Characterization of the cathodoluminescent degradation of Y202S:EU3+Y₂O₂S:EU³⁺ powder CRT phosphor

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Title:
Characterization of the cathodoluminescent degradation of Y202S:EU3+Y₂O₂S:EU³⁺ powder CRT phosphor
Physical Description:
ix, 262 leaves : ill. ; 29 cm.
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English
Creator:
Trottier, Troy Anthony, 1969-
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Subjects / Keywords:
Materials Science and Engineering thesis, Ph.D   ( lcsh )
Dissertations, Academic -- Materials Science and Engineering -- UF   ( lcsh )
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bibliography   ( marcgt )
non-fiction   ( marcgt )

Notes

Thesis:
Thesis (Ph.D.)--University of Florida, 1997.
Bibliography:
Includes bibliographical references (leaves 257-261).
Statement of Responsibility:
by Troy Anthony Trottier.
General Note:
Typescript.
General Note:
Vita.

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University of Florida
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All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 002325242
oclc - 38879437
notis - ALS8799
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Full Text












CHARACTERIZATION OF THE CATHODOLUMINESCENT DEGRADATION OF
Y202S:EU3, POWDER CRT PHOSPHOR














By

TROY ANTHONY TROTTIER


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

UNIVERSITY OF FLORIDA


1997















ACKNOWLEDGMENTS


Being the last section that I have sat down to write,

all the pages before seem a blur. To me, this section, above

all others, contains the essence of the work which is

contained within. In a few short pages, I am required to

acknowledge all those who have aided and supported me in

this effort. I cannot. For each person mentioned will have

contributed far more than I can hope to express my gratitude

for in one or two sentences. Moreover, their contributions

to my work are the last thing I would thank them for,

instead I would like to thank them for their contributions

to me. In sharing themselves with me, I have gained insight

and knowledge and grown as a person, and perhaps, if they

think about it, they have too. Each person has added

something of themselves to me, and thus to this work.

Dr. Holloway made it so easy for me to walk in and

start asking questions about any subject, and never did I

walk out thinking that the door was closing behind me. He









has mastered the secrets of the student-teacher relationship

and has been a father, a friend, and a confidant to me.

Dr. Masi's early example of what a true scientist is

will always remain with me. I cannot thank you enough for

introducing me to your friend and colleague, Paul Holloway.

Mom and Dad and Todd, through every part of my life,

you have stood by me and supported me (albeit at the time I

may have been too blind to notice!). Above all others, you

are the reason that my dreams have come to fruition. I love

you.

Jon Hack, the very definition of what a friend should

be, your friendship and presence made the trials of daily

life seem manageable. No matter what the signs say, swimming

cannot be bad for you.

Wish, it seems to me that for the past 3 years, I have

been asking favors of you and never have you hesitated. I

give you that in return. Your friendship and devils-advocate

attitude towards my research will be sorely missed. Don't

ever forget, there will always be a glass of wine waiting

for you.

Tall Joe and Little Billie, I must formally apologize

in advance for any future breakdowns of Humpty Dumpty (the

iii









new nickname for the Auger, ask Wish!). Besides that, I must

say you both have worked harder on that system than any

others during my time here, I leave it in very capable

hands.

I am indebted to Ron Petersen and Motorola Inc. for

their continued support of my research and my future.

















TABLE OF CONTENTS


ACKNOWLEDGMENTS . .


. ii


ABSTRACT . . viii

CHAPTERS


1 INTRODUCTION . . .

Motivation and Objectives . .
Scope of the Present Work . .


2
. 1



. 2


2 LITERATURE REVIEW .


Introduction . .. .. 5
Field Emission Displays . 6
Introduction to Cathodoluminescence . 8
Theoretical Aspects of Cathodoluminescence .. 11
CL Degradation . . 20
CL Degradation of Sulfides . 27
Degradation Resistant Coatings . 32
Summary and Motivation . 34

3 EXPERIMENTAL APPROACH . . 53

Introduction. .. . .. 53
Phosphor Material . . 54
Industrially Processed Yttrium Oxysulfide
Phosphor . . 54
Methods of Applying Coatings to Phosphors 55
Wet Chemistry Coatings . 55
Phosphate Coating . 56
SiO2 Coating . 57
Pulsed Laser Ablation . 59










Characterization Techniques . 60
Cathodoluminescent and Auger Electron
Spectroscopy . 60
Degradation Experiment . 62
Conversion of Time and Current Density to
Coulomb Load . 64
Turn-On voltage Experiment . 65
Scanning Electron Microscopy .. 67

4 BASELINE STUDIES OF Y202S:EU3

. . . . .. 72
Introduction . . 72
Physical and Luminescent Properties of Y202S:Eu3 73
Physical properties of Y202S:Eu3 74
Luminescent properties of Y2O2S:Eu3 75
Degradation Characteristics of Yttrium Oxysulfide
. . 77

Summary . . 87

5 EFFECTS OF OXYGEN AMBIENTS ON THE DEGRADATION OF Y202S:Eu3

. . . 101
Introduction . . 101
Degradation of Y202S:Eu3 in an Oxygen Ambient 102
Time Dependent Threshold and CL Degradation Experiment
. . . 109
Summary . . 115

6 BEAM ENERGY DEPENDANCE ON DEGRADATION RATES OF
Y202S:EU3 . . 136

Introduction . . 136
1 keV Experiment . .. 139
2 keV Experiment . .. .141
3 keV Experiment . .. .143
Discussion and Comparison of 1,2 an 3 Kev Data 146
Depth Resolved CL Studies of Y203:Eu3+ Emission 148
Summary. .... . 150










7 MODELING OF THE CL DEGRADATION OF Y202S:EU .

Introduction . . .
Modeling of Brightness as a function of Depth
CL Efficiency as a function of Depth .
Power Dissipation as a function of Depth in
Y202S:Eu3 . .
Predictions of the Brightness Model .
Modeling of Gas-Surface Interactions of Y20,S:Eu3


Introduction . ... 191
Gas Impingement On the Surface .. 192
Dissociation Efficiency of 02 by Electron Impact
. . 193


Discussion . .
Summary . .

8 DEGRADATION RESISTANT COATINGS FOR Y202S:Eu3

Introduction . .
Comparison of CL Degradation Rates .
TaSi Coated material .
Ag Coated Y22S:Eu3 . .
SiO2 Coated Y S:Eu .
Phosphate coated Y20S:Eu3 .
Summary . .

9 SUMMARY AND CONCLUSIONS . .


198
203


. 220

. 220
. 222
. 223
. 225
. 228
. 229
. 231

. 251


257

262


LIST OF REFERENCES .

BIOGRAPHICAL SKETCH .


vii


179

179
181
182

186
188

1 1















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

CHARACTERIZATION OF THE CATHODOLUMINESCENT DEGRADATION OF
Y202S:EU3 POWDER CRT PHOSPHOR

By

TROY ANTHONY TROTTIER

DECEMBER, 1997

Chairperson: Dr. Paul H. Holloway
Major Department: Materials Science and Engineering

Cathodoluminescent degradation of Y202S:Eu3 powder

phosphors has been studied by Auger electron spectroscopy

(AES) and cathodoluminescent spectroscopy (CL). The

dependance of CL degradation on pressure, ambient gas and

electron beam energy have been examined.

CL degradation occurred slowly at 1x10-8 Torr and the

combination of AES and CL spectroscopy indicated the loss of

S and C from the surface of the phosphor. Volatilization

reactions involving O containing gases, the electron beam

and the surface C and S caused a conversion of Y202S:Eu3 to


viii









a non-luminescent layer, possibly Y202SO4:Eu3. This layers

growth decreased the total brightness of the phosphor.

The conversion of Y202S:Eu3 to Y202SO4:Eu3 requires O, so

CL degradation was studied at xl10-6 Torr backfilled from

1xl108 Torr with O gas. The CL degradation rate was much

higher in the presence of 0 gas. The conversion of Y202S:Eu3"

to Y202SO4:Eu3' was not detected, but in the presence of O, a

near surface layer of luminescent Y203:Eu3+ was formed and

confirmed by CL spectroscopy.

Makhov's Law was used to predict the brightness loss of

the Y202S:Eu3. Very good correlation was found between all

experiments and the brightness loss predictions of the

model. The amount of ionized gas at the surface was modeled

using a modified ionization efficiency for electrons. Based

on these models, protective surface coatings were applied to

the phosphor particles in an attempt to slow the CL

degradation.

Coatings of TaSi and Ag were deposited by a pulsed

laser ablation and were found to reduce the CL degradation.

The TaSi coating was tenacious and may have slowed the

diffusion of S to the surface formation where it was present

on the surface. The Ag coating delayed the CL degradation

ix









but either migrated during bombardment or was desorbed

(AgS), slowly exposing the Y202S:Eu3 phosphor.

Wet chemistry coatings of SiO2 and phosphate were

applied as well, and were found not to affect the CL

degradation rate.















CHAPTER 1
INTRODUCTION




Motivation and Obiectives



The degradation of phosphor materials in

cathodoluminescent displays is one of the major concerns for

the recently developed FED (field emission display). In

order for FED's to compete with other flat panel display

technologies (EL, LCD, VFD), the luminescent lifetimes of

the currently available phosphors must be extended. The

physical phenomena that cause degradation are related to

surface volatilization and corrosion reactions induced by

residual gases in vacuum and electron beam dissociation of

these gases [Swa96].

Most manufacturers of prototype FED displays are

currently using sulfur based phosphors that have been used

in CRTs for many years. The sulfur-based phosphors (namely

ZnS:Ag, ZnS:Cu and Y202S:Eu3') used in CRTs exhibit the

highest luminous efficiencies of all the currently available

1











industrial phosphors. Unfortunately, it has been

demonstrated that the cathodoluminescent (CL) brightness of

these phosphors is sensitive to gas pressure and gas

ambient. Thus, the initial gains of high luminous efficiency

are lost too soon due to CL degradation.

In order to realize the use of sulfur based CRT

phosphors in FEDs, a basic understanding of the CL

degradation phenomena is required. The objectives of this

research therefore is the understanding of this degradation

in one phosphor: Y202S:Eu3- Once the mechanisms of CL

degradation are properly defined, methods of slowing or

stopping the CL degradation may become apparent.


Scope of the Present Work



A review of the literature (Chap.2) shows that very few

basic studies have been reported on the degradation

phenomena found in sulfur based phosphors. Evidence of

electron beam stimulated reactions causing CL degradation in

the ZnS family of phosphors has been reported [Swa96,

Ito89]. A comparison of the CL degradation rates of three

CRT phosphors has been reported and it was shown that











Y2O2S:Eu3 is the least sensitive to electron beam stimulated

reactions causing CL degradation [Tro96], but detailed

studies of the reasons have not been reported.

One objective of this study was to examine the effects

of gas pressure and type of gas on the CL degradation of

Y202S:Eu3. CL spectroscopy and Auger electron spectroscopy

(AES) were used in conjunction to correlate surface changes

on the phosphor with luminescent changes observed under

electron beam bombardment. The experimental procedure for

these types of studies are discussed in Chapter 3. The

results showed a correlation between sulfur loss from the

surface, the presence of oxygen gas and CL degradation. The

results of these experiments will be discussed in Chapter 4

and Chapter 5.

Another objective of this study was to examine the

effects of electron beam energy on the CL degradation rates

of Y2O2S:Eu3" and to attempt to model this dependance. A beam

energy dependance was observed and modeled. Good correlation

was found between the model and the experimental results of

Chapter 4 and 5. The results of the beam energy dependance

of the CL degradation rate of Y202S:Eu3 are reported in











Chapter 6. The model used to approximate the CL degradation

of Y202S:Eu3, is developed in Chapter 7.

Finally, it was determined that surface coatings

applied to the Y202S:Eu3" phosphor could be used to slow the

CL degradation phenomena. The methods used to coat the

phosphor and the types of coating materials studied are

discussed in Chapter 3. Many of the coatings that were

examined did show improved CL degradation characteristics

and the results of these studies are discussed in Chapter 8.

The conclusions from this study are given in Chapter 9.















CHAPTER 2
LITERATURE REVIEW




Introduction



For nearly 40 years, cathode ray tubes (CRT's) have

dominated the color display industry. A superior quality

picture, coupled with long lifetime, and large viewing angle

have solidified the CRT's place in the display industry. As

with all industries, the need for increased portability and

efficiency is now being demanded of the CRT. The basic

design of the CRT severely limits its ability to be reduced

in size as can be seen in Figure 2-1 [Tan85]. A certain

distance is required between the anode and cathode to

ensure proper focus and raster capability. The CRT's crisp

visual clarity and brightness are dependent upon the power

levels used (20-30kV, 1-10nA), thus a reduction in power

(increase in efficiency) compromises the visual

characteristics of the display. This interdependence of

picture quality and power level limits the CRT's ability to

5











become more portable and more efficient without sacrificing

the most important qualities of the display.


Field Emission Displays




Field emission displays (FEDs) are one solution to the

size reduction and efficiency improvement problems

associated with CRT's. Instead of using 3 electron sources

(respectively known as red, green and blue guns) and

rastering over the entire screen as in the case of the CRT,

each pixel is fitted with its own electron source. As shown

in Figure 2-2 [Sch96], the placement of an emission source

in proximity to the pixel alleviates the need for a

relatively long electron path for focus and raster. The

distance from the electron source to the phosphor screen can

now be 1 mm or less.

One of the major hurdles that the FEDs face is the

world-wide acceptance of the CRT as a standard by which

fledgling display technologies are measured. The world has

become accustomed to the chromaticity, brightness, and

overall picture quality of the CRT, and requires that any

new display technology be comparable if not better than the











CRT. The CRT and FED modes of operation

(cathodoluminescence) are very similar and this ensures the

equivalency of chromaticity, picture quality and viewing

angle when compared with passive methods of display

generation such as is found in the LCD and AMLCD. Both use a

directed, accelerated beam of electrons to induce

cathodoluminescence from a phosphor screen [Tro96]. Due to

this commonality, the FED industry has adopted the standard

CRT phosphors as starting materials for the FED display

technology. These phosphors have met with limited success in

FED's due to complications associated with the large

differences in operating parameters of the FED as compared

with the CRT. The electrons in a CRT typically strike the

phosphor screen with 20 kV or higher kinetic energies. The

FED cannot sustain such large potentials between the anode

and cathode gap because the distance is much smaller and

dielectric breakdown (arcing) can occur [Lev91]. FED's are

currently being manufactured with anode to cathode

potentials of 5 kV or less. The manufacturers are happy with

this intrinsic breakdown limitation because it reduces the

voltage requirements of the display package drivers. The

trade off becomes apparent in the brightness of the display.











In order to match the brightness of the CRT, the power

delivered to the phosphor must be the same. Thus a reduction

in voltage from 25 kV to 5 kV necessitates a fivefold

increase in screen current to obtain equivalent screen

brightness. The increased current requirements of the field

emission display are not a problem for the field emitter

tips, since they are capable of sustaining currents in

excess of 100ma/mm2 [Der94]. However, the cathodoluminescent

lifetime of the phosphor materials is directly related to

the total charge density (C/cm2) impressed upon them

[Pfa61]. Thus, the higher the current density applied to the

phosphor material, the shorter the useful lifetime of the

phosphor [Pet97].


Introduction to Cathodoluminescence



Luminescence has been described by many authors as the

conversion of energy into electromagnetic radiation over and

above black body radiation [Bla94]. The type of excitation

energy defines the luminescent phenomena. For instance,

photoluminescence is luminescence stimulated by an UV light

source. Triboluminescence defines the luminescent phenomena









9
caused by friction or fracture, as in the light generated by

rubbing two quartz crystals together. Cathodoluminescence,

naturally, is luminescence generated by cathode ray

excitation of a material.

Only very rarely does a material luminesce efficiently

in the visible region of the electromagnetic spectrum. Most

materials that are efficient emitters in the visible

spectrum have been specifically designed. Ordinarily, a

starting material is chosen which has a band gap which would

produce luminescence in or above the ultraviolet

(uV)(>4.2eV) region. This material is called the host. It

serves two purposes, as an optical window for the visible

luminescence, and as the container for the optically active

luminescent center known as the activator. The activators'

visible light emission is affected by its interaction with

the host material. The activator is intentionally introduced

into the host as a defect. Typically less than a few mole

percent of the activator is required to obtain efficient

luminescence. An example of luminescence phenomena is

depicted in Figure 2-3 [Lum78] for the case of Cr3* in A1,03.

The figure displays the absorption and emission spectra as a

function of energy in eV. For this case, only one of the











transitions (2E-4A2) is observed as visible luminescence.

Other transitions occur, as in the absorption spectra, but

the energies of these transitions produce emission in the

infrared, thus they are not shown in the luminescence

spectrum.

Due to the probablistic nature of the interaction

between an electron and the material, cathodoluminescence

has staunchly resisted theoretical descriptions. Yet by far,

it is one of the easiest luminescent mechanisms to create,

control and study experimentally. Cathode rays interact with

the phosphor system as a whole. Interactions occur with the

host, activator, surface atoms and molecules within the

penetration limits of the electron beam. The interaction is

not limited to the primary beam electrons and their effect

on the phosphor. Each primary electron creates many

secondary electrons in cascade as it is inelastically

scattered within the phosphor. This secondary cascade is one

of the major differences between cathodoluminescence and

photoluminescence. Photoluminescence primarily excites the

activator, whereas cathodoluminescence excites with gain,

the host as well as the activator. Gain is possible in

cathodoluminescence because a single primary electron can









11
generate thousands of secondary electrons which can produce

photons. In contrast, in photoluminescence a single incident

photon typically only produces a single emission photon.


Theoretical Aspects of Cathodoluminescence



At the most basic level, cathodoluminescence is defined

by the interaction of electrons with a solid. Two types of

interactions are possible between a nearly free electron

and a solid. Elastic collisions, which are collisions

between an electron and a nuclei [Yac90], produce high

energy back scattered electrons. Inelastic collisions,

composed of electron-electron and electron-plasmon

interactions, provide all the information obtainable in

electron beam instruments. These include Auger electrons,

X-rays, secondary electrons, and most importantly (for this

work), cathodoluminescence.

Figure 2-4 [Hud92] depicts the number of electrons

produced as a function of energy. The right-most peak which

is denoted by Vp are those electrons which have undergone

purely elastic collisions and exhibit no energy loss. All

electrons below Vp have undergone inelastic collisions with











the surface or bulk atoms or electrons. The large peak at

low energy are the electrons known as true secondaries.

These are electrons which have originated in the solid and

escape by gaining enough energy to exceed the work function

of the solid and leaving into vacuum.

The true secondaries which escape into vacuum are not

the electrons responsible for cathodoluminescence. Instead

it is the secondaries generated within the solid which do

not escape to vacuum which provide the energy to the

luminescent center within the phosphors. Comprehension of

this concept is vital to understanding the nature of

cathodoluminescence. A single primary electron will undergo

tens or hundreds of inelastic collisions within a solid.

Each collision can produce secondary electrons which may be

absorbed by an activator and recombine to produce visible

luminescence. Thus, in order to understand

cathodoluminescence, knowledge of inelastic scattering and

electron-solid interactions is necessary.

Inelastic scattering of primary electrons has been

described empirically by Bethe [Bet53]. The continuous power

loss of primary electrons in a material has been described

by the following simple relationship:












de -785pZ l 1.166e
I-n( ) (2.1)
ds Ae j



where de is the incremental energy loss in a unit depth da

in eV/A, A is the atomic weight in g/mol, Z the atomic

number, e the electron energy, p the material density in


g/cm3 and j is the mean ionization potential in eV, given by

the following relation for Z>13:


58.5
j=9.76*Z+ 5 (2.2)
Z0.19



For high energy electrons (5-100keV), Bethe's stopping power

equation has been shown to closely match experimental data

[Myk76, New76]. As can be seen in Figure 2-5, at low

energies (<0.5keV) the slope of Bethe's stopping power

equation becomes negative. This would imply that the primary

electron is gaining energy from the solid, which is

physically impossible. The most popular solution to this

problem has been the low energy adjustment suggested by

Rao-Sahib and Wittry [Rao74]. Their alteration used a

modified equation below the inflection point of Bethe's

original equation, while keeping the original Bethe equation









14

intact for energies greater then this inflection point. The

modified equation used by Rao-Sahib is given by:


de -785*p*Z
(2.3)
ds JA*e*j



This model extends the low energy limit to

approximately 500 volts at which point the slope of energy

loss curve continues to increase and the slope of the

experimental data of Tung et al. decreases. Figure 2-5

[Joy89] is a comparison of the original Bethe equation and

the modification proposed by Rao-Sahib and Wittry to the

experimental data found by Tung et al. As can be seen,

neither the original Bethe equation, nor the modification

applied by Rao-Sahib are good approximations for electron

energies below approximately 0.5 keV.

Joy and Luo [Joy89], in an attempt to extend the

capabilities of the original Bethe model to energies down to

50eV, rewrote the ionization potential with an energy

dependent term where j' replaces j (the average energy to

create an electron-hole pair) in the original equation and

is given by:













j=+! (2.4)
1+k-
e







where k is a fitting constant which varies from 0.7 to 0.9

depending upon the material system. It has a suggested value

of 0.85 for most materials which produces no more than a 10%

variance from experimental stopping powers. The dependence

of j' on energy has two effects. At high energies, j'

approaches j, reducing the modified Bethe equation to the

original Bethe equation where its accuracy is well

established. At low energies where e
dependance causes j' to vary linearly with e. Although the

variation of j' with e is empirically derived, some

experimental evidence for such a variation has been reported

[Lav85]. These alterations allow the modified Bethe equation

to be accurate down to approximately 50 eV. Figure 2-6

compares the theoretical stopping powers obtained by the

modified Bethe (MB) equation to experimental values for Cu

and C reported by Tung and Akkerman [Joy89]. As can be seen,









16

the predictions of the modified Bethe model proposed by Joy

and Luo closely match the experimentally determined stopping

power values found for Cu and C by Tung and Akkerman for

electron energies from 100 eV to 10,000 eV. The electron

energies used to induce cathodoluminescence range from 1 to

3 keV, thus the modified Bethe equation will be used to

approximate the power dissipation in the phosphors.

Since the Bethe equation and the modified Bethe

equation describe the continuous energy loss of a primary

electron, the total range (defined as the total travel

distance of an electron in a solid) can be determined by the

following integration:


R=RR e+f PEe 1de
ev OeV de (2.5)
ds



where RRsoev is the residual range of a 50eV electron in

Angstroms. The residual range (RRsoev) has been estimated

using the results of Nieminen [Nie88] which demonstrates

that the stopping power for any material varies as E5/2 below

50 eV. The range predicted by Joy and Luo can be used to

determine the interaction volume of the primary electrons in











a solid, as well as the amount of energy lost to the solid

at a given depth.

Kanaya and Okayama [Kan71] have derived an expression

for the total range of electrons in a solid which includes

the penetration depth of primary electrons plus a sphere

located at the maximum energy dissipation depth with a

radius equal to the diffusion length of a secondary electron

generated in the material. From Figure 2-7, it can be seen

that the range derived from the modified Bethe equation is

approximately equal to the Kanaya and Okayama range except

at extremely low primary electron voltages (<100eV).

In order for these models of primary electron energy

loss to be useful, their validity must be determined for

the specific case of cathodoluminescence. As stated earlier,

the generation of a cathodoluminescent signal is dependent

upon secondary electron generation in a phosphor. Since the

number of secondaries greatly exceeds the primary electron

density, most of the cathodoluminescent signal can be

attributed to secondary electron recombination. Donolato

[Don78], using the universal depth dose function proposed

by Everhart and Hoff [Eve71] which denotes the local

generation rate of carriers in a material, found that the









18

secondary electron-hole pair generation density was highest

inside a volume of the same order of magnitude as the

electron generation range. Figure 2-8 [Yac90] is a plot of

the excess carrier concentration (solid lines) superimposed

upon the secondary electron generation volume(dashed

circle). The solid lines depict levels of equal carrier

concentrations. The highest carrier densities are contained

within the generation volume as denoted by the relatively

large numbers within the circles. Outside of the generation

volume, the carrier density drops at an exponential rate.

It seems plausible to assume that since the primary

electrons generate the cascade of secondary electron-hole

pairs, that the electron-hole pair generation volume may be

approximated by the same volume which defines the primary

electron range, and that the secondary generation rate may

be related to the primary electron power density. These

assumptions, along with the conclusions based on the data of

Figure 2-7, that the highest density of electron hole pairs

is found within the generation range allows a first order

approximation of cathodoluminescent brightness versus depth

to be drawn qualitatively as being parallel to the power

loss curve of the primary electrons.











Makhov [Mak60], using similar assumptions, suggested

that the brightness produced at any depth in a phosphor must

be proportional to the power dissipated by the electron beam

at a given depth in the absence of carrier diffusion

processes:




dB dP
=p(x) *- (2.6)
dx dx



where dB/dx is the variation in brightness as a function of

depth x from the surface of the phosphor. dP/dx is the power

dissipated by an electron beam as a function of depth and

p(x) is the cathodoluminescent efficiency at depth x.


Typically, p(x) is assumed to be constant for all depths.

It is possible to obtain an approximation to the power

dissipated by the electron beam as a function of depth by

using the modified Bethe equation. Since average power is

equivalent to average energy dissipated per unit time, it is

possible to obtain the average power knowing the initial

kinetic energy of the electron, the distance it has traveled

and its velocity. These three quantities can all be obtained

from the modified Bethe equation.









20

Kingsley and Prener [Kin72] showed very good agreement

between theoretical and experimental brightnesses of ZnS:Cu

with known thicknesses of non-luminescent ZnS deposited on

the particles using the above assumptions by Makhov.

Theoretical and experimental brightnesses as a function of

primary electron energy shown in Figure 2-9 [Kin72] were in

very good agreement. The conclusion was that when

non-luminescent surface layers were comparable in thickness

(0.08jm to 0.4pm) to the carrier diffusion lengths in

ZnS:Cu, the cathodoluminescent brightness was dependant on

the power loss of the primary beam in the phosphors and in

the non-luminescent surface coatings [Kin72].


CL Degradation




Many phosphors [Han52, Rot54] have been shown

experimentally to decrease in brightness under prolonged

electron beam excitation. The degradation of

cathodoluminescence in phosphors has been studied in many

different materials systems, with an equal number of

proposed mechanisms for the phenomena. Introduction or

diffusion of defects in crystals, increases in optical









21
absorption, and the desorption of surface species leading to

increased surface losses have all been proposed as possible

mechanisms for CL degradation. In general, the mechanism for

cathodoluminescent degradation appears to be dependent upon

the phosphor material studied.

Rhonda et al. [Ron94] found that for high columbic

loads (>400 C/cm2), as would be typical in projection

television tubes, that LaOBr:Tb3 excited by low

energy(l-4keV) electrons exhibited increases in optical

absorption coefficients. Coupled with absorption

spectroscopy, ion desorption rates were measured and it was

found that the desorption rates of bromine was three orders

of magnitude higher than that of oxygen.

By considering the experimental data, three possible

mechanisms for phosphor degradation were proposed by the

authors [Ron94]: (1) Urbach Tail absorption (broad

absorption below the band edge), (2) the formation of color

centers (mainly F-centers, where an anion vacancy is

occupied by an electron for charge neutrality) producing

weak absorption in the visible, or (3) absorption increases

caused by chemical modification of the phosphor surface as











indicated by ion desorption of O and Br from the surface

were suggested.

The decrease in cathodoluminescence and corresponding

increase in absorption coefficient were found to be

reversible by heat treatment in a reducing atmosphere. It

was suggested by the authors that the reversibility of CL

loss by heat treatment indicated a recrystallization of the

phosphor material with a decrease in the total number of

non-radiative recombination centers [Ron94].

Klaassen and deLeeuw examined the cathodoluminescence

degradation of three different phosphors, Zn2Sio:Mn2*,

Y2SiO3:Ce3" and Sr2Al6O,1:Eu2+ as well as the degradation

characteristics of Tb3+ doped borate glass. For ZnSiO4:Mn2*,

the degradation effects were attributed to a thickening of

the dead layer by a factor of two [Kla87]. Further

experiments showed that the dead layer losses were mainly

due to an increasingly oxygen deficient layer on the surface

of the phosphor particles.

Y2Si03:Ce3+ showed changes in relative radiant

efficiency with increased saturation which was interpreted

as decreased energy flow to the luminescence center Ce3-

Degradation, in this case, was attributed to a decrease in











the active Ce3 concentration [Kla87]. Sr2Al6On:Eu2

exhibited a characteristic browning in the electron beam

exposed area. Decreased external radiant efficiency, coupled

with lower reflection values, led the investigators to

speculate that the degradation was caused by a decreased

photon escape probability [Kla87]. This self absorption was

further thought to be due to color centers in the aluminate

host. Borate glass doped with Tb3 was found to exhibit

decreased non-radiative lifetime for the excited states of

Tb3. The authors suggested that energy transfer to killer

centers created in the glass under prolonged electron beam

excitation was responsible for the decrease in decay time

[Kla87].

Some of the earliest work attempting to categorize

degradation phenomena for a large number of phosphors was

reported by Pfanhl at Bell Labs [Pfa61]. Pfanhl found that

the degradation rate of phosphors followed a simple formula:




I(N)= (2.7)
(1+C*N)









24

where I is the cathodoluminescence intensity at any electron

dose N (#e's/cm2), I0 is the initial cathodoluminescent

intensity and C is the burn parameter (cm2) Recent work by

Bechtel and associates [Bec96] has reconfirmed the validity

of this formula for anode voltages as low as 2keV, and high

current loads for the CRT phosphors of Y202S:Eu3, ZnS:Ag,Cl,

and ZnS:Cu,Au,Al.

Pfanhl studied cathodoluminescent degradation in a

large number of phosphor systems and found that the physical

phenomena leading to cathodoluminescent degradation was

attributable to one of two mechanisms. In many phosphors

studied, new non-radiative recombination sites were believed

to be created by prolonged electron beam bombardment

[Pfa61], although no experimental method of proving the

existence of these non-radiative sites was discussed or

presented. In other phosphors, deactivation of the

luminescent center (activator) by charge compensation was

believed to be the primary CL loss mechanism. For

non-activated crystals such as ZnO and CaWO4 where the

lattice ions cause luminescence, the creation of new non-

radiative recombination centers was determined to be the











cause of cathodoluminescent degradation. This would be the

only possible conclusion since deactivation of intentionally

doped activators is not possible.

It was also suggested that activated phosphors (eg.

ZnS:Ag,Cl and ZnS:Cu,Au,Al) exhibited higher degradation

rates due to the added requirement that carriers must travel

many atomic distances before a radiative recombination

occurred, unlike the non-activated phosphors mentioned above

where all sites can radiatively recombine. Many Ce3'

activated phosphors were found to be extremely sensitive to

electron beam degradation, and the author concluded that the

luminescent center was being directly affected by the

electron beam bombardment. The extremely high rate of

degradation was attributed to both increasing non-radiative

recombination and the loss of luminescent sites by charge

compensation (Ce3" to Ce2)) [Pfa61].

In an experiment on zinc oxide, a correlation between

degradation rate and phosphor preparation and vacuum

conditions was found. The zinc oxide phosphor was prepared

in three different ways: an uncoated powder degraded in a

de-mountable vacuum test stand operating at 3x10-5 Torr, an

aluminum coated powder degraded in the same test stand, and









26
an aluminum coated powder degraded in a sealed off CRT tube

operating at 1x10-7 Torr. The rate of coulomb aging for each

of the zinc oxide samples tested is shown in Figure 2-10

[Pfa61], and there are obvious differences in the rate of

degradation. All three phosphors were tested with a D.C.

beam current density of 5 mA/cm2 and a primary beam energy

of 10 kV. The uncoated zinc oxide phosphor in poor vacuum

degraded at the highest rate. The aluminum coated phosphor

in a similar vacuum condition degraded at a slower rate, and

the coated zinc oxide in the sealed-off CRT tube (best

vacuum) degraded at the slowest rate of all three phosphors

tested. The author suggested that the vacuum conditions and

the ability to conduct heat away from the phosphor, by

aluminum coating, were vital to slowing the aging process.

Data for a large number of phosphors commercially

available to researchers in 1961 are shown in Table 2-1

[Pfa61,Bli57,Gil56,Mar57,Rot55]. The author determined the

total electron dose required to reduce the initial

brightness by 50%. The column containing these values is

labeled "1/C=No. Of Coulombs necessary to reduce I to

0.5*Io". In the table, higher 1/C values are observed for











phosphors which resist degradation, with ZnSiO4 having the

largest value of 104.0 C. It is important to note that all

the phosphors listed exhibit different CL half lives under

the same electron beam conditions. This clearly indicates

that degradation is material dependent, and regardless of

the chemical makeup, all of the phosphors exhibit CL

intensity loss with higher coulomb load.


CL Degradation of Sulfides



More recent studies of CL degradation have centered

around sulfur containing phosphors, in particular ZnS:Ag,

ZnS:Cu,Au,Al and Y20OS:Eu3. These three phosphors exhibit the

highest efficiencies for CL excitation and are the current

standard phosphors for blue, green and red, respectively,

for the CRT. The energy efficiency is listed in Table 2-2 as

a percentage, and is equal to the ratio of emitted luminous

energy versus the incident electron energy. The zinc

sulfide, green and blue, efficiencies are the highest of all

the known phosphors. As shown in Table 2-2 [Oza90], the

highest efficiencies are 23% (ZnS:Ag), 21% (ZnS:Cu), and 13%

(Y2OS:Eu3) .











Itoh et al. [Ito89] studied the mechanism of

degradation of zinc sulfide and zinc cadmium sulfide

phosphors at low voltages. In-situ mass spectrometric

analysis of the gases present and XPS surface chemical

analysis were used to show the desorption of sulfur

containing species. Residual gas analysis of the various

desorbed species was performed with varying anode potentials

raised from 20 to 50 V with partial pressures of water

between 2x10-8 and ixl105 Torr. Figure 2-11 [Ito89] shows

that higher desorption rates (higher ion currents) were

found with increasing anode voltages (increasing power

densities), and increasing partial pressures of water. The

conclusions drawn from these studies were that the gas

generation phenomena was related to the power density (or

heat) from the electron beam, and to the increased

decomposition of water on the surface of the phosphors at

increased partial pressures of water.

The experiments by Itoh et al., were designed to

examine the effects of molecular species desorption on the

operation of vacuum fluorescent displays and the consequent

decrease in electron emission from oxide coated cathodes.

Despite the original intent, the experiments also provide a









29

tremendous amount of information on the mechanisms involved

in the degradation of CL phosphors. One drawback of these

experiments was that the power densities used were those of

the vacuum fluorescent display, which are generally higher

than those found in FEDs.

Similar conclusions about the effects of reactive gas

partial pressures were drawn by Swart et al. [Swa96] for

ZnS:Cu and ZnS:Ag powder phosphors in simulated FED

operating conditions. Degradation of these standard CRT

phosphors was studied by Auger electron spectroscopy (AES)

and cathodoluminescence (CL) spectroscopy. The background

pressures during experimentation were between 1x10-8 Torr

and 1x10-6 Torr. The higher pressures were obtained by

backfilling to the chosen pressure with oxygen gas. The

electron beam was operated in the D.C. mode with 1 to 2keV

primary beam energies and 2mA/cm2 current densities.

Figure 2-12 [Swa96] shows the drastic changes exhibited

by the surface of the ZnS phosphor before and after electron

beam aging. Both carbon and sulfur were depleted from the

near surface region of the phosphor while the O and Zn

surface concentrations increased. The authors suggest that

the near surface region of the ZnS phosphor was converted











to a sulfur-depleted, oxygen-rich compound, such as ZnO or

ZnSO4. Possible chemical reactions and heats of formation

were reported and are depicted below [Swa96]:




ZnS + 202 <=> ZnSO,

-46.04 + 2(0) <=> -234.9

H= -94.43 kcal/mol 02

2ZnS + 302 <=> 2ZnO + 2SO2

2(-46.04)+ 3*(0) <=> 2(-83.24) + 2(-70.94)

H=-72.09 kcal/mol 02.


Using XPS analysis, Itoh reported that ZnSO4 was formed on

the surface of ZnS and ZnCdS degraded by an electron beam,

while Swart et al reported the formation of ZnO on the

surface [Swa96].

Comparing the AES data with the CL data, Swart et al

suggested that a direct correlation existed between the CL

intensity decrease and the extent of surface reactions. The

formation of a non-luminescent ZnO surface layer was

demonstrated by sputter depth profiles taken after total

coulomb exposures of 28 C/cm2 and 38C/cm2, and found to be

18A and 30A, respectively. The oxide thickness was also









31

calculated from the Zn Auger intensity changes using Seah's

formalism [Sea83, Sea79] and were found to be consistent

with the sputtered thicknesses at 28 C/cm2. Both data show

that the thickness of the non-luminescent ZnO layer

increased with increasing coulombic loading. However, the

author concluded that niether optical absorption, nor

electron power dissipation in such a thin oxide layer could

explain the CL brightness loss.

Other authors have examined the influence of the dead

layer on CL by producing non-luminescent layers of known

thickness and studying the changes in efficiency as a

function of accelerating voltage. Figure 2-13 shows results

of Kingsley and Prener [Kin72] who examined the

cathodoluminescent efficiency of ZnS:Cu phosphor particles

on which known thicknesses of non-luminescent ZnS was

deposited. They found that for non-luminescent ZnS coatings

ranging up to 0.44m thick, the cathodoluminescent efficiency

was dominated by the power loss of the electron beam in the

non-luminescent layer. Figure 2-13 shows the luminescence

intensity as a function of accelerating voltage for the

ZnS:Cu,Au,Al phosphor with 0.0ym, 0.127pm, 0.254Mm, 0.389Mm











thick non-luminescent layers. The slope of the intensity

versus Vp remains the same, while the V at which the

response becomes linear increased with increasing dead layer

thickness. These results suggest that the intrinsic

efficiency of the ZnS phosphor did not change due to the

non-luminescent surface layer. The curve simply shifted to

the right, due to the required increase in penetration

distance of the electron beam resulting from the presence of

non-luminescent surface layers. The dependance of efficiency

on accelerating voltage is dominated by the power loss of

the electron beam in the non-luminescent layer and not by

changes in the internal efficiency of the phosphor. If the

diffusion length is greater than the dead layer thickness,

then both the power loss due to diffusion of electron-hole

pairs generated by the primary electrons and the power loss

of the primary electrons must be considered in the

efficiency calculations [Kin72].


Degradation Resistant Coatings



The sulfide and oxysulfide phosphors are currently the

most desirable phosphors available for use in FED's. Yet as











mentioned above, the degradation of these phosphors in an

FED environment (low accelerating voltage <5kV, high current

density >5mA/cm2) is unacceptably high. The apparent gains

in using the complementary set of RGB CRT phosphors

(ZnS:Ag,Cl, ZnS:Cu,Au,Al, Y202S:Eu3) is offset by the loss

in brightness associated with electron beam stimulated

surface reactions.

In order for these phosphors to be useful in an FED, a

method of slowing the degradation rate must be found and

utilized. Although many different mechanisms have been

reported for the degradation of phosphors, many have surface

phenomena in common. One method of reducing the amount of

electron beam stimulated surface reactions taking place

would be to reduce the partial pressure of reactive gas in

the flat panel. Another possibility for slowing the

degradation rate would be to surface coat the phosphors with

a material which inhibits the CL loss. In order to be

commercially viable, the coating must not be detrimental to

the handling qualities, brightness, and chromaticity of the

phosphor.

Wet chemical surface coatings of powder CRT phosphors

has been shown to be useful for pigmenting, screening,











increased electrical conductivity, and increasing low

voltage efficiency [Sil83, Hay69, Kom96, Jac95]. Very little

work to date has examined the potential effects of surface

coatings on the degradation characteristics of phosphors.

Figure 2-10 shows the results of Pfanhls' work on ZnO and

aluminum coated ZnO and it displays a remarkable reduction

in the degradation rate of the phosphors which was

attributed to increased thermal conductivity and faster

removal of heat generated by the electron beam [Pfa61].

Aluminization is a standard CRT processing step designed to

reflect light forward toward the viewer and increase the

apparent brightness of the screen. Since the lifetime of

phosphors in CRT applications was already acceptable, Pfanhl

hardly mentioned the 3-4 fold increase in lifetime of the

ZnO phosphor with aluminization over the uncoated ZnO

standard phosphor.


Summary and Motivation



Cathodoluminescent degradation is one of the major

hurdles facing the developing FED industry. The CL

degradation is related to the total charge load impressed









35
upon the phosphor screen. For optimum screen brightness and

chromaticity, the phosphors of choice are the RGB CRT

phosphors ZnS:Ag,Cl, ZnS:Cu,Au,Al, and Y202S:Eu3.

Only a few studies have investigated the kinetics of

degradation of sulfur-containing phosphors. CL degradation

of sulfur-containing phosphors has been found to be

dependent on coulomb load and vacuum ambient conditions. The

degradation is related to chemical changes on the surface of

the phosphor, typically with loss of sulfur from the

surface.

No studies have been reported for the kinetics of

degradation for Y202S:Eu3. Such studies, coupled with a

model defining the parametric relationship between coulomb

load, surface changes, vacuum conditions and CL degradation

would be instrumental to finding ways to slow or stop the

degradation phenomena. Protective coatings on the phosphors

may be one method of slowing the CL degradation and also

provide vital information to further develop the kinetic

degradation model. This work has addressed these issues.













Light
Grid I Anode otput
Grid 2 Grid 4
Heaterd f Grid 3 proe ae a o




v Phosphor
Cathode
Deflection Anodeconductor
Coils
Gun Deflection Hard vacuum



D









Figure 2-1 A cross section of a CRT tube with
arrowed line exhibiting the relatively long working
distance (labelled D above) required between the anode
and cathode for proper raster and focus [Tan85].
Typical values of D for a CRT range from 25 to 100 cm.



























\ PHOSPHOR


Figure 2-2 A cross section of a Spindt-type Field
Emission Display with vertical line, marked D,
indicating the working distance between the anode
and cathode of a field emission display. Typically D is
approximately 1mm [SCH96].


( t ~ .............................i li i ii i T7; ,t I
' i..................................... t I I
"""""......................


................ )I( I(( t( t ((
)I ) II II II ) I)( ................ )) I(

I (( I I11 II ) I) II 1 )I ( II ( ( ........... I) ( III
I II I) I I (ItI I)( ((............. .. (I I) I( .... t











S4T--------


--------TL
2T2

_T,
,, :


C


g4A2
------A, -- ---


ii


Figure 2-3 Absorption and luminescence spectra for
Cr3+ in A2103, also known as ruby. Absorption bands
exist for all transitions, but a visible luminescence
peak is only observed for the 2E-4A2 transition, as
this transition has the largest energy change, and
emits characteristic red luminescence. All other
transitions do occur, but are not in the visible region
of the spectrum [LUM78].


LIII


----~
~rrr-~r


















N(e)









V VP

Primary Voltage












Figure 2-4 Total number of electrons leaving a
surface as a function of energy under cathode
ray excitation. Incident primary electrons which
are elastically scattered are shown at energy Vp.
True secondary electrons are depicted by the
leftmost peak. Auger and Plasmon electrons are
found at energies between the true secondary peak
and the elastic peak [HUD92].









40



14
Rao-Sahib and Wittry
> 12 -

10
I-~
10


6.

. 4 Tung et al.
. 4- ^4


2 Bethe "


0.01 0.1 1 10 100

Energy (keV)









Figure 2-5 A comparison of stopping powers for
electron- solid interactions. Dotted line denoted
by Bethe depicts the stopping power calculated
using Bethe's original stopping power equation.
Dash-dot line shows the changes in low energy
stopping power predicted with Rao-Sahibs and
Wittrys' modification. These are compared to
experimetally determined stopping powers by Tung
et al. which are shown as a solid line[JOY89].




























0.3



0.1
0.01 0.1 1 10


Energy (keV)













Figure 2-6 A graph comparing the stopping
powers found experimentally in C and Cu by Tung
and Akkerman to the calculated values found by Joy
and Luo using a modified Bethe expression which
allows the ionization potential to vary linearly
(Cu-MB or C-MB)with energy for energies below IkeV
[JOY89,TUN79] .











Range from Mod. Bethe and KO Range


2 4 6 8 10


Electron Energy (keV)
Mod. Bethe
- KO Range







Figure 2-7 A comparison of Kanaya and Okayama
range (dashed line), which includes a contribution
from secondary electron-hole pair diffusion, to
the modified Bethe Range (solid line) obtained by
the Joy and Luo adaptation for low energy
electrons. Reasonable agreement is found for the
models except at very low energies (<500eV) where
Joy and Luo approaches the residual range and
Kanaya and Okayama range falls to zero.


1*10 -
0























































Figure 2-8 The distribution of excess minority
carriers(solid lines) and the electron-hole pair
generation volume(dashed line) are depicted for
the universal depth-dose function. The highest
concentration of electron hole pairs is found
within the generation volume[YAC90].









44



1.0


0.5








0. I





0.0847/ /


-10 8pr8 0.389;<


0.01 I a a i ,_0 1 a a .&
S.4 6 8 12 1 20
ACCELERATING POTENTIAL (kV)













Figure 2-9 The theoretical (solid) and
experimentally (dashed) determined CL efficiencies
of ZnS:Cu with non-luminescent ZnS coatings of
varying thicknesses are depicted and show good
agreement over a wide range of accelerating
potentials [KIN72].
















100 -

>s SEALED -OFFTUBE-
t-- s ALUM. SCREEN
z80- PRESSURE < 10" MM H6
z
DEM. STATION
S\ ALUM. SLIDE
iO\ P 3.10-5 MM HG

o

40-

w
C- OEM. STATION
NOT ALUM. SLIDE
20 P 3.10-5 MM HG





10 3 10-2 10~' i
COULOMBS /CM












Figure 2-10 Zinc oxide degradation and the
effects of vacuum and aluminization on cathode
ray-induced CL degradation. Longer CL lifetimes
are found for phosphors which have been aluminized
and/or are operated in better vacuum conditions
[PFA61] .






















10 io )3 40
ANODE VOLTAGE (V)


ANOOE VOLTAGE IV)


AIODV VOLTAGE (V)


ANODE VOLTrGE (V)


Figure 2-11 RGA (residual gas analysis) ion
currents representing the desorption of species
from ZnS under cathode ray excitation are shown.
Increased desorption rates are found for S-, SO2-,
0-' S and 02 for increasing anode potentials
indicating the desorption of surface species under
electron bombardment [IT089].












Before After









N Zn
381 eV
994 eV
510 eV Ze
Zn Za
59 e C 994 eV
272. eV


S
152 eV





0
510 eV



S
152 eV Zn
59 eV
Electron Energy (eV)




Figure 2-12 AES surface study of ZnS before and
after electron beam aging. Note the loss of S
(152eV) and C(272eV) with concurrent increases in
Zn (59eV, 994eV) and O (510eV). The surface
becomes sulfur depleted and oxygen rich[SWA96] .























0= 0.254 p
1x / / -

-0--









| 6/. 389A
S6.
Z i





0 A
0 4 8 12 16 20
ACCELERATING POTENTIAL (kV)








Figure 2-13 Luminescent intensity versus
electron accelerating potential for ZnS:Cu with
non-luminescent ZnS coatings of varying
thicknesses. The linear portions of each curve are
parallel, indicating no change in efficiency, only
the threshold value is shifted to higher energy
values by the increasing thickness of the dead
layer which electrons must first pass through to
excite cathodoluminescence [KIN72].











CL Peak Intensities Versus Coulomb Load


Y202S:Eu (Aluminum)
f (A)
(D)
Y20,S:Eu (Edge)



(C) ZnS:Cu,Au,AI

Ag,CI

Y202S:Eu Charging
... .. ....V V ..Y .. (B )


0 50


100


150


200


Electron Dose (C/cm 2)


Figure 2-14 A comparison of degradation rates
of Y202S:Eu3*, ZnS:Ag, and ZnS:Cu cathode ray
phosphors. Note that the ZnS:Ag and ZnS:Cu degrade
at higher rates than does the Y202S:Eu3. Three
separate degradation curves are shown for
Y202S:Eu3. Line A shows the degradation of
Y202S:Eu3 with good contact to an aluminum
holder. Line D shows the degradation of Y202S:Eu3
with nominal contact to the aluminum holder and
Line B shows the degradation curve for Y202S:Eu3
with no contact to the aluminum holder. The
thermal and electrical conductivity decreases with
each successive move away from the aluminum
holder, and the effects of this change are easily
seen [TR096].
















APPH


-1.5


-3.0

Before Aging


1.5


0.0
APPH


-1.5


-3.0


0 (519eV)


J v
SC (281eV)
S(158eV)


Energy(eV)


OeV)


S(158eV)


0(519eV)

4e-


Energy(eV)


After Aging


Figure 2-15 Depicts the surface state exhibited
by Y2,OS:Eu3" before and after electron beam
aging. Note the loss of C (281eV) and S (158eV)
with concurrent increases in Y (110eV) and O
(519eV) after the phosphor has been exposed to
prolonged electron beam excitation [TR096].












TABLE I
BAn Charateristics of P-ype Phlphors Excited with 10kV EecErom





6I-1
IJC-No.of FctArolurOCoastant Hiio Brn, Rn u.ion in Decay
olonbsfal Cx 10t,an1 Consiant, Time
PHiphhr PMNo. Use Cbmicual Activator Naemay to --- Cx 101------
Compoasiion Reduce These ci er
Ilo/i, Measure- Ot coitherssg

C"' ivatedSililates Flini Spot Saner (CaO).MsO. (Si0 2 Ce About i 10 10 10D
P6 Flyin( Spot Scnr CO.AlA0.SiA0 CA 10
Sulfur iinmmald. Ag and P2 Otaloowpes Zn SCdS Cu 11.9 .140 1I 1W0
SactivateKd P4 Direct ViewTV, JZS As
l("M,) XZWSCdSIZnS AS 4.5 030 0,4311 112
P Cscade Sa ZaS A 9.9 0.170 40 130
for Oscillcopl ZnCdS Cu 10.0 0.167 30 IO
PII Photography ZnS Ag 16.6 0.100W 0.w 13) U 05- 17 1I5
P14 Rdar ZnCdS Cu 16.6 0.100 25 10
P22 rojectioaColorTV ZnS Al 20.0 0.083 0.3S 30 110
ZnS A&.A 33.3- 0.050 23 175
Ogaen and Sulfur PI Scopes Radir Za2SiO4 M 104.0 0.016 0.0113 0.02",
domialed, Mo ad i N ProjcionTV CaO.MO.SIO Ti 55.5 0.030 25 65
activated 4 bldc ad white ZnO.B03SiO Mn 3.5 0.020 0.02
P4 Red TV, Projection Zn.Mg.Cd.SiOj Mn 16.6 0.100
ZnS Me 0.06"
Oxynn dominated PIS RyiuSpot(Sanr ZaO .3.5 0.050 20 100
P24 Ryig Spo Samr ZO Za 33.5 0050 0.02 20 100
PS Oicilcopes CaWO, 16.6 0.100 00 IIl 0 ISO
Other BS04 Pb About 10- 10


Table 2-1 A comparison of degradation costants for a large
number of phosphors. 1/C values denote the total coulomb load
required to reduce the initial brightness of the phosphor to half
of its value. The larger values indicate phosphors which resist
degradation [PFA61,BLI57,GIL56,MAR57,ROT55].















I'lPhosplors


iZnSAg:1CI
ZnS:(Cu:Al

YO.S:STb

l,;I()S:Th
Y2()i:Eu
YV()O:Eu
Zn:SiO(:Mn


Probability


0.66)
0.66
0.66
0.8-1.0
0.5
0.5
0.8

0,8


Eflirienrics (%)
(Ca:ulate Reporled

21 23
21 19-21
13 13
19-24 19
10 11
13 12
7 7
8 A
9 8


Firsi-trapped
Carriers

*holf
hole
electron
hole
electron
hole
electron
electrn
hole


( ,)


2
2
2

2
4
4
4


SI .L L .b


(eV)

3.8
1:.8
,1.8
4.7
4.7
4.7
4.7
4.7
4.7

5.8
4.0
5.5


(eV)

2.75
2.32
1.98
2.29

2.29
2.03
2,0
2.36


Table 2-2 External radiant efficiencies for a number of
phosphors including ZnS:Ag,Cl, ZnS:Cu,Au,Al and Y202S:Eu3
[OZA90].















CHAPTER 3
EXPERIMENTAL APPROACH




Introduction




Y2,OS:Eu3 phosphor powders, designated P22R, used in

this study were obtained from Osram Sylvania through the

Phosphor Technology Center of Excellence. P22R is the

standard, red phosphor used in the manufacture of CRT

screens. The P22R phosphor was manufactured by the sulfide

fusion industrial process [KOT95]. The powder consists of

faceted particles with an average diameter of 4.5 pm and a

range of sizes from 1 to 10pm. The details of the sulfide

fusion formation process is discussed below in greater

detail.

Two techniques were used to apply coatings to the

base phosphor material. Wet chemistry techniques, and pulsed

laser ablation were used to deposit coatings on single

particles.









54

The phosphor particles were characterized by various

techniques including cathodoluminescent spectroscopy, Auger

electron spectroscopy (AES), and scanning electron

microscopy (SEM). The phosphor manufacturing processes,

methods of coating the particles, and techniques used for

characterization are discussed in the following chapter.


Phosphor Material



Industrially Processed Yttrium Oxysulfide Phosphor


The standard industrial procedure for producing a Eu3

activated Y202S phosphor is by a sulfide fusion process

which involves a solid-melt reaction between the rare-earth

sesquioxides (Y203 and EuO03) [KOT95]. The general chemical

reaction is as follows:




(l-y)Y203 + yEu203 + S <> Y2(1-y)Eu2yO2S + % 02.


This reaction is assisted by Na2CO3 and other fluxes.

Depending on the desired Eu content of the product, y can

take on values from 0.00 to 0.04.











During this reaction, S reacts with the Na2CO3 flux

which produces polysulfides [PHA91]:




2Na2CO3 + 5S <> % Na2SO, + 3/2 Na2S3 +2CO3 +H2S +SO2.



In order to increase the efficiency of the overall reaction,

the reactants are initially ball-milled for homogenization.

The mixture is placed in an alumina crucible and heated in a

flowing nitrogen atmosphere to 1100 C for 4 hours. The

solid-melt reaction takes place and an ingot is formed. The

ingot is cooled to room temperature by an air quench, then

reduced to a fine powder again by ball milling. The final

product is washed in hot DI water and 2% HCL several times

to remove the flux, residual sulfur, and polysulfides

[YOC97].




Methods of Applying Coatings to Phosphors



Wet Chemistry Coatings


A number of coatings were applied by modified wet

chemical techniques which were developed for use in the











phosphor screening industry. Silica (SiO2), thoria (ThO,),

and phosphate (P04O3) coatings have been deposited by wet

chemical techniques which are described in the literature

[SIL83, HAY69, KOM96, JAC95]. Silica, and phosphate coatings

have been used for years in the phosphor industry as

dispersing agents prior to the application of the phosphor

to the screen and the chemistries associated with these

types of coatings are well defined.

Prior to the wet chemical coating process, a small

quantity (typically 20 grams) of industrially processed

Y202S:Eu3 from Osram Sylvania was suspended in a solution of

0.1M HCL. The powder was slurried for 1 hour using a

magnetic stir plate, decanted and filtered (#110 Fine

Buckner filter) with two methanol rinses. A final DI water

rinse was used to remove any residual impurities. The

compacted cake was placed in a 120 C oven overnight (minimum

8 hours) to bake off excess water. The compact was ground by

hand using a mortar and pestel. This powder was then used

for all wet chemistry coatings that follow.

Phosphate Coating

In order to produce a phosphate (PO03-) coating, a 0.1 M

solution of phosphoric acid was prepared. The phosphor











powder was slowly added to the acid and the mixture was

slurried for various times and temperatures, depending on

the desired coating. Slurry times of 30 minutes to 2 hours

were used, at temperatures ranging from 25 C to 80 C.

Typically, the coatings would cover more particle surface

area with longer slurry times and higher temperatures. The

chemical reaction proceeded with noticeable evolution of H2S

detected by its odor.

The mixture was removed from the stirrer and decanted

through a #110 fine Buckner filter. The powder was then

rinsed with methanol twice, and finally rinsed with DI

water. The powder compact was air baked for a minimum of 8

hours at 120 C to remove excess water. The dried compact was

hand crushed by mortar and pestel. The phosphate coated

powders were stored in a desiccator until characterized.

SiQ2 Coating

SiO2 coatings have been used for many years in vacuum

fluorescent displays (VFDs). The coating is applied in order

to keep the particles adherent to the anode of the screen.

No aluminization of the phosphor screen can be utilized due

to the low energy electron penetration limits in VFDs.











The phosphor powder was slurried in DI water for

approximately 1 hour by a magnetic stir plate and then

transferred to a sonicator. The sonicator was used to break

up agglomerates of phosphor particles prior to coating.

Typically, the slurry was sonicated for 10 minutes and

returned to the coating fume hood.

In the hood, the slurry process was re-initiated and

LUDOX was dropper fed into the slurry. LUDOX is the Dow

Chemical trade name for a SiO2 nanoparticle sol. The mixture

was slurried at room temperature for 1 hour, then decanted

and filtered through #541 fine filter paper. The compact was

DI water rinsed and methanol rinsed. Unlike the phosphate

coating, which was produced by a chemical reaction, the SiO2

coating occurs due to the attractive surface potentials

associated with the phosphor particle and the SiO2

nanoparticles.

Once decanted and rinsed, the powder compact was

transferred to an oven and air baked at 120 C for 24 hours

to remove excess water and methyl alcohol. The completed

compact was crushed by mortar and pestel and stored in a

desiccator until further characterization was required.











Pulsed Laser Ablation


Recently, pulsed laser ablation has been used to coat

particles with diameters ranging from 1 to 1000 pm [FIT97].

The pulsed laser ablation system consisted of a Lamda Physik

excimer laser and a vacuum chamber. The beam enters the

vacuum chamber through a quartz viewport where it strikes a

target material. The energy and power of the laser is

sufficient to ablate atomic and molecular species from the

target. These ablated particles produce a visible plume of

material inside the vacuum chamber. Figure 3-1 shows details

of the pulsed laser ablation system used to coat phosphor

particles.

The P22R particles were introduced into the plume and

agitated by means of a motorized shaker [FIT97]. The ablated

target material formed a thin transparent coating on the

phosphor particles. Pulsed laser ablation has been used to

coat particles with up to 70% surface coverage and

thicknesses from 5 to 30 nm in the following systems; TiO2

on A1203, Ag on A1203, YBa2Cu30O on A1203, TiO2 on SiO2 and Ag

on SiO2 [FIT97]. Pulsed laser ablation was used to produce











thin metal (Ag) and intermetallic TaSi coatings on the

surfaces of the P22R particles.




Characterization Techniques



Cathodoluminescent and Auger Electron Spectroscopy


Since both cathodoluminescent spectroscopy (CL) and

Auger electron spectroscopy (AES) data were collected

concurrently from the same spot on the same sample using the

same electron beam, both techniques will be discussed in

this section. P22R phosphor was cold pressed into a 1/4 inch

diameter dimple in a stainless steel holder, and mounted in

the AES chamber. The AES spectrometer is a Physical

Electronics model 545 with a cylindrical mirror analyzer and

a lock-in amplifier detector. The primary beam energy ranged

from 1 to 3 keV with currents between 1AA and 15AA. Data was

collected in the dN/dE mode using a 4eV peak-to-peak

modulation voltage. Auger peaks from Y (77eV-MNN), S (152eV-

LMM), C (272 eV-KLL), and 0 (502 eV-KLL) were recorded. The

chamber was rough pumped to 1*10-3 Torr by two cryo-sorption

pumps. Cross over to the high vacuum ion pump, a Perkin











Elmer Ultek DI 800 1/s, was accomplished by closing the

roughing valve and opening the poppet valve to the ion pump.

The entire system was then wrapped in aluminum foil and

baked to 200 C by heater tapes wrapped around the system for

8 hours. The bake-out is used for desorbing gases, mainly

H20, from the chamber walls and allowing those gases to be

removed from the system by the ion pump.

The system was cooled after bake-out. Vacuum pressures

were measured by a Granville Phillips ionization gauge and

controller. Base system pressures varied from 9X10-9 Torr to

2X10-8 Torr.

Typical AES operating parameters were 1, 2, or 3 kV

primary beam energies, with sample current densities of 200

to 300 lA/cm2. The AES spectra, Auger peak-to-peak heights

(APPHs) and cylindrical mirror analyzer (CMA) pass energy,

were sampled and stored on an PC compatible computer by an

analog to digital converter and software developed by the

author. Sampling delay was 0.2 seconds per point with two

channels of data being recorded simultaneously. CL data was

collected by an Oriel Instaspec IV CCD spectrometer. Maximum

CL peak height, integrated CL area (550-750nm), and spectral











distribution (intensity as a function of wavelength) were

acquired.

To determine the effects of the type and pressure of

gas in the system on phosphor degradation, the steady state

pressure was increased. For typical residual gas with no

specific composition, the poppet valve was partially closed

and the pressure rose in the chamber. The steady state

pressure could be varied or maintained by adjusting the

closure of the poppet valve. System pressures during AES and

CL analysis ranged from 2x10-8 to 1x10-6 Torr.

For high partial pressures of oxygen, a gas bottle was

attached to the system through vacuum compatible seals. The

poppet valve was slowly closed until a small pressure rise

in the system was observed. A leak valve attached between

the system and the oxygen bottle was slowly opened until the

desired pressure was achieved. This method allowed known

partial pressures of oxygen gas (2x108- to xl10-6 Torr) to be

introduced into the system.

Degradation Experiment

The phosphors were cold pressed without binder into an

stainless steel dimple and mounted onto the carousel which

was placed in the ultra high vacuum AES system. After a base











pressure of better than 2X10-8 Torr was reached, the

oxysulfide phosphors were subjected to electron beam

bombardment at pressures from 1X10-8 Torr to 1X10-6 Torr

background or backfilled gas. Figure 3-2 shows a diagram of

the experimental setup.

AES was used to characterize the surface composition

and any changes in it. Primary beam energies of 1, 2, or 3

keV with a continuous current density of 200-300 MA/cm2 was

used. Auger spectra were repetitively recorded and monitored

from 30 eV to 700 eV over the duration of the experiments

(typically 24 hours).

Cathodoluminescent data were also recorded

simultaneously by an Oriel Instaspec IV CCD camera and

monochromator. The monochromator used a 600 line/mm grating

which allowed a 200 nm scan width with 0.7nm resolution. The

spectral distribution and total area under the emission

peaks were recorded from the phosphor. CL intensity as a

function of wavelength, along with peak intensities and

total integrated area (from 550nm to 750nm) were recorded

concurrently with the AES data. The CL software was

programmed to take one 25ms exposure per minute for 24

hours.









64

Upon completion of a 24 hour experiment, the AES and CL

data were reduced and compared. Reduction of the AES data

typically required software measurements of APPHs for

carbon, oxygen, sulfur, yttrium, and any other elements

contained within the coatings.

Conversion of Time and Current Density to Coulomb Load

Typically, due to the wide variations in spot size,

beam current, sample current and time of electron beam

exposure, CL degradation experiments have used total coulomb

load (or dose) as the dependant variable for comparison.

Total coulomb load is a direct measure of the number of

electrons passing through a surface due to an electron beam.

The total coulomb load is calculable knowing the beam spot

size, the sample current (measurement of which specified

above in C/s), and the time for electron beam exposure.


C Coulomb 1
CoulombLoad( -)=Current( ulm)* (cm2) *Time(Sec)
cm2 Sec Area


By measuring the sample current with the picoammeter as

shown in Figure 3-3, and knowing the area of the spot size

by optical measurements, the time of exposure can be

calculated from the time for a repetitive auger scan or from

software settings on the CL spectrometer and the total











coulomb load can be calculated. The phosphate coated

phosphor exhibited a distinct surface darkening during

electron beam bombardment. This spot was used to estimate

the beam spot size. This size was assumed for all other

experiments.

Turn-On voltage Experiment

The primary beam voltage of the AES was set to 7 kV,

and the sample current was set at 0.2 AA/cm2. Low current

densities were chosen in order to avoid phosphor saturation

(the point at which the brightness no longer increases as

the accelerating voltage is increased at constant current)

issues at all beam voltages. Figure 3-3 shows the method

used to measure sample current by placing a 90 Volt battery

and a picoammeter in series with the sample while the

electron beam to strikes the stainless steel holder. The

battery is used to return low energy secondary electrons

which are emitted from the surface to the sample holder.

This is necessary because the sample current is the sum of

the secondary emitted electrons and the ground current.

Without the use of the battery, the sample current measured

would be only 10% of the actual sample current. After

setting the sample current, the accelerating voltage was











turned off and the sample carousel was turned to align the

phosphor with the electron beam. With the initial alignment

completed, the system was sealed so that no external light

could enter. The pressure in the system was recorded and the

ionization gauge filament was turned off. To reduce internal

shot noise, the CCD array was cooled to 27 C by an internal

thermo-electric cooler.

The CL spectrum from a 25 millisecond background

exposure was recorded and used to remove spurious signals

from the phosphor cathodoluminescent signal. The electron

beam was turned on and a single CL spectra was recorded with

a 25 millisecond exposure. The spectrometer software

calculated the area under the curve of the CL signal from

550 to 750 nm, and both the integrated area and CL spectra

were saved in a manipulable file format. The electron beam

was turned off and the accelerating voltage was adjusted to

6.5 kV and the process was repeated from 7 kV down to as low

as 0.2 kV. Each reduction in accelerating voltage required a

verification of the sample current to insure constant

current during the entire experiment. Random data points in

the specified range were sometimes recorded to determine the

actual phosphor turn-on voltage more accurately.











A graph of the normalized CL area (total brightness),

as a function of accelerating voltage was used to

extrapolate the turn-on voltage of the phosphor. The turn-On

voltage was identified as the Y = 0 intercept on the voltage

axis before saturation. Figure 3-4 shows a typical

extrapolation to determine a phosphor's turn-on voltage.

Scanning Electron Microscopy


Scanning electron microscopy (SEM), was used to

examine the surface morphology of the phosphor particles. A

JEOL 35C operating at 15kV and a working distance of 39

millimeters was used. The sample was normally tilted at 30

degrees to increase the signal from the surface and produce

a better image. To prepare a sample for SEM, a thin layer of

particles was applied to one side of double sided sticky

tape. The other side of the tape was then attached to a

standard SEM holder. Due to the highly insulating nature of

the phosphor particles and tape, a thin Au-Pd coating was

sputter deposited using a Technics D.C sputter system to

decrease charging phenomena in the SEM. The coatings were

100 to 150nm thick.























Ablation Plume


i Fluidizain S m fluidized powder
Fluidization System substrate











Figure 3-1 Depiction of Pulsed Laser ablation
system showing the laser, target, ablation plume,
motorized shaker and phosphor bed (bottom center)
[FIT97]. This apparatus was used to produce nano-
particle coatings of TaSi and Ag on the P22R
phosphor particles.


/















AES/CL Experimental Setup


Fixed lenses


M1


Gas Leak Valve


Figure 3-2 Auger electron spectrometer and
cathodoluminescent system setup. The system was
designed to be able to operate both measurement systems
concurrently. The electron beam of the Auger system
induced cathodoluminescence in the phosphor which was
measured by the CL spectrometer.











Vacuum Chamber
\e


Picoammeter


Isolated Sample
Carousel


Figure 3-3 Sample current measurement is
accomplished by connecting a 90 volt battery and
picoammeter in series with the isolated sample
carousel. The current measured is the sum of the
primary current and the secondary electron current
which has been re-attracted to the sample by the field
of the battery.


E-beam


90 V






















y= QOOB-Q1575

(141


CO




0-
Ca a2 a' ax




1200000 2 30X 4000 5000 60n 700 008



A Vdtage (eV)














Figure 3-4 Example of turn-on voltage determination
from brightness versus accelerating voltage data. A
straight line is drawn from the linear portion of
the curves and extrapolated to the intersection of y=0
on the x-axis. This voltage value is known as the
turn-on voltage and is a measure of the energy at which
the phosphor cathodoluminescence becomes measurable.
The linear equations displayed are fits of the straight
line approximations and can be used to determine the
turn-on voltages.





















CHAPTER 4

BASELINE STUDIES OF Y202S:EU3




Introduction




This chapter will discuss the results of CL degradation

experiments conducted on Y202S:Eu3 (P22R) powder phosphor.

AES and CL spectroscopy were used to study the degradation

phenomena. AES surface spectra were measured at various

coulomb doses up to ~ 20 C/cm2 using a 2keV electron beam

and a current density of 272 AA/cm2. The experiment was

conducted in the best vacuum available (typically -1xl0-8

Torr) in the AES spectrometer and Auger peak-to-peak heights

(APPH) as well as Auger peak energies for Y, C, S and O were

recorded during electron bombardment. The initial and final

CL spectra were recorded along with the total brightness,











and the wavelength for maximum emission. Threshold voltage

experiments (see Chapter 2) were conducted prior to and

after electron beam aging of the sample to examine the

relative changes in brightness at constant current as well

as to determine the change in threshold voltage due to CL

degradation.



Physical and Luminescent Properties of Y2Q2S:Eu3l



In order to better understand CL degradation, the basic

physical and luminescent properties of Y202S:Eu3 phosphor

were studied. SEM was used to examine particle size and

morphology and x-ray diffraction was used to examine the

crystal structure of the phosphor. The CL degradation

characteristics of Y202S:Eu3 were also examined using AES

analysis and CL spectroscopy. The degradation of Y2,OS:Eu3

was carried out in typical FED operating conditions to

obtain a baseline for comparison with other experiments

reported in future chapters.











Physical properties of Y2Q2S:Eu3


SEM images, as shown in figure 4-1, shows that

Y202S:Eu3+ particles were faceted with sizes ranging from 1

to 10 pm with an average particle size of 4.5ym. X-ray

powder diffraction was used to determine the crystal

structure of the Y202S:Eu3 particles and Figure 4-2 shows

the X-ray intensity as a function of 2e angle. The upper

portion of Figure 4-2 is the experimental diffraction

spectrum obtained from the powder, and the lower portion is

the expected peak positions and intensities for a hexagonal

Y202S structure obtained from the JCPDS files (card #:24-

1424). The peaks in the X-ray spectrum have been assigned

Miller indices as shown in figure 4-2. The excellent

correlation between the experimental data and the JCPDS file

data suggests that the Y202S:Eu3 powder has a hexagonal

structure with lattice parameters of a=3.284Aandc=6.509A.

Figure 4-3 depicts 1/3 of the aforementioned unit cell.

Three such rhombehedral subunits combine to form the

hexagonal Y202S unit cell. Each rhombehedral structure

contains one molecule of Y22S:Eu3+.











Luminescent properties of Y2Q2S:Eu3


The CL intensity spectrum as a function of wavelength

from an unaged (as received) Y202S:Eu3 sample was measured

using a 2kV accelerating voltage and 200pA/cm2 current

density. Figure 4-4 shows the relative CL intensity

distribution as a function of wavelength. The strongest

transition occurs at 626nm which is the 5D0o-F2 transition of

the Eu3 ion [KAD91]. Secondary emission peaks,

corresponding to other transitions of the Eu3" ion, can be

seen at 616nm and 702nm. The CL spectrum matches the

published spectra for Y202S:Eu3 powder phosphors. A very

small peak is found at 611nm which corresponds with the main

emission peak in Y203:Eu3 The luminescent transitions found

in Y202S:Eu3 are extremely narrow line emissions. This is

due to the transitions being localized on the europium ion.

The relative brightness of the starting material has

been studied as a function of accelerating voltage at a

constant current density of 20/a/cm2. Relative brightness

was obtained by determining the area under the CL spectra as

a function of wavelength (500nm to 700nm) at a given

accelerating voltage. A representative threshold curve is











shown in Figure 4-5 where brightness is plotted as a

function of accelerating voltage. Two important aspects of

the luminescent characteristics of the phosphor are obtained

from the brightness as a function of accelerating voltage

(threshold) data. The first, threshold voltage, is defined

to be the intercept of the extrapolation of the linear

portion of the B-V curve. The dotted line in figure 4-5 is a

fit to the linear region portion of the B-V curve. The x-

intercept at the y=0 crossing for the phosphor is the

threshold voltage. The crossing value is found to be 871 V

for this unaged sample.

Secondly, a qualitative, relative brightness efficiency

for the phosphor can also be determined from the slope of

the linear portion of the curve. In this case the linear

region extends from 2000 volts to 4000 volts. The slope

for this phosphor is found to be 2.3x10-4 brightness(a.u)

units per volt.

The luminescent characteristics obtained from the

threshold voltage curve can be used to examine changes in

the phosphor due to coulomb aging. In the literature, the

threshold voltage is attributed to a surface layer with

reduced or no luminescence which an electron must penetrate











before luminescence will occur. In this study, it will be

shown that the change in threshold voltage after coulomb

aging may indicate one or a combination of several changes

including a change in the thickness of any non-luminescent

surface layer, a changed secondary electron emission

coefficient resulting in a different surface region

charging, or a change in radiative efficiency near the

surface. A change in radiative efficiency can be determined

by a change in the slope of the threshold curve in the

linear portion of the curve which would indicate a change in

radiative efficiency of the phosphor powder. Probable causes

of changed efficiencies are decreased radiative transition

probabilities or de-activation of luminescent centers in the

phosphor.

Degradation Characteristics of Yttrium Oxysulfide


Previous studies by Swart et al [SWA96] and Itoh

[IT089] showed that the CL degradation of ZnS powder

phosphors were sensitive to overall vacuum pressure and

types of gas. In considering previous studies on sulfur

containing phosphors, it was decided that initial studies on

yttrium oxysulfide should be completed under good vacuum











conditions to minimize the degradation effects due to

ambient. A 2keV primary electron beam with DC current

density of ~ 272 gA/cm2 was used for the aging studies of

the phosphor.

Figure 4-6 shows the loss in CL brightness (integrated

area under the CL spectrum) as a function of coulomb dose

for a P22R phosphor in a vacuum ambient of 1.4x10-8 Torr, as

measured by a Bayrd-Alpert hot cathode ionization gauge. The

degradation experiment left a dark beam spot which was used

to estimate the electron beam spot area (0.011 cm2) and

calculate current densities. The spot was found to be

spherically shaped by microscopic analysis. As shown in

figure 4-6, a rapid decay in brightness occurs at the

beginning of the cathodoluminescent degradation. This

decrease is typically attributed to either charge buildup on

the surface of the phosphor due to its extremely low

electrical conductivity (1012-1014 mhos for Y202S) or to

thermal quenching of luminescence due to electron beam

heating of the phosphor [Has90]. This rapid loss will not be

considered in determining the extent of CL degradation due









79

to large variations observed in the initial loss from sample

to sample.

The initial rapid drop is followed by a slow increase

in brightness(from 0.57 to 0.60) until a maximum is reached

at 4-5 C/cm2' and then a slow steady decrease in CL

brightness until the end of the experiment at 22 C/cm2 (from

0.6 to 0.5). The phosphor does not degrade while the

electron beam is turned off.

In order to determine if chemical changes in the

phosphor were responsible for the changes observed in the CL

brightness, AES was conducted concurrently with CL

spectroscopy. The AES peak heights and shapes, as well as

energy shifts were recorded continuously throughout the

experiment. Figure 4-7 shows the AES peak to peak height

changes (APPH) which occurred during the experiment for 0,

C, S and Y. Changes in APPHs are qualitative indicators of

the relative amount of an element present which are sampled

within the AES probed volume (0.011 cm2 by ~30A depth). As

can be seen, the signals from oxygen and yttrium increased

slightly from 0 to 5-10 C/cm2 and then remained relatively

constant, while the AES signals for carbon and sulfur











dropped dramatically over the first C/cm2 (as did the CL

brightness in figure 4-6) and then stabilized for the

duration of the experiment. Although the S APPH is low, the

signal is still present in the final AES spectra. The C

APPH, however, has fallen below the detection limit of the

AES spectrometer.

It is evident that the chemical changes near the

surface of the phosphors, observed by AES analysis, can be

correlated with the slight increase in CL brightness to ~ 5

C/cm2 in figure 4-7, but this determination could be errant

due to the volatility of the species studied (O,C, and S).

These species can be removed from the surface by desorption

or volatilization reactions. A better way to examine the

data would be to normalize the peak intensity of O, C, and S

to the Y AES signal. The vapor pressure of yttrium is

extremely low (-10-18 Torr at room temperature) and therefore

it is unlikely that the total amount of Y sampled by the

electron beam will change due to the loss of yttrium from

the surface of the phosphors. Figure 4-8 shows the APPH

ratios for oxygen, carbon and sulfur normalized to the

yttrium APPH. The CL brightness as a function of coulomb

load is also shown. The APPH ratios for C/Y and S/Y exhibit











a rapid initial decrease over the first C/cm2 and a

subsequent stabilization to nearly constant values beyond -

3 C/cm2, while the O/Y ratio is nearly constant or

decreasing slightly from 0 to 22 C/cm2. The S/Y decrease

corresponds to the slow increase in CL brightness observed

from 0 to 5 C/cm2. Apparently the carbon at the surface is

rapidly removed during the early stages of degradation.

Another possibility to explain the increase in CL up to 5

C/cm2 is the modification of the phosphor work function due

to the chemical modifications associated with the C and S

removal, resulting in a change in secondary electron yield

which could affect the CL brightness.

Not only did the APPH values for Y,O,C and S change

during the aging experiment, but also the peak energies

shifted as well. Figure 4-9 illustrates the initial (0.5

C/cm2) and final (19.9 C/cm2) AES scan. The spectrum shows

the AES peaks for Y, S, C and 0 as a function of electron

energy. The changes in peak height are small for Y, 0 and S.

The C signal is barely detectable at 0.5 C/cm2 and has

fallen below the AES detection limit (~1012 atoms/cm2) by

19.9C/cm2.









82

A measurable shift (-1.5 eV) to higher energies of the

Auger peaks of Y and 0 as well as the secondary emission

peak (lowest energy peak) is observed from 0.5 C/cm2 to 19.9

C/cm2 (Figure 4-9). The Y, 0, and S peaks all exhibit

positive shifts in energy from the expected AES peak

energies 14, 11, and 7 V ,respectively. Since surface

charging and/or changes in surface charging during

degradation could be significant, the AES energies for Y, O,

C and S were measured throughout the experiment to determine

if these energy shifts could be correlated with the surface

chemical changes. The shifts are small in relation to the

energy of each Auger peak, so linear trend lines were fitted

to the real data to discern any measurable patterns as shown

in figure 4-10, consistent with figure 4-9, Y and O exhibit

shifts to higher energies (indicated by a positive slope

denoted in trend line equation) while carbon and sulfur

exhibit slight decreases (negative slope). The C energy

shifts can only be examined until 2 C/cm2 due to the

carbon surface concentration falling below the minimum

detectable limit for AES analysis. The positive shift in

energies associated with Y and O could be consistent with a

charging shift caused by the removal of the conductive C









83

surface species. The C and S peak energies shift negatively

throughout the experiment.

The APPH data shows the loss of surface C and S. It is

possible that the electron beam interaction with the surface

and with gas species impinging on the surface are

volatilizing the weaker bond C and S, leaving a sulfur

depleted, oxygen rich surface. A conversion of Y202S to Y203

or Y202SO4 may be occurring. If the conversion of Y202S to

Y203 or to Y202SO4 is real, peak shifts or new peaks in the

cathodoluminescent spectra may become visible. This

postulate assumes that the Eu in the forming phase is still

active for luminescence and remains in yttrium sites in the

lattice. Figure 4-11 is a graph of the initial and final CL

spectra (0.5 C/cm2 and 19.9C/cm2) as a function of

wavelength in nanometers. The peaks are all known emissions

from Y202S:Eu3 as indexed in figure 4-4. The overall

decrease in intensity due to coulomb aging is found on all

emission peaks, but no shifts or new peaks are immediately

evident.

Since the changes in intensity are small in comparison

to the peak intensities, the differences between these

spectra can be enhanced by subtraction of the initial from











the final spectrum. Figure 4-12 shows the results of this

manipulation. The wavelength scale (x-axis) has been

expanded to enhance the differences found between the

spectra. Negative changes on the CL difference line

indicates a peak in the original Y202S:Eu3" spectra which has

decreased in intensity after the degradation has occurred.

These include the 612, 617, 626 and 632 nm peaks. Positive

deflections in the CL difference spectra indicate new peaks

or peaks which have increased in intensity. It is clear that

the lower integrated CL brightness largely results from

reduced detected emission from Y202S:Eu3. Increased emission

peaks are found at 618, 629 and 632 nm and the peaks at 618

and 632 nm correspond with known emissions of Y202SO4:Eu3

[OZA91], possibly indicating its presence on the surface.

The 618, 629, and 632nm peaks could also be caused by

spectral broadening of the main emissions of Y202S:Eu3. The

presence of Y2O2SO4:Eu3+ would be surprising since the

temperature required for oxidation in air of Y202S:Eu3+ to

produce Y202SO,:Eu3+ is about 750-975C [OZA91]. Since these

temperatures are clearly not achieved by electron beam

heating, the phase conversion of Y202S:Eu3 to Y202SO4:Eu3 or











Y203:Eu3' by an electron beam assisted reaction is

suggested.

Figure 4-13 shows the overall change in brightness as a

function of accelerating voltage threshold voltage data for

unaged versus Y20OS:Eu3 aged to 19.9 C/cm2. The relative

brightness for the aged and unaged phosphor are normalized

to their maximum value over the voltage range studied

(typically maximum brightness is found at maximum voltage),

so the slope of the linear region (proportional to

luminescent efficiency) and threshold voltage can be

compared. The undegraded spot has a threshold of 539 V,

whereas the degraded spot has a threshold of 640 V. These

data show conclusively that an increase in the threshold

voltage (which would be attributable to the growth of the

over layer of Y202SO4:Eu3, or to increased charging of the

surface) directly affects the brightness of the phosphor. In

the linear region of the curve, the loss in brightness is ~

4% at a current density of 20uA/cm2 and a 2 keV accelerating

voltage. The loss in brightness is nearly constant

throughout the linear portion of the curves (2keV to

4.5keV). This, along with the linearity (parallel slopes for

aged and unaged between 2.5 keV and 5 keV) in the









86

non-saturated region, suggests that the Eu3 residing in the

excited volume of the Y202S host has not undergone any

radiative efficiency losses. At the lower accelerating

voltages (0.5 keV to 2.5 keV), the slopes of the aged and

unaged spot diverge. The aged spot has a lower slope,

indicating a loss in brightness efficiency. Clearly, the

surface changes measured by AES and CL spectroscopy indicate

an altered near surface region of the phosphor which results

in an altered low voltage brightness efficiency.

By considering the increase in range associated with

the increase in threshold voltage, figure 2-7 can be used to

estimate the increase in dead layer thickness. The range of

a 539 eV electron is 0.057Am. The range of a 640 eV

electron is 0.07Am. The difference in the range of the

electrons can be used as an approximation of the thickness

of the dead layer found near the surface of the Y202S

particles and is 130A.












Summary



Initial experiments in a vacuum of -~xl108 Torr have

shown that CL degradation may occur by the partial surface

conversion of Y20OS:Eu3 to Y20OSO4:Eu3+ or Y203:Eu3+. The CL

degradation observed is small and may be a combination the

new phase formation and surface charging induced by the

chemical changes. A possible surface reaction driving the CL

degradation is:

Y202S + 202 <> Y202SO4.

This reaction occurs in the presence of oxygen or oxygen-

containing species and only takes place under electron beam

stimulation.

The CL degradation proceeds by the initial removal of

C. Once the carbon is removed by surface reaction, the

detrimental surface reaction (loss of S) takes place and CL

degradation occurs. AES peak energy shifts to higher

energies for Y and 0, and to lower energies for C and S. The

CL degradation is marked by an increase in threshold

voltage.























































Figure 4-1 SEM image of Y202S:Eu3 particles. The
photomicrograph was taken at 25kV and magnification of
2600X. The white bar at bottom right of image indicates a
scale of 10 im. Particles are faceted with an average
diameter of 4.5 pm and a distribution from 1 to 10 Am.











O.90j
X1O










0.40|
0,80

0,7 0
O,'i5


0
o-


CI~0"
V %.., \.
_I P3 t


24
?4,


10 0 .0 30,0 40.0 so.0 B0.0
100.0 .
80.0
40.0 ,

10.0 20.0 ,0 40.0 .o0 0.0


Figure 4-2 X-Ray powder diffraction spectra for
Y202S:Eu3 as a function of 26 angle. The Miller Indices
of the crystallographic planes are indicated on the
upper graph. The lower graph is the matching JCPDS
(#24-1424) file displaying the expected relative
intensities for Y202S.


















Oxygen



O Yttrium




A















Figure 4-3 Schematic of the crystal structure of
Y202S:Eu3. 3 rhombehedral units make up the full
hexagonal structure for the material. Lattice
parameters are shown above with a=3.284A and c=6.509A.