powder surface. Spatially resolved electron energy loss
was calculated to predict the cathodoluminescence intensity versus beam energy and
incident angle relative to the local surface normal. A modified Bethe stopping power
equation was initially used to predict the luminescence from the coated phosphors. This
was found to overestimate the attenuation of the luminescence at beam energies below
about 3 keV. To provide a more accurate fit to the low energy region, a new energy loss
equation based on a form similar to a Makhov power loss equation was introduced. By
using the new energy loss equation, the cathodoluminescence intensity of the coated
phosphors could accurately be predicted for all energies up to 4 keV.
The model developed in this work was used to fit luminescence losses from as
low as 4.71% (at 4 keV) for a 10 nm coating of AI2O3 on Y2Ü3:Eu to as large as 27.4%
(at 4 keV) for a 43 nm MgO coating on Y2SiOs:Tb. No change in the surface
recombination rate was observed. The coatings were also found to have no effect on the
decay states under pulsed excitation, except those attributed to beam energy loss.
x

CHAPTER 1
INTRODUCTION
Cathode ray tubes (CRTs) have dominated the visual display market since their
invention in the 1920s.1 However, due to their large size, they are not suitable for use in
many modem applications that require compact screens. To fill this role, a class of
devices known as flat panel displays (FPDs) has been invented. These displays are very
thin in comparison to the CRT, which makes them the ideal display media for portable
electronic devices. So far, liquid crystal displays (LCDs) have led the market in the field
of FPDs.2 However, new types of FPDs are being developed to improve upon the LCD.
At the forefront of the new FPDs is the field emission display (FED). It offers
advantages in terms of its wide operating temperature range, wide viewing angle, fast
response time, low power consumption, high brightness, high durability, low weight, and
scalability.2'12 For these reasons, the FED is likely to challenge the LCD for dominance
in the FPD market.
The FED excites luminescence from phosphors deposited on a glass faceplate,
similar to a CRT. However, unlike a conventional CRT that relies on just three electron
guns that create beams that are rastered across the phosphor screen, the FED uses
thousands of tiny emitters behind each phosphor pixel. The depth of a CRT is
determined by the distance necessary for deflection of the electron beam over the entire
screen surface. The FED removes this limitation due to the fact that the emitters are
1

2
located directly behind the pixels and can be turned on and off, unlike scanning in the
CRT, in which the electron gun is continuously on. Therefore, the device can be made
very thin (the typical spacing between the emitter array and phosphor screen is around
10 mm).
The proximity of the emitters to the pixels also allows much lower voltage to be
used in the FED (4 or 5 keV, compared to 25 keV in a CRT). The problem with using a
lower voltage, however, is a reduction in the luminescence intensity from the phosphors.
To counteract this loss in intensity, the current from the emitters is increased. The
increased current helps regain the lost luminescence, but it also leads to more rapid
degradation.13
Due to the operating conditions of FEDs (low voltages and high currents),
phosphor degradation is one of the limiting factors in the lifetime of the device. Coatings
have been shown to greatly improve the degradation resistance of the phosphors.14 One
of the drawbacks to using coatings is that they also cause a reduction in the
cathodoluminescence from the phosphor. Under certain circumstances, however,
coatings have been shown to improve the low-voltage efficiency of phosphors.15 If the
efficiency of the phosphors is increased at low voltages, then the power consumption of
the device and the rate of degradation can be reduced.
The focus of this work has been on examining the effects on cathodoluminescence
of wide-bandgap oxide coatings, specifically MgO and A1203, deposited by pulsed laser
deposition onto two commercially available phosphors, Y203:Eu and Y2Si05:Tb, used in
FEDs. A review of the literature available on selected aspects of the cathodoluminescent
process is given in Chapter 2. Also included in the chapter is a discussion of the

3
mechanisms behind the degradation process and how the use of coatings can slow or
eliminate this loss. Near the end of the chapter, the characteristics and merits of pulsed
laser deposition are presented, along with the reason for choosing this technique to
deposit the coatings.
The experimental procedures used to deposit the coatings and collect the
cathodoluminescence from the phosphors are discussed in Chapter 3. This chapter also
includes a description of ellipsometry, transmission electron microscopy, Auger electron
spectroscopy, and scanning electron microscopy, all of which were used to characterize
the coatings.
Results of the cathodoluminescent measurements on the coated samples showed
that the coatings attenuated the luminescence from the phosphors. The effect was larger
at lower voltages due to the interaction volume of the electron beam. Thicker coatings
were found to produce a larger reduction in the cathodoluminescent intensity. These
results are discussed in Chapter 4.
A model was developed to explain the loss in luminescence based on a dead layer
approximation. The model relied on calculation of the energy loss of the electron as it
travels through the coating. Resultant energy after passing through the coating was then
used to find the luminescence. This model allows for calculation of the
cathodoluminescence over the entire energy range up to 4 keV. Spatially resolved energy
loss was found using a cosine dependent coating based on a uniform deposition flux onto
spherical phosphor powders. An improved energy loss equation was incorporated to
account for the extended range of the low energy electrons. Calculated results from this
model were found to accurately predict the cathodoluminescence from the coated

4
phosphors for all measured thicknesses. Derivation of the model is given in Chapter 5.
A discussion of the applicability of the model is presented in Chapter 6.
The conclusions from this work are given in Chapter 7. Appendices A through M
contain the various Mathcad programs used during the calculations.

CHAPTER 2
LITERATURE REVIEW
Introduction
Electronics have become an ever-increasing part of everyday human life. The
ability to process information is continually advancing as new and faster ways of
computing are developed. One important aspect of this trend is the ability of the machine
to transmit data to a human operator. Shown in Figure 2-1 are the many different means
of interfacing between a computer and a person. It can clearly be seen that visual
communication is currently one of the fastest means of data transfer, at around 300
Mb/sec. Many different types of visual signal media can be used to accomplish this goal.
Historically, cathode ray tubes (CRTs) have dominated the visual display market.
However, new types of displays have recently evolved. Most of these new technologies
have been designed to serve a more specific purpose that allows them to compete with
the CRT. For example, the development of smaller, more portable electronic devices
created a need for compact screens. Liquid crystal displays (LCD) have filled this role
thus far, with a 90% market share in 1995.2 However, in demanding applications such as
military uses, medical instruments, vehicles, and dusty environments, to name a few, the
weaknesses of the LCD (one of which is its inability to operate over wide temperature
ranges) can have severe consequences. It is in these areas that new devices are being
developed to challenge the dominance of the LCD. All of these new devices can be
5

6
lumped under the general heading of flat panel displays (FPDs) because of their slim
profile when viewed from the side. Each type of display has certain advantages and
disadvantages when used for different applications. Table 1 lists the various FPDs and
some of the attributes of each. Portable applications are not the only market for FPDs;
their strengths over other visual media will allow them to compete in all arenas of display
technology, from tiny head-mounted displays to huge billboard-size screens.
Among the various types of FPDs, field emission displays (FEDs) offer many
advantages, such as wide viewing angle, fast response time for video refresh rates and
“instant-on” capability, low power consumption, high brightness, durability, wide
operating temperature range, low weight, and scalability to a larger size. Each of these
attributes makes FEDs desirable for a wide variety of applications.2'12
Field Emission Display (FED)
Cathode ray tubes (CRTs), the displays found in all conventional televisions, rely
on cathodoluminescence for their operation. Figure 2-2 shows the basic design of a CRT.
An electron gun accelerates electrons toward a phosphor screen. These electrons are
deflected by means of plates or magnetic fields and are then raster scanned across the
screen. Red, green, and blue phosphors are deposited on the screen in small holes known
as pixels. (Actually, each hole that contains a single color phosphor is called a subpixel.
Three subpixels together, one of each color, make up a pixel.) In a full color display,
three separate guns are used, each specific to an individual color.
One of the biggest problems with CRTs is their depth, which is needed in order to
have sufficient distance of travel for deflection of the 20 keV electron beams across the

7
entire screen. FEDs remove this size restriction by using thousands of individual electron
sources behind each subpixel. Figures 2-3 and 2-4 show typical FED setups. Electrons
are extracted from very sharp tips by tunneling when high fields are present. These
electrons are then accelerated toward the faceplate, which contains the phosphors. The
emitter tips are usually located only about 10mm away from the phosphor screen, thus
allowing the display to be extremely thin.
Emitter arrays can consist of up to thousands of tips per pixel. They are usually
made from molybdenum, tungsten, platinum, or silicon2, although carbon has recently
been found to be an effective cathode material as well. Carbon emitters are not grown
like traditional tips and are either diamond thin films or are in the form of
nanotubes.4’12-16'18 Regardless of the type of material chosen for the emitters, electron
extraction from the tip is carried out in the same fashion. By applying an appropriate
bias, dependent upon the type of material and the specific device setup, electric fields can
be generated to cause tunneling of electrons out of the tip. This process is accurately
described by the Fowler-Nordheim tunneling theory for electrons.6-7-9-19'21 Figure 2-5
depicts the field with and without applied bias. As can be seen from the picture, with an
applied bias, the potential energy barrier on the surface has been reduced in height and
has a finite width. The applied field is magnified at the tip because of its extreme
sharpness (its radius is approximately 50 nm). This results in more tunneling current at
lower voltages. Due to the finite barrier and large effective fields near the surface, it is
possible for the electrons to quantum mechanically tunnel out of the tip and enter the
vacuum as field-emitted electrons.22 For a more detailed discussion of the use of the
Fowler-Nordheim equation and suggestions for improving the description of electron

8
emission from sharp tips, the reader is referred to the previously-cited paper by Cutler
and associates.19
One of the challenges faced in applying this technology in a commercial flat panel
display is achieving uniform emission from the thousands of tips. To combat the
problem of nonuniform emission, resistors have been fabricated in series with the tips to
help control tip-to-tip nonuniformity in current.23 This has provided a partial solution to
the problem, but it is not a perfect solution. The best method would be to fabricate a
constant-current source to the tip to regulate current more effectively. A group from
Japan has reported doing this using silicon tips grown as part of a metal-oxide-
semiconductor field-effect-transistor (MOSFET structure).23 Using such a device
structure allows for uniform and precise control of electron emission. With these
advances in cathode technology, research attention is shifting to the improvement of
other aspects of the FED.
The phosphor screen is an integral part of the FED. In terms of the phosphors,
some of the hurdles that still need to be overcome in order to realize a commercially
viable full color display include: better low voltage efficiency, enhanced chromaticity,
reduced saturation, and lower degradation.3
Cathodoluminescence (CL)
When some materials are impacted with certain forms of energy, they will emit
photons in excess of thermal radiation. This process is known as luminescence, and it
can be categorized based on the type of excitation source. These categories include
photoluminescence, cathodoluminescence, chemiluminescence, triboluminescence, X-ray

9
luminescence, and electroluminescence, among others.24’25 Photoluminescence refers to
excitation by photons, usually from a UV lamp. Cathodoluminescence is excitation from
a beam of energetic electrons, also known as cathode rays. Chemiluminescence results
from energy released during a chemical reaction. Triboluminescence is a result of
mechanical energy, such as friction or fracture, and can also be seen during processes
such as grinding. X-ray luminescence comes from X-ray excitation, as the name
suggests. Finally, electroluminescence is produced by an electric field resulting from an
applied voltage across the material. It should be noted however, that
thermoluminescence does not refer to thermal excitation from the ground state. Instead,
this term is used to describe the thermal stimulation of electrons from excited state traps
that can then recombine and produce luminescence. The applied heat does not actually
excite the electrons; this occurred during a prior excitation event. Rather, the heat only
gives the electrons the energy needed to surmount the energy barrier holding them in the
trapped state.25
In addition to the characterization of luminescence based on the different types of
excitation, there is a distinction made according to the length of the delay between the
excitation and emission of photons. Materials can exhibit either fluorescence or
phosphorescence. “Fluorescence” is the term used when the emission of photons occurs
o
within about 10’ seconds after excitation. “Phosphorescence” pertains to any material
that displays luminescence for longer than this. These phosphorescent materials are
called phosphors, and they can display luminescence lifetime ranging from 10‘7 seconds
to hours after the excitation source is removed.24

10
Phosphors have many technologically important roles, especially in the display
industry. They are responsible for converting incident electrons into light in devices such
as CRTs and FEDs. However, their luminescent decay rate must be considered in
determining their suitability in a particular application. In a display, the phosphor must
continue to luminesce long enough to display an image, but also extinguish quickly
enough to allow for fast video refresh rates. This is accomplished by choosing suitable
phosphor materials and processing, which allows the decay time of the phosphor to be
tailored.26 This is not usually a concern because most phosphors display a suitable
response time without any special processing. However, in applications such as the new
high-definition TVs, in which very fast video refresh rates are needed, the decay times of
various phosphors under different types of processing will need to be taken into
consideration.
When an electron beam enters a solid, it will undergo collisions with the host
atoms. These collisions are either elastic or inelastic. During elastic collisions, the
incident electron interacts with a nucleus of the atom and is deflected with little or no
energy loss. If the electron is deflected back toward the surface, it can be lost from the
solid as a backscattered electron. Inelastic collisions involve interaction between the
incident electron and the electrons of the atom. In these collisions, the incident electron
loses part of its energy to the atom. When the electrons in an atom return from the
excited state to their ground state, a host of signals can be produced. Among them are
Auger electrons, X-rays, secondary electrons, photons, and phonons (thermal effects such
as electroacoustic signals).24 For this study, the generated photons
(cathodoluminescence) are of primary interest.

11
The incremental energy lost (dE) over a distance of travel (ds) due to an inelastic
collision can be found from the Bethe equation14-24- 27-30;
dE -785pZ (LI 66E^
- -In
v J j
[2.1]
ds A* E
where A is the atomic weight in g/mol, Z is the atomic number, E is the electron energy,
p is the density in g/cm3, and J is the mean ionization potential in eV. The latter is the
average energy loss per interaction (for all possible energy loss processes), and for Z > 13
it can be expressed as:
58.5
J = 9.76Z + -
r0.19 ‘
[2.2]
These equations have been shown to be accurate for E > 6.34 multiplied by the mean
ionization potential.14-24-28'30 Below this energy, the modification by Rao-Sahib and
Wittry needs to be considered30:
dE _ -785*p*Z
ds 1.2588*A*y/E*J '
This equation corrects for the low energy region while mimicking the behavior of the
original Bethe equation at high energy. This new equation can be used for energies as
low as approximately 500 V. Attempting to reduce this limit even further, Joy and Luo
suggested a new expression for the mean ionization potential using an energy dependent
term14- 29,31:
r=
J
1 + k
(r
[2.4]

12
where J’ is the new mean ionization potential, J is still the average energy loss per
interaction, and k is a fitting parameter. The constant k varies from 0.7 to 0.9, but it is
usually around 0.85.
The stopping power equations cited above provide a measure of how the electrons
lose energy in the solid, but they do not give an accurate indication of the interaction
volume. During the electron’s trip through the lattice, it will also undergo elastic
collisions. These will not result in an energy loss, but rather will lead to a change in the
direction of the electron’s travel. Equation 2.1 can be integrated to find the length of an
individual electron “random walk” trajectory. This is known as the Bethe range.
However, the interaction volume will be much smaller due to scattering. Typically, the
interaction volume is known as the Gruen, electron beam, or penetration range. It can be
expressed as:
K =
v K y
[2.5]
where p is the density, Eo is the electron beam energy, k' depends on the atomic number
and is a function of energy, and a depends on the atomic number and E. Various
equations have been given for the solution to this problem, but the one most widely
accepted was proposed by Kanaya and Okayama:
0.0276A
Pz
0.889
1.67
[2.6]
measured in pm, where E0 is in keV, A is in g/mol, p is in g/cm3, and Z is the atomic
number.24 Figure 2-6 demonstrates how this interaction volume moves deeper into the
material as the beam energy is increased.

13
Every incident electron gives off energy to the lattice in multiple steps.
Therefore, it is possible for one electron to generate a multitude of secondary electrons.
The number of electron-hole pairs generated per incident electron is known as the
generation factor; it can be expressed as:
G = Mzl) [2.7]
E,
where y represents the fractional electron beam energy lost due to backscattered
electrons. E¡ is the ionization energy, the energy needed to form a single electron-hole
pair. The ionization energy is related to the bandgap of the material by:
E, = 2.8 Et + M [2.8]
where M is between 0 and 1 eV, depending on the material in question. For a more
detailed discussion, the reader is referred to Yacobi and Holt’s work on the subject.24
Once the incident electron has transferred its energy to the solid, this energy can
be used to produce visible light. Luminescence from inorganic phosphors usually takes
place at an impurity, which is referred to as an activator.24-32-33 However, not all
impurities are activators. Those that do not lead to luminescence are referred to as
quenchers or killers.32 Since the activators are an imperfection in the crystal lattice, they
will usually exhibit energy levels between those of the conduction and valence bands of
the host lattice. These states can become populated either through direct excitation or
trapping of secondary electrons generated in the host lattice. When the activator returns
to its ground state through recombination of an electron and hole, the energy can be
released as luminescence.24- 25>32_36 This is known as a radiative transition. When the
atom relaxes without the emission of a photon, it is called a nonradiative transition.

14
Competition between these two types of transitions determines the intensity of the
emitted light.25
When an electron and hole radiatively recombine, the emitted photon will be
characteristic of the energy levels involved, which is determined by the electronic states
of the activator. The photon energy (hv) is given by:
hv = Ef - E, [2.9]
where Ef and E¡ are the energy of the final and initial states, respectively. In the literature
on the subject, this energy is often referred to instead by its wavelength equivalent.
There is a simple relation relating the two:
1.2398
E
[2.10]
where E is the photon energy in eV and X is in pm.24-36 For most materials, the
recombination of the electron and hole will be the rate- limiting step to the luminescent
process.37
Phosphors
Phosphors are responsible for converting incident energy into photons (light).
Without them, visual display media would not be possible. There are a plethora of
different materials that exhibit this behavior. The following is only a partial list of
phosphors that can be used to generate the three primary colors (red, blue, and green) of a
common display. For red light, there are Y202S:Eu, Y203:Eu, CaS:Eu2+, SrS:Eu2+, and
YV04:Eu. For green light, there are ZnS:Cu:Au:Al, Zn2Si04:Mn, Y2SiOs:Tb,
Gd202S:Tb, La202S:Tb, ZnO:Zn, Y3(Al,Ga)5Ol2, Y2Ge05:Pr, Y202S:Pr, ZnGa204:Mn,
Y3Al50i2:Ce, and SrGa2S4:Eu. For blue light, there are ZnS:Ag:Cl, Zn2Si04:Ti,

15
Y2Si05:Ce, ZnGa204, and SrGa2S4:Ce.3-38 For this work, Y203:Eu and Y2Si05:Tb were
chosen to be representative of the red and green groups, respectively. This allowed for
examination of not only the coating effects, but also the consequences of the underlying
host material. The attributes of each will be discussed below.
Y203:Eu
Y203:Eu shows a main emission peak in the visible spectrum at 611 nm (red)
under cathodoluminescent excitation. This results from a 5Do - 7F2 transition of the Eu+3
site.25.39-43 Figure 2-7 shows such a transition. Eu is a rare earth ion and is
characterized by an incompletely filled 4f shell. The 4f6 shell is shielded by the filled 5s2
and 5p6 shells.25’41 Shielding allows the Eu ion to retain its atomic character and have
energy levels close to that of a free ion.25-44 As a result, the emission is very sharp at the
transmission wavelength.44 This is also the reason that Eu is usually able to keep its
characteristic luminescence, regardless of the host material. There are two possible
crystallographic configurations for the Eu atoms, either C2 or C3¡.39'41 Figure 2-8 shows
these. The main emission from the phosphor is due to transitions in the C2 sites. Energy
absorbed into the C3¡ sites is effectively transferred into the C2 site for recombination 40
In the literature, C3¡ symmetry is often referred to as Só symmetry.39
Of importance in the Y203 host is the charge transfer to the luminescent center.
The highest occupied levels of the ground state in the host lattice are the 2p orbitals of
oxygen, while the lowest unoccupied levels are a mixture of the 3s orbitals of oxygen and
the 4d orbitals of yttrium. When electrons are promoted to an excited state under electron
bombardment, they can be transferred into the 5D levels of Eu.25 The configurational

16
coordinate diagram can be used to represent such a transition. For a fixed central atom,
the diagram represents the potential energy (E) curves of the atom as a function of the
distance between it and its neighbors (R). Figure 2-9 shows such a diagram. The solid
lines in the figure represent the ground states and excited states of the activator, while the
dashed line represents another excited state of the activator known as the charge-transfer
state. The parabolic shape results from the assumption that the restoring force on the
neighboring atoms is proportional to their displacement from equilibrium. The charge-
transfer state is one which excited electrons can easily be fed into because of allowed
transitions from the ground state of the activator or from the host lattice. The
luminescent state is not directly accessible because the transitions are forbidden. In
Figure 2-9 (a), the minimum in the charge-transfer band causes excited electrons to be
effectively transferred into the luminescent states. This is the case for Y2C>3:Eu. If,
however, the charge-transfer band had a minimum located closer to the ground state
bands, as shown in Figure 2-9 (b), then excited electrons would be effectively transferred
into the ground state through nonradiative relaxation. Due to the location of the charge-
transfer band in Y2C>3:Eu, excited electrons populate the luminescent states, giving this
phosphor its high quantum efficiency.45
The efficiency of different FED phosphors are shown in Table 2.2 Y202S:Eu has
a larger efficiency than Y2C>3:Eu, but it has other disadvantages in terms of degradation
that make it a less desirable material for FED applications.14

17
Y2S¡05:Tb
Y2Si05:Tb is a green phosphor with a main emission at 538 nm.46 This is a result
of a 5D4-7F5 electron transition on the Tb3+ site, as shown in Figure 2-7. The other three
peaks seen in a typical Tb-doped spectrum are the result of transitions to the other Fn
states.47 As with the Eu atom in yittria, the Tb in Y2SÍO5 is well shielded by electrons in
the outer shells. In a similar fashion, this usually makes the resulting spectrum invariant
to host lattice composition.44'48 It also makes the transitions exhibit little spread in the
emitted wavelength.44 Also similar to Y203:Eu, Y2SiOs:Tb has a charge-transfer band
that efficiently deposits excited electrons into the luminescent states, as shown in
Figure 2-9 (a). In the Y2SiOs:Tb system, this is the 4f75d band of Tb, and it results in a
relatively high quantum efficiency.25
One of the properties that makes Y2SiOs:Tb favorable over other green phosphors
is its saturation behavior. In a FED application, as compared to a CRT, lower voltages
are used (hundreds to a few thousand volts versus 10 to 20 kV). To regain brightness,
higher currents are employed (up to 1000 A/cm2 versus 0.5 A/cm2).11-43 Most phosphors
will begin to show saturation effects as the current is increased, especially at these low
voltages.3 This is speculated to result from ground state depletion.49-50 As the ground
state of the activators becomes depleted, the probability of exciting an electron into a
luminescent state is reduced. The result is an increase in the nonradiative recombination
rate, which leads to a reduction in the efficiency of the phosphor (see section on
efficiency in this chapter). However, this is not as prevalent in Y2SiOs:Tb because of the
fast decay rate of the excited state. Because the excited electrons recombine so quickly,
they refill the ground state and are thus available for excitation again.49-50 Because

YaSiOsiTb shows little or no saturation effects, it is gaining considerable interest in the
display market.
18
Efficiency
A phosphor is “a solid which converts certain types of energy into
electromagnetic radiation over and above thermal radiation.”25 In other words, when an
electron beam strikes a phosphor, it can impart some of its energy to exciting electrons
into higher energy levels. The rest of the energy is lost in collisions with the host
material. The excited electrons can then recombine with a hole in a radiative or
nonradiative transition. Radiative transitions give off electromagnetic radiation, whereas
nonradiative transitions transfer energy to Auger electrons, ionizations, or phonons.25
Efficiency is defined as the brightness per unit of input power and is measured in
lumens per watt.3-25-39-51 In a phosphor, charge transfer to a luminescent center is
critical to achieving high efficiency. If the electron and hole are not effectively
transferred to the luminescent impurities, then they will have a high probability of
recombining nonradiatively. This is one of the major problems in FEDs, where lower
voltages are used, as compared to CRTs. As the accelerating voltage is decreased, the
efficiency also decreases.1 '•35-51'53 Table 3 and Figure 2-10 depict this situation. From
the table, it is apparent that the reduced efficiency at low voltage is one of the primary
disadvantages to FED operation. In order to regain high brightness, larger currents need
to be used. These larger currents, in turn, lead to saturation effects and enhanced
phosphor degradation and thus short device lifetime.3

19
Many researchers have attempted to devise ways of improving efficiency in this
low voltage (under 4 kV) range. One method involved placing the phosphor material
directly on top of the gate electrode, as seen in Figure 2-11. Other researchers have tried
using quantum confinement to enhance luminescence by spatially confining electrons and
holes. This method usually requires phosphor particles smaller than 10 nm in diameter,
as compared with the 5 pm phosphors currently in production.54'59
The problem with most of the aforementioned approaches is that they tend to be
very difficult to implement. The fundamental problem to solve in order to improve
efficiency is that of how to transport charge to luminescent centers while decreasing the
probability of the charge reaching nonradiative sites. In order to realize this goal,
charging in the phosphor during electron bombardment needs to be controlled.
Charging
Surface recombination refers to the return of excited electrons to the ground state
by recombination with an available hole at the phosphor surface. Due to the disruption in
the phosphor lattice at the surface, there will be a large number of defects in this region.
These defects provide efficient nonradiative pathways for electron-hole recombination.
In order to alleviate this problem, surface passivation is often used to regain
luminescence. Surface passivation involves coating the surface with another material so
that charge is effectively radiated and not allowed to reach the nonradiative surface
states.60'63 These surface states become critical when considering charging of the
phosphor. This is especially important in a FED environment, where low voltages are
used. As the accelerating voltage of the incident electrons is decreased, the generation of

20
electron-hole pairs will occur closer to the surface due to the lower penetration of the
incident electrons.50 Figure 2-6 shows a schematic of the electron beam penetration into
the phosphor at various beam energies. From the figure, it can be seen that at lower
voltages the electrons are localized closer to the surface. Therefore, surface states will
have a larger influence on the generated secondary electrons at these lower electron beam
voltages.
While surface recombination is possible, other effects can occur upon electron
bombardment of phosphors. Yoo and Lee attributed decreased luminescence on the basis
of trapped electrons on the surface. They postulate that low energy incident electrons are
trapped on the surface. These trapped electrons create an electrostatic potential barrier
for subsequent primary electron bombardment and therefore decrease luminescence.63
Ozawa further contributes to this explanation with what he calls secondary bound
electrons. When an incident electron enters a material, it creates secondary electrons in
the near surface region during interaction with the host. If these electrons are within the
mean free path of secondary electrons, then they can be ejected from the material. The
secondary electron coefficient (5) is the number of ejected secondary electrons per
incident electron. Typical values range from 1.5 to 3 for a primary beam energy of about
1 keV, meaning that more secondaries escape the surface than enter it. These leave
behind holes, which give the surface a net positive charge. If the secondaries do not have
sufficient energy to escape or reenter the crystal, then they can become trapped a short
distance above the surface. An electron cloud is thus produced above the surface, known
as a space charge, and it acts as a barrier for subsequent incident electrons.35

21
Bennewitz and associates studied the surface potential by looking at desorption of
F* ions from a CaF2 sample under 1 keV electron bombardment. In their work, they
point out that the positive surface potential is a result of secondary electron loss from the
near surface region. Therefore, it does not depend strongly on the primary beam current,
but only on the secondary electron distribution. The primary beam current will influence
this potential when there are current pathways to the surface from other pathways, such
as desorption of positive ions from the surface, or a leakage current between the
irradiated spot and the sample holder.64
Seager et al. have postulated that instead of trapping electrons outside the surface,
internal fields lead to charging during electron beam bombardment.52 Utilizing
secondary electron emission energies and carbon Auger peak shifts during irradiation
with electron beams of energies from 0.5 to 5 keV, they were able to provide evidence of
surface charging. Four phosphors were studied: ZnS:Ag, SrGa2S4:Eu, ZnO:Zn, and
Y2C>3:Eu. All were powders sedimented onto ~1 cm2 Au foil. Particle diameters varied
from 1 to 10 pm, and the layers were about 5 to 10 particles thick. When the researchers
changed the potential of the plate that the samples were mounted on, surface potential on
the front of the sample (closest to the beam) varied. Figure 2-12 shows results for
Y203:Eu, which is representative of the other phosphors. The figure shows that the
energy of the C Auger electrons changed linearly with application of the applied bias.
This suggests that the backing plate bias led to a change in the surface potential that
altered the escape energy of the Auger electrons. Similar shifts were seen for the
secondary electrons. Thus, it is possible to change the potential of the phosphor with
application of a backing plate bias.

22
Seager et al. also used dual electron beams to irradiate the sample during
excitation. The second electron flood beam was used to control surface potential of the
phosphor. Their results led them to believe that secondary electrons are not recaptured
and used to build an electron cloud, as postulated by Ozawa, Yoo, Lee, and others; rather,
an internal mechanism is responsible for bringing the phosphor into a steady state
condition. They postulated that this internal mechanism is a buildup of positive charge
near the surface, with a corresponding negative charge near the end of range of the
incident electrons.
One problem with the experiments of Seager et al. is the fact that the potential of
the front surface of the phosphor was unpinned. This made application of a known field
within the phosphor difficult. To circumvent this problem, Seager applied a metallic
mesh to the front surface of the phosphor to pin the surface potential at electrical ground.
Electric fields were then applied across the phosphor layer to either enhance or retard the
cathodoluminescence, depending on polarity.65 Two phosphors — Y203:Eu and
Y2Si05:Tb — were sedimented onto brass plates. Total thickness of the phosphor layer
was around 25 pm. Metal mesh with square holes (7.5 pm) was attached to the top
surface of each sample. These grids allowed electric fields to be induced within the
phosphor layer while still allowing incident electrons to reach the phosphor surface.
Figure 2-13 shows this sample configuration. It was found that the maximum voltage
that could be applied before breakdown was between 400 and 450 V. This voltage is
important because it gives a measure of the dielectric strength of the material.
Initial results suggested that internal fields could be used to sweep generated
charges away from or toward the surface. This would, in turn, lead to an increase or

23
decrease in cathodoluminescence because of losses to surface recombination.65
However, further experimentation led to the conclusion that the results seen could be
caused by surface potential fluctuations. Instead of changing the internal fields of the
material, these fluctuations caused the energy of the incident electrons to be altered. The
most likely explanation for the unpinned potential of the surface is that there was an air
gap between the phosphor and grid as a result of the roughness of the powder.66 Thus,
intimate contact is needed if the internal fields of a material are to be altered in a
controlled fashion. Coatings applied directly to the phosphors could provide such a
situation.
Recently, Pantano et al. published an explanation of charging that takes into
account the internal fields, with the addition of allowing for a negative surface potential.
They rely on a model proposed by Cazaux67 which depends on the stored charges per unit
area, q+ and q., and the charge densities, p+ and p.:
t s J06t
q+=J0St p+=-^—
ds
q- = J„(i-nt) p.-^
dP
where Jo is the incident primary-beam current, 8 is the secondary-electron emission
coefficient, r| is the backscatter coefficient, t is the time, ds is the thickness of the
positively charged surface layer, and dp is the penetration depth of the incident
electrons.68 If 1 - q ) 8, the total negative charge q. will exceed q+ in time and the surface
will charge up negatively. The negative surface charge will then act as a potential barrier
for incoming electrons. This can only occur for 8 < 1, such as for beam energies below

24
about 1 kV. The reduced energy of the incoming electrons will limit the luminescence
from the material. If 8 > 1 - r|, the positive surface charge q+ will exceed the negative
end of range charge q.. This process will be self-regulating because as the positive
surface potential increases, the low energy secondary electrons will not have enough
energy to escape. The result is a limited, but stable, positive surface.68 Under such
conditions, the excited electrons generated within the phosphor are swept toward the
surface, where there is a high probability of nonradiative recombination.
One possible method from the presented models above for affecting the
luminescence is to change the velocity of the incoming electrons. Yoo and Ozawa claim
that charge buildup on the surface produces such a velocity change.35-63 Internal fields
could also build up similar surface fields if 8 < 1, as shown in the models by Seager and
by Pantano et al. As a consequence, incident electrons are repelled by an electrostatic
potential at the surface, which lowers their velocity. Due to their lower incident energy,
the electrons that make it into the material have less energy with which to excite
luminescence. Based on calculations of the dielectric strength of the phosphors, as
discussed below, however, this does not seem to be a reasonable model.
Dielectric strength is a measure of the maximum electric field that can be applied
across a dielectric before breakdown. The equation is as follows:
where £ represents the electric field, V represents the voltage, and d represents the
distance. Most materials have a dielectric strength of 105 to 107 V/cm.69-70 For
Y2C>3:Eu, it has been reported that breakdown occurred at 450 V over a 25 pm film.65
This would correspond to a dielectric strength of 1.8* 105 V/cm, which is within the

25
expected range. The value calculated is lower than the bulk dielectric strength because
the numbers were taken from a powder sample. Therefore, the number has a larger
contribution from the lower surface dielectric strength than from the higher bulk
dielectric strength. Given the above dielectric strength and that the penetration depth of
the electrons within the phosphor surface is on the order of 1000 Á (probably an
overestimate), breakdown would be expected at around 1.8 V. Even if one assumed a
dielectric strength one order of magnitude larger, breakdown would be expected at 18 V.
When this is compared to the energy of the incident electrons (500 - 4000 V), it does not
seem possible that these small voltages could be affecting the velocity of the incident
electrons enough to cause a large decrease in luminescence.
Considering the above argument, the most plausible explanation of charging is
that internal fields are causing excited secondary electrons to be swept toward the
surface, where they have a high probability of nonradiative recombination or emission.
These internal fields are a result of a positive surface charge due to secondary electron
emission and a negative charge region near the end of range of the primary electrons. In
order to reduce charging in phosphors, the internal fields need to be minimized or
removed so that generated secondary electrons remain in the bulk of the phosphor, where
they have a higher probability of radiative recombination. This, in turn, will lead to a
higher overall efficiency.
Cathodoluminescent Degradation
One of the main problems that needs to be overcome in order to make an FED
device for mass production is that of increasing FED lifetime. This is especially

26
important for a low voltage device. As the accelerating voltage of the electrons is
reduced, the brightness of the device decreases rapidly, due to a loss in efficiency. To
attempt to regain the original luminescence, higher currents are used.43 In turn, these
higher currents cause operational problems in the device. Besides saturation, as
discussed earlier, degradation is greatly enhanced due to the high current densities
employed. When the lifetime of the device is considered, the degradation of both the
phosphor and field emitters needs to be taken into account. Both can lead to a reduction
in the cathodoluminescence seen from the FED.
In a CRT, it is relatively easy to maintain a good vacuum over the lifetime of the
device. This is due to two reasons. First, the relatively small surface-to-volume ratio of
CRTs makes it easy to initially pump the device. Second, there is a large area over which
to apply a getter material, giving a large ratio of active getter area to system volume. The
FED does not have either of these benefits. Due to its closely spaced anode and cathode,
it has a large surface-to-volume ratio. This makes it difficult to initially pump the device
due to the conductance between the plates. There is also little area over which to apply
the getter material.71 Some improvements to the pumping could be realized by
increasing the gap between the plates, but this creates problems with the focusing of the
electrons and would require the insertion of a focusing grid into the device. Placing the
getter between the anode and cathode could help with the pumping, but this too would
cause problems with the actual operation of the display.71 New techniques are being
explored for gettering, such as placing a non-evaporable getter around the sides of the
package or attaching a getter to the back with small holes to allow pumping. Although

27
these might help in the maintenance of the vacuum within the FED, the role of gases
needs to be considered when examining the device operation.
There are two main sources of background gas in a FED device, those resulting
from desorption from the device structure and those released from the phosphor due to
electron beam impingement. Gases released from the phosphors depend on the specific
material in question and will be discussed later in this section. All other gases are mainly
a result of outgassing from the structural components of the device, such as the spacers,
cathode, and black matrix. Other sources of background gas are present — for example,
permeation — but these usually contribute a negligible amount to the overall composition
of gases in the device. The main outgassing products inside a FED are H2, H2O, CO,
CO2, and hydrocarbons such as methane, ethane, and propane.72-73 Depending on the
relative amount of these gases and the specific material, various processes can lead to
decreased cathodoluminescence. If the environment is dominated by carbon-containing
gas molecules, then carbon deposition on the phosphor surface will most likely
result.74'76 Under steady-state conditions, gas molecules are constantly physisorbing on
and desorbing off the phosphor surface. When an electron beam is present, it can impart
energy to these molecules and cause them to crack (i.e., break the bonds holding them
together) to atomic species. The carbon is then free to form bonds with the surface, while
the other components combine with each other and are released back to the gas phase.
The rate at which this will occur depends on the residual vacuum pressures of the specific
atoms and molecules. As the carbon layer grows, it will substantially reduce the
luminescence from the phosphor particle in the electron beam exposed area.74-76 After

28
exposure in such an environment, a dark spot is usually present that can be seen with the
naked eye.
If hydrogen and water, rather than carbon compounds, dominate the FED
environment, then degradation is still present, but it often will be a result of a change in
the internal efficiency of the phosphor.77 Although different materials degrade in
different ways and at different rates, certain generalizations can be made for various types
of phosphors. Some of the most efficient high voltage phosphors currently known
contain sulfur. Examples include Y2C>2S:Eu for red, ZnS:Ag:Cl for blue, and
ZnS:Cu:Al:Au for green. Under the electron beam, dissociation of residual gas
molecules can occur. These atoms can then form bonds with the atoms in the outer layer
of the phosphor. Due to the volatile nature of the sulfur compounds such as SOx, FUS,
etc., they desorb back into the vacuum. As a result, sulfur is leached from the
surface.13-78'83 It would be expected that this phenomenon would be limited to the near
surface region, however, the lack of sulfur causes a diffusion gradient that brings more
sulfur to the surface region.68 As the sulfur is removed, the particle surface is converted
into its oxide equivalent. Swartz et al. showed that when ZnS is subjected to an electron
beam, its surface is converted into the non-luminescent ZnO.,3>78’80-82 Trottier
demonstrated that Y202S:Eu is transformed into the less efficient Y2Ü3:Eu on the surface
under an electron beam. In addition to the decrease in luminescence, a peak shift was
observed for Y202S:Eu that was due to the emission from the Y2Ü3:Eu layer.13-14 These
types of degradation reactions have been termed electron-stimulated surface chemical
reactions (ESSCR) because of their mechanism.13-14

29
It would seem, based on the mechanism discussed above, that oxide-based
phosphors would not be susceptible to electron beam degradation. However, this is not
the case. Oxides usually degrade because of electron-stimulated desorption on the
surface. This causes the introduction of lattice defects in the material.68-84 Because they
are a disruption in the lattice symmetry, defects tend to increase the nonradiative
recombination rates. As the rate of nonradiative transitions increases, the efficiency of
the phosphor decreases and, thus, so does the cathodoluminescent brightness. Although
this is fundamentally a surface phenomenon, the electron interaction proceeds deep into
the material. Once the outermost surface is changed, an activity gradient exists to drive
the diffusion of species in subsurface regions. Thus, the damage continues into the
phosphor. Compounds most affected by this are ones in which the cation and anion have
Pauling electronegativity differences greater than 1.7, a category that includes many
oxides.68 In a way, this is similar to the sulfide case described above. Even though
ESSCR causes the majority of the degradation in sulfide-based materials, defect
introduction is occurring simultaneously to further decrease the luminescence.
In all of the scenarios discussed, the electron beam caused the degradation to take
place. It seems natural, therefore, that the length of exposure determines the extent of
degradation. By convention, this is measured as the coulomb load to which the phosphor
has been exposed. It can be expressed as:
Coulomb load (Coulombs/cm2) = (I * t) / A
where I is the sample current (Coulombs/sec), t is the time of exposure (sec), and A is the
area of the beam spot (cm2). For this reason, this type of degradation is often referred to
as Coulombic aging.85'87 There have been different formulas presented to represent the

30
loss in luminescence with Coulomb load, but the one most widely referenced was derived
by Pfanhl at Bell Labs. It simply states that the intensity as a function of dose can be
expressed as:
Jo
(1 +
where I is the cathodoluminescent intensity at any given dose N (number of
electrons/cm2), Io is the initial cathodoluminescent intensity, and C' is the bum parameter
(cm2).14-71 The bum parameter C' will often be replaced by a quantity l/Q5o% in which
Q50% is equal to the charge dose, for which the initial intensity of the phosphor is
halved.88 Not all phosphors behave in such an ideal way. Sometimes a phosphor will
show an initial rapid decrease in luminescence that is followed by a slight rise and
eventual inflection and further decrease in intensity. To account for this rise, Cappels and
associates have derived an equation that uses two decay curves similar to the equation
above.89 In essence, their equation is just the superposition of two simple inverse decay
curves.
Holloway, et al. modeled the degradation they observed in sulfide-based
phosphors based on an ESSCR sulfur removal rate. Their model predicted that the
concentration of sulfur on the surface is exponentially dependent on the dose. It is a
more in-depth study of the exact mechanisms behind the degradation process. Their
equation is as follows:

31
where K' is a constant, Prg is residual gas pressure, and N is the electron dose. However,
they pointed out that within experimental error, the data could be made to fit their
equation or Pfahnl’s.13
At the beginning of this section, it was mentioned that the electron beam can not
only degrade the phosphor, but can lead to degradation of the emitter tips as well.
Usually, this is an indirect contamination due to volatile species being ejected from the
phosphors. It is well established that sulfide-containing phosphors “poison” the emitter
tips during excitation with an electron beam.1 *-90-93 This is suspected to be a result of
the ejected volatile species traveling through the vacuum and reacting with the emitters.
Upon reaction, these species can change the work function of the tip material. Due to the
change in work function, the emission characteristics will be altered.3-94 Therefore, not
only a sulfide, but any phosphor has the potential to degrade the cathode emitter tips.
Consequently, the more volatile species have a much larger effect on field emission from
the cathode than their more stable counterparts. For this reason, oxides have received
considerable attention. However, due to efficiencies lower than sulfides or oxysulfides,
they represent a trade-off in performance.
Coatings for Phosphors
Key issues in designing phosphors for low voltage FED applications are improved
efficiency and reduced degradation. Many researchers have attempted to use coatings to
accomplish these goals.
In CRT manufacturing, an aluminum backing layer is applied to the phosphor
screen. Usually, a lacquer film is first deposited to provide a planer surface on which to

32
deposit the aluminum film. Then the lacquer is baked out. The aluminum reflects
generated light back toward the front of the device to improve brightness and contrast.
Even though the aluminum is an effective energy barrier for the incoming electrons, it is
not a problem due to the high voltages used in CRTs.71-95
Many groups have looked at coating for protecting phosphor particles from
electron degradation. Some of the earlier work in this area was performed by Kingsley
and Prener. They used sol-gel processes to coat ZnS:Cu particles with non-luminescent
ZnS layers. Thickness of the layers was determined based on weight gain and size
distribution of the powder after processing. Figures 2-14 and 2-15 show results of their
work. As can be seen from the figures, the coatings did not change the slope of the
luminescence versus voltage curves; they only changed the turn-on voltage. The turn-on
voltage is found by taking an extrapolation from the linear portion of the luminescence
versus voltage curve. The voltage at which the linear extrapolation crosses the axis (i.e.,
when the intensity is zero) is the turn-on voltage. The fact that the coatings only changed
the turn-on voltage suggests that the coating did not alter the luminescent states in the
phosphor, but only caused a decrease in the incoming electron energy. (A layer that
exhibits this type of behavior is often referred to as a “dead layer” because the energy lost
in the layer is no longer available to excite luminescence.) Using this assumption,
Kingsley and Prener were able to model the decrease in luminescence with a Makhov
power loss equation with corrections for the spherical surface and surface
recombination.96 The Makhov power loss equation is given by:
P(x,j0,V0)=j0V0exp(-X2-X0-9)

33
where P is the beam power per unit area, jo is the beam current density at the surface, Vo
is the accelerating potential, and X is a normalized depth in the material given by:
X(x,V0) = -zr^
p-i *c*yQn
where p is the density, C and n are materials constants, and x is the depth in the
material.96
As noted above, sulfide-based coatings may be degraded under electron beam
exposure due to ESSCR at the surface. For this reason, other materials have been studied
as potential capping layers. Bechtel reports on the use of a phosphate coating to improve
the degradation characteristics of ZnS-based phosphors.97 Dmitrienko suggests the use
of stable oxides such as SÍO2, MgO, or AI2O3, however, he notes that the optimal
thickness and deposition techniques still need to be determined.98
One of the first studies to show the protection offered by applying coatings to
phosphors was done by Trottier and Fitz-Gerald. They demonstrated that coatings of
TaS¿2 and Ag deposited by pulsed laser ablation onto Y202S:Eu, a highly efficient red
phosphor, could be effective at reducing the degradation of this phosphor. Figure 2-16
shows the results from their work. The pulsed laser deposited coatings of TaSÍ2 and Ag
were compared against wet-chemistry coatings of SÍO2 and phosphate. The pulsed laser
coating was better at slowing degradation and exhibited far less loss in brightness after
aging to 20 C/cm2. Of the two laser ablated coatings, TaS^ protected the underlying
phosphor better than Ag. This is believed to be due to Ag migration or volatilization
under the electron beam.14*99
Recent interest in phosphor coatings has concentrated not only on their protective
properties, but also on their use to increase cathodoluminescent efficiency at low

34
voltages. The group of Yang and Yokoyama showed that coating CuxS on ZnS:Ag:Cl led
to an increase in the efficiency of the phosphor.100 However, sulfides are susceptible to
ESSCR in a FED environment. It seems that wide-band oxides are the preferred choice
for increasing the efficiency of the phosphors.101 The group of Kominami et al. showed
that coating ZnS:Ag:Cl with In2C>3 led to improved efficiency at voltages below 500 eV.
They attributed this to an increase in the conductivity of the phosphor, which allowed
charge to be dissipated. An increase in the phosphor lifetime was also reported.102
Villalobos and associates coated Si02 on ZnS:Ag particles using a newly designed spray
coating process, in which the host particle is suspended in a liquid mixture and then
sprayed into a hot zone where the coating gels on the surface. The particles are then heat
treated as a final step. Using this technique, increases in both the brightness and
degradation resistance were reported.103
Attempts to model these phenomena have been made by groups at the Georgia
Institute of Technology.15-93-104 They used a 1-D discrete computer model to calculate
the efficiency of the phosphor with varying thicknesses of coating applied. A correction
term was added to account for surface recombination velocity and the band offset of the
coating compared to the host material. Experimental results were collected for Si02
coatings on ZnS:Cu:Al deposited by sol-gel methods. If the pH was kept around 6.5 and
the silica concentration at 1 wt%, then enhancement of the cathodoluminescent efficiency
was possible below around 2 keV. However, the enhancement was only seen for coatings
less than 40 nm thick. Beyond this value, electron stopping in the coating was reported to
overshadow the surface recombination gains.15-93'104

35
It should be noted that processing of the phosphors will have an impact on their
cathodoluminescent response. Many commercially available phosphors have undergone
a series of growth, grinding, and annealing steps. Many researchers have shown that
improved cathodoluminescence is possible by tailoring these processes to produce a more
uniform and less defective particle surface.105-106 The reader should be aware of this
when examining some reports on the effects of coatings. Some authors will apply
coatings to phosphors that have been grown in their own laboratory. The coated
phosphors will then be compared against commercially available materials of the same
composition. This is an unfair comparison because the commercial powders may have a
lower CL response than the laboratory-grown phosphors. Thus, it is difficult to separate
the effects of the coating from those of the phosphor processing.
Another issue to consider when coating phosphors is the consequences of the
coating on the ability to apply the phosphor to the anode faceplate. Certain materials will
cause aggregation of the phosphor particles in the slurry before screening, while others
will help them stay dispersed. In order to create screens with the highest possible
brightness, dispersion of the particles needs to be maintained, and thus the coatings must
help maintain this condition.107
Pulsed Laser Deposition (PLD)
Advances in PLD have made it an increasingly popular method for applying
coatings to materials.108-109 In this technique, short laser pulses are used to evaporate
material from a target. Several different things happen during the laser pulse. Lowndes
describes these as “rapid heating and vaporization of the target; increasing absorption by

36
the vapor until breakdown occurs to form a dense plasma; and absorption of the
remainder of the laser pulse to heat and accelerate the plasma.”110 The particles in this
plasma undergo collisions, which in turn produce a highly directional expansion away
from the target. All of this is occurring in the Knudsen layer above the target.110 These
evaporated materials undergo gas phase collisions and are subsequently deposited onto a
substrate. By controlling the ambient gas mixture, background pressure, substrate
temperature, and laser energy and duration, films can be grown to desired specifications.
Due to the method of ablation, the flux of material leaving the target is strongly peaked in
the direction perpendicular to the target surface.110-111 However, recent advances in the
use of target rotation and apertures have made uniform deposition over larger areas
possible.112
Of particular interest are the advances being made in the application of PLD to
optical materials. An example includes the deposition of ultrathin (<10 nm) indium tin
oxide (ITO) films. ITO is one of the most widely used transparent conductive coatings in
electro-optic applications.113 PLD is particularly good for growing oxides due to the fact
that the material transfer from target to substrate is normally stoichiometric, meaning that
the deposited film has the same composition as the target.110’114 This is a result of the
high initial rate of heating and highly nonthermal target erosion.110 Large bandgap
semiconductors and dielectric materials usually have large optical absorption at short
wavelengths. Therefore, large amounts of the laser energy are deposited in a small
volume close to the surface. As a result, the sublimation temperature of the material is
attained at depths greater than the thermal diffusion distance of the constituents. Target

37
components are not able to segregate, therefore they leave the target in proportion to their
bulk concentrations. This results in deposits with the same composition as the target.39
By varying the conditions under which thin film phosphors are deposited, their
optical properties can be improved. Y203:Eu is a good example of these effects. It has
been shown that by increasing the oxygen partial pressure during deposition, rougher
films are produced. This leads to an increase in brightness due to forward scattering by
anomalous diffraction.39-115 Microcrystallites of II-IV compounds such as CdTe and
CdS have been grown in a similar fashion.116 Temperature of the substrate can also
affect film properties. Models suggest that higher substrate temperature will lead to
rougher films.117 The type of substrate will also determine whether amorphous,
polycrystalline, or epitaxial single-crystal films are grown.110 In one experiment,
Y203:Eu was grown on bare (100) Si wafers and on diamond-coated Si wafers. The
diamond coating was prepared using a hot filament chemical vapor deposition. The
phosphor films were grown under identical conditions on the two substrates. There was a
substantial improvement in luminescent brightness from the diamond-coated substrate.
This was attributed to the higher roughness of the surface; see Figures 2-17 and
2-18.41-118
Sapphire substrates have also been used to improve the brightness of Y203:Eu
thin films. The increase in brightness is believed to be due to the low absorption and low
refractive index of red light in sapphire and to the improved growth of grains with
unidirectional orientation on the (0001) sapphire substrates.119
Finally, Y203:Eu films have been epitaxed onto LaA103 substrates. These are
good substrates because the lattice mismatch is only 0.8% with an orientation of

38
[110] Y203||[ 100]LaA103 and [-110]Y203||[010]LaA103. Z-contrast scanning
transmission electron microscopy (STEM) was used to demonstrate the absence of
precipitates of Eu in the deposited films.120 This is advantageous in a luminescent film
because the activators are thus spread out.
Besides its use to deposit phosphor thin films, PLD has also been used to coat
phosphor powders to improve luminescence and reduce degradation. Fitz-Gerald
deposited Y203:Eu onto powders of Si02 and A1203. The powders were agitated in a
fluidized bed setup so that the deposited coating would be uniformly distributed over the
powder surface.121'123 The photoluminescent spectra showed the 612 nm peak from the
5D0-7F2 transition in Y203:Eu; so the coatings were effectively deposited onto the core
particles.122 Transmission electron microscopy (TEM) was used to confirm that the
coatings were continuous.123 Next, coatings of TaSi and Ag were deposited on
Y202S:Eu, a highly efficient red phosphor, to help reduce degradation under electron
beam excitation. As discussed in the previous section, degradation curves clearly showed
the advantage of these two coatings for protecting the phosphor.14-99
Motivation for this Work
Coatings have been shown to be effective in slowing the degradation of
phosphors, especially at the low voltages and high currents of a FED environment.
Improvements in the efficiency of cathodoluminescence at low voltages have also been
demonstrated. Thus far, most of the work on coatings has focused on putting a coating
around the entire phosphor particle, using such techniques as sol-gel processing or spray
pyrolysis. These steps add considerably to the cost of making the powder and must be

39
monitored closely to ensure proper results. In this work, the commercially available
phosphors Y203:Eu and Y2SiC>5:Tb were sedimented onto Mo substrates and coated using
pulsed laser deposition. This is a much simpler process that can be applied to large area
screens using existing coating techniques.
To ensure that all results observed were due to the coating and not the processing
of the screen (such as deposition, CL geometry, etc.), half of the screen was masked
during deposition. This allowed side-by-side comparison of coated and uncoated
phosphor material. Coatings of MgO and A12C>3 were deposited at various thicknesses to
examine their effects on cathodoluminescence. These materials were chosen because
they both have large work functions compared to the host materials. This should lead to
enhanced efficiency due to repulsion of electrons from the particle-coating interface and
thus a lower surface recombination velocity.
Phosphor powders were chosen for this study instead of thin films because they
represent real world devices. Thin films provide an excellent surface on which to study
the basics of electron interactions, but they can introduce other problems. Lattice
mismatch between the luminescent film and substrate can lead to a drastic reduction in
cathodoluminescence. This can be alleviated in some instances by the careful choice of
substrate. (For example, LaAlC>3 for growth of Y2C>3:Eu.124) However, this is often a
difficult challenge. There is also the problem of a much lower intensity due to greater
internal reflection in the thin film.39 Because the initial intensity from the thin films is
low, detecting changes in the brightness becomes more difficult. Because phosphor
powders are used in FED devices and offer high initial brightness, they were chosen for
this study.

40
CPU
Figure 2-1
Transducer
Direct
Connection
to Brain
Senses
(300 Mb/sec)
(20 kb/sec)
(10 b/sec)
(4 b/sec)
(0.1 b/sec)
The different means of interfacing between a human and a computer. Also
shown are the speeds of each type of transfer.6

41
Table 1
The different types of flat panel displays and the advantages of each.
Technology
Type
Size
Power
Advantages
Barriers
AMEL
emissive
small
moderate
Rugged, high-
resolution, high¬
brightness
Pixel size limit, lack
of suitable phosphor
for full-color.
ÓEL
emissive
small
low
High luminous
efficiency, low
drive voltage
Lithography
incompatibilities,
instability, short
lifetime, limited
temperature range
Plasma
emissive
large
high
Mature
technology, high
brightness
Hign-power, slow
refresh rates, limited
military market
Laser
Projection
light
valve
large,
scaleable
moderate
High brightness,
display size is
scaleable
Still in early stages
of development, no
full-scale prototype
MEMS
Projection
light
valve
large,
scaleable
moderate
Some
commercially
available (TI-
DMD), scaleable
Development of new
technologies costly,
digital artifacts may
be a problem
Reflective
LCD
light
valve
small to
medium
very low
Very low power,
bistable
Limited temperature
range,
manufacturability,
durability
FED
emissive
small to
medium
low
Low power, high
brightness, wide
viewing angle
Manufacturability,
lifetime
Reproduced from reference 8.
The acronyms in the table stand for the following:
AMEL- Active Matrix Electroluminescent
OEL - Organic Electroluminescent
LCD - Liquid Crystal Display
FED - Field Emission Display

42
Phosphor
Screen
Figure 2-2 Typical cathode ray tube (CRT), consisting of an electron gun and deflection
plates to raster the electron beam across the phosphor screen.6

43
Figure 2-3 A typical FED setup, consisting of the cathode assembly, which houses the
field emitter tips, and the anode assembly, a glass plate on which the
phosphors are deposited. Spacers are used to separate the two halves of the
device.6

44
Figure 2-4 Schematic cross section of a typical FED. Not drawn to scale.
9

45
Energy
O ♦
Figure 2-5 Potential energy curves of an electron near a metal surface. The “Image
Potential” is with no external field. An externally applied field that is
electrically negative to the surroundings can be represented by the “Applied
Potential” curve. The resultant “Total Potential” will then be the
superposition of the two curves. Due to the lower energy barrier at the
surface, electrons in the metal have a finite probability of quantum
22
mechanically tunneling into the vacuum.

46
primary electron
beam
Figure 2-6 Electron penetration in a phosphor at various beam energies. Incident beam
energy is increasing from A to B to C. This also demonstrates how the
interaction volume moves closer to the surface as the beam energy is
decreased. When surface recombination is considered, the interaction
volume in A will clearly have a stronger influence from the surface than that
in C. As a result, surface effects become more pronounced at lower beam
voltages.35

47
47
Figure 2-7 Expected transitions in various rare-earth dopants used for FED phosphors,

48
Figure 2-8 The two different crystallographic configurations of Eu in a Y2O3 host lattice.
41
The Se symmetry is often referred to as C3¡ symmetry.

49
V E cl E
Figure 2-9 The dashed curves represent the charge-transfer (CT) state. In (a), the CT
state helps feed the emissive 5D levels because of its position relative to the
7F ground state. This is the situation found in Y203:Eu and Y2Si05:Tb. In
(b), the offset of the CT state causes the electrons to be returned to the ground
25
state, thus reducing the luminescence of the material.

50
Table 2
Composition, color and efficiency at low-voltage operation of selected
FED phosphors
Composition
Color
Efficiency (1m W ')
500 V
ZnO:Zn
Green
10.7
Gd202S:Tb
Green
7.9
Y,A!sOlS;lFb
Green
2.0
Y202S:Eu
Red
3.5
Y202:Eu
Red
2.2
YV04:Eu
Red
0.4
300 V, 131 p,A
CaS:Ce
Green
3.10
SrGa2S4:Eu
Green
3.00
La202S:Tb
Green
5.20
Gd202S:Tb
Green
3.52
Y202S:Eu
Red
2.20
Y20,:Eu
Red
1.57
Y2Si05:Ce
Blue
0.25
Y2SiOs:Tb
Green
1.05
LaOBr:Tb
Green
1.95
LaOCI:Tb
Green
0.36
LaOBr:Tb
Blue
0.54
LaOBr:Tm
Blue
0.17
Reproduced from reference 2.

51
Table 3
Classification of FED structures
Type
Anode
voltage
(kV)
Anode-cathode
separation
(mm)
Advantages
Disadvantages
High
-voltage
4-10
2-3
• CRT phosphors S
processes applicable
•High efficiency
•Focussing electrodes required
•Spacers with a high aspect ratio
•Breakdown
Medium
-voltage
1-4
0.2-0.8
•Simple structure
•Fair efficiency
•Spacers with a relatively high
aspect ratio
Low
-voltage
0.4-1
0.1-0.2
•Simple structure
•High reliability
•Low cost
•Limited efficiency
Reproduced from reference 11.

acceleration voltage [volts]
Figure 2-10 Efficiency versus acceleration voltage for Y203:Eu.38

53
\\ \ \ \ \
v Faceplate
- V
Electrons
Phosphor
Figure 2-11 FED with phosphor on the gate electrode to double the light output from the
device.6

54
-COLLECTION BIAS ( V )
Figure 2-12 Carbon Auger peak shifts after changes in the bias of the sample mounting
plate.52

55
MESH
PHOSPHOR
LAYER
\ I
BRASS BACK
PLATE
Figure 2-13 Setup for metal mesh placed on the phosphor surface for taking internal field
re 0
measurements under electron excitation.

56
Figure 2-14 Luminescence versus accelerating potential for ZnS-coated (0.127 pm,
0.254 pm, or 0.389 pm thick) ZnS:Cu. The similar slope of the lines
suggests that the coating did not alter the luminescent states of the
phosphor, but only decreased the incoming electron energy.96

57
Figure 2-15 Luminescence versus accelerating potential for ZnS-coated (0.0508 pm,
0.0847 pm, or 0.188 pm thick) ZnS:Cu. The similar slope of the lines
suggests that the coating did not alter the luminescent states of the
phosphor, but only decreased the incoming electron energy.96

CL Brightness (a.u)
58
Coulomb Load (C/cm2)
Figure 2-16 Cathodoluminescence degradation of Y202S:Eu with various coatings. The
Si02 and phosphate coatings were applied by wet chemistry techniques,
while the Ag and TaSi coatings were deposited by pulsed laser ablation in a
fluidized bed setup. “Original” refers to the uncoated powder.14

59
x
z
10.000 iWdiv
3.000 VM/div
UK
Eu:Yz03 Film
on Silicon
EurYzOi Film
on Diamond
X 10.000 im/diu
Z 3.000 ON/dlv
Figure 2-17 Atomic force microscopy (AIM) images of Y2C>3:Eu film grown on bare Si
substrates and on diamond-coated Si substrates. The images clearly show
41
the higher roughness of the films on the diamond-coated material.

60
300 400 500 600 700
DepositionTemperature (°C)
Figure 2-18 Photoluminescence from Y20.3:Eu films grown on bare Si and diamond-
coated Si substrates. The curves show the higher luminescence attained
with the diamond-coated substrate. This is attributed to the higher
roughness of these films.41

CHAPTER 3
EXPERIMENTAL PROCEDURES
Introduction
The effects of coatings on field emission display (FED) phosphors were studied in
this work. Coatings of AI2O3 and MgO were deposited by pulsed laser deposition (PLD)
onto screens of Y203:Eu and Y2Si05:Tb. These screens were prepared by sedimentation
from a phosphor slurry. During deposition of the coatings, a Si shadow mask was used to
provide coating on only half of the sample. This allowed for direct comparison of coated
and uncoated phosphor material.
Many different types of characterization techniques were used to measure the
samples. Cathodoluminescence measurements were taken under steady-state and pulsed
conditions to examine the response of the phosphors to an electron beam. Ellipsometry,
transmission electron microscopy (TEM), and Auger electron spectroscopy (AES) were
used to determine the thickness of the coatings. Finally, AES and a scanning electron
microscope (SEM) with a fiber optic attachment used for measuring CL maps were used
to examine coating uniformity.
Sedimentation of Phosphor Screens
The phosphor screens of Y2C>3:Eu and Y2SiÜ5:Tb were prepared by sedimentation
of the powder from a slurry. The apparatus consists of plastic beakers with a hole drilled
61

62
in the bottom. Attached through this hole is a connector that allows for a tube to be
inserted onto the bottom of the beaker. A vise-style clamp on this tube controls the flow
of the liquid out of the container. Samples are held down on the bottom of the vessel by
spring clips that are attached along the bottom edge.
The first step in the sedimentation process is preparing the slurry mixture. The
phosphor is weighed and added to a magnesium nitrate hexahydrate and isopropyl
alcohol mixture. This is then placed in a sonicator for 30 minutes to ensure complete
dissociation of the phosphor throughout the slurry. Natural heating also takes place
during the agitation, which helps hold the powder in suspension. At this time, the
substrates are cleaned by rubbing them with a methanol-soaked swab and then immersing
them in methanol and placing them in the sonicator for about five minutes. After
cleaning, the substrates are attached to the bottom of the beaker. The hose is then
attached and clamped off. With all of this in place, the slurry is poured into the beakers.
It takes many hours for the phosphor to settle out of solution and coat the samples.
Usually, the beakers are covered and left overnight. In the morning, the liquid is slowly
drained from the beaker by loosening the clamp on the hose. The samples are then air
dried in the container for several hours.
Both of the phosphors used for this work, Y2C>3:Eu and Y2SiOs:Tb, were standard
Nichia powders. These were chosen because they were readily available and would be
typical for use in a FED. Molybdenum was chosen as a substrate material for all of the
samples because of its good electrical conduction and high melting temperature.
(Originally, gold was used as a substrate material because it is inert to most materials and

63
has a very high electrical conductivity. Unfortunately, the gold would not endure the
750° C temperature needed during the pulsed laser deposition of the coating materials.)
Pulsed Laser Deposition (PLD) of Coatings
Since PLD has been shown to be particularly good for growing oxides due to the
fact that the material transfer from target to substrate is generally stoichiometric110- *14, it
was chosen as the means of depositing the AI2O3 and MgO coatings for this work. The
apparatus used consists of a stainless steel chamber pumped by a Pfeiffer Balzers TPU
450 H corrosive turbomolecular pump with a TCP305 controller and backed by a MD41
diaphragm pump. A Leybold Trivac mechanical pump model # D4A was used for
roughing the chamber. To control background pressure, a gate valve was used between
the pump and chamber to adjust the pumping speed. Precision leak valves provide
further control of pressure through backfilling of the chamber with a variety of gasses.
Ablation energy comes from a KrF (k=248 nm) 1 Watt Lambda Physik Lasertechnik
LPX300 excimer laser. Pulse width was set at 10 nanoseconds and laser energy kept at
about 350 mJ for all depositions. Pulse frequency was variable between 1 and 100 Hz,
but 10 Hz was chosen for all experiments. Laser light enters through a viewport on the
side of the chamber. Control of the laser dimensions and position is accomplished with
collimating and focusing lenses located on an optics bench positioned between the laser
and vacuum chamber.
Directly across from the laser entry viewport inside the chamber is the target
holder. The holder is positioned so that the laser strikes at 45° from the surface normal,
which in turn generates a plume perpendicular to the target surface.110-111 The target

64
holder is connected to a rotary motion feedthrough to allow target rotation during
deposition, thus allowing better utilization of the targets. Target rotation also helps
minimize deposition of large particulates from the target onto the growing film.125
Substrates are mounted on a sample holder located directly across from the target.
Target-to-substrate distance was set such that the samples were located near the end of
the plasma plume, which was around 3 centimeters in the chamber used for this work.
Plasma expansion models suggest that this is the optimal position for the substrate.114
The sample was not rotated during deposition. Heating of the samples was accomplished
using a quartz lamp located within the sample holder. This allowed for a range of 150°C
to 1000°C, although 750°C was used throughout these experiments. A stainless steel
heater plate was used to transfer heat to the substrates, which were mounted to this plate.
A thermocouple attached to the plate provided accurate control of the deposition
temperature throughout the run.
The AI2O3 and MgO targets were made from their powders. First, approximately
five grams of the powder was placed in a hardened stainless steel die. The die used was
designed to produce targets an inch and a half in diameter. A hydraulic press was used to
compress the powder in the die to around 120 psi for 4 minutes. The green powder
compacts of these two materials were found to hold together very well. Both materials
came out of the die without excessive force, so no lubrication of the die was necessary.
Had such lubrication been used, it could have been a source of contamination for the
target. For similar reasons, no flux was used to help hold the compacts together, although
a flux is occasionally added to increase the sintering of the powder in the compact.
During firing of the compacts, the flux is supposed to evaporate, but if it does not, it can

65
also be a source of contamination. The compacts were then placed in a tube furnace at
1200°C for 12 hours. This anneal is used to greatly increase the density of the target.
Finished targets were then silver pasted onto holders used to attach them to the rotary arm
of the target holder.
Phosphor screens were attached to the stainless steel heating plate of the sample
holder by silver paint. A piece of Si was placed over half of the screen to act as a shadow
mask. A spring clip attached to the side of the sample holder was used to hold the mask
in place. The target and substrate were then inserted into the vacuum chamber. Once
sealed off, the chamber was evacuated to a base pressure of around 1*10'5 Torr.
Temperature of the substrate was then raised to 750°C. At this point, the gate valve was
partially closed and the precision leak valve opened to adjust the background pressure of
oxygen in the system. Oxygen was fed to the leak valve from ultra-high purity
compressed gas cylinders. A MKS model 247C mass flow controller set at 80 seem was
used between the gas cylinder and leak valve. Background pressure of 50 mTorr was
used for the majority of the experiments, with 200 mTorr used for the remainder (as
noted later in the experimental results section).
Contrary to most other deposition techniques, such as electron beam, thermal
evaporation, and sputtering, PLD does not show a strong decrease in deposition rate with
increasing pressure above a few mTorr. This is due to the inherent quasi-free jet
expansion of the plume during pulsing. The particles in the plume impart momentum to
the background gas molecules. Therefore, the background gas and plume travel together
toward the substrate. This greatly reduces the scattering of deposition species due to

66
collisions with gas molecules. The result is a very long mean-free path for the particles,
which allows deposition at higher pressures.111
Control of the coating thickness was achieved by closely monitoring the
deposition time. Materials ablate at different rates, so the coating time had to be adjusted
for each of the material systems used. For the system used for this work, it was found
that the deposition rate at 50 mTorr was around 65 Á/min for AI2O3 and 18.2 Á/min for
MgO.
Characterization Techniques
Many different characterization techniques were used to determine the coatings’
thickness and effect on the luminescence of the phosphor. The techniques include
steady-state and pulsed cathodoluminescence (CL), ellipsometry, transmission electron
microscopy (TEM), Auger electron spectroscopy (AES), and scanning CL in a scanning
electron microscope (SEM).
Figure 3-1 shows the energy distribution of electrons emitted from a sample
surface under electron bombardment. Electrons in various energy regions on this curve
are measured in some of the aforementioned characterization techniques. The secondary
electrons are used for secondary electron (SE) imaging in the SEM. The Auger electrons
are measured in AES. The elastically backscattered electrons are used for diffraction
analysis in the TEM.

67
Steady-State Cathodoluminescence
Cathodoluminescence (CL) is the process of generating light by electron beam
excitation of a phosphor material. Incident electrons impart some of their energy to the
phosphor material through electron excitation. These excited electrons can then
recombine with an available hole (the absence of an electron) to produce visible
light.25-35
Cathodoluminescence measurements were carried out in a stainless steel vacuum
chamber pumped with a CTI-Cryogenics cryopump and a Varían ion pump. Roughing
was accomplished using a rotary-vane mechanical pump. Base pressure for this
combination of pumps was between 5* 109 and 1*10'8 Torr with no bake-out. Although
baking the chamber would have allowed for a lower pressure by removing some of the
desorbed gas on the walls, it was not an option due to a fiber optic feedthrough.
However, these pressures are more than adequate for taking CL measurements. Electron
excitation was from a Kimball Physics EFG-7F electron gun with a EGPS-7H power
supply and RGDU-3C raster generation unit. The electron gun was capable of energies
up to 5 keV and currents from 0.01 to 500 pA. Focus was set to maintain a spot size of
approximately 2 mm.
Phosphor luminescence was collected using a fiber optic connected to an Ocean
Optics S2000 spectrometer. This spectrometer uses a diffraction grating and a charge
couple device (CCD) array detector, which allows luminescent spectra to be collected
over the entire visible wavelength range. To further improve wavelength resolution, a
homemade slit was fashioned on the end of the fiber before it entered the spectrometer.
This greatly reduced the broadening of the luminescent peaks. This is especially helpful

68
in materials with closely spaced peaks that can become indistinguishable due to peak
overlap.
To improve signal-to-noise in the collected spectra, the raster unit was utilized to
redirect the electron beam off-axis. Due to the electron gun design, the filament used to
generate electrons within the gun is visible through the exit aperture. This causes a bright
white spot of light on the sample directly in front of the gun. Using a mechanical
manipulation system, this filament light can be redirected away from the area of interest
on the sample. This greatly reduces the background signal detected by the spectrometer.
By setting the appropriate bias on the raster unit, voltage was applied to deflection plates
on the electron gun to redirect the electron beam to the center of interest on the sample.
Accurate current measurement requires the collection of secondary as well as
primary electrons. For this reason, a +100 V bias from a Fluke 343A DC Voltage
Calibrator power supply was applied to the electrically isolated sample carousel. A
Keithley 619 Electrometer/Multimeter was connected in series between the power supply
and the carousel to measure the current. Background subtraction was used to account for
any stray currents present in the system. A switch was installed between the meter and
carousel to allow the carousel to be shorted to ground. This is necessary during
measurement of the luminescence because the sample bias will impart extra energy to the
incoming electrons.
The sample carousel held samples perpendicular to the incident electron beam.
Samples were inserted such that the uncoated half of the powder screen was above the
coated half, instead of beside it. In this arrangement, the electron beam could be moved
from an uncoated to a coated area by adjusting the vertical position of the carousel.

69
Using only vertical repositioning assured that the electron beam-to-sample and sample-
to-fiber alignment would not be changed. It is critical to keep these alignments constant
because changing them will result in a change in the measured luminescence.
Pulsed Cathodoluminescence
Pulsed cathodoluminescence measurements are used to study the decay of the
luminescence from a phosphor under a pulsed electron beam. The same chamber was
utilized as in the steady-state CL measurements above. The Kimball Physics electron
gun has a fast pulsing option. A grid located inside the gun is used to blank the beam.
By applying a large negative bias to the grid, a potential barrier for the electrons is
established. The magnitude of this potential barrier, adjusted through the grid voltage,
determines how many electrons can make it past the grid. If an electron has enough
energy to surmount this barrier, then it will proceed through the gun as normal. Pulsing
is accomplished by sending a positive voltage pulse along the grid supply line to reduce
the grid retarding voltage. By selecting the appropriate grid bias, the positive pulse can
be used to turn the electron beam on and off. Thus, the characteristics of the positive
pulse determine the electron beam characteristics. Kimball Physics supplies a box that
can be installed in the grid voltage line for such an operation. This box allows the pulse
generator to be connected to the grid line and prevents the grid voltage from being sent
back through the line to the pulse generator. Due to the design of the electron gun, the
grid voltage (typically 100 to 250 V) is added to the accelerating voltage (typically 0.5 to
4 keV) before being transmitted down the grid line. During operation, it was noticed that

70
the pulsing box would not give consistent pulsing. To correct this, the circuit was
redesigned and a new box built to provide a stable, low noise operation.
A Hewlett Packard (HP) 8112A Pulse Generator was used to provide the positive
voltage pulses to turn the electron gun on. Pulse width was set to 10 ps with a delay of
65 ns and a period of 100 ms. Amplitude was adjusted to produce a 16 volt turn-on
pulse.
Luminescence was collected using a fiber optic mounted inside the vacuum
system. The other end of the fiber was connected to a Hamamatsu Photomultiplier Tube
(PMT). A 1150 V power supply was used to power the PMT. A box was used to house
the PMT with a fitting for the fiber on the outside. During operation, a thick black piece
of fabric was wrapped around the box to reduce the amount of stray light. A holder was
designed for the inside of the box to allow filters to be placed between the fiber and PMT.
Filters were used to select specific wavelength regions. This gave the ability to study the
decay of single peaks (different decay states) of the phosphors. Filters used included a
535 nm bandpass and Coming glass 2-73, 3-70, and 5-58 highpass filters. Figures 3-2 to
3-4 show the transmission of these Coming filters. The output of the PMT was
connected through a “T” junction to an oscilloscope and a HP 3478A Multimeter. The
multimeter was used to look at the voltage from the PMT due to background light in the
chamber, most of which was due to filament light from the electron gun. During actual
measurement, the multimeter was disconnected to reduce the amount of noise in the
signal. The oscilloscope used was a 500 MHz 2 GS/s Hewlett Packard Infinium
Oscilloscope Model 54825A. A built-in disk drive was used to save files and transfer
them to another computer.

71
Sweep rate on the oscilloscope was chosen to ensure that the entire decay pulse
was measured. This meant making sure that the after-pulse luminescence returned to
background level. Once the sweep rate was set, the sample averaging was adjusted to
give 25044 data points per scan. To further improve signal-to-noise, 3000 scans were
collected for every set of conditions. The oscilloscope automatically averaged these as
they were collected. Therefore, every data file consisted of a 25044x2 matrix of time and
intensity. Comparison of multiple files of this size is very computer processor intensive,
so further averaging was done. Files were loaded into Mathcad, where a sliding time
average was applied. The program takes a preset number of data points and finds an
average, which it assigns to that interval. It then moves on to the next interval and
repeats the process. In this fashion, the entire data set is averaged to produce a more
manageable size. See Appendix A for a more detailed description of the program.
Typically, the data interval was set for 25 points and the last 44 points discarded, which
produced a 1000 point file.
Ellipsometry
Ellipsometry, also known as polarimetry and polarization spectroscopy, can be
used to obtain the thickness and optical constants of thin films.126 Some of the
advantages of this technique over others is that it is simple to operate, nondestructive, and
requires no vacuum system. Measurements can be taken in a vacuum system, in air, or in
a liquid.36 Figure 3-5 shows the typical experimental arrangement. Collimated
monochromatic light is passed through a polarizer (Glan-Thomson calcite prism) and a
quarter-wave compensator (mica plate with 45° retardation) to give elliptically polarized

72
light. This is reflected off the sample surface into a second polarizer (Glan-Thomson
calcite prism) that acts as an analyzer. The polarizer and analyzer are then rotated to find
the maximum extinction of the reflected light.127
It is important to know the angle of incidence of the incoming light and its
wavelength and to keep these constant throughout the measurement. Once the initial set
of polarizer and analyzer settings is found, the two are rotated to find a second set of
extinction conditions. The polarizer is adjusted to 90° plus the original polarizer angle.
The analyzer is adjusted to 180° minus the original analyzer angle. Both are then rotated
to find maximum light extinction. In order to ensure accurate measurements, the new
angles should not differ from the calculated positions by more than four degrees.128 With
these data, the values of psi (XF) and delta (A) can be determined from the relationships:
,,, 180° -fo -A)
2
A = 360° -(/> +P2)
where A2, A], P2, and Pi are the second and first set of analyzer and polarizer angles,
respectively.127’128
Light is an electromagnetic wave and therefore must obey Maxwell’s equations.
As a consequence, there are certain relations that must be obeyed when light encounters a
boundary between two media. First, the angle of incidence must equal the angle of
reflectance. Second, in the case of one material on top of another, Snell’s Law must
nx sin 0, = n0 sin 0O
hold:

73
where njand no are the complex indices of refraction in material 1 and material 0 and 0i
and 02 are the angles from the surface normal in material 1 and material 0.36 Third, the
Fresnel reflection coefficients are given by:
_ n0 cos G0 - nt cos 0,
n0 cos0o + nx cos0,
n, cos0o - n0 cos0,
p n0 cos0o+n, cos0[
where s refers to the light vector component perpendicular to the plane of incidence and p
the parallel component.127 The plane of incidence is defined by the incident and reflected
beams and the surface normal.36 It is these last two relations that are important for the
ellipsometry measurement. They are related to the 4' and A values obtained earlier by the
following equation:
— = (tan 4*)e'A .
r.
These equations can also be related to the reflection coefficients through the following:
R. =
r + r e 2<^
'OU '\sub,sc
1 + r r
1 T 'Ol.s'lsub.s^
r + r e 2i^
^ _ '01,p ^ 'lsub,pc
p ~ l + r r e~2‘p
1 T 'Ol.p'\sub,p^
where the subscripts 0, 1, and sub represent the measurement medium, film, and
substrate, respectively. The film thickness, dj, can be found from the equation for the
phase angle, p:
P = 271
^ \
n, cos0,.
\

74
It is this equation that relates the index of refraction to the thickness, as well as to the
phase changes due to reflection at the interface.36’126 There are many computer
programs available that use these relations to determine film thickness or index of
refraction from the T and A values found from the measured analyzer and polarizer
settings. Due to time and space constraints, a more detailed discussion of the equations
will not be attempted here. However, it is possible to analyze multiple films using this
technique, although such analysis is very complex.127’129
For the current work, two different types of ellipsometry equipment were
utilized. The first was a Gaertner Scientific Corp. ellipsometer with a HeNe laser
(632.8 nm) and manual polarizer and analyzer. For this apparatus, the angle was set at
70°, and the sample positioned to give reflection into the detector. Once this was set,
polarizer and analyzer readings could be collected. The second piece of ellipsometry
equipment used was a fully automated rotating analyzer ellipsometer. In this
arrangement, a fixed polarizer is used and the analyzer is rotated to determine extinction
values. Advantages of the latter include multiple wavelength capabilities and more
accurate measurement of the analyzer and polarizer relative positions. The computer
program also allows for multiple angles to be used to further improve analysis. Built-in
data libraries for most elements and compounds make data analysis much simpler and
more precise.
Transmission Electron Microscopy (TEM)
TEM is a powerful technique for obtaining information about the atomic structure
of a material. The main requirement is that the sample is thin enough to transmit

75
electrons.36’126 Figure 3-6 shows a typical TEM setup. For the current work, TEM was
used to determine coating thickness from the Si shadow masks used during deposition.
One of the most difficult aspects of using TEM is sample preparation. Since coating
thickness was desired in the present study, cross-sectioned samples were prepared. This
is done by first thinly slicing the sample using a diamond saw. Two of these thin slices
are then laid flat with the coated sides touching. Wax is used to hold the sample together.
In order to obtain a region thin enough for electrons to propagate through, the sample is
placed in an ion-mill. This machine uses energetic ions to sputter thin the sample. Once
a hole appears, the sample is ready for analysis. By looking at regions on the periphery
of the hole, films sufficiently thin for analysis yield information about the sample.
In the microscopy, electrons are scattered as they pass through the samples. It is
the nature of the scattering that determines the type of information that is obtained. There
are two types of scattering events: elastic and inelastic scattering. Elastic scattering is a
result of Coulombic interactions of the incoming electron and the potential field of the
ion cores, and it results in no loss in energy to the electron. This type of process is
known as Rutherford scattering and gives rise to diffraction patterns. The magnitude of
the interaction scales with the charge on the nucleus, and thus with the atomic number.
Inelastic scattering is the interaction between the primary electron beam and electrons in
quantum states around the nuclei or in the solid. Energy is transferred during the
scattering, giving rise to spatial variation in the intensity of the transmitted beam
dependent upon defects and heterogeneities.126 By examining the intensity of the
transmitted electrons, different layers and their interfaces are visible. From this, the
thickness of the coating layer can be determined.

76
Auger Electron Spectroscopy (AES)
AES is a very useful tool for looking at surface compositions. By utilizing ion
beam sputtering, composition versus depth is attainable. Figure 3-7 shows a typical AES
setup. Ultra-high vacuum (UHV) conditions are needed to reduce surface contamination
during analysis. Sample characterization takes place as follows. Energetic primary
electrons (~5 keV) are focused by an electron gun onto the sample surface. These
primary electrons lose energy as they traverse the sample. Similar to the process
described for the TEM, some of this lost energy goes into exciting ground state electrons
into empty quantum states of the atoms or into continuum energy states. The atoms have
two options for recombining an electron with an available hole and returning to the
ground state. These are to produce an (1) X-ray or (2) Auger electron; see Figure 3-8.
Both of these processes happen simultaneously, but this analysis is concerned only with
the Auger electrons. In addition, for de-excitation energies less than approximately
2000 eV, Auger electron emission dominates over X-ray emission.
Auger electrons get their energy from the atom when it relaxes back to its ground
state. Therefore, they are characteristic of the energy levels of that specific atom. Once
Auger electrons are ejected from the atom, they must make it through the material and be
ejected without energy loss. Figure 3-9 shows the mean free path of various atomic
species. Immediately obvious from this graph is the fact that none of the elements listed
have an Auger electron escape depth over 30 Á. This is the reason that AES
characterizes the near surface region. With the use of computers, data can now be
collected directly in N(E) versus E mode and then manipulated to obtain the differential

77
dN(E)/dE versus E. Historically, the data was collected in differential form due to the
use of an ac modulation on the signal and detection with lock-in amplifiers, as shown in
Figure 3-7, but with modem detectors and computers, this is no longer necessary. Saving
the data in its non-differentiated form presents advantages in noise and processing.36
However, the differential form is best for viewing the data since it highlights the Auger
peaks.22’ 36>126
For the current work, a Perkin-Elmer PHI660 Scanning Auger Multiprobe system
was utilized. Electron beam energy was kept at 5.0 keV with a 25 pA current. Pressure
in the system was around 3*10'8 Torr during sputter profiling. A 3x3 raster was used on
the 3 keV Ar+ ion gun during sputter analysis. Initial surface scans were collected from
coated and uncoated parts of the sample to find peaks for analysis during depth profiling.
The surface scans of the coated materials can give information about the uniformity of
the coating within a thickness range equal to small multiples of the escape depth of the
Auger electrons. Depth profiling was used to determine thickness of the coating on
powders as compared to complementary coatings on Si masks.
Scanning Cathodoluminescence (CL) in a Scanning Electron Microscope (SEM)
Uniformity is also thought to be important to the success of the coatings. A SEM
with a fiber optic attached to a spectrometer was used for producing CL maps of the
sample surface. The SEM is a useful tool for magnifying the sample surface (about 10X-
300,000X).36 To produce an image, a focused electron beam is rastered across the
sample surface. When the electrons enter the sample, they lose energy through inelastic
collisions, as discussed above. This inelastic energy loss is transferred to the host lattice

78
and gives rise to a multitude of different electron energies that leave the surface.
Figure 3-1 shows a plot of these different electrons. In addition, other signals can be
produced, such as X-rays, light, and heat. Due to the inherent roughness of a sedimented
powder screen, there are intensity variations in the CL map of the surface. In order to
provide a complementary image of the surface being studied, a secondary electron (SE)
image was collected. Uniformity of the coating could then be checked by comparison of
the two images. The SE image provided a picture of the different phosphor surfaces.
The corresponding surfaces on the CL image could then be located and studied.

79
ELECTRON
YIELD
SECONDARY ELASTIC BACK
ELECTRONS SCATTERED
ELECTRONS-^
AUGER /
EL^ONS^ J
2000
ELECTRON ENERGY (eV)
â– o
Figure 3-1 Energy distribution of electrons emitted from a sample surface under electron
bombardment. The secondary electrons are used in secondary electron (SE)
imaging in a scanning electron microscope (SEM). The Auger electrons are
used in Auger electron spectroscopy (AES). The elastic energy is used for
126
diffraction analysis in the transmission electron microscope (TEM).

80
Figure 3-2 Transmission of light through a Coming glass 2-73 filter. Taken from data
sheet supplied with filter.

81
Figure 3-3 Transmission of light through a Coming glass 5-58 filter. Taken from data
sheet supplied with filter.

82
Figure 3-4 Transmission of light through a Coming glass 3-70 filter. Taken from data
sheet supplied with filter.

83
SUBSTRATE
FILTER
LIGHT SOURCE
POLARIZER
ANALYZER
TELESCOPE
COLLIMATOR
DETECTOR
(EYE OR MICROPHOTOMETER)
Figure 3-5 Schematic of a typical ellipsometer. Monochromatic light is passed through a
polarizer and a quarter-wave compensator to give elliptically polarized light.
This is reflected off the sample into the analyzer (a second polarizer). The
polarizer and analyzer are rotated to determine maximum extinction of the
reflected light.126

84
Column vacuum block
35 mm Roll film camera
Focussing screen
Plate camera
16 cm Main screen
Electron gun
Anode
Gun alignment coils
Gun airlock
1st Condenser lens
2nd Condenser lens
Beam tilt coils
Condenser 2 aperture —
Objective lens
Specimen block
Diffraction aperture
Diffraction lens Kt-
iril
Intermediate lens jul- «•«
1 st Projector lens
2nd Projector lens
Figure 3-6 A typical transmission electron microscope. The top portion of the
instrument above the specimen block is used to generate a focused high-
energy electron beam. The apertures and lenses below the specimen block
are used to select specific regions of the diffracted electron beam for imaging.
Most instruments allow for collection of generated images on a photographic
film.36

85
Figure 3-7 A typical Auger electron spectroscopy (AES) setup, based on a cylindrical
mirror electron energy analyzer (CMA).22

86
Characteristic X-ray Auger Electron
Fluorescence Emission
hv - E - E E - E -E -E
K L2 KLL K L2 L9
*L1
n(E)
Figure 3-8 Schematic representation of the processes of X-ray fluorescence and Auger
electron production. A KLL transition is shown as an example. Initial
excitation comes from ejection of a core shell electron by an incident
electron. Both processes occur in a material under electron beam
bombardment.

87
Figure 3-9 Electron escape depth as a function of initial kinetic energy. This is a
measure of the average distance the electron will travel before undergoing an
inelastic collision with the host lattice. It is also commonly referred to as the
22
electron’s inelastic mean free path.

CHAPTER 4
EXPERIMENTAL RESULTS
Introduction
Results are separated into four main categories. These include the steady-state
cathodoluminescence (CL) spectra, which show the luminescence over the visible
wavelength range and at multiple beam energies. Next, the effects of beam energy on
steady-state CL intensities at specific wavelengths are reported. Third, pulsed CL decay
curves at various beam energies and wavelengths are reported. Fourth, the coating
thickness and uniformity data are presented. A list of all samples is given in Table 4.
Cathodoluminescent Spectra
Figure 4-1 shows the cathodoluminescent spectra of Y2Ü3:Eu as reported in the
Phosphor Technology Center of Excellence: Low Voltage Phosphor Data Sheets.38 This
figure shows the relative radiant intensity (i.e., cathodoluminescence intensity) as a
function of wavelength at 1 keV and 1 pA/cm2. The intensity has been normalized to the
value of the 611 nm peak. Figures 4-2 through 4-5 depict the cathodoluminescent spectra
for Y203:Eu with coatings of MgO deposited for 4 or 8 minutes, and AI2O3 deposited for
1.2 or 2.4 minutes, respectively. These curves show the intensity as a function of
wavelength from 520 to 720 nm. Spectra are displayed for uncoated and coated samples
88

89
at beam energies of 0.8 keV, 1.4 keV, 2.5 keV, and 4 keV. Current density was kept
constant at 0.16 pA/cm2 for each measurement. From the data, it can be seen that the
coating reduces the cathodoluminescent intensity over the entire wavelength range.
However, no measurable change occurred in the wavelength dependence of the
luminescent peaks, as will be discussed later in this section. Another feature of these
graphs is that coatings of the same material deposited for longer times (which were
therefore thicker) led to a larger attenuation of the intensity at all beam energies from 0.8
to 4 keV. This can be seen by comparing the intensity ratio of coated to uncoated
phosphors at any given beam energy for the same coating material and time.
Figure 4-6 shows a typical cathodoluminescent spectra for Y2SiOs:Tb from 400 to
720 nm measured at 1 keV, as reported in the literature.49 The intensity has been
normalized to the main emission peak. Figures 4-7 through 4-10 are spectra of uncoated
and coated Y2Si05:Tb. Coatings of MgO, deposited for 4 and 8 minutes, and AI2O3,
deposited for 2.4 and 5 minutes, are presented. No spectra are reported for the 1.2 minute
AI2O3 coating on Y2SiOs:Tb because the data files were corrupted. The intensity as a
function of wavelength from 460 nm to 680 nm at beam energies of 0.8 keV, 1.4 keV,
2.5 keV, and 4.0 keV are shown. Similar to the Y2C>3:Eu samples, the MgO and AI2O3
coatings caused a reduction in the cathodoluminescent intensity over the entire
wavelength range. Also, coatings of the same material deposited for longer times (which
were therefore thicker) attenuated the luminescence more strongly. Wavelength
dependence of the luminescent peaks was unaffected by the coatings.
No calibrated light source was available inside the chamber, so luminescence in
Figures 4-2 to 4-5 and 4-7 to 4-10 was plotted in arbitrary units. During measurement,

90
the integration time of the spectrometer was adjusted to provide a main peak intensity
signal that was between 80 and 90 percent of the maximum allowable intensity. This
helps increase the signal-to-noise ratio. To further improve this ratio, 64 scans were
collected and averaged. Sixty-four background spectra scans were also collected and
averaged before each run and were subtracted from the luminescent spectra in real time.
Averaging of the background spectra is needed in order to keep from introducing noise
during subtraction.
All spectra in a given figure have been corrected for any differences in integration
time and can therefore be directly compared against one another. However, care must be
taken when comparing curves from different figures because not all figures have been
normalized to the same overall integration time. This was necessitated by small changes
in sample positioning and light collection between experiments. In order to compare
curves from different figures, the ratio of the coated to uncoated spectra needs to be used.
Since all powders of each type of phosphor are the same, the luminescence of the
uncoated side should be the same for all samples of that phosphor. This is one of the
main reasons for masking half of the phosphor screen during deposition. Therefore, by
comparing the ratio of intensities at different beam energies, the effects of the coating can
be obtained. Using this method, it can be concluded from the figures that coatings of a
given material deposited for longer times show larger attenuation of the
cathodoluminescence intensity, as stated in the previous paragraph.
In order to better understand the beam energy and coating effects on the CL peak
positions and relative heights, spectra for beam energies from 0.8 to 4 keV were
normalized to the intensity at the main emission peak. These are shown in Figures 4-11

91
to 4-18. It can be seen that the coatings had no effect on the peak positions and very little
effect on relative peak intensities. Various luminescent peaks arise due to transitions
from different energy levels (states) within the phosphor activator and host, as discussed
in Chapter 2. If the coatings had altered these states, then the energy released during
relaxation (recombination of excited electrons with holes to return to the ground state)
would be different than that released by an uncoated phosphor. This change in energy
would have caused a shift in wavelength of the emitted light. Since no such shift was
observed, it can be concluded that the coatings did not alter the electronic configuration
of the activators.
In addition to examining peak positions on the normalized graphs, peak height
must be considered. If the peak ratios remain the same for all energies, then looking at a
single peak is sufficient to see the beam energy effects for the entire spectrum. However,
if the ratios change, as in the Y203:Eu samples, then each individual set of peaks must be
analyzed because the amount of attenuation will depend upon wavelength. This will be
important later when the decay of different luminescent states is measured.
The peak height ratio of the 5Di / 5D0 transitions (591 nm / 611 nm) in the
Y203:Eu samples changed not only with the coatings present, but also for the uncoated
phosphor samples. This suggests that the changes are due to the phosphor itself and are
not caused by the coating. The most likely explanation for the change in peak height
ratios with beam energy is a change in Eu concentration near the surface of the phosphor.
Tseng et al. have shown that the 5Ü! / 5D0 intensity ratio decreases with increasing Eu
concentration.130 As the beam energy is reduced, the interaction volume moves closer to
the surface. Since the 5Di / 5D0 intensity ratio decreases with decreasing energy in

92
Figures 4-11 to 4-14, the Eu concentration is higher near the surface than in the bulk of
the phosphor. This is consistent with studies on commercially prepared Y2C>2S:Eu35 and
pulsed laser deposited thin films of Y203:Eu39.
Beam Energy Effects on Luminescence
From the luminescent spectra shown in Figures 4-2 to 4-5 and 4-7 to 4-10, it can
be determined that coatings attenuate the luminescence from phosphors. By plotting the
intensity of luminescence versus beam energy at a specific wavelength for the coated and
uncoated phosphor, the decrease in luminescence is more easily seen. By fitting curves
to this data, the luminescence at any given beam energy can be predicted. Luminescence
intensity values were taken from the spectra in the previous section. To account for any
change in peak ratios, graphs were made for both the main peak and the next largest
distinctly separate peak. For Y203:Eu samples, these were the 611 nm and 591 nm peaks.
For the Y2Si05:Tb samples, these were the 547 nm and 487 nm peaks. A large difference
is noticeable in the Y203:Eu curves because of a change in the ratio of main to side peak
height; see spectra in Figures 4-11 to 4-14 for clarification. Those for Y2SiOs:Tb are
nearly identical because, within the range of experimental error, the normalized intensity
does not change with beam energy over the wavelengths of interest.
Curves for Y203:Eu samples are shown in Figures 4-19 through 4-26. Plotted is
the intensity, measured in arbitrary units, on a logarithmic scale, versus the electron beam
energy from 0.8 to 4 keV, also on a logarithmic scale, for both coated and uncoated
samples. Similar curves for the Y2SiOs:Tb samples can be seen in Figures 4-27 through
4-35. As reported above, no spectra are available for the 1.2 minute A1203 coating on

93
Y2SiC>5:Tb. However, the intensity of the main 547 nm peak was recorded during
measurement of these samples, and these values were used to plot Figure 4-31. This was
the only peak recorded during measurement, and therefore there is no corresponding
487 nm luminescence data for this sample. The following expression shows the second
order polynomial that was found to give the best fit to the data:
iogio M = a0 + a, log10 (x)+ a2 [logI0 (x)]2.
Based on these data, two conclusions can be drawn. First, the relative attenuation of the
luminescence by the coating decreases at higher energy, i.e., the coated and uncoated data
converge at higher beam energy. This presumably is due to the larger interaction volume
of the incident electrons at higher beam energy. As a result, the coating consumes a
smaller percentage of the electron interaction volume at higher beam energy. Second,
coatings deposited for longer times (which are therefore thicker) attenuate the
luminescence more strongly. These effects are explained further in Chapter 5.
Pulsed Cathodoluminescence
Pulsed cathodoluminescence measurements can be used to do time-resolved
studies of a phosphor’s luminescence. Through the use of filters, the decay of individual
sets of luminescent states can be observed. For Y2C>3:Eu, the two peaks of interest lie at
535 nm and 611 nm; see Figures 4-2 through 4-5. The 535 nm line results from a
transition from the 5Di state to the 7Fj ground state of Eu3+ in Y203. The 611 nm
emission is the result of a 5D0 state to 7F2 ground state transition. Figure 2-7 shows these
energy levels with the 611 nm transition depicted. These peaks were chosen because
they represent luminescence from the two lowest lying excited states of Eu3+. A 535 nm

94
bandpass and Coming glass 2-73 filter were used to select these peaks respectively; see
Figure 3-2 for the Coming filter transmission versus wavelength.
For similar reasons, the two peaks of interest in Y2SiOs:Tb were 420 nm and
547 nm. The 420 nm emission arises from the 5Ü3 to 7F5 transition of Tb3+ in Y2SÍO5,
while the 547 nm peak comes from the 5D4 to 7F5 state. Figure 2-7 shows these energy
states and the 547 nm transition. Coming glass 5-58 and 3-70 filters were used
respectively to look at these regions; see Figures 3-3 and 3-4 for transmission curves of
these filters.
Even though the set of peaks around 591 nm in Y2C>3:Eu showed slightly different
attenuation than the main 611 nm peak, they were not chosen for study under pulsed
conditions because these sets of peaks arise from the same initial state as the 611 nm
peak. The only difference between the transitions is in the ground state. To examine the
effect of different energy state transitions, a transition with the same ground state and a
different initial state was studied. Thus, the 535 nm peak of Y2C>3:Eu was the ideal
choice. The fact that the ratio of peak height of the 611 nm peak to the 535 nm peak did
not change was also beneficial in that only one attenuation curve was required for
comparison. Any difference between the curves from the two filters should be a direct
result of difference in the initial states of the electrons.
Figures 4-36 through 4-53 were plotted on a log-linear scale. The log scale shows
the change in luminescent intensity, while the linear scale shows time. Sample current
was not precisely controlled during collection of the pulsed measurements. Instead, the
curves were normalized to the intensity at the end of the pulse to account for a difference
in current. Pulse width was controlled by the pulse generator and was set at 10 p,s. To

95
get an accurate end-of-pulse luminescence, the raw data files were used (after averaging
the 3000 scans together, but before averaging using the Mathcad program; see
Chapter 3). Files contained both time and intensity columns, which allowed for finding
the correct intensity value. Background intensity found during averaging in Mathcad (see
Appendix A) was subtracted from this intensity. This is necessary because the
background had been subtracted from the rest of the data, and, if not accounted for,
would give erroneous results.
Through the use of filters, spectral resolution of the CL decay measurements is
possible. Figures 4-36 to 4-43 show these results for coatings of AI2O3 and MgO on
Y2Ü3:Eu. Each coating corresponds to two graphs, one for each of the two filters. The
data for the 2-73 filter is shown in Figures 4-36, 4-38, 4-40, and 4-42. These represent
the luminescence at 611 nm originating from the 5Do state. The rise after the end of pulse
and the curve maximum are attributed to feeding of this state from higher energy states
(the 5D¡ states where i is greater than or equal to 1, as shown in Figure 2-7 and discussed
in Chapter 2). Therefore, the increase in peak height and maximum at higher incident
beam energies suggests that the lifetime of the upper excited states is longer at higher
energies. Another interesting characteristic of these curves is that the decay behavior at
long times is very similar for different beam energies. All of the beam energy
dependence is contained in the peak rise and decay immediately following the end of
pulse. The coatings do not have an effect on the decay rate of the luminescent states,
except for that due to beam energy effects. Beam energy changes due to the coating will
be discussed in Chapter 5.

96
The 535 nm bandpass filter data is shown in Figures 4-37, 4-39, 4-41, and 4-43.
These represent luminescence from the 5Di states of Eu3+. These curves tend to be
noisier than the others, especially at lower energies. This is due to their much lower
intensity, which is only about three percent of the intensity found at 611 nm (see
Figures 4-11 to 4-14.) By injecting more current into the phosphor, the luminescence can
be increased, but there is a limit to the amount of current that can be used. As the current
is increased, the phosphor will exhibit saturation (see Chapter 2) and the background
level will rise. This adds considerably to the noise in the signal and was therefore
avoided. At higher energies, this was not a problem because the luminescence could be
increased to desired levels before saturation. The rise in intensity after the end of pulse
was much lower than for the 2-73 filter data, suggesting that the population of the upper
excited states was lower than for the 5D0 state. The curves also peaked sooner, which
suggests that the upper excited states decay faster than lower lying states. Beam energy
effects for the 535 nm data were similar to those of the 611 nm data. Lower beam
energies exhibited less rise after end of pulse, peaked sooner, and decayed quicker
initially. The changes in the curves due to the coatings can be attributed to a change in
the incident energy of the incoming electrons.
The time-resolved intensity at various beam energies for coatings of AI2O3 and
MgO on Y2Si05:Tb are presented in Figures 4-44 through 4-53. As in the Y203:Eu
samples, there are two graphs for each coating, one for each filter. The curves from the
3-70 filter represent luminescence originating in the 5D4 state of Tb3+ in Y2Si05; these are
shown in Figures 4-44, 4-46, 4-48, 4-50, and 4-52. As with the yittria samples, some of
the curves are noisy at lower beam energies. Also similar is the rise in intensity after the

97
end of pulse. This is attributed to feeding of the state from higher energy states. The
number of electrons transferred shows a strong dependence on beam energy. Lower
excitation energy produces a luminescence that peaks at shorter times and at a lower
intensity, with a corresponding quicker initial decay. This is similar to the Y203:Eu data,
but there is a much larger variation in the Y2Si05:Tb samples. Luminescence decay data
from the next highest excited state, 5D3 corresponding to 420 nm emission, are shown in
Figures 4-45, 4-47, 4-49, 4-51, and 4-53. In contrast to all previous data, these show
almost no rise in luminescence after the end of pulse. This suggests either that higher
energy states decay on a time scale much shorter than the pulse width of 10 psec or that
5D3 state emission does not rely on electrons from higher excited states. Although all of
these curves start decaying from the end of pulse, beam energy effects are still
discemable. Lower incident beam energies result in a larger decay rate. This causes the
curves to diverge after the end of pulse. Similar to before, the changes seen as a result of
the coatings can be attributed to changes in the incident beam energy. The incident beam
energy reduction as a result of the coating will be quantified in Chapter 5.
Coating Thickness and Uniformity
To determine the effect of the coating on the cathodoluminescence generated in
the phosphor particles, an accurate measurement of the thickness and uniformity of the
coating is needed. Table 5 shows the thicknesses for all samples. In cases where the
thickness was derived from more than one method, both values are included. These are
the thicknesses derived for a uniform deposition flux onto a planar substrate, the Si mask
in this case. Due to the shape of the phosphor particles, the thickness of the coating will

98
not be uniform across the surface. However, results derived from the Si masks will be
used to determine the thickness on the phosphors in Chapter 5.
For the initial set of samples, MgO (4 min.)/Y203:Eu, MgO (8 min.)/Y203:Eu,
Al203(1.2 min.)/Y203:Eu, AI2O3 (2.4 min.)/Y203:Eu, and Al2C>3(1.2 min.) A^SiC^Tb,
the thickness was determined by assuming a linear growth rate of the film during PLD.
Films of MgO and AI2O3 were grown on Si substrates for five minutes under identical
conditions as during the coating of the phosphor screens. Ellipsometry was then
performed using the single wavelength setup described in Chapter 3. The thickness of
the AI2O3 film was determined to be 325 Á, while the MgO film was only 91 Á. Based
on these numbers, the deposition times of the different materials were chosen to produce
films of comparable thickness. The extrapolated thickness of the 4 min. MgO coating
was 73 Á, 8 min. MgO was 146 Á, 1.2 min. AI2O3 was 78 A, and 2.4 min. AI2O3 was
156 Á.
During the coating of the second set of samples, the 2.4 min. AI2O3 coating had to
be deposited in a new chamber due to a mechanical failure with the turbo pump on the
old chamber. Deposition time was adjusted to 5 min. to account for the difference in
deposition rate between the two chambers, thereby producing a coating with a thickness
comparable to that of a coating grown for 2.4 min. in the old chamber. The coating was
otherwise deposited under the same conditions as the rest of the samples.
For the second set of samples, MgO (4 min.)/Y2Si05:Tb, MgO (8 min.)/
Y2Si05iTb, AI2O3 (5 min.)/Y2Si05:Tb, and Al203(5 min. in new chamber)/Y2SiOs:Tb,
the Si mask used to cover the phosphor screen during coating was used to obtain
thickness measurements. It was initially hoped that having the masks from each run

99
would allow for more accurate determination of the coating thickness from the first set of
samples, but the new set of samples was inadvertently run at a higher oxygen partial
pressure than the first set. The first set of coatings had been deposited at 50 mTorr
oxygen background, but the new samples were deposited at 200 mTorr oxygen. The
higher pressure resulted in films much thicker than desired.
To characterize the thickness of the coatings on the Si masks from the second set
of samples, ellipsometry was taken using both a fixed wavelength and a variable
wavelength setup. The variable wavelength equipment provides for higher precision by
using multiple wavelengths and angles. The fitting software of the multiple wavelength
machine also allowed for the inclusion of a native oxide layer (18 Á was chosen) between
the Si substrate and the film during film thickness calculation.
Performing thickness measurements on the single wavelength setup gave results
of 268 Á for 4 min. MgO, 494 Á for 8 min. MgO, and 489 Á for 5 min. (in new chamber)
AI2O3. No meaningful measurement could be taken from the mask on the 5 min. AI2O3
sample because of a thickness gradient across the surface. Next, the multiple wavelength
setup (500 to 800 nm in 10 nm steps) was used to try and improve upon the accuracy of
the ellipsometric measurements taken at a single wavelength. Results were similar to
those found earlier: 290 Á for 4 min. MgO, 549 Á for 8 min. MgO, and 545 Á for 5 min.
(in new chamber) AI2O3. As with the previous measurements, no meaningful data could
be collected from the 5 min. A1203 mask.
Since no meaningful ellipsometry data could be collected from the mask used
during the 5 min. coating of AI2O3 on Y2Si05:Tb, a Dektak profilometer was used to
attempt a measurement from this particular mask. During the coating, a metal spring clip

100
was used to secure the mask in place. Consequently, there was a small region of
uncoated Si on the mask. By scanning from this region out into the coated area, a step
profile was generated; this is shown in Figure 4-54. From this, the height of the coating
was deduced to be approximately 1100 Á. As noted earlier in this section, however, the
thickness varied across the mask surface.
To confirm the validity of the thickness values obtained from ellipsometry, the
5 min. coatings of MgO and AI2O3 on Si substrates deposited at 50 mTorr (used to
extrapolate thickness values for first set of samples) were examined using transmission
electron microscopy (TEM). Cross-sections were prepared so that the thickness of the
layer could be determined. Figures 4-55 and 4-56 show the results. The measured
thickness of the AI2O3 coating was around 340 A, which is comparable to the 325 A
value found through ellipsometry. Another important aspect of Figure 4-55 is the
uniformity of the layer. The MgO coating in Figure 4-56 showed discontinuous islands
and a much rougher interface with the Si substrate. This may be due to the lower
deposition rate of the MgO compared to the AI2O3. No diffuse scattering of the laser was
observed during ellipsometry, so the numbers collected from this method are assumed to
be accurate. AES data collected on a MgO coated sample, as discussed below, showed
no signal from the underlying phosphor material. This suggests that the coating was
continuous for that sample. The most likely explanation is that growth started out as
islands and switched to layers once the islands had reached a critical density.
As a check of the uniformity of the coating, the 1.2 min. Al203-coated Y2Ü3:Eu
sample was examined in an SEM using CL and secondary electron (SE) imaging.
Figures 4-57 and 4-58 show the uncoated Y203:Eu CL and SE scans, respectively.

101
Figures 4-59 and 4-60 show the AI2O3 coated Y2Ü3:Eu CL and SE scans, respectively.
All scans were collected at 1.4 keV and 2 pA. This low voltage was used so that the
effect of the coating could be seen if present. At higher voltages, the interaction volume
is so large that the coating’s effect on the cathodoluminescence becomes negligible. It is
apparent from Figure 4-57 that the CL is not uniform over the entire scan area for the
uncoated material. This is most likely due to varying geometry between the surface and
detector, which causes the number of collected photons to vary. By comparing the CL
image to the SE image (Figure 4-58) taken over the same area, the contrast seen in the CL
surface map matches changes in the surface seen in the SE image. From the images, it
appears that the CL intensity variations can be correlated with changes in the surface
topography. Results for the coated sample show similar behavior.
If changes in the CL intensity can be correlated to the SE image, then to examine
the coating uniformity, concentration should be focused on a region that, from the SE
image, would be expected to show little contrast in the CL image. If the coating were
discontinuous, there should be pinholes of brighter intensity in regions that are otherwise
uniform. Since no such bright spots were seen, it can be assumed that the coating is
continuous within the resolution of this technique, estimated to be around 0.25 pm.
All of the coating thicknesses have been determined based on measurements
taken from layers grown on Si substrates and masks. To examine the coatings on the
phosphor powders, sputter Auger was performed for the 8 min. coating of MgO on
Y2Si05:Tb. Figure 4-61 shows a surface scan of the MgO layer on the Si mask. Only the
Mg and O peaks are present, with no trace of the Si (the Cl and C are from surface
contamination during handling). This shows that the coating is continuous and uniform

102
at least over a thickness approximately equal to the escape depth of an Auger electron.
Figure 4-62 is a sputter profile using a 3x3 raster on the ion gun. It can be seen that after
about 70 minutes, the beam penetrated through the top layer to the underlying Si
substrate. This corresponds to a sputter rate of approximately 7.7 Á/min. The carbon
signal is seen to drop immediately upon sputtering, which confirms that the C signal in
Figure 4-61 is due to surface contamination. Figure 4-63 is a surface scan after sputtering
for 95 minutes (the end of the depth profile). No trace of the MgO remains, and only the
Si peak is present. Figure 4-64 is a surface scan on the coated Y2Si05:Tb powder. As
before, only the Mg and O peaks are present. In this case, the absence of a Y peak is
indicative of a continuous coating. Again, the small C peak is due to surface
contamination during handling. Figure 4-65 is a surface scan of the uncoated Y2SiC>5:Tb
powder. The spectrum shows a large drop at electron energies below 100 eV and is
noisy, most likely due to sample charging by the electron beam. However, the Y peak is
still visible, along with the O peak. Figure 4-66 is a depth profile on the coated area of
the powder, using a 3x3 raster on the ion gun similar to the previous measurement. Due
to the roughness of the surface and the incident angle of the ion gun, there was
shadowing of areas on the sample. This resulted in a less sharp transition from the
coating material to the underlying phosphor. A 5 sec. delay was also used between
sputtering and collecting the peak heights in order to allow time for the sample to
dissipate any stored charge as a result of sputtering. Figure 4-67 shows the surface scan
after the 125 min. depth profile. Both the Y and Mg peaks are present. This is due to
shadowing of the ion sputter beam during removal of the coating. In the Auger system
used for this work, the ion sputter beam is located at a 30° angle from the sample, while

103
the electron beam is directly above the sample; see Figure 4-68. As a result, when a
rough surface is analyzed, the electron beam may sample a small portion of the surface
that is not in the direct line of sight of the ion gun. Therefore, a small signal from the
original non-sputtered surface is not unexpected during each subsequent Auger analysis.
Due to the gradual transition in the powder sample shown in Figure 4-66, no exact
correlation is possible with the mask data shown in Figure 4-62. However, since the
transitions in both figures happened after about 60 minutes of sputtering, it is reasonable
to assume that the thickness data from the masks can be used to determine the coating
thickness on the powder samples. For a more exact representation of the coating on the
powder surface, see the discussion of deposition from a uniform flux onto a spherical
surface presented in Chapter 5.

104
Table 4
List of all Samples
Laser
Energy
Rep.
Rate
Pulse
Duration
Oxygen
Background
Pressure
MgO (4 min) / Y203:Eu
350 mJ
10 Hz
10 nsec
50 mTorr
MgO (8 min) / Y203:Eu
350 mJ
10 Hz
10 nsec
50 mTorr
A1203 (1.2 min) / Y203:Eu
350 mJ
10 Hz
10 nsec
50 mTorr
A1203 (2.4 min) / Y203:Eu
350 mJ
10 Hz
10 nsec
50 mTorr
Al203 (1.2min)/Y2Si05:Tb
350 mJ
10 Hz
10 nsec
50 mTorr
A1203 (2.4 min) / Y2Si05:Tb
(actually 5 min in new chamber)
350 mJ
10 Hz
10 nsec
200 mTorr
A1203 (5 min) / Y2Si05:Tb
350 mJ
10 Hz
10 nsec
200 mTorr
MgO (4 min) / Y2Si05:Tb
350 mJ
10 Hz
10 nsec
200 mTorr
MgO (8 min) / Y2Si05:Tb
350 mJ
10 Hz
10 nsec
200 mTorr

relative radiant energy
105
Figure 4-1 Cathodoluminescent spectrum of Y203:Eu over the visible wavelength range
from 400 to 720 nm. The main emission peak is at 611 nm. Beam energy
was 1 keV with a current density of 1 pA/cm2.38

INTENSITY (ARB. UNITS)
106
MgO (4 min) coated Y203:Eu
0.8 keV 0.8 coated 14keV 1.4 coated
2.5 keV 2.5 coated 4keV 4 coated
520 540 560 580 600 620 640 660 680 700 720
WAVELENGTH (nm)
Figure 4-2 Cathodoluminescence intensity as a function of wavelength for a coating of
MgO deposited for 4 minutes on Y203:Eu powder. Both coated and uncoated
results are shown. The beam current was kept constant at 0.16 pA/cm2, while
the accelerating voltage was varied from 0.8 to 4 keV.

INTENSITY (ARB UNITS)
107
MgO (8 min) coated Y203:Eu
0 8 keV 0 8 coated 1.4 keV 1.4 coated
2.5 keV 2.5 coated 4.0 keV 4.0 coated
520 540 560 580 600 620 640 660 680 700 720
WAVELENGTH (nm)
Figure 4-3 Cathodoluminescence intensity as a function of wavelength for a coating of
MgO deposited for 8 minutes on Y203:Eu powder. Both coated and uncoated
results are shown. The beam current was kept constant at 0.16 pA/cm2, while
the accelerating voltage was varied from 0.8 to 4 keV.

INTENSITY (ARB. UNITS)
108
Al203(1.2 min)/Y203:Eu
0.8 keV 0 8 coated 14keV 1.4 coated
2.5 keV 2.5 coated 4.0 keV 4.0 coated
WAVELENGTH (nm)
Figure 4-4 Cathodoluminescence intensity as a function of wavelength for a coating of
AI2O3 deposited for 1.2 minutes on Y203:Eu powder. Both coated and
uncoated results are shown. The beam current was kept constant at
0.16 pA/cm2, while the accelerating voltage was varied from 0.8 to 4 keV.

INTENSITY (ARB. UNITS)
109
AI203(2.4min)/Y203:Eu
0.8 keV 0.8 coated 1.4 keV 1.4 coaled
2.5 keV 2.5 coated 4keV 4 coated
WAVELENGTH (nm)
Figure 4-5 Cathodoluminescence intensity as a function of wavelength for a coating of
AI2O3 deposited for 2.4 minutes on Y203:Eu powder. Both coated and
uncoated results are shown. The beam current was kept constant at
0.16 pA/cm2, while the accelerating voltage was varied from 0.8 to 4 keV.

Relative Intensity
no
Figure 4-6 Cathodoluminescence spectrum for Y2SiOs:Tb measured at 1 keV over the
visible wavelength range from 400 to 720 nm.49

INTENSITY (ARB. UNITS)
111
MgO (4 min) / Y2Si05:Tb
0 8 keV 0 8 coaled 1.4keV 1.4 coated
2.5 keV 2.5 coaled 4.0 keV 4.0 coated
460 480 500 520 540 560 580 600 620 640 660 680
WAVELENGTH (nm)
Figure 4-7 Cathodoluminescence intensity as a function of wavelength for a coating of
MgO deposited for 4 minutes on Y2Si05:Tb powder. Both coated and
uncoated results are shown. The beam current was kept constant at
0.16 pA/cm2, while the accelerating voltage was varied from 0.8 to 4 keV.

INTENSITY (ARB UNITS)
112
MgO (8 min) / Y2Si05:Tb
0.8 keV 0.8 coated 1.4keV 1.4 coated
2 5 keV 2.5 coated 4.0 keV 4.0 coated
460 480 500 520 540 560 580 600 620 640 660 680
WAVELENGTH (nm)
Figure 4-8 Cathodoluminescence intensity as a function of wavelength for a coating of
MgO deposited for 8 minutes on Y2Si05:Tb powder. Both coated and
uncoated results are shown. The beam current was kept constant at
0.16 pA/cm , while the accelerating voltage was varied from 0.8 to 4 keV.

INTENSITY (ARB. UNITS)
113
Al203(2.4 min)/Y2Si05:Tb
0 8keV 0.8 coated 1.4keV 14 coated
2.5 keV 2.5 coated 4.0 keV 4.0 coated
WAVELENGTH (nm)
Figure 4-9 Cathodoluminescence intensity as a function of wavelength for a coating of
AI2O3 deposited for 2.4 minutes on Y2Si05:Tb powder. Both coated and
uncoated results are shown. The beam current was kept constant at
0.16 pA/cm , while the accelerating voltage was varied from 0.8 to 4 keV.

INTENSITY (ARB. UNITS)
114
Al203(5 min)/Y2Si05:Tb
0 8 keV 0 8 coaled 1.4keV 1 4 coated
2 5 keV 2.5 coaled 4.0 keV 4.0 coated
WAVELENGTH (nm)
Figure 4-10 Cathodoluminescence intensity as a function of wavelength for a coating of
AI2O3 deposited for 5 minutes on Y2Si05:Tb powder. Both coated and
uncoated results are shown. The beam current was kept constant at
0.16 pA/cm2, while the accelerating voltage was varied from 0.8 to 4 keV.

INTENSITY (ARB UNITS)
115
MgO (4 min) coated Y203:Eu
0.8 keV 0.8 coaled 1.4 keV 14 coaled
2.5 keV 2.5 coated 4keV 4 coated
520 540 560 580 600 620 640 660 680 700 720
WAVELENGTH (nm)
Figure 4-11 Cathodoluminescence intensity normalized to the intensity of the main
611 nm peak as a function of wavelength for a coating of MgO deposited
for 4 minutes on Y203iEu powder. Both coated and uncoated results are
shown. The beam current was kept constant at 0.16 pA/cm2, while the
accelerating voltage was varied from 0.8 to 4 keV.

INTENSITY (ARB UNITS)
116
MgO (8 min) coated Y203:Eu
0 8 keV 0 8 coated 14keV 1A coated
2.5 keV 2.5 coated 4 0 keV 4,0 coated
520 540 560 580 600 620 640 660 680 700 720
WAVELENGTH (nm)
Figure 4-12 Cathodoluminescence intensity normalized to the intensity of the main
611 nm peak as a function of wavelength for a coating of MgO deposited
for 8 minutes on Y203:Eu powder. Both coated and uncoated results are
shown. The beam current was kept constant at 0.16 pA/cm , while the
accelerating voltage was varied from 0.8 to 4 keV.

INTENSITY CARB UNITS)
117
Al203(1.2 min)/Y203:Eu
0.8 keV 0.8 coated 1.4keV 1.4 coated
2 5 keV 2 5 coated 4.0 keV 4.0 coated
WAVELENGTH (nm)
Figure 4-13 Cathodoluminescence intensity normalized to the intensity of the main
611 nm peak as a function of wavelength for a coating of AI2O3 deposited
for 1.2 minutes on Y203:Eu powder. Both coated and uncoated results are
shown. The beam current was kept constant at 0.16 pA/cm , while the
accelerating voltage was varied from 0.8 to 4 keV.

INTENSITY (ARB. UNITS)
118
Al203(2.4 min) / Y203:Eu
0,8 keV 0.8 coated 14keV 1.4 coated
2.5 keV 2.5 coated 4keV 4 coated
520 540 560 580 600 620 640 660 680 700 720
WAVELENGTH (nm)
Figure 4-14 Cathodoluminescence intensity normalized to the intensity of the main
611 nm peak as a function of wavelength for a coating of AI2O3 deposited
for 2.4 minutes on Y203:Eu powder. Both coated and uncoated results are
shown. The beam current was kept constant at 0.16 pA/cm2, while the
accelerating voltage was varied from 0.8 to 4 keV.

INTENSITY (ARB UNITS)
119
MgO (4 min) / Y2Si05:Tb
0 8keV 0.8 coated 14keV 14 coated
2 5 keV 2 5 coated 4 0keV 4.0 coated
WAVELENGTH (nm)
Figure 4-15 Cathodoluminescence intensity normalized to the intensity of the main
547 nm peak as a function of wavelength for a coating of MgO deposited
for 4 minutes on Y2SiOs:Tb powder. Both coated and uncoated results are
shown. The beam current was kept constant at 0.16 p.A/cm2, while the
accelerating voltage was varied from 0.8 to 4 keV.

INTENSITY (ARB. UNITS)
120
MgO (8 min) / Y2Si05:Tb
0 8 keV 0 8 coated 14keV 14 coated
2.5 keV 2.5 coated 4 0keV 4.0 coated
WAVELENGTH (nm)
Figure 4-16 Cathodoluminescence intensity normalized to the intensity of the main
547 nm peak as a function of wavelength for a coating of MgO deposited
for 8 minutes on Y2Si05:Tb powder. Both coated and uncoated results are
shown. The beam current was kept constant at 0.16 pA/cm2, while the
accelerating voltage was varied from 0.8 to 4 keV.

INTENSITY (ARB. UNITS)
121
Al203(2.4 min)/Y2Si05:Tb
0 8 keV 0 8 coated 14keV 1.4 coated
2.5 keV 2 5 coated 4 0 keV 4.0 coated
460 480 500 520 540 560 580 600 620 640 660 680
WAVELENGTH (nm)
Figure 4-17 Cathodoluminescence intensity normalized to the intensity of the main
547 nm peak as a function of wavelength for a coating of A1203 deposited
for 2.4 minutes on Y2SiOs:Tb powder. Both coated and uncoated results
are shown. The beam current was kept constant at 0.16 pA/cm2, while the
accelerating voltage was varied from 0.8 to 4 keV.

INTENSITY (ARB UNITS)
122
Al203(5 min)/Y2Si05:Tb
0.8 keV 0.8 coated 1.4keV 1.4 coated
2.5 keV 2.5 coated 4 0keV 4 0 coated
460 480 500 520 540 560 580 600 620 640 660 680
WAVELENGTH (nm)
Figure 4-18 Cathodoluminescence intensity normalized to the intensity of the main
547 nm peak as a function of wavelength for a coating of AI2O3 deposited
for 5 minutes on Y2SiOs:Tb powder. Both coated and uncoated results are
shown. The beam current was kept constant at 0.16 jiA/cm2, while the
accelerating voltage was varied from 0.8 to 4 keV.

INTENSITY (ARB. UNITS)
123
MgO (4 min) coated Y203:Eu
POWDER SAMPLE (TOP HALF COATED)
NO COAT A MgO(4)
200000
100000
10000
1000
0.7 1 5
611 nm PEAK
j i L
VOLTAGE (keV)
Figure 4-19 Cathodoluminescence intensity of the 611 nm peak as a function of beam
energy for a coating of MgO deposited for 4 minutes on Y203:Eu powder.
Both coated and uncoated results are shown. The beam current was kept
constant at 0.16 pA/cm2, while the accelerating voltage was varied from 0.8
to 4 keV. The slight divergence of the curves at high energy is a result of
the curve fitting and is not indicative of the phosphor or coating.

INTENSITY (ARB UNITS)
124
30000
10000
1000 r
100
MgO (4 min) coated Y203:Eu
POWDER SAMPLE (TOP HALF COATED)
A MgO(4)
NO COAT
591 nm PEAK
0.7
VOLTAGE (keV)
Figure 4-20 Cathodoluminescence intensity of the 591 nm peak as a function of beam
energy for a coating of MgO deposited for 4 minutes on Y203:Eu powder.
Both coated and uncoated results are shown. The beam current was kept
constant at 0.16 pA/cm2, while the accelerating voltage was varied from 0.8
to 4 keV.

INTENSITY (ARB UNITS)
125
MgO (8 min) coated Y203:Eu
POWDER SAMPLE (TOP HALF COATED)
+ NO COAT A MgO(8)
611 nm PEAK
200000
100000
10000
1000
100
07 1 5
VOLTAGE (keV)
Figure 4-21 Cathodoluminescence intensity of the 611 nm peak as a function of beam
energy for a coating of MgO deposited for 8 minutes on Y203iEu powder.
Both coated and uncoated results are shown. The beam current was kept
constant at 0.16 pA/cm2, while the accelerating voltage was varied from 0.8
to 4 keV.

INTENSITY'(ARB UNITS)
126
MgO (8 min) coated Y203:Eu
POWDER SAMPLE (TOP HALF COATED)
+ NO COAT A MgO(8)
591 nm PEAK
30000
10000
1000
100
50
0.7 1 5
VOLTAGE (keV)
Figure 4-22 Cathodoluminescence intensity of the 591 nm peak as a function of beam
energy for a coating of MgO deposited for 8 minutes on Y203:Eu powder.
Both coated and uncoated results are shown. The beam current was kept
constant at 0.16 pA/cm2, while the accelerating voltage was varied from 0.8
to 4 keV.

INTENSITY (ARB UNITS)
127
Al203(1.2 min)/Y203:Eu
POWDER SAMPLE (TOP HALF COATED)
NO COAT A AI203(12)
611 nm PEAK
200000
100000
10000
1000
VOLTAGE (keV)
Figure 4-23 Cathodoluminescence intensity of the 611 nm peak as a function of beam
energy for a coating of AI2O3 deposited for 1.2 minutes on Y2Ü3:Eu
powder. Both coated and uncoated results are shown. The beam current
was kept constant at 0.16 pA/cm2, while the accelerating voltage was varied
from 0.8 to 4 keV.

INTENSITY (ARB UNITS)
128
Al203(1.2 min)/Y203:Eu
POWDER SAMPLE (TOP HALF COATED)
+ NO COAT A AI203(12)
591 nm PEAK
VOLTAGE (keV)
Figure 4-24 Cathodoluminescence intensity of the 591 nm peak as a function of beam
energy for a coating of A1203 deposited for 1.2 minutes on Y203:Eu
powder. Both coated and uncoated results are shown. The beam current
was kept constant at 0.16 pA/cm , while the accelerating voltage was varied
from 0.8 to 4 keV.

INTENSITY (ARB UNITS)
129
AI203(2.4min)/Y203:Eu
POWDER SAMPLE (TOP HALF COATED)
NO COAT A AI203(2.4)
200000
100000
10000
1000
07 1 5
611 nm PEAK
-A‘
,A
A
X
—I 1 1 I I L
VOLTAGE (keV)
Figure 4-25 Cathodoluminescence intensity of the 611 nm peak as a function of beam
energy for a coating of AI2O3 deposited for 2.4 minutes on Y2Ü3:Eu
powder. Both coated and uncoated results are shown. The beam current
was kept constant at 0.16 pA/cm2, while the accelerating voltage was varied
from 0.8 to 4 keV.

INTENSITY (ARB UNITS)
130
AI203(2.4min)/Y203:Eu
POWDER SAMPLE (TOP HALF COATED)
NO COAT A A1203(2.4)
20000
10000
1000
100
0.7 1 5
591 nm PEAK
a
A
A
_1 I I I 1 L
VOLTAGE (keV)
Figure 4-26 Cathodoluminescence intensity of the 591 nm peak as a function of beam
energy for a coating of AI2O3 deposited for 2.4 minutes on Y203:Eu
powder. Both coated and uncoated results are shown. The beam current
was kept constant at 0.16 pA/cm2, while the accelerating voltage was varied
from 0.8 to 4 keV. The slight divergence of the curves at high energy is a
result of the curve fitting and is not indicative of the phosphor or coating.

INTENSITY (ARB UNITS)
131
MgO (4 min) / Y2SiOs:Tb
POWDER SAMPLE (TOP HALF COATED)
+ NO COAT A MgO(4)
547 nm PEAK
1000000
100000
10000
1000
0.7 1 5
VOLTAGE (keV)
Figure 4-27 Cathodoluminescence intensity of the 547 nm peak as a function of beam
energy for a coating of MgO deposited for 4 minutes on Y2SiOs:Tb powder.
Both coated and uncoated results are shown. The beam current was kept
constant at 0.16 pA/cm , while the accelerating voltage was varied from 0.8
to 4 keV.

INTENSITY (ARB UNITS)
132
MgO (4 min) / Y2Si05:Tb
POWDER SAMPLE (TOP HALF COATED)
NO COAT A MgO(4)
486.7 nm PEAK
300000
100000 r
10000
1000
0.7
VOLTAGE (keV)
Figure 4-28 Cathodoluminescence intensity of the 486.7 nm peak as a function of beam
energy for a coating of MgO deposited for 4 minutes on Y2Si05:Tb powder.
Both coated and uncoated results are shown. The beam current was kept
constant at 0.16 pA/cm2, while the accelerating voltage was varied from 0.8
to 4 keV.

INTENSITY (ARB UNITS)
133
MgO (8 min) / Y2SiOs:Tb
POWDER SAMPLE (TOP HALF COATED)
NO COAT
A MgO(8)
547 nm PEAK
VOLTAGE (keV)
Figure 4-29 Cathodoluminescence intensity of the 547 nm peak as a function of beam
energy for a coating of MgO deposited for 8 minutes on Y2SiOs:Tb powder.
Both coated and uncoated results are shown. The beam current was kept
constant at 0.16 [xA/cm2, while the accelerating voltage was varied from 0.8
to 4 keV.

INTENSITY (ARB UNITS)
134
MgO (8 min) / Y2Si05:Tb
POWDER SAMPLE (TOP HALF COATED)
NO COAT A MgO(8)
486.7 nm PEAK
200000
100000
10000
1000
500
0.7 1 5
VOLTAGE (keV)
Figure 4-30 Cathodoluminescence intensity of the 486.7 nm peak as a function of beam
energy for a coating of MgO deposited for 8 minutes on Y2SiOs:Tb powder.
Both coated and uncoated results are shown. The beam current was kept
constant at 0.16 pA/cm , while the accelerating voltage was varied from 0.8
to 4 keV.

INTENSITY'(ARB UNITS)
135
Al203(1.2 min)/Y2Si05:Tb
POWDER SAMPLE (TOP HALF COATED)
NO COAT A AI203(12)
547 nm PEAK
200000
100000 -
10000
1000
100 1—1—1 1 1 1
07 1 5
VOLTAGE (keV)
Figure 4-31 Cathodoluminescence intensity of the 547 nm peak as a function of beam
energy for a coating of AI2O3 deposited for 1.2 minutes on YaSiOsTb
powder. Both coated and uncoated results are shown. The beam current
was kept constant at 0.16 pA/cm2, while the accelerating voltage was varied
from 0.8 to 4 keV.

INTENSITY (ARB UNITS)
136
Al003(2.4 min)/Y2Si05:Tb
POWDER SAMPLE (TOP HALF COATED)
NO COAT A A1203(2.4 eq)
547 nm PEAK
800000
100000 r
10000 r
1000
0.7
VOLTAGE (keV)
Figure 4-32 Cathodoluminescence intensity of the 547 nm peak as a function of beam
energy for a coating of AI2O3 deposited for 2.4 minutes on YaSiOsTb
powder. Both coated and uncoated results are shown. The beam current
was kept constant at 0.16 pA/cm2, while the accelerating voltage was varied
from 0.8 to 4 keV.

INTENSITY (ARB UNITS)
137
AI203(2.4min)/Y2Si05:Tb
POWDER SAMPLE (TOP HALF COATED)
NO COAT A AI203(2.4 eq)
486.7 nm PEAK
300000
100000
10000
1000
500
0.7 1 5
VOLTAGE (keV)
Figure 4-33 Cathodoluminescence intensity of the 486.7 nm peak as a function of beam
energy for a coating of AI2O3 deposited for 2.4 minutes on Y2SiÜ5:Tb
powder. Both coated and uncoated results are shown. The beam current
was kept constant at 0.16 pA/cm2, while the accelerating voltage was varied
from 0.8 to 4 keV.

INTENSITY (ARB UNITS)
138
Al203(5 min)/Y2S¡05:Tb
POWDER SAMPLE (TOP HALF COATED)
NO COAT A A1203(5)
547 nm PEAK
800000
100000
10000
1000
100
0 7
5
VOLTAGE (keV)
Figure 4-34 Cathodoluminescence intensity of the 547 nm peak as a function of beam
energy for a coating of AI2O3 deposited for 5 minutes on Y2Si05:Tb
powder. Both coated and uncoated results are shown. The beam current
was kept constant at 0.16 pA/cm2, while the accelerating voltage was varied
from 0.8 to 4 keV.

INTENSITY (ARB UNITS)
139
Al203(5 min)/Y2Si05:Tb
POWDER SAMPLE (TOP HALF COATED)
+ NO COAT A A1203(5)
486.7 nm PEAK
300000
100000
10000
1000
100
07 1 5
VOLTAGE (keV)
Figure 4-35 Cathodoluminescence intensity of the 486.7 nm peak as a function of beam
energy for a coating of AI2O3 deposited for 5 minutes on YaSiOsTb
powder. Both coated and uncoated results are shown. The beam current
was kept constant at 0.16 pA/cm2, while the accelerating voltage was varied
from 0.8 to 4 keV.

INTENSITY (ARB. UNITS)
140
MgO (4 min) coated Y203:Eu
2-73 Filter
0.8 keV 0.8 coated 1.4 keV 1.4 coated
2 5 keV 2.5 coated 4 keV 4 coated
TIME (SEC)
Figure 4-36 Pulsed cathodoluminescence intensity as a function of time for a coating of
MgO deposited for 4 minutes on Y2Ü3:Eu powder. Data were collected
through a Coming glass 2-73 filter. Both coated and uncoated results are
shown for beam energies ranging from 0.8 to 4.0 keV. Data have been
normalized to the intensity at the end of the pulse.

INTENSITY (ARB. UNITS)
141
MgO (4 min) coated Y203:Eu
535 nm Bandpass Filter
0.8 keV 0.8 coated 14keV 1.4 coated
2.5 keV 2.5 coated 4keV 4 coated
TIME (SEC)
Figure 4-37 Pulsed cathodoluminescence intensity as a function of time for a coating of
MgO deposited for 4 minutes on Y2C>3:Eu powder. Data were collected
through a 535 nm bandpass filter. Both coated and uncoated results are
shown for beam energies ranging from 0.8 to 4.0 keV. Data have been
normalized to the intensity at the end of the pulse. The curve for the coated
sample at 0.8 keV is offset due to difficulty in accurately determining the
end of pulse intensity because of a small signal-to-noise ratio.

INTENSITY (ARB UNITS)
142
MgO (8 min) coated Y203:Eu
2-73 Filter
0.8 keV 0.8 coated 14keV 1.4 coated
2 5 keV 2.5 coated 4keV 4 coated
2 00
1.00
NORMALIZED TO TRUE END OF PULSE
Ü. IV
0.0000 0.0005 0.0010 0.0015 0.0020
TIME (SEC)
0.0025
0.0030
0.0035
Figure 4-38 Pulsed cathodoluminescence intensity as a function of time for a coating of
MgO deposited for 8 minutes on Y2Ü3:Eu powder. Data were collected
through a Coming glass 2-73 filter. Both coated and uncoated results are
shown for beam energies ranging from 0.8 to 4.0 keV. Data have been
normalized to the intensity at the end of the pulse.

INTENSITY (ARB UNITS)
143
MgO (8 min) coated Y203:Eu
535 rim Bandpass Filter
0 8 keV 0.8 coated 14keV 1.4 coaled
2 5 keV 2.5 coated 4keV 4 coaled
TIME (SEC)
Figure 4-39 Pulsed cathodoluminescence intensity as a function of time for a coating of
MgO deposited for 8 minutes on Y203:Eu powder. Data were collected
through a 535 nm bandpass filter. Both coated and uncoated results are
shown for beam energies ranging from 0.8 to 4.0 keV. Data have been
normalized to the intensity at the end of the pulse.

INTENSITY (ARB UNITS)
144
AI203(1.2 min)/Y203:Eu
2-73 Filter
0.8 keV 0.8 coaled 14keV 1 4 coated
2.5 keV 2.5 coaled 4keV 4 coated
TIME (SEC)
Figure 4-40 Pulsed cathodoluminescence intensity as a function of time for a coating of
AI2O3 deposited for 1.2 minutes on Y2C>3:Eu powder. Data were collected
through a Coming glass 2-73 filter. Both coated and uncoated results are
shown for beam energies ranging from 0.8 to 4.0 keV. Data have been
normalized to the intensity at the end of the pulse.

INTENSITY (ARB UNITS)
145
Al,03(1.2 min) / Y?03:Eu
535 nm Bandpass Filter
0.8 keV 0 8 coaled 1.4keV 1.4 coated
2 5 keV 2 5 coaled 4keV 4 coaled
TIME (SEC)
Figure 4-41 Pulsed cathodoluminescence intensity as a function of time for a coating of
AI2O3 deposited for 1.2 minutes on Y2Ü3:Eu powder. Data were collected
through a 535 nm bandpass filter. Both coated and uncoated results are
shown for beam energies ranging from 0.8 to 4.0 keV. Data have been
normalized to the intensity at the end of the pulse.

INTENSITY (ARB UNITS)
146
AI203(2.4min)/Y203:Eu
2-73 Filter
0.8 keV 0.8 coated
2.5 keV 2.5 coated
1.4 keV 1 4 coated
4 keV 4 coated
TIME (SEC)
Figure 4-42 Pulsed cathodoluminescence intensity as a function of time for a coating of
AI2O3 deposited for 2.4 minutes on YaC^iEu powder. Data were collected
through a Coming glass 2-73 filter. Both coated and uncoated results are
shown for beam energies ranging from 0.8 to 4.0 keV. Data have been
normalized to the intensity at the end of the pulse.

INTENSITY (ARB UNITS)
147
AI203(2.4min)/Y203:Eu
535 nm Bandpass Filter
0 8 keV 0.8 coated 14keV 1.4 coated
2 5 keV 2.5 coated 4keV 4 coated
TIME (SEC)
Figure 4-43 Pulsed cathodoluminescence intensity as a function of time for a coating of
AI2O3 deposited for 2.4 minutes on Y2C>3:Eu powder. Data were collected
through a 535 nm bandpass filter. Both coated and uncoated results are
shown for beam energies ranging from 0.8 to 4.0 keV. Data have been
normalized to the intensity at the end of the pulse. The curve for the coated
sample at 0.8 keV is offset due to difficulty in accurately determining the
end of pulse intensity because of a small signal-to-noise ratio.

INTENSITY (ARB. UNITS)
148
MgO (4 min) / Y2Si05:Tb
3-70 Filter
0 8 keV 0.8 coated 14keV 14 coated
2 5 keV 2 5 coated 4 0keV 4.0 coated
TIME (SEC)
Figure 4-44 Pulsed cathodoluminescence intensity as a function of time for a coating of
MgO deposited for 4 minutes on Y2SiOs:Tb powder. Data were collected
through a Coming glass 3-70 filter. Both coated and uncoated results are
shown for beam energies ranging from 0.8 to 4.0 keV. Data have been
normalized to the intensity at the end of the pulse.

INTENSITY (ARB UNITS)
149
MgO (4 min) / Y2Si05:Tb
5-58 Filter
0 8 keV 0 .8 coated 1.4keV 1 4 coaled
0.00000 0.00002 0.00004 0.00006 0 00008 0.00010
TIME (SEC)
Figure 4-45 Pulsed cathodoluminescence intensity as a function of time for a coating of
MgO deposited for 4 minutes on Y2Si05:Tb powder. Data were collected
through a Coming glass 5-58 filter. Both coated and uncoated results are
shown for beam energies ranging from 0.8 to 4.0 keV. Data have been
normalized to the intensity at the end of the pulse.

INTENSITY (ARB UNITS)
150
MgO (8 min) / Y2SiOs:Tb
3-70 Filter
0.8 keV 0.8 coated 1.4keV 1.4 coated
2 5 keV 2.5 coated 4 0keV 4.0 coated
TIME (SEC)
Figure 4-46 Pulsed cathodoluminescence intensity as a function of time for a coating of
MgO deposited for 8 minutes on Y2Si05:Tb powder. Data were collected
through a Coming glass 3-70 filter. Both coated and uncoated results are
shown for beam energies ranging from 0.8 to 4.0 keV. Data have been
normalized to the intensity at the end of the pulse.

INTENSITY (ARB. UNITS)
151
MgO (8 min) / Y2Si05:Tb
5-58 Filter
0 8 keV
2 5 keV
0.8 coaled
â–  1 4 keV
1 4 coated
2 5 coated
4.0 keV
4 0 coated
TIME (SEC)
Figure 4-47 Pulsed cathodoluminescence intensity as a function of time for a coating of
MgO deposited for 8 minutes on Y2Si05:Tb powder. Data were collected
through a Coming glass 5-58 filter. Both coated and uncoated results are
shown for beam energies ranging from 0.8 to 4.0 keV. Data have been
normalized to the intensity at the end of the pulse.

INTENSITY (ARB. UNITS)
152
Al203(1.2 min)/Y2Si05:Tb
3-70 Filter
0 8 keV 0 8 coated 14keV 1.4 coated
2 6 keV 2.5 coated 4 0keV 4.0 coated
TIME (SEC)
Figure 4-48 Pulsed cathodoluminescence intensity as a function of time for a coating of
AI2O3 deposited for 1.2 minutes on Y2SiOs:Tb powder. Data were collected
through a Coming glass 3-70 filter. Both coated and uncoated results are
shown for beam energies ranging from 0.8 to 4.0 keV. Data have been
normalized to the intensity at the end of the pulse.

INTENSITY (ARB UNITS)
153
Al203(1.2 min)/Y2Si05:Tb
5-58 Filter
O BkeV 1.4 keV 1 4 coated 2 5 keV
0.00000 0 00002 0.00004 0 00006 0 00008 0.00010
TIME (SEC)
Figure 4-49 Pulsed cathodoluminescence intensity as a function of time for a coating of
AI2O3 deposited for 1.2 minutes on Y2Si05:Tb powder. Data were collected
through a Coming glass 5-58 filter. Both coated and uncoated results are
shown for beam energies ranging from 0.8 to 4.0 keV. Data have been
normalized to the intensity at the end of the pulse.

INTENSITY (ARB UNITS)
154
Al003(2.4 min)/Y0Si05:Tb
3-70 Filter
0 8 keV 0 8 coated 1.4 keV 1 4 coated
00000 00008 0.0016 0.0024 0 0032 0.0040 0.0048 0.0056 0.0064
TIME (SEC)
Figure 4-50 Pulsed cathodoluminescence intensity as a function of time for a coating of
AI2O3 deposited for 2.4 minutes on Y2SiOs:Tb powder. Data were collected
through a Coming glass 3-70 filter. Both coated and uncoated results are
shown for beam energies ranging from 0.8 to 4.0 keV. Data have been
normalized to the intensity at the end of the pulse.

INTENSITY (ARB UNITS)
155
Al O (2.4 min)/Y SiO :Tb
5-58 Filter
0.8 keV 0.8 coated 1.4 keV 14 coated
0.00000 0.00002 0 00004 0 00006 0.00008 0.00010
TIME (SEC)
Figure 4-51 Pulsed cathodoluminescence intensity as a function of time for a coating of
AI2O3 deposited for 2.4 minutes on Y2Si05:Tb powder. Data were collected
through a Coming glass 5-58 filter. Both coated and uncoated results are
shown for beam energies ranging from 0.8 to 4.0 keV. Data have been
normalized to the intensity at the end of the pulse.

INTENSITY (ARB. UNITS)
156
Al203(5 min)/Y2Si05:Tb
3-70 Filter
0.8 keV 0.8 coated 1.4keV 1.4 coated
0.0000 0.0008 0.0016 0 0024 0.0032 0.0040 0.0048 0.0056 0.0064
TIME (SEC)
Figure 4-52 Pulsed cathodoluminescence intensity as a function of time for a coating of
AI2O3 deposited for 5 minutes on Y2SiOs:Tb powder. Data were collected
through a Coming glass 3-70 filter. Both coated and uncoated results are
shown for beam energies ranging from 0.8 to 4.0 keV. Data have been
normalized to the intensity at the end of the pulse.

INTENSITY (ARB. UNITS)
157
Al203(5 min)/Y2S¡05:Tb
5-58 Filter
0.8 keV 1.4 keV 14 coated
0.00000 0 00002 0.00004 0.00006 0.00008 0.00010
TIME (SEC)
Figure 4-53 Pulsed cathodoluminescence intensity as a function of time for a coating of
AI2O3 deposited for 5 minutes on Y2SiOs:Tb powder. Data were collected
through a Coming glass 5-58 filter. Both coated and uncoated results are
shown for beam energies ranging from 0.8 to 4.0 keV. Data have been
normalized to the intensity at the end of the pulse. No measurable signal
could be obtained from the coated sample at 0.8 keV.

158
Table 5
Thickness of coatings on samples based on measurements taken from Si shadow masks.
Thickness (Á)
MgO (4 min) / Y203:Eu
73 (a)
MgO (8 min) / Y203:Eu
146 (a)
AI2O3 (1.2 min) / Y2Ü3:Eu
78 (a)
AI2O3 (2.4 min) / Y2Ü3:Eu
156 (a)
AI2O3 (1.2 min) / Y2Si05:Tb
78 (a)
AI2O3 (5 min) / Y2Si05:Tb
(grown in new chamber)
545 (b) and 489 (c)
A1203 (5 min) / Y2Si05:Tb
~ 1100 (d)
MgO (4 min)/Y2Si05:Tb
290 (b) and 268 (c)
MgO (8 min) / Y2Si05:Tb
549 (b) and 494 (c)
(a) Based on a linear growth rate extrapolation of measurements taken from films of the
same materials grown on Si substrates for 5 minutes under identical conditions.
AI2O3 film (5 minutes) = 325 Á from ellipsometry and about 340 Á from TEM
MgO film (5 minutes) = 91 Á from ellipsometry
(b) Data taken on multiple wavelength ellipsometer and fit assuming an 18 Á SÍO2 film
between the substrate and film due to oxidation of the Si.
(c) Data taken on single wavelength ellipsometer.
(d) Approximation made from step height on stylus profilometer.

159
Direction ->
Repeats: 1
Force: 10 mg
Level:
Meas*
9000A
8000A
7000A
8000A
5000A
4000A
JOOOA
2000A
1000A
0 0
s
Cursors
Left Right
Meas 812.0 1117
Delta 838.0 1133
Level 0.000 106.8
Delta 26.45 131.3
Ref: 0 000
L Height: 1127.2 A
R Height: 1216.8 A
St Height:89.7 A
Width: 299.7 pm
Tift 3074.1 A
•
i
i
L
Liu-
i
J
LiuUJ
UÃœL-
168 336 503 671 839 1007 1175 1342 1510 1878
Scan Length (pm)
Figure 4-54 Stylus profilometer output showing the step between bare Si and AI2O3 film
on a shadow mask used during 5 minute deposition of AI2O3 onto
Y2Si05:Tb powder. The measured height is approximately 1150 Á.

160
Si A1203 Epoxy
Figure 4-55 Transmission electron microscopy (TEM) image of an AI2O3 film on a Si
substrate deposited by PLD at 50 mTorr oxygen. This thickness was used
to confirm the ellipsometry results, and these measurements were used to
extrapolate the thicknesses for the earlier samples. The darkest region is the
Si substrate. The light gray region on the right side of the image is the
epoxy used during cross-section sample preparation. The film is located in
the middle and is a medium gray in color.

161
40 inn
Figure 4-56 Transmission electron microscopy (TEM) image of a MgO film on a Si
substrate deposited by PLD at 50 mTorr oxygen. The dark islands are the
MgO. The medium gray region at the bottom is the Si substrate. The light
gray region at the top is the epoxy used during cross-section sample
preparation.

162
Figure 4-57 A 4000X cathodoluminescence scan of an uncoated Y203:Eu powder taken
in a scanning electron microscope. Beam energy was 1.4 keV. For a size
marker, see Figure 4-58.

163
Figure 4-58 A 4000X secondary electron image of an uncoated YaC^Eu powder taken in
a scanning electron microscope. Beam energy was 1.4 keV.

164
Figure 4-59 A 4000X cathodoluminescence scan of an AI2O3 coating deposited for
1.2 minutes onto Y2Ü3:Eu powder. The image was taken in a scanning
electron microscope. Beam energy was 1.4 keV. For a size marker, see
Figure 4-60.

165
Figure 4-60 A 4000X secondary electron image of an AI2O3 coating deposited for
1.2 minutes onto YaC^Eu powder. The image was taken in a scanning
electron microscope. Beam energy was 1.4 keV.

166
Figure 4-61 Auger electron spectroscopy (AES) surface scan of a MgO film, deposited
for 8 minutes using PLD, on a Si shadow mask.

PEAK HEIGHT
167
0 10 20 30 40 50 GO 70 80 90
SPUTTER TIME. «in.
Figure 4-62 Sputter AES depth profile of a MgO film on a Si shadow mask. The area
analyzed is the same as in Figure 4-61. A 3x3 raster was used on the ion
sputter gun.

168
KINETIC ENERGY. eV
Figure 4-63 AES surface scan after depth profiling on a MgO-coated Si shadow mask.
This is the same area as in Figure 4-62. The trace shows complete removal
of the MgO film.

169
Figure 4-64 AES surface scan of a MgO film deposited for 8 minutes on Y2ÍOs:Tb
powder. The scan shows that the coating is continuous and uniform within
the escape depth of Auger electrons.

170
KINETIC ENERGY, eV
Figure 4-65 AES surface scan of uncoated Y2SiOs:Tb powder. The large decrease in the
signal at low energies is due to charging of the sample under the electron
beam.

PEAK HEIGHT
171
SPUTTER TIME, nin.
Figure 4-66 AES depth profile of MgO deposited for 8 min on Y2Si05:Tb powder. A
3x3 raster was used on the ion sputter gun. A 5 second delay was used after
sputtering and before collection of the Auger signal to improve the noise.

172
Figure 4-67 AES surface scan after the sputter depth profile of Figure 4-66 was taken.
The Mg signal that is still present is due to shadowing of the sputter ion
beam. The shadowing effect is a result of the uneven height of the particles.

173
Figure 4-68 Schematic depicting shadowing of the ion sputter beam due to the spherical
shape of the phosphor powder surface. The area depicted in the image will
not be sputtered by the ion beam, however, it will be detected in the AES
surface profile. As a result, there will be a small signal from the coating
constituents in all of the AES measurements regardless of sputter time.

CHAPTER 5
MODELING OF RESULTS
Introduction
In Chapter 4 and the literature review, it was demonstrated that coatings applied
to phosphor powders attenuate their luminescence under electron beam excitation. In this
chapter, a model will be presented to explain and predict this decrease in luminescence.
First, a simple “dead layer” model will be assumed. Then corrections will be added to
enhance the accuracy of the initial model. Corrections will take into account (1) the
contribution to the luminescence of the incident angle of incoming electrons, (2) the
uniformity of the coating, and (3) the electron energy loss at low energies. Consideration
will be given to the backscattering coefficient due to the coating and the index of
refraction, although it was found that neither was necessary for accurate prediction of the
luminescence from the coated phosphors.
Energy Loss in Dead Layer
When an electron enters a solid, it loses energy through inelastic collisions with
electrons of host lattice atoms. This energy is used to excite electrons into upper energy
levels. These excited electrons can then recombine with an available hole to produce
radiation, heat, X-rays, Auger electrons, etc. In a luminescent material, some of the
excited electrons will recombine at an activator site to produce visible light. In a
174

175
non-luminescent material, the energy released during recombination will be used in one
of the other processes mentioned above. A non-luminescent material on top of a
luminescent one is therefore called a “dead layer” because it absorbs energy that could be
used in the luminescent material to generate light.
Initially, a calculation is made to determine the energy of an electron at any point
along its trajectory into the solid. Empirically, it is found that the cathodoluminescent
(CL) intensity, L, follows the relationship:
L = B* E0y [5.1]
where Eo is the incident beam energy, y is a material constant usually in the range of 1.2
to 2.0 (its exact value will not be important for this model), and B is a constant dependent
upon the specific luminescent material. This equation allows for calculation of the
intensity of the light emitted from the sample.
According to the Rao-Sahib and Wittry correction to the Bethe stopping power
equation (cited in Chapter 2), the energy loss per incremental electron travel distance
(i.e., the stopping power) can be expressed as:30
dE_
dz
>-y
3 a
[5.2]
where z is the distance along the trajectory, E is the energy at point z along the primary
trajectory, and a is a constant dependent upon the material. By taking the integral of
Equation 5.2, the energy E at any point z along the trajectory can be related to the
incident energy Eo through:
E =
(
p y2 _ i!
a
v
[5.3]

176
Given the above three equations, the change in luminescence along the trajectory can be
found to be:
dL_2yB(
a
v
dz 3a
This can be checked for validity by integrating over the entire trajectory range. This
range, which will be referred to as zr, can be found by determining when the electron
energy has reached zero. Equation 5.3 then becomes:
[5.4]
0 =
V
3/ 7
JO/2
a
or
Zr=aE/\ [5.5]
The total luminescence is then found by integrating the change in luminescence from the
surface (z = 0) to the end of range zr:
Solving the integral after plugging in Equations 5.4 and 5.5 yields:
L-
2 yB
3 a
y z ^
F n __
2yÁf
\
a
-3 a
2y
\
J
= B*En
[5.6]
Comparison of Equations 5.1 and 5.6 reveals that they are identical. Thus Equation 5.4 is
self-consistent.
In order to calculate the effect of the coating on the luminescence of the
underlying phosphor, it was assumed that the coating only acts to reduce the energy of

177
the incoming electron before it can reach the phosphor/coating interface. It then becomes
necessary to determine the energy of an electron upon exiting a material of a known
thickness. Equation 5.3 provides just such an answer. The parameters that will be
needed are the path length in the dead layer (zrcj), the incident energy (Eo), and the
constant (a) for the coating material. By Equation 5.5, the constant (a) is related to the
incident energy and the total range of an electron in a bulk sample of the coating material
(zrb). Using Equations 5.3 and 5.5, the electron energy upon exiting the dead layer (Ei)
can be found to be:
£,=
l-
J rd
[5.7]
< Zrb
This equation was found to give an accurate prediction of the energy loss in the coatings
for energies above about 3 keV, depending on coating material and thickness. An
improved energy loss equation will be presented below to provide a better fit for lower
beam energies.
Incident Angle Contributions and Coating Uniformity
In order to accurately calculate the energy loss in the coating material, the path
length of the electron is required. In this section, the fractional contribution to the
luminescence from a given incidence angle will be calculated. This quantity will be
referred to as the incident angle probability for the purposes of this study. Some of the
equations derived in this calculation will be used to derive a more realistic representation
of the coating thickness and account for its non-uniformity. The fractional contribution
will then be recalculated based on the non-uniform coating.

178
As a first approximation, a uniform coating thickness will be assumed. It is also
assumed that the particles are spherical, all incoming electrons are traveling from the top
parallel to the mean surface normal of the particle, and the electron distribution is
uniform in the mean plane of the surface. The assumption of spherical particles is an
approximation to the shape of commercially processed phosphor powders. A phosphor
grown from a sol-gel process also generally exhibits a spherical shape. This assumption
of spherical particles also greatly reduces the complexity of the calculation. A uniform
electron beam traveling along a trajectory parallel to the mean surface normal of the
particles is valid for an electron gun located directly above the sample with low angular
divergence. (If the electron gun were located in some other orientation with a low
angular divergence, the treatment would be the same. However, knowledge of the
location of the neighboring particles would be necessary to account for any small changes
due to shadowing of the electrons by these particles.)
For this calculation, the center axis will be defined as the line that passes through
the center of the sphere and is parallel to the mean surface normal vector. The first step
in the calculation is to find the relationship between the angle 0 from the center axis
(which will also be the incident angle for a uniform coating) and the radius r from the
center axis of the particle. Figure 5-1 shows a cross-sectional view of the spherical
particle. From the figure, it can be seen that:
sin(0) =—-— [5.8]
r0+t
0 =sin‘
< r '
rn +t
or

179
where t is the thickness of the coating. The derivative of Equation 5.8 will be needed
later in the calculation, so it is presented here:
r = (r0 +f)sin(0)
dr = (r0 + /)d[sin(0)] = (r0 +1)cos(0)¿/0 . [5.9]
Next, an incremental area of the incoming electron beam is calculated. The electrons are
assumed to be uniform in the plane above the sphere. The particle’s projection onto this
plane will be a disk with a radius r, where r is the same as used above. Figure 5-2 shows
a representation of this disk with an arbitrary incremental area depicted. This incremental
area can be found by multiplying its two sides to give:
dAdisk =r(dr)(d Taking the integral over d(J) gives:
dAdisk = 2nr(dr).
Plugging in Equations 5.8 and 5.9 allow this incremental area to be related back to the
incident angle on the spherical particle:
dAdisk = 2n(r0 +t)2 sin(0) cos(0)i/0. [5.11]
Dividing Equation 5.11 by d0 will give the incremental area for any given 0. To find the
probability of an electron striking the particle at this angle, the equation must also be
divided by the area of the sphere. However, since the electrons can only strike the top
half of the particle, only the top half need be considered when finding the surface area:
AHemisphere = Mr0 + . [5.12]
Dividing Equation 5.11 by d0 and Equation 5.12 gives the probability P(0) for any given
incident angle 0 as:

180
1hemisphere
[5.13]
The above equation can also be derived by relating an incremental area on the
sphere to the incident electron flux. For this method, the first step is to find the
incremental area on the sphere. Figure 5-3 depicts this area. The equation is obtained by
multiplying the two sides that bound the box:
= (rd(t))[(r0 + t)dQ\= (r0 +1)~ sin(0)i/0i/(|).
dA
[5.14]
"sphere
Again, taking the integral over d0 gives 2n. Combining this with Equation 5.12 and
dividing by d0 results in the following equation:
[5.15]
This may seem contradictory to the probability found earlier, but the direction of the
incident electrons still needs to be considered. Thus far, spherical symmetry has been
assumed. If this were the case, the electrons would have to be incident along the surface
normal for any point on the sphere, meaning that the electrons would have to be
uniformly distributed over the surface of the sphere and all traveling toward the center.
For the model, it is assumed that all electrons are traveling in one direction, parallel to the
mean surface normal of the particle. To account for this, a relation must be found
between an incremental area of the incoming electrons and the corresponding area on the
sphere. As in the previous calculation, the incremental area of the electrons is equivalent
to that on a disk and is given by Equation 5.10. Similarly, the incremental area on the
sphere is represented by Equation 5.14. Combining these equations yields:

181
dAdisk _ r{dr\d dAsphere K^O +t X^X^) + t V6 '
Plugging in Equation 5.9 gives:
disk = cos(Q). [5.16]
dAsphere
This is the incremental area on the particle that corresponds to the same incremental area
of incident electrons. Multiplying Equations 5.15 and 5.16 gives the correct equation for
the probability:
HA
Where
A hemisphere d®
dAdisk
dA sphere
dA
—— = sin(0) cos(0).
Ahemisphered®
Therefore, both methods yield similar results for the probability, as can be see by
comparing the above equation with Equation 5.13. (The second method was included
because some of the equations derived from it will be used to calculate a more accurate
representation of the coating thickness.) A plot of P(0) versus 0 is shown in Figure 5-4.
The curve exhibits a maximum at 45° of 0.5. This shows that the majority of the
luminescence from the particles with a uniform coating comes from electrons that are
incident at 45° from surface normal. The fractional contribution goes towards zero as 0
goes towards zero or 90. The large angle dependence is due to the curvature of the
particle. The small angle dependence is a result of the diminishing area of interest as 0
goes towards zero.
Until now, it was assumed that the coating layer was uniform in thickness over
the surface of the powder particle. For an incident coating flux, uniform in the plane
perpendicular to the mean surface normal and deposited onto a spherical particle surface,

182
the resultant coating will not be uniform. The spherical shape of the powder will lead to
a thicker coating at the top of the particle and a thinner coating near the edges. This is
similar to considerations given during calculation of the incident angle probability for the
incoming electrons given above. From Equation 5.16, the ratio of an incremental area of
the deposition flux to the same area on the spherical surface shows a cosine dependence.
If the maximum thickness (at the top of the particle where 0 = 0) is defined as t, then the
thickness of the coating, as measured radially from the center of the particle at any given
angle 0, will be given by t*cos(0). See Figure 5-5 for a schematic.
A new incident angle probability will be calculated with the cosine dependent
thickness. All other assumptions are the same as before. The relationship between 0 (the
angle from the center axis) and r (the distance from the central axis to the coating surface)
is given by:
r = (r0 +tcos0)sin0 [5.21]
dr = [r0 cos 0 + t(cos 2 0 - sin 2 0 )Jc/0 .
An incremental area of the coating flux can be found to be:
dAdisk = 2n(r ){dr). [5.22]
The area of the top half of the sphere with the coating can be found by taking the integral
of the incremental area on the sphere from zero to 7t/2. The result is:
¿hemisphere = +r0t+3t2)- [5-23]
Plugging in Equations 5.21, 5.22, and 5.23, the incident angle probability can be found to
be:

183
F(0) =
dA-disk
Ahemisphere
[5.24]
or
P(0) =
r02sin0cos0 + 2r0tsin0cos2 0-rotsin3 0 + t2 sin0cos3 0-r2 sin3 0cos0
ro+rot + lt2
Figure 5-6 (derived in Appendix B) shows a plot of P(0) versus 0 for a variety of
thicknesses. As can be seen from the graph, the maximum of this function moves to
lower angles as the thickness is increased. This is a result of the 0 dependence of the
coating.
In the uniform coating model, 0 was not only the angle from the center axis, but
also the incidence angle from surface normal. This is no longer the case for the cosine
dependent coating because surface normal does not point radially out from the coating.
The curvature of the coating would have to be spherical for this to be the case. To find
the angle to surface normal (\j/), the slope of the surface normal vector is needed. This is
the negative reciprocal of the slope of the surface tangent. From Figure 5-5, it can be
seen that the height of the coated particle can be expressed as:
h = (r0 +rcos0)cos0. [5.25]
The slope of the tangent line can be expressed as:
dh dh^dQ
dr dQ dr
This can be found using Equation 5.21 and the derivative of Equation 5.25. The slope of
the surface normal is the negative reciprocal of the tangent line because the lines are
perpendicular. The slope of the surface normal therefore can be expressed as:

184
dr _ rocos0 + ícos20-/sin20 ^
dh rosin0 + 2/cos0sin0
The angle of the incident electron to the surface normal is the cotangent of the slope of
the surface normal. By Equation 5.26, the angle is:
[5.27]
Figure 5-7 (derived in Appendix B) shows a plot of the relationship between \j/ and 0.
The incident angle probability versus the angle to surface normal is shown in Figure 5-8.
It can be seen that all of the curves have a maximum around 45°, similar to the results
from the uniform coating.
Path Length of Electrons in the Coating
To determine the energy loss in the coating, a measure of the path length of the
electron in the coating is needed. A new angle (y) must be defined to facilitate this
computation. Figure 5-5 shows this schematically. The angle y will be defined as the
angle from the center axis (defined as before) to the point on the spherical surface
directly below the intersection of the radial line with the coating surface at angle 0. The
relationship between y and r can be expressed as:
r = r0siny. [5.28]
Combining this with Equation 5.21 gives:
y = sin
./ (r0 + rcos0)sin0
[5.29]

185
The coating thickness parallel to the center axis is found by subtracting the height of the
particle from the height of the particle and coating:
d = (r0 +1 cos 0)cos0-ro cosy.
Figure 5-9 (derived in Appendix C) shows the coating at various thicknesses. The
undercutting at the edge of the particle is due to shadowing by the coating during
deposition. Figure 5-10 (derived in Appendix C) shows the vertical coating thickness d
as a function of angle for multiple thicknesses. Assuming no diffuse scattering of the
electron as it passes through the coating, the distance d is the path length of the electron
in the coating. One interesting aspect of the thickness d is that it is almost constant for all
angles, except near the edge of the particle where the coating becomes nearly vertical.
The nearly constant value of d implies that the electron travels the same distance in the
coating irrespective of where it impinges on the surface. Thus, only one angle needs to
be considered for calculation of the energy loss in the coating layer. It should also be
pointed out that the thicker coatings in Figures 5-6 through 5-10 and Appendices B and C
are very large compared to the coatings applied for this work. These large values were
chosen to exaggerate and illustrate the effects, but they should not be considered an
accurate portrayal of the coatings studied here.
Calculation of the Cathodoluminescence from Coated Phosphors
To calculate the cathodoluminescence from a coated phosphor, the energy loss in
the coating needs to be determined. From the discussion above, the path length in the
coating will be equal to the thickness at 0 = 0, regardless of where the electron strikes the
particle. Given this path length, the next step is to determine the energy loss along this

trajectory. To start, the modified Bethe stopping power equation is used. Bethe’s
original equation is given in Equation 2.1 as:
dE -785pZ ( 1.166£'n\
— = —In
ds A* E ^ J )
where A is the atomic weight in g/mol, Z is the atomic number, E is the electron energy,
p is the density in g/cm3, and J is the mean ionization potential in eV.14-24> 27'30
Rao-Sahib and Wittry suggested a correction to the original equation to account for the
inflection at low energies. Their new equation was a parabolic extrapolation of the
stopping power from the inflection point at E = 6.338 J down to E = 0. The modified
equation is given in Equation 2.3 as30:
dE _ -785*p*Z
ds 1.2588*A*jE*J '
The low energy correction by Joy and Luo of the mean ionization potential is also used; it
is given in Equation 2.4 as14-29-31:
r=
i+*
where J’ is the new mean ionization potential, J is the average energy loss per interaction,
and k is a fitting parameter around 0.85. Appendix D shows a plot of the various
stopping power equations. The stopping power suggested by Love et al. is also shown
for comparison.131 The stopping power equation of Rao-Sahib and Wittry was chosen
for calculating the energy loss in the coating because it had the correct form and is widely
accepted as being more accurate. Equation 5.7 is the integrated stopping power equation.
This is the same equation used in the energy loss calculations.

187
The integrated stopping power equation requires knowledge of the incident
energy of the electrons, range in the coating, and the total path length of an electron in a
bulk sample of the coating material. As a first attempt, Monte Carlo simulations were
used to find the total path length in bulk, zrb.132’133 These simulations provided a
convenient method of finding the ranges at a few selected energies, but they were not
realistic for determining the ranges over the entire energy interval of interest. To
calculate the path length at any given incident energy, the stopping power equation was
integrated and the total range defined as the point where the energy of the electron went
to zero. Thus the total range can be expressed as:
¿rb =
252* A* J1* E0*
2355 *p*Z
[5.30]
where the energy values are expressed in keV and the range is in microns. The mean
ionization potential J given by Equation 2.2 is valid for Z > 13; since both of the coatings
used in this work had an average Z = 10, the following equation was chosen instead:
J = 11.5*Z [5.31]
where J is measured in eV. This equation is valid for Z < 12.29
The luminescence versus beam energy curves for the uncoated and coated
phosphors shown in Figures 4-19, 4-21, 4-23, 4-25, 4-27, 4-29, 4-31, 4-33, and 4-35 were
converted to a linear intensity versus energy scale to be more consistent with results
displayed in the literature. These are shown in Figures 5-11 through 5-19. These
luminescence curves were chosen because the main emission peak of the phosphor will
dominate emission from the sample. The curves have been fit using a power law
equation of the form:

188
L = aO + al* Ea2
where aO, al, and a2 are the fit parameters; they are displayed on the graphs.
Using all of the above parameters, the energy loss, based on the Rao-Sahib and
Wittry stopping power equation, is calculated as the electron passes through the coating.
The energy of the electron as it passes from the coating into the phosphor is then used to
find the luminescence by plugging this new lower energy into the fit equation for the
uncoated phosphor’s luminescence. The Mathcad programs used to calculate the
luminescence are presented in Appendices E through M. The calculated luminescence
versus beam energy curves are shown in Figures 5-20 through 5-28, along with the
measured luminescence from Figures 5-11 through 5-19.
New Energy Loss Equation
Examination of the calculated luminescence in Figures 5-20 through 5-28 shows
that the Rao-Sahib and Wittry equation does not give an accurate fit to the measured
curves at low energy. A second energy equation was used to improve the accuracy of the
fit. Many authors have shown that the Bethe stopping power equation overestimates the
energy loss at low electron energies. To correct for this, they used a cutoff energy in the
interval and added a residual range to the calculated result.28-29 Joy, based on results by
Nieminen, further suggested that the stopping power for all materials varies as E5/2 below
about 50 eV.29 Other authors have used the power loss function derived by Makhov that
says that the electron beam power per unit area at a depth x is given by the following:
[5.32]

189
where jo is the beam current density at the surface, Vo is the accelerating potential, and X
is the normalized depth given by:
where p is the density of the material and C and n are fitting parameters that are material
dependent. R(V0) is referred to as the electron range and is equal to the point when
P = j0V0 exp(-2) ,96-134 Kingsley and Prener also suggested a simple exponential
excitation function:
[5.33]
P(x,j0,V0) = 7o^oexP ——
that fit their data just as well as the Makhov function. This equation has a form similar to
Equation 5.32, with equal exponents. It is also similar to an equation given by Dowling
and Sewell.96
To provide a better fit to the measured luminescence from the coated phosphors,
Equation 5.33 was used. Dividing by the current gives the energy of an electron after
traveling a distance x through the coating. The resulting equation is:
[5.34]
where a' and k' are fitting parameters for the material of interest. One of the biggest
potential sources of error in implementing the new model involved finding an accurate
measurement of the coating thickness. To obtain an accurate value, the thickness used
for the model was adjusted slightly so that the attenuation predicted by the Bethe
stopping power equation matched the measured luminescence from the coated sample in
the higher energy region (around 4 keV). This method was chosen because the Bethe

190
stopping power equation has been shown by many authors to be an accurate prediction of
energy loss at energies of ~ 4 keV. The fitting parameters in Equation 5.34 were then
found so that the energy loss would match the Bethe equation at higher energies while
still accurately predicting the luminescence from the coated sample at low energies.
Once the constants a' and k' were found for one coating thickness, the energy loss
equation (5.34) could then be used to accurately predict the luminescent attenuation for
all of the other coating thicknesses for the same coating material. Plots of the energy of
the electron as it exits the coating that were made using the Bethe equation and
Equation 5.34 are shown in Figures 5-29 through 5-37 for each of the coatings used for
this study. The Mathcad programs used to derive these figures are presented in
Appendices E through M. The calculated and measured luminescence from the coated
sample is shown in Figures 5-20 through 5-28. Their derivation is also shown in
Appendices E through M. Comparison of the calculated attenuation in the luminescence
to the measured luminescence shows that this method allows for accurate prediction of
the luminescence from a coated particle.
Backscattering Coefficients
When a coating is placed on top of a phosphor, it will change the number of
electrons backscattered from the material. The coating material and its thickness could
play an important role in determining the backscattering coefficient for a given beam
energy. To study the effects on the backscattering coefficient, Windows-based Monte
Carlo simulation codes were used.132-133 Required parameters for the simulations
included average atomic number, average atomic weight, and density for each of the

191
materials. Table 6 gives a list of these. The simulations provided an output of the
backscattering coefficient for the current run directly on the plot. To find this number,
the program keeps track of the number of electrons lost from the top surface of the
material. It then compares this against the total number of incident electrons to calculate
the percentage backscattered. Multiple runs of 100,000 trajectories were done to ensure
sufficient averaging. Proceeding in such a manner, simulations were done for uncoated
Y2C>3:Eu and coatings of MgO and AI2O3 on Y203:Eu . For each of these, simulations
had to be completed for multiple beam energies at various coating thicknesses. Table 7 is
a compilation of all of the calculated backscattering coefficients. No simulations were
performed for the Y2SiOs:Tb samples because the thickness was so large that the
simulations predicted that only a small fraction, if any, of the incident electrons would
reach the phosphor material. This is due to the overprediction of the stopping power
based on the Bethe equation, as discussed above.
Incorporation of a correction into the luminescence model would require an
assumption that the difference in backscattering coefficient is due to electrons that
remained in the phosphor material as a result of the coating. Although the coating could
have contributed to a small change in the number of electrons lost from the phosphor, it is
not feasible to assume that the numbers calculated with the Monte Carlo simulations are
an accurate measure of these. Due to the agreement of the calculated
cathodoluminescence with the experimental data, no correction was used to account for a
change in the backscattering due to the presence of a coating layer.

192
Index of Refraction
If there were a large difference in the index of refraction between the film and
phosphor, then critical angle issues would have to be considered for a smooth surface.
When electromagnetic radiation passes from one medium to another, its direction is
changed. Snell’s Law describes this change135:
n, sin(91) = n2sin^2)
where n is the index of refraction and 0 is the angle from surface normal. If the light
travels from a medium with a higher index to one with a lower index, then there is an
angle, 0C, at which all of the light will be internally reflected. This is called the critical
angle and is given by the following70:
where ni and n2 are the higher and lower index of refraction, respectively. Since the
phosphor’s index is larger than the air around it, the light generated in the phosphor will
exhibit a critical angle dependence. Applying a coating to the surface of the phosphor
can affect the light rays and change this angular dependence. Table 8 shows the index of
refraction for the coatings and phosphors at the wavelength of interest for the light
produced by the phosphors. It can be seen from the table that the indices of the coatings
and phosphors are similar. Thus, there will be little effect on the light traveling from the
particle into the coating. Also, the critical angle of the light leaving the coating will be
nearly the same as for the uncoated powder. For these reasons, no corrections were made
in the model to account for a difference in the index of refraction.

193
Figure 5-1 Cross-sectional schematic of spherical phosphor with a uniform coating layer
of thickness t. The angle from the center axis, 0, will be equal to the incident
electron angle from surface normal because of the uniform coating.

194
Figure 5-2 Schematic of incremental area of the incident electrons. This also represents
the planar projection of the spherical particle as seen by the incoming
electrons.

195
136
Figure 5-3 Schematic of the phosphor surface with an incremental area depicted.

Area Weighted Contribution to
196
I
3
0.6
0.5
0.4
0.3
0.2
0.1
0
T 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
J I I I I I I I I I I I I I I I I
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
8 (degrees)
Figure 5-4 Area weighted contribution to the luminescence versus the angle from the
center axis of the particle for a uniform coating. The center axis has been
defined as the line passing through the center of the particle parallel to the
mean surface normal.

197
Figure 5-5 Schematic of phosphor particle with a non-uniform coating that varies in
thickness as t*cos(0), where 0 is the angle from the center axis. All other
variables are defined in the text.

Area Weighted Contribution to the Luminescence
198
Figure 5-6 Area weighted contribution to the luminescence as a function of the angle
from the mean surface normal for a cosine-dependent coating thickness. The
thickness values shown are for 0 = 0.

199
Figure 5-7 The angle from surface normal versus the angle from the mean surface
normal for a cosine-dependent coating thickness. The thickness values
shown are for 0 = 0.

Area Weighted Contribution to the Luminescence
200
Figure 5-8 Area weighted contribution to the luminescence as a function of the angle of
incidence from surface normal for a cosine-dependent coating thickness. The
thickness values shown are for 0 = 0.

201
Figure 5-9 The height of the coating as a function of the distance from the center axis for
a uniform coating flux deposited onto a spherical particle. The center axis
has been defined as the line passing through the center of the particle parallel
to the mean surface normal. The undercutting of the coating is due to
shadowing during the deposition process. The thickness values shown are
the maximum thickness of the coating, which will be located at the top of the
particle.

distance through coating (¿¿m)
202
Figure 5-10 The vertical distance through the coating as a function of the angle from the
mean surface normal for a cosine-dependent coating thickness. The
thickness values shown are for 0 = 0.

INTENSITY (ARB UNITS)
203
160
-
L=aO+al*E"a2
140
-
Unco ated
_
aO = -7.62236313516689 e+3
120
al - 1.83137820423457 e+4
a2 - 1.43141730761361
100
Coated
yf S'
aO - -9.9356600351823 e+3
S' S'
â– 
al = 1.49033861347459 e+4
80
a2 = 1.52742562074871
S'
60
yf .
s
40
UNCOATED
20
A MgO(4)
n
- A-n 1 1 1
i i 1 i
1 2 3 4 5
VOLTAGE (keV)
Figure 5-11 The cathodoluminescence intensity of the 611 nm peak as a function of
beam energy for a coating of MgO deposited for 4 minutes on Y2C>3:Eu
powder. Both coated and uncoated results are shown. The beam current
was kept constant at 0.16 pA/cm2, while the accelerating voltage was varied
from 0.8 to 4 keV. The data were fitted to a power law equation, with the
fit parameters shown on the graph.

204
120
(A
t
GQ
OC
<
100 -
80
us
~u
c
ID
UD
3
O
.c
t-
60
40
20
L=aO+al*EAa2
Uncoated
aO = -2.1553761365568e+003
al = 7 8235646626320le+003
a2 = 1.78566539748617
Coated
aO = -2 0661370398617e+003
al =2 29888121748175e+003
a2 = 2.4293699844816
/ /
/ /
/
+ UNCOATED
A MgO(8)
l i
VOLTAGE (keV)
Figure 5-12 The cathodoluminescence intensity of the 611 nm peak as a function of
beam energy for a coating of MgO deposited for 8 minutes on Y2C>3:Eu
powder. Both coated and uncoated results are shown. The beam current
was kept constant at 0.16 pA/cm2, while the accelerating voltage was varied
from 0.8 to 4 keV. The data were fitted to a power law equation, with the
fit parameters shown on the graph.

205
co
CD
DC
<
CO
z
LU
Cfl
T3
C
(0
CO
3
o
b
120
100
80
60
40
20 -
L-aO+a l*E"a2
Uncoated
aO = -7.2SS51157499782e+003
al = 1.42940056423308e+004
a2 - 1.33770010631825
Co ated
aO
al
a2
-6.261633176897 Ole+003
9.6051329759772le+003
1.58503483410552
X
/
/
X
x
X
x
UNCOATED
A1203(1.2)
VOLTAGE (keV)
Figure 5-13 The cathodoluminescence intensity of the 611 nm peak as a function of
beam energy for a coating of AI2O3 deposited for 1.2 minutes on Y2C>3:Eu
powder. Both coated and uncoated results are shown. The beam current
was kept constant at 0.16 pA/cm2, while the accelerating voltage was varied
from 0.8 to 4 keV. The data were fitted to a power law equation, with the
fit parameters shown on the graph.

206
150
Vi
b
m
DC
Vi
u¡
â– o
c
<0
u>
13
o
120 -
90
60 -
30 -
L=aO+al*E''a2
Uncoated
aO = -3.6413299776774Se+003
al = 1.22897133336396e+004
a2= 1 61115422022399
Coated
aO = -6 70266215869945e+003
al = 8.2079281 3658368e+003
a2 = 1 79982256009043
/
UNCOATED
A A1203 (2.4)
2 3
VOLTAGE (keV)
Figure 5-14 The cathodoluminescence intensity of the 611 nm peak as a function of
beam energy for a coating of AI2O3 deposited for 2.4 minutes on Y2C>3:Eu
powder. Both coated and uncoated results are shown. The beam current
was kept constant at 0.16 pA/cm2, while the accelerating voltage was varied
from 0.8 to 4 keV. The data were fitted to a power law equation, with the
fit parameters shown on the graph.

207
V)
b
CD -O
< to
E|
z
LU
900
800
700
600
500
400
300
200
100
0
-
L=aO+al*E~a2
-
Uncoated
X /
-
aO = -3.3539296789554e+004
/
/
A
-
al = 1.06999152628346e+005
-
a2 = 1.47933368154553
-
Coated
/ /
-
aO = -7.14397526126849e+004
x /
-
al = 6.64654699205777e+004
x X
-
a2 = 1 719254605608
X X
jS
x
x
x
+
UNCOATED
-
X X
-
A
MgO(4)
-
^X yX
-
xX ^
kZL 1 1 1 i
1 I
_| I
VOLTAGE (keV)
Figure 5-15 The cathodoluminescence intensity of the 547 nm peak as a function of
beam energy for a coating of MgO deposited for 4 minutes on Y2SiOs:Tb
powder. Both coated and uncoated results are shown. The beam current
was kept constant at 0.16 jxA/cm2, while the accelerating voltage was varied
from 0.8 to 4 keV. The data were fitted to a power law equation, with the
fit parameters shown on the graph.

208
co
co
ir
<
to
TD
C.
aS
to
3
o
CO
z
LU
600
500
400
300
200
100 -
L=aO+al*E‘'a2
Uncoated
aO = -1.07141 806607 322e+004
al = 5.31865450116782e+004
a2 = 1.53937175151185
Coated
aO = -2.51103277212396e+004
al = 2.05371456793615e+004
a2 = 2.0320565403483
_L
2 3
VOLTAGE (keV)
/
s.
t UNCOATED
A MgO(8)
Figure 5-16 The cathodoluminescence intensity of the 547 nm peak as a function of
beam energy for a coating of MgO deposited for 8 minutes on Y2Si05:Tb
powder. Both coated and uncoated results are shown. The beam current
was kept constant at 0.16 [xA/cm2, while the accelerating voltage was varied
from 0.8 to 4 keV. The data were fitted to a power law equation, with the
fit parameters shown on the graph.

209
200
160
r—.

b
z
m ? 120
< te
>: o
»t 80
Z
LU
I
40
0
1 2 3 4 5
VOLTAGE (keV)
Figure 5-17 The cathodoluminescence intensity of the 547 nm peak as a function of
beam energy for a coating of A1203 deposited for 1.2 minutes on Y2Si05:Tb
powder. Both coated and uncoated results are shown. The beam current
was kept constant at 0.16 pA/cm , while the accelerating voltage was varied
from 0.8 to 4 keV. The data were fitted to a power law equation, with the
fit parameters shown on the graph.
L=aO+al*E/va2
Uncoated
aO = -4 5186519683078 e+3
al = 9.61525630600558 e+3
a2 = 1.83509379303954
Coated
aO = -7.73226472803259 e+3
al = 6.32789998475401 e+3
a2 = 2.0881981625911
/
/ ,
/
/
+ UNCOATED
A AI203(12)
L

210
cn
m
ac
<
E
V)
z
LU
tfj
•O
to
o
8UU
L=aO+al*EAa2
y /
X /
700
Uncoated
-
a0 = -7.19918149270744 e+4
X /
/ /
600
al = 1.38768408800792 e+5
X /
/
a2 = 1.22985327091371
Coated /
500
ao = -4.33461658805846 e+4 X
-
al = 2.64210300173364 e+4
X
400
-
a2 = 2 18952712892998
S
/
/
300
-
/ /
“
UNCOATED
200
-
/ /
A AI203(2.4 eq)
100
0
1 _ -A'i 1 1 1
. i •
1 2 3
4 5
VOLTAGE (keV)
Figure 5-18 The cathodoluminescence intensity of the 547 nm peak as a function of
beam energy for a coating of AI2O3 deposited for 5 minutes on Y2SiOs:Tb
powder in new chamber. Both coated and uncoated results are shown. The
beam current was kept constant at 0.16 pA/cm2, while the accelerating
voltage was varied from 0.8 to 4 keV. The data were fitted to a power law
equation, with the fit parameters shown on the graph.

211
800
700
en
H
3 ~
w
m -o
< <0
El
(ft c.
|
600
500
400
300
200
100
0
' L=aO+al*E/'a2 + UNCOATED
Uncoated l aipcwri
• aO = ■ 1.75187745172519 e+3 ( )
- al = 5.74573949151565 e+4
- a2 = 1 67325236613149
_ Coated
. aO = 3.39760988734409 e+2
al = 9.22481210844051 e+2
a2 = 3.28254555216493
0 1 2 3 4 5
VOLTAGE (keV)
Figure 5-19 The cathodoluminescence intensity of the 547 nm peak as a function of
beam energy for a coating of AI2O3 deposited for 5 minutes on Y2SiÜ5:Tb
powder. Both coated and uncoated results are shown. The beam current
was kept constant at 0.16 pA/cm\ while the accelerating voltage was varied
from 0.8 to 4 keV. The data were fitted to a power law equation, with the
fit parameters shown on the graph.

Luminescence Intensity (arb. units)
212
Figure 5-20 Luminescence intensity as a function of beam voltage for a 4 minute coating
of MgO on Y2Ü3:Eu powder. Curves are shown for the measured
cathodoluminescence from both the coated and uncoated phosphor.
Calculated cathodoluminescence intensity from the coated sample using
both the Rao-Sahib and Wittry stopping power equation and the new energy
loss equation are also shown. From the graph, it can be seen that the new
energy loss equation allows for an improved calculation of the
cathodoluminescence at low energies. Markers on the curves are for curve
differentiation only and do not represent actual measured values.

Luminescence Intensity (arb. units)
213
Incident Beam Energy (keV)
Figure 5-21 Luminescence intensity as a function of beam voltage for a 8 minute coating
of MgO on Y2C>3:Eu powder. Curves are shown for the measured
cathodoluminescence from both the coated and uncoated phosphor.
Calculated cathodoluminescence intensity from the coated sample using
both the Rao-Sahib and Wittry stopping power equation and the new energy
loss equation are also shown. From the graph, it can be seen that the new
energy loss equation allows for an improved calculation of the
cathodoluminescence at low energies. Markers on the curves are for curve
differentiation only and do not represent actual measured values.

Luminescence Intensity (arb. units)
214
Figure 5-22 Luminescence intensity as a function of beam voltage for a 1.2 minute
coating of AI2O3 on Y2 cathodoluminescence from both the coated and uncoated phosphor.
Calculated cathodoluminescence intensity from the coated sample using
both the Rao-Sahib and Wittry stopping power equation and the new energy
loss equation are also shown. From the graph, it can be seen that the new
energy loss equation allows for an improved calculation of the
cathodoluminescence at low energies. Markers on the curves are for curve
differentiation only and do not represent actual measured values.

Luminescence Intensity (arb. units)
215
Incident Beam Energy (keV)
Figure 5-23 Luminescence intensity as a function of beam voltage for a 2.4 minute
coating of AI2O3 on Y2C>3:Eu powder. Curves are shown for the measured
cathodoluminescence from both the coated and uncoated phosphor.
Calculated cathodoluminescence intensity from the coated sample using
both the Rao-Sahib and Wittry stopping power equation and the new energy
loss equation are also shown. From the graph, it can be seen that the new
energy loss equation allows for an improved calculation of the
cathodoluminescence at low energies. Markers on the curves are for curve
differentiation only and do not represent actual measured values.

Luminescence Intensity (arb. units)
216
Incident Beam Energy (keV)
Figure 5-24 Luminescence intensity as a function of beam voltage for a 4 minute coating
of MgO on Y2SiC>5:Tb powder. Curves are shown for the measured
cathodoluminescence from both the coated and uncoated phosphor.
Calculated cathodoluminescence intensity from the coated sample using
both the Rao-Sahib and Wittry stopping power equation and the new energy
loss equation are also shown. From the graph, it can be seen that the new
energy loss equation allows for an improved calculation of the
cathodoluminescence at low energies. Markers on the curves are for curve
differentiation only and do not represent actual measured values.

Luminescence Intensity (arb. units)
217
Incident Beam Energy (keV)
Figure 5-25 Luminescence intensity as a function of beam voltage for a 8 minute coating
of MgO on Y2Si05:Tb powder. Curves are shown for the measured
cathodoluminescence from both the coated and uncoated phosphor.
Calculated cathodoluminescence intensity from the coated sample using
both the Rao-Sahib and Wittry stopping power equation and the new energy
loss equation are also shown. From the graph, it can be seen that the new
energy loss equation allows for an improved calculation of the
cathodoluminescence at low energies. Markers on the curves are for curve
differentiation only and do not represent actual measured values.

Luminescence Intensity (arb. units)
218
Incident Beam Energy (keV)
Figure 5-26 Luminescence intensity as a function of beam voltage for a 1.2 minute
coating of AI2O3 on Y2Si05:Tb powder. Curves are shown for the
measured cathodoluminescence from both the coated and uncoated
phosphor. Calculated cathodoluminescence intensity from the coated
sample using both the Rao-Sahib and Wittry stopping power equation and
the new energy loss equation are also shown. From the graph, it can be
seen that the new energy loss equation allows for an improved calculation
of the cathodoluminescence at low energies. Markers on the curves are for
curve differentiation only and do not represent actual measured values.

Luminescence Intensity (arb. units)
219
Figure 5-27 Luminescence intensity as a function of beam voltage for a 5 minute coating
in the new chamber of AI2O3 on Y2SiOs:Tb powder. Curves are shown for
the measured cathodoluminescence from both the coated and uncoated
phosphor. Calculated cathodoluminescence intensity from the coated
sample using both the Rao-Sahib and Wittry stopping power equation and
the new energy loss equation are also shown. From the graph, it can be
seen that the new energy loss equation allows for an improved calculation
of the cathodoluminescence at low energies. The poor agreement between
the calculated and measured luminescence is due to the fitting equation
found for the measured luminescence. Markers on the curves are for curve
differentiation only and do not represent actual measured values.

Luminescence Intensity (arb. units)
220
Incident Beam Energy (keV)
Figure 5-28 Luminescence intensity as a function of beam voltage for a 5 minute coating
of AI2O3 on Y2Si05:Tb powder. Curves are shown for the measured
cathodoluminescence from both the coated and uncoated phosphor.
Calculated cathodoluminescence intensity from the coated sample using the
new energy loss equation is also shown. No luminescence is predicted over
this energy range using the Rao-Sahib and Wittry energy loss calculation.
Markers on the curves are for curve differentiation only and do not
represent actual measured values.

Energy After Passing Through Coating (keV)
221
Figure 5-29 Energy of the electron after passing through coating layer as a function of
incident beam energy for a 4 minute coating of MgO on Y2Ü3:Eu. Shown
are the energies based on the Rao-Sahib and Wittry stopping power
equation and the new energy loss equation. The fitting parameters in the
new energy loss equation were chosen so that the energy loss would match
that from the Rao-Sahib and Wittry equation at high energies (above about
3 keV), while allowing for a better fit to the measured luminescence at low
energies.

Energy After Passing Through Coating (keV)
222
Figure 5-30 Energy of the electron after passing through coating layer as a function of
incident beam energy for a 8 minute coating of MgO on Y203:Eu. Shown
are the energies based on the Rao-Sahib and Wittry stopping power
equation and the new energy loss equation. The fitting parameters in the
new energy loss equation were chosen so that the energy loss would match
that from the Rao-Sahib and Wittry equation at high energies (above about
3.5 keV), while allowing for a better fit to the measured luminescence at
low energies.

Energy After Passing Through Coating (keV)
223
Incident Beam Energy (keV)
Figure 5-31 Energy of the electron after passing through coating layer as a function of
incident beam energy for a 1.2 minute coating of AI2O3 on YaC^Eu.
Shown are the energies based on the Rao-Sahib and Wittry stopping power
equation and the new energy loss equation. The fitting parameters in the
new energy loss equation were chosen so that the energy loss would match
that from the Rao-Sahib and Wittry equation at high energies (above about
3 keV), while allowing for a better fit to the measured luminescence at low
energies.

Energy After Passing Through Coating (keV)
224
Figure 5-32 Energy of the electron after passing through coating layer as a function of
incident beam energy for a 2.4 minute coating of AI2O3 on Y2C>3:Eu.
Shown are the energies based on the Rao-Sahib and Wittry stopping power
equation and the new energy loss equation. The fitting parameters in the
new energy loss equation were chosen so that the energy loss would match
that from the Rao-Sahib and Wittry equation at high energies (above about
3 keV), while allowing for a better fit to the measured luminescence at low
energies.

Energy After Passing Through Coating (keV)
225
Figure 5-33 Energy of the electron after passing through coating layer as a function of
incident beam energy for a 4 minute coating of MgO on Y2SiOs:Tb. Shown
are the energies based on the Rao-Sahib and Wittry stopping power
equation and the new energy loss equation. The fitting parameters in the
new energy loss equation were chosen so that the energy loss would match
that from the Rao-Sahib and Wittry equation at high energies (above about
3 keV), while allowing for a better fit to the measured luminescence at low
energies.

Energy After Passing Through Coating (keV)
226
Figure 5-34 Energy of the electron after passing through coating layer as a function of
incident beam energy for a 8 minute coating of MgO on Y2SiOs:Tb. Shown
are the energies based on the Rao-Sahib and Wittry stopping power
equation and the new energy loss equation. The fitting parameters in the
new energy loss equation were chosen so that the energy loss would match
that from the Rao-Sahib and Wittry equation at high energies (above about
3.5 keV), while allowing for a better fit to the measured luminescence at
low energies.

Energy After Passing Through Coating (keV^
227
Figure 5-35 Energy of the electron after passing through coating layer as a function of
incident beam energy for a 1.2 minute coating of AI2O3 on Y2SiOs:Tb.
Shown are the energies based on the Rao-Sahib and Wittry stopping power
equation and the new energy loss equation. The fitting parameters in the
new energy loss equation were chosen so that the energy loss would match
that from the Rao-Sahib and Wittry equation at high energies (above about
3 keV), while allowing for a better fit to the measured luminescence at low
energies.

Energy After Passing Through Coating (keV)
228
Figure 5-36 Energy of the electron after passing through coating layer as a function of
incident beam energy for a 5 minute coating in the new chamber of AI2O3
on Y2SiC>5:Tb. Shown are the energies based on the Rao-Sahib and Wittry
stopping power equation and the new energy loss equation.

Energy After Passing Through Coating (keV)
229
Figure 5-37 Energy of the electron after passing through coating layer as a function of
incident beam energy for a 5 minute coating of AI2O3 on Y2SiOs:Tb.
Shown are the energies based on the Rao-Sahib and Wittry stopping power
equation and the new energy loss equation. The Rao-Sahib and Wittry
stopping power predicts that no electrons will penetrate this coating over the
energy range from 0 to 4 keV.

230
Table 6
Average atomic number, density, and average atomic weight of the different materials
used in this work.
Material
Average Atomic
Number (Z)
Density
g/cm3
Average Atomic
Weight
AI2O3
10
3.97
20.3922
MgO
10
3.6
20.152
Si
14
2.3290
28.086
Y2O3
20.4
5.03
45.162
Y2Si05
16.5
4.43
35.7368
C (graphite)
6
2.5
12.011
Atomic number and atomic weight numbers tabulated from periodic table. Density
values for all elements except Y2SÍO5 taken from CRC Handbook. Density value for
Y2SÍO5 taken from reference 51.

231
Table 7
Backscattering coefficients calculated using Monte Carlo simulations.
Backscattering Coefficient (%)
Material
1.4 keV
2.5 keV
4.0 keV
Y2O3
36.865
37.19
37.425
MgO (73 A) /Y203
36.3
37.215
37.825
MgO (146 Á) /Y203
26.03
34.835
37.195
AI2O3 (78 A)/Y203
32.7
36.77
37.54
A1203 (156 Á) /Y2O3
25.94
29.67
36.6
No backscattering simulations were performed for the Y2SÍO5 samples.

232
Table 8
Optical constants for selected materials.
Material
N
k
Si02(reference)
1.46
0C
MgO
1.736a and 1.752 b
0C
A1203
1.765 a
0C
Y203:Eu
1.93
0C
Si
3.85
-.02
a data taken from CRC Hand
o
o
b Based on a linear fit to data from OPTICAL WAVES IN LAYERED MEDIA by Pochi
Yeh 500 nm - n=1.76; 1000 nm - n=1.72
c Actual values are on the order of 10'7, which is insignificant in the calculations used for
this work

CHAPTER 6
DISCUSSION
Introduction
In Chapter 4, cathodoluminescence data were presented for coatings of MgO and
AI2O3 ablated onto powders of Y2Ü3:Eu and Y2SiOs:Tb by pulsed laser deposition. It
was seen that the coatings caused a reduction in the cathodoluminescent intensity from
the phosphors. Also, thicker coatings caused a larger attenuation of the intensity. In
Chapter 5, a model was presented to help explain and predict the loss in luminescence
from the coated samples. In this chapter, the validity of the model and its implications
for the effects of the coatings will be discussed.
Initial Uniform Coating Model
Before development of the model discussed in Chapter 5, a model had been
developed to predict the cathodoluminescence from a coated sample, based on the
assumption of a uniform coating. This earlier model was designed to predict the loss in
luminescence at a single beam energy and coating thickness. Corrections for the
fractional contribution to the luminescence based on incident angle and backscattering
coefficient were incorporated to help improve the accuracy of the calculation. As with
the model presented in Chapter 5, it was assumed that the coatings were acting as dead
layers, i.e., they were absorbing energy from the incident electrons and therefore
233

234
reducing the energy available to produce luminescence in the phosphor. An energy loss
calculation was performed based on Rao-Sahib and Wittry’s modified Bethe stopping
power equation. The ranges used in the calculation were obtained by measuring
trajectories in a Monte Carlo simulation program. Data were fit by comparison of the
energy determined by the calculation to that determined from the luminescence intensity
versus beam energy curves.
Although the model appeared to give reasonable fits to the energy loss in the
coating layers, there were a few drawbacks that limited its applicability. First, it only
allowed for prediction of the energy loss (and therefore the luminescence) at a single
thickness and energy. If an additional energy or thickness was desired, the calculation
had to be repeated, which meant running new Monte Carlo simulations with the updated
parameters. In order to get accurate statistical averaging, many trajectories had to be
measured with the simulations. This caused the energy loss calculation to be very time-
consuming, and therefore prediction of the coated phosphor’s luminescence at more than
a few selected energies was unfeasible.
Another drawback to this initial model was that it only worked for coatings less
than about 175 Á. For thicker coatings, the model could not predict the energy loss
accurately. This was especially noticeable with the thicker coatings on the Y2SiOs:Tb
powders from the second set of samples (see Coating Thickness and Uniformity section
in Chapter 4). The model predicted that electrons should not be able to penetrate a
coating that was thicker than their path length. Therefore, no luminescence should be
present at beam energies below the point at which the electron penetration range equals
the thickness of the coating. This was not the case with the thicker set of coatings, which

235
continued to display luminescence well below this point. The reason for this discrepancy
and the method used to fix it will be discussed in the new model below.
One of the biggest problems with modeling the effects of the coatings on the
phosphor luminescence is getting an accurate determination of the thickness of the
coating on the phosphor. In this study, two different methods were used for determining
coating thickness. The thickness on the first set of samples was determined by measuring
the thickness of films grown, under the same conditions as those for the powders, for
5 minutes and using a linear growth rate extrapolation. These included the following
samples: MgO (4 min.)/Y203:Eu, MgO (8 min.)/Y203:Eu, A12C>3 (1.2 min.)/Y2C>3:Eu,
A1203 (2.4 min.)/Y203:Eu, and A1203 (1.2 min.) /Y2Si05:Tb. Thickness for the second set
of samples was found from the Si masks used to cover these samples during deposition.
These included the following samples: MgO (4 min.)/Y2SiOs:Tb, MgO (8 min.)/
Y2Si05:Tb, Al203 (5 min.)/Y2SiOs:Tb, and Al203(5 min. in new chamber)/Y2SiOs:Tb.
Values obtained from the second set are assumed to be more accurate since they were
taken from the masks used to cover the samples during the actual deposition. In the
discussion of the model below, it is shown that the thicknesses for the first set of samples
may be greater than originally believed, based on the linear growth rate extrapolation.
This initial model assumed a uniform coating on the phosphor surface. Since
there was a uniform deposition flux over a spherical particle surface, this is not a realistic
depiction of the coating thickness. Therefore, results could not predict the luminescence
from the coated sample accurately. The use of the Rao-Sahib and Wittry stopping power
equation also reduced the accuracy of the calculation at energies below about 3 keV, as
discussed below. A new model was developed to account for the non-uniform coating.

236
Validity of the Energy Loss Model
To overcome the problems of the initial model, a new model was developed; this
new model was discussed in Chapter 5. The new model allows for fast and accurate
calculation of the cathodoluminescence intensity from a coated phosphor over the entire
energy range up to 4 keV. In this model, the coating thickness was assumed to have a
cosine dependence, as predicted from the incremental area of the spherical phosphor
surface as compared to that same area of the incident coating flux. From the non-uniform
coating thickness, it was found that the electron path length through the coating is nearly
the same irrespective of where the electron enters the coating, assuming that there is no
diffuse scattering. The electron energy loss was then predicted for beam energies from
zero to 4 keV using Rao-Sahib and Wittry’s modified Bethe stopping power equation.
Luminescence for the coated particle was then calculated based on this new lower energy
electron. Calculated curves agreed well with the measured luminescence from the coated
phosphors at energies above about 3 keV, but they overestimated the attenuation of the
luminescence at lower energies. To provide a more accurate fit, a new energy loss
equation was introduced based on a modification of the Makhov power loss equation for
thin films.
The modification of the Bethe stopping power equation proposed by Rao-Sahib
and Wittry has been widely referenced as an accurate prediction of the energy loss at
beam energies of about 4 keV and up, although some suggest that the low energy limit is
closer to 1 keV. When used to predict the luminescence from the second set of coated
samples, this equation gave accurate results for the high-energy (about 3 keV and up,

237
depending on the sample) region of the curves. However, at low energies, the
luminescence predicted was lower than that measured from the samples. The calculated
luminescence for the first set of samples found using the linear extrapolation thicknesses
was much lower than the measured luminescence. The most probable explanation for
this is that the samples used to measure the thicknesses were deposited under slightly
different conditions than those used for the coatings ablated onto the phosphors. This is
possible since the films grown on the Si to determine the thicknesses were deposited after
the coatings were ablated onto the phosphors, and there is no way to ensure that the two
were exactly the same thickness.
For the modeling of the data, it was assumed that the modified Bethe stopping
power equation was an accurate prediction of the energy loss for beam energies from
about 3 keV and higher. The thickness of the coatings was then adjusted so that the
calculated and measured luminescence from the coated samples matched over this energy
range. The thickness values determined in this manner closely agreed with those found
from ellipsometry for the second set of samples. However, the values for the first set of
samples was almost double that found from ellipsometry, for reasons mentioned in the
previous paragraph. For a comparison of the thickness values, see Table 9 at the end of
this chapter.
As stated earlier, the luminescence at low energies calculated through application
of a modified Bethe stopping power equation predicted a larger attenuation than was
measured from the samples. The Bethe stopping power equation is based on an average
stopping power for the electrons. It does not include the cross-section for scattering of
the electrons as they travel through the material. In applications of beam energies above

238
about 4 keV, this is not a problem because the low energy region of the electron
trajectory does not contribute greatly to the overall process. However, for beam energies
below this value, the low energy electrons need to be taken into account to get an
accurate representation of the electron stopping power. The Rao-Sahib correction to the
original Bethe equation is a parabolic extrapolation that allows for energies below the
inflection point of the original equation at E = 6.338 J, where E is the electron energy and
J is the mean ionization potential for all energy loss processes. Therefore, it does not
specifically address the low energy electron stopping power, but rather retains the form of
the original equation. Therefore, the Rao-Sahib stopping power equation overestimates
the stopping power for low energy electrons.
To provide a more accurate fit to the low energy electrons, the scattering of the
electrons as they traverse the coating layer should be considered. The screened
Rutherford scattering cross section is widely used for computation of the scattering of
electrons in solids. However, at energies below 20 keV or for materials with high atomic
numbers, the Mott cross section is needed to give an accurate representation of the
scattering of the energetic electrons with the lattice.24 The Mott cross section equation is
more accurate in these situations because it is derived from a more exact treatment of the
elastic scattering using quantum mechanics. However, there is no straightforward
method of calculating the Mott scattering cross section based on materials parameters
alone. Some Monte Carlo simulations utilizing these expressions rely on databases of
measured scattering at incremental energies and use various extrapolation procedures to
determine the intermediate energies. Others use expressions that give an approximation
of the Mott scattering based on the Rutherford scattering expression, although these still

239
require knowledge of the scattering from the material of interest to find the fitting
parameters.31-13 137
Since calculation of the Mott scattering cross sections required knowledge of the
scattering in the two coating materials, it was beyond the scope of this research project.
Instead, an improved energy loss calculation was sought that would account for the
extended range of the low energy electrons. Researchers such as Kanaya et al. and Love
et al. have suggested stopping power equations that extend the useful range of the Bethe
equation to lower energies. The problem with most of these equations is that they require
the use of multiple fitting parameters and, in cases where more than one beam energy
dependent term is used, do not allow for convenient determination of the electron energy
loss upon integration. To keep the number of fitting parameters to a minimum and still
provide an accurate fit to the energy loss at low beam energies, an energy loss equation
similar to one suggested by Kingsley and Prener was chosen.96 The new energy loss
equation is:
E = E0 exp —
f ~2x ^
a * Ek
¿0 J
where E is the energy at a point x along the path length, Eo is the incident beam energy,
and a' and k' are fitting parameters for the material of interest. For a coating applied to a
phosphor, the fitting parameters should only be dependent upon the properties of the
coating material since the reduction in luminescence comes from energy loss processes in
the coating layer.
Results of the calculated cathodoluminescence intensity as a function of beam
energy using the above energy loss equation are shown in Figures 5-20 through 5-28.

240
Comparison with the measured luminescence from coated samples shows that the new
equation allows for very accurate prediction over the entire energy range up to 4 keV.
The fitting parameters were found to be 0.064 for a' and 1.36 for k' when the coating
thickness is measured in micrometers and the energy terms are in keV. These constants
were found to be appropriate for all coating thicknesses studied and for both coating
materials, although the constants are not expected to be the same for all coating materials.
The similarity in density, average atomic number, and average atomic weight of MgO
and AI2O3 could account for the fact that the same fitting parameters worked for both
materials.
Due to the need to find the two fitting parameters (a' and k') for a given coating
material, one coating will have to be deposited onto a phosphor. The Mathcad program
can then be used to find the values of a' and k' for this coating material. Once these are
determined, the coating’s effect on the cathodoluminescence can be predicted for any
other thickness simply by changing the thickness value in the Mathcad program. This
allows for very fast and accurate prediction of the attenuation of the luminescence
intensity at any desired thickness and any beam energy up to 4 keV. Given the trade-off
between loss in intensity and reduced degradation with thicker coatings, this program can
be used, along with knowledge of the degradation resistance at various coating
thicknesses, to determine the optimal coating thickness for a particular application.
It is claimed that this model is accurate for beam energies less than 4 keV. This
upper limit was chosen because no data were available at higher energies due to limits
imposed by the electron gun used to excite the luminescence. However, if the energy
loss function is examined for higher energies, it is found to closely follow the Bethe

241
energy loss function. As mentioned earlier in this section, the Bethe energy loss function
is believed to be accurate for beam energies above about 4 keV. Therefore, since the two
functions are almost identical in this high-energy range, the model presented in this
section should also be accurate for energies above 4 keV.
No further corrections were added to the model because it provided an accurate fit
of the cathodoluminescence data from the coated samples over the energy range of
interest. Possible corrections could have included: backscattering coefficient, scattering,
surface recombination velocity, varying activator concentration in the phosphor, and
surface roughness. These are discussed below.
Backscattering Coefficient
In Chapter 5, it was shown that the coated phosphors displayed a different
backscattering coefficient than the uncoated phosphors. The difference in backscattering
was dependent on the coating material, thickness, and incident beam energy. Monte
Carlo simulations were used to determine the value of the backscattering coefficient at
selected beam energies and coating thicknesses. At higher energies, the backscattering
coefficients of the coated and uncoated phosphor were similar. This is due to the
increased interaction volume at higher energies, which means that the coating contributes
less to the overall electron solid interaction. Therefore, any correction to the model is
only a concern at low energies (below about 2 keV, dependent on the material). It is
possible that the new energy loss expression accounts for the change in the backscattering
coefficient due to the fitting procedure. However, this does not seem reasonable, given
that the same fitting parameters were used for both of the coating materials, although

242
their backscattering coefficients will be different. Also, as discussed in Chapter 5, the
backscattering coefficients are measured as the electrons leave the surface of the coating.
To accurately account for a change in backscattering, the number of electrons that leave
the phosphor, and not the coating layer, is needed. This value could not be found using
the simulations in their present form. Since the model showed good agreement between
the calculated and measured luminescence without a backscattering correction, and since
the backscattering coefficient from the Monte Carlo simulations is not an accurate
measure of a change in the number of electrons in the phosphor itself, this correction was
not included into the model.
Scattering
Scattering of the incident electrons will change their path length in the coating
layer. At higher beam energies, the mean free path of the electron in a solid is larger, and
therefore scattering is not as much of a concern as it is at low energies. Although a
correction was not explicitly included in the model to account for scattering, the new
energy loss equation could inherently be taking the low energy scattering into account. It
was assumed that the Bethe stopping power equation did not provide an accurate fit of
the energy loss at low beam energies because it overestimated the stopping power of
these electrons. The explanation of this is that the Bethe stopping power equation
assumes an average path length based on scattering of the electrons. At low energies, the
expression is no longer an accurate depiction because the process is dominated by
electrons with a longer than average path length. To account for this low energy region,

243
the new energy expression was incorporated into the model. Therefore, the scattering of
the low energy electrons is included in the model through the energy loss calculation.
Surface Recombination and Charging
Surface recombination refers to the loss of excited electrons from the bulk of the
phosphor to nonradiative de-excitation at surface states. As discussed in Chapter 2,
certain coatings have been shown to increase the efficiency of phosphors at low voltages.
It has been suggested that this is a result of band misalignment that repels generated
secondary electrons from the surface and therefore reduces charging of the phosphor. If
this is the case, then large bandgap materials, such as those used for this work, should
exhibit this behavior under ideal conditions. However, no enhancement was observed
over the entire voltage range studied. Comparison of the measured luminescence curves
found in Figures 5-20 through 5-28 with those of Kingsley and Prener, shown in
Figures 2-14 and 2-15, show a similar low-voltage behavior. Since the curves in
Figures 2-14 and 2-15 are for undoped ZnS coated onto ZnS:Cu, no enhancement from
surface recombination velocity reduction is expected because the bandgaps are the same.
Since the curves measured on the MgO- and Al203-coated Y203:Eu and Y2Si05:Tb
displayed a nearly identical functional dependence at low voltages, it is assumed that no
reduction in surface recombination velocity is occurring in these samples.
The time-resolved cathodoluminescence results shown in Figures 4-36 through
4-53 also reveal that no surface recombination velocity change resulted from the coating.
All changes in the curves in these figures can be attributed to a reduction in the beam
energy on the incident electrons. If the coating had reduced the surface recombination,

244
then the non-radiative decay rate would be reduced. Since the overall decay rate is the
sum of the radiative and non-radiative decay rates, a reduction in the non-radiative rate
would have led to a reduction in the overall rate. This would have shown up as a smaller
slope on the pulsed CL curves. No such change could be detected. Therefore, no
reduction in the charging of the phosphors was observed with the pulsed laser deposited
coatings used in this work.
Surface Segregation
In Chapter 4, it was found that the Eu concentration in the Y203:Eu samples was
higher in the near surface region of the phosphor. This was based on a change in the
peak height ratios of the normalized luminescence versus wavelength plots of
Figures 4-11 through 4-14. No such problem was found in the Y2Si05:Tb samples,
shown in Figures 4-15 to 4-18. Since the model was able to accurately predict the
luminescence from coated samples of both types of phosphor, the surface segregation
correction was assumed to be small. However, it could improve the accuracy of the
model slightly for the low-voltage region of the Y2C>3:Eu samples if included. To
implement such a change would require knowledge of the Eu concentration as a function
of depth and the luminescent efficiency as a function of Eu concentration in Y2C>3.
Similar curves would be necessary if the model were to be applied to a different
phosphor. Since the goal of this model was to allow prediction of the coating’s effect on
the luminescent intensity from a wide range of materials systems, and since inclusion of
the activator concentration quenching would only slightly improve the accuracy of the
calculation, it was not included.

245
Surface Roughness
Rough surfaces have been shown to increase the light radiated from a phosphor
surface. This is presumably due to enhanced coupling of the light transmitted from the
phosphor because of scattering at the surface. For the present work, commercially
available phosphors were chosen. These undergo a series of processing steps, such as
ball milling, which result in a slightly rough surface. Therefore, the coating is unlikely to
cause a large increase or decrease in the surface roughness for these materials. However,
if this model were applied to phosphors grown under controlled conditions to produce
very smooth surfaces, then increases in surface roughness due to a coating process might
need to be considered.

246
Table 9
Coating thickness based on ellipsometry measurements and fitting in the Mathcad
programs.
Thickness from
Ellipsometry (Á)
Thickness from
Mathcad program (Á)
MgO (4 min) / Y2Ü3:Eu
73 (a)
130
MgO (8 min) / Y203:Eu
146 (a)
390
A1203 (1.2 min) / Y203:Eu
78 (a)
100
A1203 (2.4 min) / Y203:Eu
156 (a)
210
A1203 (1.2 min) / Y2Si05:Tb
78 (a)
140
A1203 (5 min) / Y2Si05:Tb
(grown in new chamber)
545 (b) and 489 (c)
510
A1203 (5 min) / Y2Si05:Tb
~ 1100 (d)
2300
MgO (4 min) / Y2Si05:Tb
290 (b) and 268 (c)
290
MgO (8 min) / Y2Si05:Tb
549 (b) and 494 (c)
430
(a) Based on a linear growth rate extrapolation of measurements taken from films of the
same materials grown on Si substrates for 5 minutes under identical conditions.
Al203 film (5 minutes) = 325 A from ellipsometry and about 340 Á from TEM
MgO film (5 minutes) = 91 Á from ellipsometry
(b) Data taken on multiple wavelength ellipsometer and fit assuming an 18 Á Si02 film
between the substrate and film due to oxidation of the Si.
(c) Data taken on single wavelength ellipsometer.
(d) Approximation made from step height on stylus profilometer.

CHAPTER 7
SUMMARY AND CONCLUSIONS
Coatings of MgO and AI2O3 were deposited onto sedimented screens of Y203:Eu
and Y2Si05:Tb by pulsed laser deposition. A Si shadow mask was used during
deposition to cover half of the sample. This allowed side-by-side comparison during
characterization of coated and uncoated phosphors that had experienced the same
processing. Cathodoluminescence data were collected at a constant current density of
0.16 pA/cm2 for beam energies of 0.8, 1.4, 2.5, and 4.0 keV. The spectra revealed that
the coatings caused a decrease in the cathodoluminescence for all wavelengths in the
visible range. Coatings deposited for longer times, which were therefore thicker,
attenuated the luminescence more strongly. Based on ellipsometry measurements from
the Si shadow masks, the thickness of the measured coating varied from 73 to 550 Á.
This is close to the thickness estimated for powders based on modeling of the
luminescence data; the estimated thicknesses range from about 100 to 510 Á.
Spectra from the phosphors showed that the coatings had no effect on the
wavelength dependence of the emission peaks. Luminescence is generated from an
activator site when an excited secondary electron recombines with an available hole to
return to the ground state. The wavelength of the emitted light will depend on the
difference in the energy levels of the activator site. Therefore, if the energy levels of the
activator are disturbed, the wavelength of the emitted light will be shifted. Since no such
247

248
shift was observed in the coated samples, it was concluded that the coatings did not alter
the electronic states of the activator sites in the phosphors.
The cathodoluminescence spectra were also normalized to the intensity of the
main emission peak. This allows for examination of the intensity ratio of all other peaks
to the main peak. A change in this ratio can give information about the phosphor and
coating. The Y203:Eu samples showed an increase in the ratio of the 5Di / sDo peak
heights (591 nm / 611 nm) with increasing beam energy in both coated and uncoated
powders. Based on work by Tseng et al.130 and the increased interaction volume of
electrons at higher beam energy, the spectra suggest an increase in the Eu concentration
near the surface of the phosphor. The effect is not attributed to the coating, but instead to
segregation during synthesis. The normalized Y2SiOs:Tb spectra showed no change in
the peak height ratios over the entire range of interest (460 to 680 nm). This suggests a
uniform Tb concentration in the phosphor.
A concern when depositing coatings onto phosphor materials is the deactivation
of luminescent centers. This can be caused by one of the following two processes: the
Eu can diffuse out of the phosphor into the coating, or one of the coating constituents can
diffuse into the phosphor. Either of these processes would lead to a change in the
concentration of active luminescent sites, which would show up as a change in the peak
height ratios on the normalized CL graph. Neither of these processes occurred in the
present study.
A model was developed to predict the luminescence from the coated samples. It
relied on the assumption that the coating was acting as a dead layer on top of the
phosphor. As electrons passed through the coating, they lost energy that was then

249
unavailable for excitation of luminescence in the phosphor. The model was designed to
allow for prediction of the luminescence over the entire energy range up to 4 keV and at
any given thickness. A cosine-dependent coating thickness was used because it is a
realistic approximation of a coating deposited from a uniform flux onto a spherical
surface. From this approximation of the coating thickness, it was found that the path
length through the coating is the same irrespective of the incident point of the electron,
assuming no diffuse scattering. Therefore, only one angle needed to be considered for
calculating the coating’s effect on the luminescence.
The Rao-Sahib and Wittry modification to the Bethe stopping power equation was
used in the first attempt to calculate the energy loss in the coating. It was found to only
provide an accurate fit for the higher energy region of the curves. Below about 3 keV,
depending on the sample, the calculation overestimated the attenuation of the
luminescence intensity. This was attributed to the averaging of the electron scattering in
the stopping power equation. To provide a more precise energy loss equation for the low
energy electrons, a new formula was incorporated. It had the following form:
( - 2x ^
E = E0 exp —7—^7
\a*E° j
where E is the electron’s energy at a distance x into the material, Eo is the incident beam
energy, and a' and k' are fitting parameters for the material of interest. By fitting the
constants a' and k' for a given material, the attenuation in the cathodoluminescence
intensity could be predicted for any given thickness. The a' and k' values found for MgO
and AI2O3 were the same and were equal to 0.064 and 1.36, respectively. The fact that
both materials displayed the same constants is likely due to the similarity in density,

250
average atomic number, and average atomic weight of these materials; it cannot be stated
as a general conclusion.
The model presented in this work allows for accurate prediction of the
cathodoluminescence from coated phosphors. Application to a different materials system
would require the fitting of the luminescence intensity versus beam energy for one
coating of known thickness to determine the constants in the energy loss equation (a' and
k'). Afterwards, the luminescence could be predicted for any other thickness.
Since the model was developed to explain the effect of wide-bandgap oxide
coatings, no absorbance of the emitted light was included. If the model were to be
applied to a material with a bandgap comparable to or smaller than the emitted light, then
absorbance would need to be included. This requires knowledge of the absorption
coefficient of the coating material and the thickness of the film.
It has been shown recently that certain coatings, when applied to phosphors, can
lead to an increase in the low voltage efficiency of the phosphors. This is presumably
due to a band offset between the coating and phosphor. Generated secondary electrons in
the phosphor are then repelled from the coating-phosphor interface to minimize or avoid
surface recombination. A reduction in the surface recombination velocity gives a higher
brightness.15-93’104 Although the use of wide-bandgap oxides potentially could have
produced such results in the present study, no enhancement of the low voltage efficiency
was observed.
In order to realize a commercially available FED, device lifetime needs to be
improved. Phosphor degradation is one of the major hurdles to be overcome to reach this
goal. As suggested by numerous studies, applying a coating to the phosphors may be the

251
best way of extending their lifetime. Although deposition of a coating onto a phosphor
may reduce degradation and therefore increase lifetime, results of this study and other
research indicate that coating phosphors reduces their cathodoluminescence intensity.
The selection of coating material type and thickness for a given application should
therefore be based on the trade-off between a loss in intensity and slower degradation.
Information gained from the model developed in this work should be used in making
such a decision.

APPENDIX A
MATHCAD DATA AVERAGING PROGRAM
This is a Mathcad program to convert data files from the Hewlett-Packard
Infinium oscilloscope using a sliding interval averaging calculation. The data are
reduced to a/g points with the moving average. A two-point log time derivative and slope
calculations for sections of the decay curve are also included.
a := 25000 number of data points in ASCII files
g := 25 number of data points in interval for moving average
g
y := 1,2.. e final data point index
Z := 0,1.. (g - 1) floating avgerage index
M := data datafile
The Infinium oscilloscope records a number ol
data points before the pulse trigger. The
average of these is used to find a background
value. This background value is then
subtracted from the rest of the signal.
f := 20 number of points for zero calculation
Mf o =-5.310 used to ensure zero calculation is in prepulse region
f
y = o
d =-1.9-10
-4
dark CL value
252

253
M
g-y + Z’O
^Mgy-f-z,l
g
moving time average
¿y
+ d
Take negative of the intensity to give positive values and then
subtract background
WRITEPRN ("test.pm") := A
Writes time averaged data to a .pm file in the
winmcad directory for use in slidewrite (data is
properly zeroed).
The beam energy is 1.4 keV
The sample is Y2Si05:Tb coated with Al203 (1.2 min)
3-70 filter in PMT box

254
Ag = 1.4744-10"4
k := 1,2.. e- 1 index for two point log time derivative
(^k - +• l)’2
Bi, := -7 .—, r negative of two point log time derivative
(Nk+l,0-Nk,o)-(Ak+i + Ak)
V
B.
k
o o ó
-1
0.009
0.0081
0.0072
0.0063
0.0054
0.0045
0.0036
0.0027
0.0018
9 10
-4
—O
o
0
A
o
n r\
o
ó
_ O
o
i—. Q
0
CK
o
O
c
o J
3
3
0^
o
o
o
o
O
o
O
o
O °
O
O
O
o
CO®0
o o
o
o
° a
Oo
q
o (¿)
fe> °
0 c
3
°o
o
qOUD
orw, jtvoo
o t
O°o
°a„
^O
O
.cP
0);
O _ fi
o c
o
® 0 vbc®>o° u T-’
—o
o
o a
no „.c
0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01
Nk,0
Below is the code to calculate the decay constant over a given data interval.
S := 900 start data point
q := 998 end data point
N$ q = 8.99627-10 ^ starttime
Nq q =9.97627-10 ^ end time
j := 0,1.. (q - s)
Pr'K-Vj) -j ;= Ns+j,o
h :=-(slope(r,p))'
h = 2.881148 • 10_ 3
average decay time constant at
long times

The following is a repeat of the earlier code over an interval at the beginning of the pulse.
U := 80 start
t := 120 end
Nu o =7.9627’10 4 start time
Nt 0 = 1.19627 -10'3 end time
m := 0,1.. (t - u)
Vn ln( Aj +- m)
h := -(slope(s ,1))
h = 1.956926-10"3
u + m,0
average decay time
constant at short times

APPENDIX B
LUMINESCENCE DUE TO THE INCIDENT ANGLE OF THE ELECTRONS
This is a Mathcad program to compute the modified probability function taking
into account the thickness variation across a spherical particle coated with a uniform
incident flux from the top. Curves are plotted for coating thicknesses of 0, 0.2, 0.4, 0.6,
0.8, 1, and 2 pm. It should be noted that the coatings used for the research project are
thinner than those shown in this program. These larger coatings were chosen to represent
the behavior of the equations over a range of coating thicknesses. Refer to Figure 5-11
for a schematic of the particle and coating. The derivation of these equations is explained
in Chapter 5.
0 := 1,2.. 90 angle from center axis of particle
t is the coating thickness at 0 = 0, measured in pm
R :=5 radius of particle in pm
A(0,t):
. [R~ sin(0 deg)-cos(9 deg) + 2-R t sin(0 deg)-(cos(9 deg))2- R t-(sin(6 deg))3]
R2 -t- R-t + llL
B(0,t):
â– _[(t)2-sin(9 deg)-(cos(9 deg))3- (t)2-(sin(0deg))3cos(0deg)]
R2+R-t-|-lÍL
P(0>t) :=A(0,t)-t-B(0,t)
area weighted contribution to luminescence
at given angle© and thickness t
256

257
This is a plot of the area weighted contribution to the luminescence at an angle 0 from the center
axis of the particle. The second term in the brackets is the coating thickness in ^m. As can be
seen from the plot, the angle at which the contribution is maximum decreases with increasing
coating thickness. This is due to the shape of the coating.
The angle from the center axis, 0, is no longer equal to the angle between the incident electron
and surface normal as in the uniform coating. The angle from surface normal as derived in
Chapter 5 is given by the following.
\|/(0,t) :=acot
Reos (0 deg) +1•( cos (0 deg))2 - t•( sin( 0 deg) )2
Rsin( 0 deg) +â–  2-t-cos (0 deg) -sin( 0 deg)
angle from surface
normal at given©

258
H/( 0,0)-57.296
>l<( 0,0.2)-57.296
j j j
V( 0,04)-57.296
i|/( 0,0.6) -57.296
V( 0,0.8)-57.296
f~\
\|/( 0,1)-57.296
V( 0,2)-57.296
0
This is a plot of the angle from surface normal of the electron beam as a function of e for various
coating thickness. The second term in the brackets is the coating thickness measured in ^.m.

259
This is a plot of the area weighted contribution at a given angle as a function of the angles.
Similar to Figure 5-4 for a uniform coating thickness, the contribution is maximum at about 45
degrees.

APPENDIX C
NONUNIFORM COATING SHAPE
This is a Mathcad program to determine the shape of the coating based on a
uniform incident particle flux onto a spherical surface. Refer to Figure 5-11 for a
schematic. The derivation of the equations is given in Chapter 5.
r:=5 radius of particle in
0 :=0,1.. 90 angle from the center axis of the particle
Y(Q t) ;=asin (R + t-cos(O-deg)) sin(O-deg)
L R
r(0,t) := (R)-sin(y (0,t))
h(0,t) :=R cos(Y(0,t)) height of particle
c( 0, t) :=(R+1 cos (0 deg)) -cos (0 deg) height of particle with coating
260

261
h(0,O)
a. 8,0.2)
+-H-
c(0,O.4)
â–¡no
QOO
c( 0,0.6)
c(0,O.8)
UUU
c(0,l)
vA/v/
A. AA
c(0,2)
2
1 2 3 4 5
r(0,0),i< 0,0.2 ),r( 0,0.4 ),r( 0,0.6 ),r( 0,0.8 ),r(0,1 ),i{ 0,2)
Cross-section of various thicknesses of coating on a spherical particle. The thicknesses are listed
as the second term in the parentheses and are measured in ^m.

262
d(0,t) :=c(0, t) — h(0,t) distance through the coating measured parallel to center axis
Distance through the coating, as measured parallel to the center axis, as a function of 0 for
various coating thicknesses. The thickness is given as the second term in the brackets. The
curves increase at the very end due to the curvature of the particle. For the coatings used in this
research, the distance through the coating is nearly constant for all angles, as determined from
the graph above.

APPENDIX D
STOPPING POWER
This is a Mathcad program to compare the stopping power equations of Bethe,
Rao-Sahib and Wittry, and Love et al. The curves have been computed for AI2O3, but
they are similar for other materials.
p :=3.97 density
Z := 10 average atomic number
A := 20.3922 average atomic weight
Ei := 0,0.01.. 5 incident beam energy
mean ionization potential in keV
k :=0.85 fitting parameter for ionization potential correction
J2(Ei) :=
J
low energy correction of ionization potential by Joy and Lou
Bethe stopping power equation
A-Ei
1.26A-(EiJ)0'5
Rao-Sahib and Wittry parabloic extrapolation of Bethe stopping
power equation
S3( Ei) :=
p-Z-10*4
7T I I5 ~05 Zk T Love et al. stopping power equation
A-j. 1.18-10 -(J-Ei)+1.47-10 EiJ a H
263

All equations are the absolute value of the stopping power. The stopping power is a negative
quantity because the atom is losing energy to the lattice as it travels through the material.
However, the absolute value allows the behavior of the functions to be seen more easily.
264
All stopping powers are measured in keV / ^m
Ei
This is a plot of the stopping power equations as a function of incident beam energy. The red cun/e
is the Bethe stopping power equation. The blue curve is the parabolic extrapolation proposed by
Rao-Sahib and Wittry. The green curve is the stopping power equation of Love et al.

APPENDIX E
MATHCAD PROGRAM FOR MgO (4 min) / Y203:Eu
This is a Mathcad program to compute the luminescence as a function of beam
energy from a 4 minute coating of MgO on Y203:Eu. The coating was applied by pulsed
laser deposition.
t := .013 thickness in microns of coating at top of particle
R := 5 radius of phosphor particle in microns
p :=3.6 density
Z := 10 atomic number
A :=20.152 average atomic weight
0 :=0,1.. 90 angle from center axis to coating's surface
Y(0) :=asin
(R+t cos(0 deg)) sin(0deg)
R
angle from center axis to phosphor's surface
r(0) :=(R)-sin(y (0)) distance from center axis
h( 0) :=Reos (y (0)) height of phosphor particle
c(0) :=(R-i-t-cos(0 deg))-cos(0 deg) height of phosphor and coating
265

266
d(0) :=c(0)- h(0) distance traveled through the coating at given©
0.02
0.0175
0.015
0.0125
d(6) 0.01
0.0075
0.005
0.0025 ~
0
till
1 1 1 1
A
-
1
-
-
1 1 1 1
1 1 1 1
0 10 20 30 40 50 60 70 80 90
0
Thickness is nearly constant for all 0, so take 0 = 0 for subsequent calculations.
Ei := 0,. 1.. 4 incident beam energy
J :=( 11.5-Z)-10-3 mean ionization potential in keV
k :=0.85 fitting parameter for ionization potential correction
J2(Ei) :=
Ei+ 10'50/
low energy correction of ionization potential
j_ 3
D(Ei) 100T.262A( J2(Bi))2-(Ei)2
2355-pZ
range in bulk of coating material at given incident
energy Ei based on Rao-Sahib & Wittry
D( 1.4) = 0.033
D(2.5) = 0.079 range in microns at beam energy (keV) inside brackets
D(4) = 0.161

267
E( 0, Ei) :=Ei- 1 —
2
d(9) \3
D( Ei) /
energy after traversing coating from Rao-Sahib & Wittry
E2(0,Ei) :=Ei-exp -2
d(0)
0.064-Í Ei)136
fitted energy dependence
This is a plot of the electron's energy after traveling through the coating as a funciton of incident
beam energy. The red curve is based on the calculation using the stopping power equation of
Rao-Sahib and Wittry. The blue curve is based on the new fitted energy loss equation.

268
aO :=-7.62236313516689 O5
fitting coefficients for a power law fit to the uncoated
al := 1.8313782042345-1 (f phosphor luminescence, taken from SlideWrite program
a2:= 1.43141730761361
L(0,Ei) :=aO+al-(E(0,Ei))a2
luminescence of coated material based on Rao-Sahib and
Wittry energy loss equation
„ „ . a2 luminescence of coated material based on new
L2(0,Ei) :=aO+al-(E2(0,E)) .
^ ; t v >> energy loss equation
Li(0,Ei) :=a0+ al-(Ei)a2 luminescence of uncoated material
am0:=-9.935660035182105
ami := 1.49033861347453 0*
am21=1.52742562074871
Lrr(0,Ei) :=am0+ arnl-(Ei)am2 measured luminescence from the coated phosphor
fitting parameters for measured luminescence from
coated material, taken from SlideWrite program
The following graph shows the cathodoluminescence intensity as a function of beam
energy. The black curve is the uncoated phosphor. The green curve is the measured
luminescence from the coated phosphor. The red curve is the luminescence as
calculated based on the Rao-Sahib modification of the Bethe stopping power equation.
The blue curve is the luminescence based on the new energy loss equation. From the
graph, it can be seen that the Rao-Sahib and Wittry equation overestimates the
attenuation of the luminescence at low energies. The new energy loss equation
accurately predicts the luminescence over the entire energy range up to 4 keV.
Measurement of the thickness of the coating on the phosphor is the largest source of
error for the calculation. To provide a more accurate analysis, it was assumed that the
Rao-Sahib and Wittry equation is valid for the high energy region of the curve. The
thickness value was then optimized to give agreement between the calculated and
measured curves around 4 keV. For further discussion of this correction, see the
discussion of the second model in Chapter 6.

269
The following is the SlideWrite plot used to determine the curve fiting coefficeints for the measured
luminescence from the coated and uncoated phosphor.
z
LL)
160
140
120
D V 100
m "o
(fl
D
80
£ £
(fí c-
60
40
20
0
L-a0+al*EAa2
Unco ated
aO = -7.62236313516609 e+3
al = 1.03137020423457 e+4
a2 - 1.43141730761361
Coated
aO - -9.9356600351823 e+3
al = 1.49033861347459 e+4
a2 = 1.52742562074871
s
/
UNCOATED
MgO(4)
VOLTAGE (keV)
5

APPENDIX F
MATHCAD PROGRAM FOR MgO (8 min) / Y203:Eu
This is a Mathcad program to compute the luminescence as a function of beam
energy from a 8 minute coating of MgO on Y203:Eu. The coating was applied by pulsed
laser deposition.
t := .039 thickness in microns of coating at top of particle
r :=5 radius of phosphor particle in microns
p :=3.6 density
Z := 10 atomic number
A :=20.152 average atomic weight
6 := o, 1.. 90 angle from center axis to coating's surface
Y(0) :=asin
(R+ t-cos(0deg)) sin(0deg)
R
angle from center axis to phosphor's surface
r(0) :=(R)-sin(Y(0)) distance from center axis
h( 0) :=Reos (Y (0)) height of phosphor particle
c(0) :=(R+tcos(0deg))-cos(0deg) height of phosphor and coating
270

271
d(0) :=c(0) — h(0) distance traveled through the coating at givene
Thickness is nearly constant for all e, so take 0 = 0 for subsequent calculations.
Ei :=0,. 1.. 4 incident beam energy
J :=( 11.5-Z)-10"3 mean ionization potential in keV
k :=0.85 fitting parameter for ionization potential correction
J2( Ei) :=
Ei+ 10'50/
low energy correction of ionization potential
1 3
D(Ej) 100T.262-A-(J2(Ei))2-(Ei)2
2355pZ
range in bulk of coating material at given incident
energy Ei based on Rao-Sahib & Wittry
D( 1.4) = 0.033
D(2.5) = 0.079 range in microns at beam energy (keV) inside brackets
D(4) = 0.161

272
E(0,Ei) :=Ei-(l-.d(9I
\ D(Ei)
energy after traversing coating from Rao-Sahib & Wittry
E2(0,Ei) :=Eiexp -2
d(0)
0.064(Ei)136
fitted energy dependence
This is a plot of the electron's energy after traveling through the coating as a funciton of incident
beam energy. The red curve is based on the calculation using the stopping power equation of
Rao-Sahib and Wittry. The blue curve is based on the new fitted energy loss equation.

aO:=-2.155376136556aO?
fitting coefficients for a power law fit to the uncoated
al :=7.82356466263201 Or1 phosphor luminescence, taken from SlideWrite program
a2:= 1.78566539748617
UG.Ei) :=aO+al (E(0,Ei))a2
luminescence of coated material based on Rao-Sahib and
Wittry energy loss equation
L2( 6, Ei) :=aO-t- a 1 â– ( E2( 0, Ei) )a2
luminescence of coated material based on new
energy loss equation
Li( 0, Ei) :=a0+ al-(Ei)a2 luminescence of uncoated material
amO:=-2.066137039861:J03
ami := 2.29888121748173 G*
am2 ¡=2.4293699844816
fitting parameters for measured luminescence from
coated material, taken from SlideWrite program
Lm(0,Ei) :=am0+ aml-(Ei)am2 measured luminescence from the coated phosphor
The following graph shows the cathodoluminescence intensity as a function of beam
energy. The black curve is the uncoated phosphor. The green curve is the measured
luminescence from the coated phosphor. The red curve is the luminescence as
calculated based on the Rao-Sahib modification of the Bethe stopping power equation.
The blue curve is the luminescence based on the new energy loss equation. From the
graph, it can be seen that the Rao-Sahib and Wittry equation overestimates the
attenuation of the luminescence at low energies. The new energy loss equation
accurately predicts the luminescence over the entire energy range up to 4 keV.
Measurement of the thickness of the coating on the phosphor is the largest source of
error for the calculation. To provide a more accurate analysis, it was assumed that the
Rao-Sahib and Wittry equation is valid for the high energy region of the curve. The
thickness value was then optimized to give agreement between the calculated and
measured curves around 4 keV. For further discussion of this correction, see the
discussion of the second model in Chapter 6.

274
Ei
The following is the SlideWrite plot used to determine the curve fiting coefficeints for the measured
luminescence from the coated and uncoated phosphor.
t
z
D
GO
DC
<
05
(0
T3
C
!C
UD
=3
O
120
100
80
60
40
20
L=aO+al*EAa2
Uncoated
aO = -2 1553761365568e+003
al = 7 82356466263201e+003
a2 = 1.78566539748617
Coated
aO = -2 0661370398617e+003
al = 2 29888121748175e+003
a2 = 2.4293699844816
UNCOATED
A MgO(8)
—I i
VOLTAGE (keV)
5

APPENDIX G
MATHCAD PROGRAM FOR A1203 (1.2 min) / Y203:Eu
This is a Mathcad program to compute the luminescence as a function of beam
energy from a 1.2 minute coating of A1203 on Y203:Eu. The coating was applied by
pulsed laser deposition.
t := .01 thickness in microns of coating at top of particle
r := 5 radius of phosphor particle in microns
p :=3.97 density
Z := 10 atomic number
A :=20.3922 average atomic weight
0 :=0,1.. 90 angle from center axis to coating's surface
Y (9) :=asin
(R+t-cos(0 deg)) sin(0 deg)
R
angle from center axis to phosphor's surface
r( 0) :=(R) -sin(Y (0)) distance from center axis
h(0) :=R-cos(Y(0)) height of phosphor particle
c( 0) := (R +1 'cos (0 deg)) cos (0 -deg) height of phosphor and coating
275

276
d(0) :=c(6)- h(0) distance traveled through the coating at givene
0.0175 -
0.015 -
0.0125 -
d( 0) 0.01 4
0.0075 “
0.005 -
0.0025 “
0I I I I I I I I I
0 10 20 30 40 50 60 70 80 90
6
Thickness is nearly constant for all 0, so take 0 = 0 for subsequent calculations.
Ei :=0, .1.. 4 incident beam energy
J :=( 11.5Z)10"3 mean ionization potential in keV
ionization potential correction
low energy correction of ionization potential
3
mn. 1 A / rv\2 ,_. . 2 range in bulk of coating material at given incident
DUS# :=iy.L2M'A (12IE'» (E|) energy Ei based on Rae-Sahib & Wittry
2355-pZ
D( 1.4) = 0.03
D(2.5) = 0.072 range in microns at beam energy (keV) inside brackets
k := 0.85 fitting parameter for
J
J2( Ei) :=.
1 + k-
Ei+ 10
>-50
D(4) =0.147

277
E( 0, Ei) :=Ei-
energy after traversing coating from Rao-Sahib & Wittry
E2(0,Ei) :=Ei-exp -2
d(0)
0.064-(Ei)‘ 36
fitted energy dependence
This is a plot of the electron's energy after traveling through the coating as a funciton of incident
beam energy. The red curve is based on the calculation using the stopping power equation of
Rao-Sahib and Wittry. The blue curve Is based on the new fitted energy loss equation.

278
aO :=-7.2555115749978-2O5
al := 1.42940056423308O4
a2:= 1.33770010631825
fitting coefficients for a power law fit to the uncoated
phosphor luminescence, taken from SlideWrite
program
L(0,Ei) :=a0+ al-(E(0,Ei))a2
luminescence of coated material based on Rao-Sahib and
Wittry energy loss equation
. luminescence of coated material based on new
L2(0,Ei) :=a0-(-al-(E2(0,Ei)) . ,.
^ T v ^ ’ ” energy loss equation
Li( 0, Ei) :=a0+ al-(Ei)a2 luminescence of uncoated material
amO:=- 6.261633176897GI103
ami := 9.6051329759772103
am2:= 1.58503483410552
fitting parameters for measured luminescence from
coated material, taken from SlideWrite program
Ln<0,Ei) :=am0+- aml-(Ei)am2
measured luminescence from the coated phosphor
The following graph shows the cathodoluminescence intensity as a function of beam
energy. The black curve is the uncoated phosphor. The green curve is the measured
luminescence from the coated phosphor. The red curve is the luminescence as
calculated based on the Rao-Sahib modification of the Bethe stopping power equation.
The blue curve is the luminescence based on the new energy loss equation. From the
graph, it can be seen that the Rao-Sahib and Wittry equation overestimates the
attenuation of the luminescence at low energies. The new energy loss equation
accurately predicts the luminescence over the entire energy range up to 4 keV.
Measurement of the thickness of the coating on the phosphor is the largest source of
error for the calculation. To provide a more accurate analysis, it was assumed that the
Rao-Sahib and Wittry equation is valid for the high energy region of the curve. The
thickness value was then optimized to give agreement between the calculated and
measured curves around 4 keV. For further discussion of this correction, see the
discussion of the second model in Chapter 6.

279
The following is the SlideWrite plot used to determine the curve fiting coefficeints for the measured
luminescence from the coated and uncoated phosphor.
120
L=aO+al*EAa2
/
100
Unco ated
s'
aO - -7.25551157499782e + 003
yy
S/
S'
1—
al - 1.42940056423308e+004
a2 = 1.33770010631825
zz
yy
z
D
80
Co ated
yV
A
â–¡â–¡

T3
aO = -6.26163317689701e+003
yy
rr
S
>-
c
CO
(A
3
60
al = 9.6051329759772le+003
a2 - 1.58503483410552
X/
F-
<0
1
z
LU
h-
40
+
UNCOATED
z
â–²
A1203(12)
20
n
' | i
i i
I l
0
1 2
3
4 5
VOLTAGE (keV)

APPENDIX H
MATHCAD PROGRAM FOR A1203 (2.4 min) / Y203:Eu
This is a Mathcad program to compute the luminescence as a function of beam
energy from a 2.4 minute coating of A1203 on Y203:Eu. The coating was applied by
pulsed laser deposition.
t := .021 thickness in microns of coating at top of particle
r :=5 radius of phosphor particle in microns
p :=3.97 density
Z := 10 atomic number
A :=20.3922 average atomic weight
0 :=0,1.. 90 angle from center axis to coating's surface
Y(0) :=asin
(R+ t-cos(0 deg)) sin(0deg)
R
angle from center axis to phosphor's surface
r(0) :=(R)-sin(y(0)) distance from center axis
h(0) :=Reos(Y (0)) height of phosphor particle
c(0) :=(R+t-cos(0 deg))-cos(0 deg) height of phosphor and coating
280

281
d(0) :=c(0)- h(0) distance traveled through the coating at givene
Thickness is nearly constant for all 0, so take e = 0 for subsequent calculations.
Ei :=0,. 1.. 4 incident beam energy
J :=( 11.5-Z)â–  10-3 mean ionization potential in keV
k :=0.85 fitting parameter for ionization potential correction
J2( Ei) :=
eí+ icr50/1
low energy correction of ionization potential
1 2
D(E¡) 100T.262-A-(J2(Ei))2-(Ei)2
2355p-Z
range in bulk of coating material at given incident
energy Ei based on Rao-Sahib & Wittry
D( 1.4) = 0.03
D(2.5) = 0.072 range in microns at beam energy (keV) inside brackets
D( 4) =0.147

282
BCe.EO :=
E2(e,Ei):
E(O.Ei)
| | |
E2(0,Ei)
ÜUÜ
2
d(9) \3
D(Ei)/
energy after traversing coating from Rao-Sahib & Wittry
= Ei-exp
-2-
d(0)
0.064(Ei)
1.36
fitted energy dependence
This is a plot of the electron's energy after traveling through the coating as a funciton of incident
beam energy. The red curve is based on the calculation using the stopping power equation of
Rao-Sahib and Wittry. The blue curve is based on the new fitted energy loss equation.

aO :=-3.64132997767743O3
al := 1.22897133336396 (f
a2 : = 1.61115422022399
fitting coefficients for a power law fit to the uncoated
phosphor luminescence, taken from SlideWrite
program
L(0,Ei) :=a0+ al-(E(6,Ei))a2
luminescence of coated material based on Rao-Sahib and
Wittry energy loss equation
L2(0,Ei) :=a0-t- al (E2(0,Ei))a2
luminescence of coated material based on new
energy loss equation
Li(0,Ei) :=a0-i- al-(Ei)a2 luminescence of uncoated material
am0:=-6.70266215869943 03
ami := 8.20792813658368 05
am21=1.79982256009043
Lnt0,Ei) :=am0+ amHEi)81"2 measured luminescence from the coated phosphor
fitting parameters for measured luminescence from
coated material, taken from SlideWrite program
The following graph shows the cathodoluminescence intensity as a function of beam
energy. The black curve is the uncoated phosphor. The green curve is the measured
luminescence from the coated phosphor. The red curve is the luminescence as
calculated based on the Rao-Sahib modification of the Bethe stopping power equation.
The blue curve is the luminescence based on the new energy loss equation. From the
graph, it can be seen that the Rao-Sahib and Wittry equation overestimates the
attenuation of the luminescence at low energies. The new energy loss equation
accurately predicts the luminescence over the entire energy range up to 4 keV.
Measurement of the thickness of the coating on the phosphor is the largest source of
error for the calculation. To provide a more accurate analysis, it was assumed that the
Rao-Sahib and Wittry equation is valid for the high energy region of the curve. The
thickness value was then optimized to give agreement between the calculated and
measured curves around 4 keV. For further discussion of this correction, see the
discussion of the second model in Chapter 6.

284
The following is the SlideWrite plot used to determine the curve fiting coefficeints for the measured
luminescence from the coated and uncoated phosphor.
t
z
D _
• <0
CD TJ
cr c
—* w
é I
z
in
£
150
120
90
60
30
- L=aO+al*E^a2
. Uncoated
/ /
aO = -3.64132997767748e+003
al = 1.22897133336396e+004
y y
a2 = 1.61115422022399
//
/ x
' Coated
- aO = -6 70266215869945e+003
- al = 8.20792813658368e+003
x
. a2 = 1.79982256009043
y
-
yZ y/
X X
/ /
UNCOATED
A
>T z-’
-i- 1 1 1 1
A AI203(2.4)
-j 1 i
VOLTAGE (keV)

APPENDIX I
MATHCAD PROGRAM FOR MgO (4 min) / Y2Si05:Tb
This is a Mathcad program to compute the luminescence as a function of beam
energy from a 4 minute coating of MgO on Y2SiOs:Tb. The coating was applied by
pulsed laser deposition.
t :=.029 thickness in microns of coating at top of particle
r := 5 radius of phosphor particle in microns
p :=3.6 density
Z := 10 atomic number
A:=20.152 average atomic weight
0 :=0,1.. 90 angle from center axis to coating's surface
7(0) :=asin
(R+t cos(0 deg)) sin(0 deg)
R
angle from center axis to phosphor's surface
r(0) :=(R) sin(Y(0)) distance from center axis
h( 0) :=Reos (7(0)) height of phosphor particle
c( 0) := (R +1-cos (0 deg)) cos (0 -deg) height of phosphor and coating
285

286
d(6) :=c(0) — h(0) distance traveled through the coating at given0
Thickness is nearly constant for all e, so take 0 = 0 for subsequent calculations.
Ei:=0,.l„ 4 incident beam energy
J :=( 11.5-Z)-10“3 mean ionization potential in keV
k :=0.85 fitting parameter for ionization potential correction
J2( Ei) :=
Ei+ 10'50/
low energy correction of ionization potential
1 3
D(Ei) 100T.262A( J2(Ei))2(Ei)2
2355pZ
range in bulk of coating material at given incident
energy Ei based on Rao-Sahib & Wittry
D( 1.4) = 0.033
D(2.5) = 0.079 range in microns at beam energy (keV) inside brackets
D(4) =0.161

287
B(0,Ei) :=
E2C0.EÍ)
E(0,Ei)
I I |
E2(0,Ei)
ÜUÜ
Ei-fl-iíil)3
\ D(Ei)/
energy after traversing coating from Rao-Sahib & Wittry
= Ei-exp
d(0)
0.064-CEi)136
fitted energy dependence
This is a plot of the electron's energy after traveling through the coating as a funciton of incident
beam energy. The red curve is based on the calculation using the stopping power equation of
Rao-Sahib and Wittry. The blue curve is based on the new fitted energy loss equation.

aO:=-3.3539296789554^
al := 1.0699915262834ÃœO5
a2 := 1.47933368154553
fitting coefficients for a power law fit to the uncoated
phosphor luminescence, taken from SlideWrite
program
L(0,Ei) :=a0-t-al-(E(0,Ei))a2
luminescence of coated material based on Rao-Sahib and
Wittry energy loss equation
*2 luminescence of coated material based on new
L2( 0, Ei) aO + a 1 •( E2( 0, i)) energy |0SS equation
Li(0,Ei) :=a0+ al-(Ei)a2 luminescence of uncoated material
amO :=- 7.14397526126849 Cf4
ami := 6.6465469920577104
am21=1.719254605608
fitting parameters for measured luminescence from
coated material, taken from SlideWrite program
Lm( 0, Ei) :=am0+ aml-(Ei)am2 measured luminescence from the coated phosphor
The following graph shows the cathodoluminescence intensity as a function of beam
energy. The black curve is the uncoated phosphor. The green curve is the measured
luminescence from the coated phosphor. The red curve is the luminescence as
calculated based on the Rao-Sahib modification of the Bethe stopping power equation.
The blue curve is the luminescence based on the new energy loss equation. From the
graph, it can be seen that the Rao-Sahib and Wittry equation overestimates the
attenuation of the luminescence at low energies. The new energy loss equation
accurately predicts the luminescence over the entire energy range up to 4 keV.
Measurement of the thickness of the coating on the phosphor is the largest source of
error for the calculation. To provide a more accurate analysis, it was assumed that the
Rao-Sahib and Wittry equation is valid for the high energy region of the curve. The
thickness value was then optimized to give agreement between the calculated and
measured curves around 4 keV. For further discussion of this correction, see the
discussion of the second model in Chapter 6.

289
The following is the SlideWrite plot used to determine the curve fiting coefficeints for the measured
luminescence from the coated and uncoated phosphor.

3
OD
CD T3
S,
2
900
7 7
-
L=aO+al*E''a2
800
-
Uncoated
X /
-
aO = -3.3539296789554e+004
x
700
-
al = 1 06999152628346e+005
x
-
a2 = 1.47933368154553 /
x
600
Coated y'
aO = -7.14397526126849e+004
500
al = 6.64654699205777e+004 X X
a2 = 1.719254605608 X X
400
yX /
300
+
UNCOATED
200
A
MgO(4)
x^ X
100
x^
0
eT 1-^ 1 1 1 1 i
_l i
0 1 2 3 4 5
VOLTAGE (keV)

APPENDIX J
MATHCAD PROGRAM FOR MgO (8 min) / Y2Si05:Tb
This is a Mathcad program to compute the luminescence as a function of beam
energy from a 8 minute coating of MgO on Y2SiOs:Tb. The coating was applied by
pulsed laser deposition.
t := .043 thickness in microns of coating at top of particle
R :=5 radius of phosphor particle in microns
p :=3.6 density
Z := 10 atomic number
A :=20.152 average atomic weight
0 :=0,1.. 90 angle from center axis to coating's surface
Y(0) :=asin
(R+t-cos(0 deg)) sin(0deg)
R
angle from center axis to phosphor's surface
r(0) := (R)-sin(y (0)) distance from center axis
h( 0) :=Reos (y (0)) height of phosphor particle
c(0) :=(R+1-cos(0 deg)) cos(0 deg) height of phosphor and coating
290

291
d(0) :=c(0)- h(0) distance traveled through the coating at given©
Thickness is nearly constant for all 0, so take 0 = 0 for subsequent calculations.
Ei :=0, .1.. 4 incident beam energy
J :=( 11.5-Z)-10-3 mean ionization potential in keV
k :=0.85 fitting parameter for ionization potential correction
J2( Ei) :=
1 + k.
\Ei+ 10
\-50
low energy correction of ionization potential
1 3
D(Ei) 100T.262-A-(J2(Ei))2(Ei)2
2355-pZ
range in bulk of coating material at given incident
energy Ei based on Rao-Sahib & Wittry
D( 1.4) = 0.033
D(2.5) = 0.079 range in microns at beam energy (keV) inside brackets
D(4) = 0.161

292
E( 0, Ei) :=
E2( 0, Ei) :
E(0,Ei)
| j j
E2(0,Ei)
non
uuu
d(9) \3
D(Ei)/
energy after traversing coating from Rao-Sahib & Wittry
= Ei-exp -2
d(0)
0.064(Ei)136
fitted energy dependence
This is a plot of the electron's energy after traveling through the coating as a funciton of incident
beam energy. The red curve is based on the calculation using the stopping power equation of
Rao-Sahib and Wittry. The blue curve is based on the new fitted energy loss equation.

aO^-l.OVMlSOóóOTSiatí*
al :=5.3186545011678-2(J1
a2 := 1.53937175151185
fitting coefficients for a power law fit to the uncoated
phosphor luminescence, taken from SlideWrite
program
L(0,Ei) :=aO+al-(E(0,Ei))a2
luminescence of coated material based on Rao-Sahib and
Wittry energy loss equation
L2(0,Ei) :=aO+al-(E2(0,Ei))a2
luminescence of coated material based on new
energy loss equation
Li( 0, Ei) :=a0-t- al-(Ei)a2
luminescence of uncoated material
amO:=- 2.51103277212396 0*
aml:=2.0537145679361-3tf*
am21=2.0320565403483
fitting parameters for measured luminescence from
coated material, taken from SlideWrite program
Ln<0,Ei) :=am0+ aml-(Ei)31112
measured luminescence from the coated phosphor
The following graph shows the cathodoluminescence intensity as a function of beam
energy. The black curve is the uncoated phosphor. The green curve is the measured
luminescence from the coated phosphor. The red curve is the luminescence as
calculated based on the Rao-Sahib modification of the Bethe stopping power equation.
The blue curve is the luminescence based on the new energy loss equation. From the
graph, it can be seen that the Rao-Sahib and Wittry equation overestimates the
attenuation of the luminescence at low energies. The new energy loss equation
accurately predicts the luminescence over the entire energy range up to 4 keV.
Measurement of the thickness of the coating on the phosphor is the largest source of
error for the calculation. To provide a more accurate analysis, it was assumed that the
Rao-Sahib and Wittry equation is valid for the high energy region of the curve. The
thickness value was then optimized to give agreement between the calculated and
measured curves around 4 keV. For further discussion of this correction, see the
discussion of the second model in Chapter 6.

294
Ei
The following is the SlldeWrlte plot used to determine the curve fiting coefficelnts for the measured
luminescence from the coated and uncoated phosphor.
cn
600
500
z
D
CD
cc
<
(A
TD
C
(A
ZJ
o
C/3
Z
LU
400
300
200
100
- L=aO+al*E''a2
7"
/
s
Uncoated
" aO = • 1.07141806607322e+004
. al = 5.31865450116782e+004
s' /
a2 = 1.53937175151185
7T X
Coated
s /
. aO =-2.51103277212396e+004
s s
al = 2.05371456793615e+004 /
x
“ a2 = 2.0320565403483
/
/
/
s
/
S X
s' s''
s' s'
UNCOATED
â–  s'
s' s'
A MgO(8)
>s .s
1 1 1
_j 1 i
0 1 2 3 4 5
VOLTAGE (keV)

APPENDIX K
MATHCAD PROGRAM FOR A1203 (1.2 min) / Y2Si05:Tb
This is a Mathcad program to compute the luminescence as a function of beam
energy from a 1.2 minute coating of A1203 on Y2SiOs:Tb. The coating was applied by
pulsed laser deposition.
t := .014 thickness in microns of coating at top of particle
r :=5 radius of phosphor particle in microns
p:=3.97 density
Z := 10 atomic number
A :=20.3922 average atomic weight
6 :=0,1.. 90 angle from center axis to coating's surface
Y(0) :=asin
(R-ht-cos(9deg)) sin(9 deg)
R
angle from center axis to phosphor's surface
r(0) :=(R)-sin(Y(0)) distance from center axis
h( 0) :=Reos ( y (0)) height of phosphor particle
c( 0) := (R +1 cos (0 deg)) cos (0 deg) height of phosphor and coating
295

296
d(9) :=c(0) — h(0) distance traveled through the coating at given0
Thickness is nearly constant for all 0, so take 0 = 0 for subsequent calculations.
Ei :=0,. 1.. 4 incident beam energy
J :=( 11.5-Z) • 10-3 mean ionization potential in keV
k :=0.85 fitting parameter for ionization potential correction
J2( Ei) :=
1 + k-
J
/ J \
\Ei+ 10'50 /
low energy correction of ionization potential
1 3
D(Ei) 100-1.262-A-( J2(Ei))2-(Ei)2
2355-p Z
range in bulk of coating material at given incident
energy Ei based on Rao-Sahib & Wittry
D( 1.4) = 0.03
D( 2.5) = 0.072 range in microns at beam energy (keV) inside brackets
D(4) = 0.147

297
E(e,Ei) :=
E2( 6, Ei) :
E(0,Ei)
+-+-+
E2(0,Ei)
uuu
2
d(fl) \3
D( Ei) /
energy after traversing coating from Rao-Sahib & Wittry
= Ei-exp
-2-
d(0)
0.064-( Ei)1 36
fitted energy dependence
This is a plot of the electron's energy after traveling through the coating as a funciton of incident
beam energy. The red curve is based on the calculation using the stopping power equation of
Rao-Sahib and Wittry. The blue curve is based on the new fitted energy loss equation.

298
aO :=-4.518651968307a03
fitting coefficients for a power law fit to the uncoated
al :=9.6152563060055S03 phosphor luminescence, taken from SlideWrite program
a2:= 1.83509379303954
U0,Ei) :=a0+ al-(E(0,Ei))a2
luminescence of coated material based on Rao-Sahib and
Wittry energy loss equation
L2(0,Ei) :=a0-|-al-(E2(0,Ei))a2
luminescence of coated material based on new
energy loss equation
U0,Ei) :=aO-t-al-(Ei)a2
luminescence of uncoated material
am0:=-7.732264728032550?
ami := 6.3278999847540105
am21=2.0881981625911
fitting parameters for measured luminescence from
coated material, taken from SlideWrite program
Lm(0,Ei) :=am0+ aml-(Ei)ara2 measured luminescence from the coated phosphor
The following graph shows the cathodoluminescence intensity as a function of beam
energy. The black curve is the uncoated phosphor. The green curve is the measured
luminescence from the coated phosphor. The red curve is the luminescence as
calculated based on the Rao-Sahib modification of the Bethe stopping power equation.
The blue curve is the luminescence based on the new energy loss equation. From the
graph, it can be seen that the Rao-Sahib and Wittry equation overestimates the
attenuation of the luminescence at low energies. The new energy loss equation
accurately predicts the luminescence over the entire energy range up to 4 keV.
Measurement of the thickness of the coating on the phosphor is the largest source of
error for the calculation. To provide a more accurate analysis, it was assumed that the
Rao-Sahib and Wittry equation is valid for the high energy region of the curve. The
thickness value was then optimized to give agreement between the calculated and
measured curves around 4 keV. For further discussion of this correction, see the
discussion of the second model in Chapter 6.

299
The following is the SlideWrite plot used to determine the curve fiting coefficeints for the measured
luminescence from the coated and uncoated phosphor.
00
t
z
D
tfi
CD
T3
cc
s
S
>-
CO
U)
3
o
t
00
z
LU
É.
|
200
160
120
80
40
0
L=a0+al*E~a2
Uncoated
//
_ aO = -4.5186519683078 e+3
//
al = 9.61525630600558 e+3
//
- a2 = 1.83509379303954
y/
Coated
y y
“ aO = -7 73226472803259 e+3
//
al = 6.32789998475401 e+3
yy
a2 = 2.0881981625911
yy
UNCOATED
.--+'
A AI203(1.2)
—+— i ■ i
I i
1 2 3 4 5
VOLTAGE (keV)

APPENDIX L
MATHCAD PROGRAM FOR A1203 (2.4 min equiv.) / Y2Si05:Tb
This is a Mathcad program to compute the luminescence as a function of beam
energy from a 2.4 minute equivalent coating of A1203 on Y2SiOs:Tb. The coating was
applied by pulsed laser deposition. The poor accuracy of the calculated luminescence is
due to the fitting of the measured luminescence at low voltages.
t := .051 thickness in microns of coating at top of particle
r :=5 radius of phosphor particle in microns
p :=3.97 density
Z := 10 atomic number
A :=20.3922 average atomic weight
0 := o, 1.. 90 angle from center axis to coating's surface
Y (0) :=asin
(R+t-cos(0deg)) sin(0 deg)
R
angle from center axis to phosphor's surface
r(0) :=(R) sin(Y(0)) distance from center axis
h(0) :=R cos(Y(0)) height of phosphor particle
c( 0) :=(R +1-cos (0 deg)) -cos (0 deg) height of phosphor and coating
300

301
d(0) :=c(0) — h(0) distance traveled through the coating at givene
Thickness is nearly constant for all 0, so take 0 = 0 for subsequent calculations.
Ei := 0,. 1.. 4 incident beam energy
J :=(11.5-Z)-10-3 mean ionization potential in keV
k :=0.85 fitting parameter for ionization potential correction
J2(Ei) :=
J
1 + k-
J \
jEi+-10'50/
low energy correction of ionization potential
2 £
D(Ei) ,_10ai.262 A-(J2(Ei))2-(Ei)2
2355pZ
range in bulk of coating material at given incident
energy Ei based on Rao-Sahib & Wittry
D( 1.4) = 0.03
D(2.5) = 0.072 range in microns at beam energy (keV) inside brackets
D(4) =0.147

302
E( 0, Ei) :=Ei-
1-
d(9)
D(Ei)
energy after traversing coating from Rao-Sahib & Wittry
E2(0,Ei) :=Eiexp -2
d(0)
0.064(Ei)
1.36
fitted energy dependence
This is a plot of the electron's energy after traveling through the coating as a funciton of incident
beam energy. The red curve is based on the calculation using the stopping power equation of
Rao-Sahib and Wittry. The blue curve is based on the new fitted energy loss equation.

aO ¡=-7.19918149270744 Cf*
fitting coefficients for a power law fit to the uncoated
al := 1.3876840880079-205 phosphor luminescence, taken from SlideWrite program
a2:= 1.22985327091371
„ a2 luminescence of coated material based on Rao-Sahib and
L(0, i) .-a +al-(E(0, 0) wittry energy loss equation
L2(0,Ei) :=a0-j- al-(E2(0,Ei))a2 luminescence of coated material based on new
L<0,Ei) :=a0+ al-(Ei)a2
am0:=-4.3346165880584d0*
ami := 2.642103001733640*
am2 ¡=2.18952712892998
Lm( 0, Ei) :=am0-t- aml-(Ei)am2
energy loss equation
luminescence of uncoated material
fitting parameters for measured luminescence from
coated material, taken from SlideWrite program
measured luminescence from the coated phosphor
The following graph shows the cathodoluminescence intensity as a function of beam
energy. The black curve is the uncoated phosphor. The green curve is the measured
luminescence from the coated phosphor. The red curve is the luminescence as
calculated based on the Rao-Sahib modification of the Bethe stopping power equation.
The blue curve is the luminescence based on the new energy loss equation. From the
graph, it can be seen that the Rao-Sahib and Wittry equation overestimates the
attenuation of the luminescence at low energies. The new energy loss equation
accurately predicts the luminescence over the entire energy range up to 4 keV.
Measurement of the thickness of the coating on the phosphor is the largest source of
error for the calculation. To provide a more accurate analysis, it was assumed that the
Rao-Sahib and Wittry equation is valid for the high energy region of the curve. The
thickness value was then optimized to give agreement between the calculated and
measured curves around 4 keV. For further discussion of this correction, see the
discussion of the second model in Chapter 6.

304
The following is the SlideWrlte plot used to determine the curve fiting coefficeints for the measured
luminescence from the coated and uncoated phosphor.
C/2
t
z
D ^
m -S
< U)
El
C/2 t
Z
LU
VOLTAGE (keV)

APPENDIX M
MATHCAD PROGRAM FOR A1203 (5 min) / Y2Si05:Tb
This is a Mathcad program to compute the luminescence as a function of beam
energy from a 5 minute coating of A1203 on Y2SiOs:Tb. The coating was applied by
pulsed laser deposition. Due to the thickness of the coating, no accurate fit could be
obtained. However, the following shows a fit based solely on the new energy loss
equation.
t := .23 thickness in microns of coating at top of particle
R :=5 radius of phosphor particle in microns
p :=3.97 density
Z := 10 atomic number
A :=20.3922 average atomic weight
0 :=0,1.. 90 angle from center axis to coating's surface
Y(0) :=asin
(R-l-t-cos(0-deg)) sin(0 deg)
R
angle from center axis to phosphor's surface
r(0) := (R) -sin(y (0)) distance from center axis
h( 0) :=Reos ( y (0)) height of phosphor particle
c(0) :=(R-M-cos(0-deg))-cos(0-deg) height of phosphor and coating
305

306
d(0) :=c(0) - h(0) distance traveled through the coating at given0
0.4
0.35
0.3
0.25
1 1 1 1
1 1 1 1
d( 0) 0.2
-
0.15
-
0.1
-
0.05
1 1 1 1
_l 1 1 1
0 10 20 30 40 50 60 70 80 90
0
Thickness is nearly constant for all 0, so take 0 = 0 for subsequent calculations.
Ei:=0,.l.. 4 incident beam energy
J :=(11.5Z)10"3 mean ionization potential in keV
k :=0.85 fitting parameter for ionization potential correction
J2^El') 7 j T low energy correction of ionization potential
1 +■ k- ¡
\Ei+ 10'50 j
1 3
D(Ei) 100-1.262 A (J2(Ei))2-(Ei)2
2355p-Z
D( 1.4) = 0.03
D(2.5) = 0.072 range in microns at beam energy (keV) inside brackets
range in bulk of coating material at given incident
energy Ei based on Rao-Sahib & Wittry
D( 4) = 0.147

307
ECe.Ei) :=Ei
1-
d(0)
D(Ei)
energy after traversing coating from Rao-Sahib & Wittry
E2(0,Ei) :=Ei-exp -2
d(0)
0.064(Ei)
1.36
fitted energy dependence
This is a plot of the electron's energy after traveling through the coating as a funciton of incident
beam energy. The red curve is based on the calculation using the stopping power equation of
Rao-Sahib and Wittry. The blue curve is based on the new fitted energy loss equation.

308
a0 :=-1.7518774517251903
fitting coefficients for a power law fit to the uncoated
at ¡=5.7457394915156304 phosphor luminescence, taken from SlideWrite program
a2:= 1.67325236613149
Lie.Ei) :=aO+al-(E(0,Ei))a2
luminescence of coated material based on Rao-Sahib and
Wittry energy loss equation
, *2 luminescence of coated material based on new
L2(0,Ei) ¡=a0+al-(E2(0,Ei)) . *•
^ T y ^ ’ ” energy loss equation
L'i 0, Ei) :=aO-t-al-(Ei)a2 luminescence of uncoated material
amO :=- 3.39760988734409 02
ami := 9.2248121084405102
am2:= 3.28254555216493
Lm(0,Ei) :=am0+ ambfEi)31"2 measured luminescence from the coated phosphor
fitting parameters for measured luminescence from
coated material, taken from SlideWrite program
The following graph shows the cathodoluminescence intensity as a function of beam
energy. The black curve is the uncoated phosphor. The green curve is the measured
luminescence from the coated phosphor. The luminescence as calculated based on
the Rao-Sahib modification of the Bethe stopping power equation is not present over
this energy range. The blue curve is the luminescence based on the new energy loss
equation. To find a thickness value for this calculation, the same energy loss
equation as in the other fitting programs was used. The thickness was then varied
until agreement was obtained between the predicted and measured luminescence
from the coated phosphor. Due to the thickness of this coating and the limit of the
beam energy imposed by the electron gun, no accurate prediction is possible for this
sample.

309
Ei
The following is the SlideWrite plot used to determine the curve fiting coefficeints for the measured
luminescence from the coated and uncoated phosphor.
3
w
T3
C
-
S «
CD
o:
C/5
Z
LU
O
800
700
600
500
400
300
200 -
100 -
0
L=aO+al*E/'a2
Uncoated
aO = 1.75187745172519 e+3
al = 5.74573949151565 e+4
a2 = 1.67325236613149
Coated
aO = 3.39760988734409 e+2
al = 9.22481210844051 e+2
a2 = 3.28254555216493
+
â–²
UNCOATED
AI203(5)
-h—L
VOLTAGE (keV)

1
2
3
4
5
6
7
8
9
10
11
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BIOGRAPHICAL SKETCH
William Joseph Thornes, Jr. was bom on July 21, 1974, in Richmond, Virginia.
He attended John Randolph Tucker High School, where he graduated with honors in
1992. The next four years were spent at the College of William and Mary in
Williamsburg, Virginia, where he obtained his Bachelor of Science degree in physics and
mathematics. His senior thesis, entitled “Development of a Small Scale Gridded Energy
Analyzer,” was awarded honors by the Physics department. Upon completion of his
undergraduate education in May 1996, he came to the University of Florida to study
under the guidance of Dr. Paul Holloway. The next several years were spent learning
about the field of Materials Science. He received his Master of Science degree in
May 1999. That summer, he traveled to Albuquerque, New Mexico to do collaborative
research with Dr. Carl Seager and Dr. Dave Tallant at Sandia National Laboratories.
322

I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
/f
Paul H. Holloway, Chairman
Professor of Materials Science and
Engineering
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
Ataoil. i£. S21 íá,í—
Josebh H. Simmons
Professor of Materials Science and
Engineering
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
Rajiv K. Singh
Professor of Materials Science and
Engineering
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy
iophy.
JL
Rolf E. Hummel
Professor of Materials Science and
Engineering

I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
Fred Sharifi
Associate Professor of Physics
This dissertation was submitted to the Graduate Faculty of the College of
Engineering and to the Graduate School and was accepted as partial fulfillment of the
requirements for the degree of Doctor of Philosophy.
August 2000
Dean, College of Engineering
Winfred M. Phillips
Dean, Graduate School

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