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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.
|
|
PAGE 1
CHARACTERIZATION OF THE CATHODOLUMINESCENT DEGRADATION OF Y2O2S: 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
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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 ii
PAGE 3
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
PAGE 4
new nickname for the Auger, ask Wish!). Besides that, I mus t 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. iv
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TABLE OF CONTENTS ACKNOWLEDGMENTS ABSTRACT CHAPTERS 1 INTRODUCTION Motivation and Objectives Scope of the Present Work 2 LITERATURE REVIEW Introduction ........... Field Emission Displays ..... Introduction to Cathodoluminescence Theoretical Aspects of Cathodoluminescence CL Degradation CL Degradation of Sulfides .. Degradation Resistant Coatings Summary and Motivation .... 3 EXPERIMENTAL APPROACH I I ii viii 1 1 2 5 5 6 8 11 20 27 32 34 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 V
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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+ 7 4 Luminescent properties of Y202S :Eu3+ 75 Degradation Characteristics of Yttrium Oxysulfide . . . . . . . ... 77 Summary . . 87 5 EFFECTS OF OXYGEN AMBIENTS ON THE DEGRADATION OF Y202S: Eu3+ Introduction. 101 101 Degradation of Y202S: Eu3+ in an Oxygen Ambient 102 Time Dependent Threshold and CL Degradation Experiment . . . . . .. 109 Summary 6 BEAM ENERGY DEPENDANCE ON DEGRADATION RATES OF Introduction ..... 1 keV Experiment 2 keV Experiment 3 keV Experiment Discussion and Comparison of Depth Resolved CL Studies of Summary vi 1,2 an 3 Kev Data Y203 : Eu3 + Emission 115 136 136 139 141 143 146 148 150
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7 MODELING OF THE CL DEGRADATION OF Y202S: EU3+ 179 Introduction . . . . . . . 1 79 Modeling of Brightness as a function of Depth 181 CL Efficiency as a function of Depth 182 Power Dissipation as a function of Depth in Y202S: Eu3+ 186 Predictions of the Brightness Model . . 188 Modeling of Gas-Surface Interactions of Y202S: Eu3+ . . . . . . . 191 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 Y202S: Eu3+ Si02 Coated Y202S: Eu3+ Phosphate coated Y202S: Eu3+ Summary 9 SUMMARY AND CONCLUSIONS LIST OF REFERENCES BIOGRAPHICAL SKETCH vii . I 198 203 220 220 222 223 225 228 229 231 251 257 262
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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 1x10s Torr and the combination of AES and CL spectroscopy indicated the loss of sand C from the surface of the phosphor. Volatilization reactions involving O containing gases, the electron beam and the surf ace C and S caused a conversion of Y202S: Eu3+ to viii
PAGE 9
a non-luminescent layer, possibly Y2O2SO4 : Eu3 This layers growth decreased the total brightness of the phosphor. The conversion of Y2O2S: Eu3 to Y2O2SO4 : Eu3 requires O, so CL degradation was studied at 1x10-6 Torr backfilled from 1x10-s Torr with Ogas. The CL degradation rate was much higher in the presence of O gas. The conversion of Y2O2S: Eu3 to Y2O2SO4 : Eu3 was not detected, but in the presence of O, a near surf ace layer of 1 uminescent Y2O3 : Eu3 was formed and confirmed by CL spectroscopy. Makhov's Law was used to predict the brightness loss of the Y2O2S: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
PAGE 10
but either migrated during bombardment or was desorbed (Ag2S) slowly exposing the Y202S: Eu3 phosphor. Wet chemistry coatings of Si02 and phosphate were applied as well, and were found not to affect the CL degradation rate. X
PAGE 11
CHAPTER 1 INTRODUCTION Motivation and Objectives 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 Y2O2S:Eu3+ ) used in CRTs exhibit the highest luminous efficiencies of all the currently available 1
PAGE 12
2 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 therefor 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
PAGE 13
Y202S: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. 3 Another objective of this study was to examine the effects of electron beam energy on the CL degradation rates of Y202S: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
PAGE 14
Chapter 6. The model used to approximate the CL degradation of Y202S:Eu3+ is developed in Chapter 7. 4 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.
PAGE 15
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-lOnA), 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
PAGE 16
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 6 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
PAGE 17
7 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.
PAGE 18
8 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
PAGE 19
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 Al2O3 The figure displays the absorption and emission spectra as a function of energy in eV. For this case, only one of the
PAGE 20
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. 10 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 s ~udy 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
PAGE 21
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 V P are those electrons which have undergone purely elastic collisions and exhibit no energy loss. All electrons below VP have undergone inelastic collisions with
PAGE 22
12 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:
PAGE 23
13 de ds -785pzln( l.166e) Ae j ( 2. 1) where de is the incremental energy loss in a uni t depth ds 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>l3: -9 76 Z 58 5 J +--z O 1 9 ( 2. 2 ) For high energy electrons (5-lOOkeV), 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.SkeV) 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 impossi b le. The most popular sol ution to thi s 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
PAGE 24
14 intact for energies greater then this inflection point. The modified equation used by Rao-Sahib is given by: de -785*P*Z -ds JA* e j ( 2. 3) 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:
PAGE 25
15 j '=--j __ l+k* l e (2. 4) 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
PAGE 26
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 +J PEev 1 de SOe V 50eV ( de ) ds ( 2 5) 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 1 2 below 50 eV. The range predicted by Joy and Luo can be used to determine the interaction volume of the primary electrons in
PAGE 27
a solid, as well as the amount of energy lost to the solid at a given depth. 17 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 (
PAGE 28
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 l ines) 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.
PAGE 29
19 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) *dx dx ( 2 6) 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.
PAGE 30
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 (0.08m to 0.4m) 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
PAGE 31
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
PAGE 32
indicated by ion desorption of O and Br from the surface were suggested. 22 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, Zn2SiO4 : Mn2+ Y2Si03 : Ce3+ and Sr2Al6O11: 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. Y2SiO3 : 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
PAGE 33
23 the active Ce3+ concentration [Kla87] Sr2Al6011: 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: Io I(N)=---( 1 + C *N) (2. 7 )
PAGE 34
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 CaW04 where the lattice ions cause luminescence, the creation of new nonradiative recombination centers was determined to be the
PAGE 35
25 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 3x10s Torr, an aluminum coated powder degraded in the same test stand, and
PAGE 36
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 O.S*I0". In the table, higher 1/C values are observed for
PAGE 37
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 27 More recent studies of CL degradation have centered around sulfur containing phosphors, in particular ZnS:Ag, ZnS: Cu, Au, Al and Y 2O2S : 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% (Y202S: Eu3+)
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28 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 2x10s and 1x10s 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
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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 1x10a 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
PAGE 40
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 + <=> -46.04 + 2 ( 0) <=> -234.9 H= -94.43 kcal/mol 02 2ZnS + <=> 2ZnO + 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]. 30 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
PAGE 41
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 0.4m 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 O.Om, 0.127m, 0.254m, 0.389m
PAGE 42
32 thick non-luminescent layers. The slope of the intensity versus V P remains the same, while the V P 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
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33 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,
PAGE 44
34 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
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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.
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36 Cathode Gun Deflection 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.
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D .,,,,, ,,, ..... ,,,,,, , ,,., ,,,11,I, I,,,, ,,,,, .... ,, ,,,,,, , , ,,,,11,, , , ,,,,, ,,,,,,., '''''' ,,,, , ,,,,,,,,, ,,,,,,,,,,,.,,,, ,,1,,,,,,,,,,1,,,,, ' ' ' ' ' . ' ' ' ' ... . . 'GL,-rl' ..... ,,,,,,,,,,,,,.,,., ,,,, , , 1,,1,1,,. , , IW, , ,,, ,,,,,,,,,,,11111,1111 ,,,, .,,,,,,,,,,,,,,,,,,, "PHOSPHOR Figure 2-2 A cross section of a Spindt-type Field Emission D isplay with vertical line, marked D, indicating the working distance between the anode 3 7 and cathode of a field emission display. Typically D i s appr oximat e l y 1mm [SCH96].
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-> 38 -----4Tt --------4 3 r, ---tr2 r ----2 t I r, 'E --_. C lg j: !~ ii ( I 3 0 4A 2 -. .. -~---------------Figure 2-3 Absorptio n and luminescence spectra for Cr3+ in Al203 also known as ruby. Absorption bands exist for all transitions, but a visible luminescence peak is only observed for the 2 E-4A2 transition, as this transition has the largest energy change, and emits characteristic red luminescence. All other transitions d o occur, b u t are not in the visible region of the spectrum [LUM78].
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39 N(e) V 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 V P. 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].
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> a, -r C. a. 0 Cl) 14 1 2 10 8 6 4 2 ' \ Tung et al. \ ' \ Rao-Sahib and Wittry \ 1', 'l \ ...... / ',,:., / l: : ~. B:the ,,. _ ,~ 0 -t--r-"T""T'TTTI~-,-.,...,..TTTT.,,...--r-'T""T"'l"'TT'I'~ .,.,,,~ --,.,-~""" ~"" 0 .01 0.1 1 Energy (keV) 1 0 100 40 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].
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-'< -:.i ,>. ., :> e.D = Q. .... ':J') 41 30-,--------------------10 3 --o-Cu-,JB 1 Cu-Tung C-~IB C-Al..kt-rman 0.3 0.1 ....------,,---""T"-----.--------~--~ 0.01 0.1 l 10 Energ~ (ke\") 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 lkeV [JOY89,TUN79].
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E ::, .._... Q) C) C: ro 0:: 42 Range from Mod. Bethe and KO Range 10,----------------------------~ 0 1 0 .01 13._: ------------------~-----' 0 2 4 6 8 10 12 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.
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43 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].
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c,, 1---:z =:> 44 >ca:: .,_. ci:i 0.1 12:. -=C 0 .IBBJL O.O I 2 4 6 8 12 16 20 ACCELERATING POTENTIAL (kV) Figure 2-9 The t heoretical (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]
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100 >.... In ~80 .... z 20 Figure 2-10 ()--, 0 ---0--SEALED-OFF' TUBE ALUM SCREEN PRESSURE < 10-7 MM HG OEM. STATION ALUM. SLIDE P 3.10-S MM HG OEM. STATION NOT ALUM. SLIDE P -3.10-5 MM HG Zinc oxide degradation and the 45 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]
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...... .4. J .. X ..... I-2 z LIJ 11 1 u ~o e .... 1"'4 'l w Cl [l t j u 'Z 0 0~ 46 $ 1Me=16 tK.E. ,on o+) r M/~-c32 (K.E. 1rn st) 0 1 1 21~,u -1[, ,rr] l I 1 'iX 10'1 [1011] r 1 ~ XlC 1 [Ttrr] "" w I I 0~10-. [ T' ,1r) I( \ t J u 10 2U )0 40 so z 0 0 1 0 ~o ~o ,o .so ANODE VOL TAG~ (V) A~0Dt VOLTAGE IVl ;, ~ r,~321stro/l 0 M ~064 (SO/) I" Y. r Z 1 w [. ) (.) 7-. CJo 0 ~NODE VOLT AGE IV) ~NO D E VOLTAGE IV) 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 S02 o s + and 02+ for increasing anode potentials indicating the desorption of surface species under electron bombardment [IT089].
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Zn 59 e C 272 eV s 152 eV 0 510 eV Before Zn 99" eV s 152 eV Zn 59 eV Electron Energy ( e V) 0 510 eV 47 After Zn 994 eY 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].
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48 2 0 t 1 8 UNCOATf PHOSPHOR 16 ~'/ 1--:z O.i 27 fL, ::::, >14 er: a:: t:: i2 ca a:: >10 1--U) ;z: l.&..I a t-= u,J 0 :z:: 6 '-L.J <...,;) U) LA.J :z: 4 1-::E ::=i __J 2 00 -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 firs t pass throug h to excite cathodoluminescence [KIN72].
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::::, ..!!. en C Cl) C ....I 0 1 0 49 CL Peak Intensities Versus Coulomb Load 0 50 Y202S:Eu (Aluminum) (A) (D) Y 202S:Eu (Edge) ZnS:Cu,Au,AI (E) ZnS :Ag ,CI Y202S:Eu Charging (B) 100 150 200 Electron Dose (Clem 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 seprate degradation curves are shown for Y202S: Eu3+ Line A shows the degradation of Y 202S: 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].
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50 1.5 APPH S(158eV) -3. 0 Energy(eV) Before Aging APPH After Aging 1.5 -1.5 C~V) S(158eV) -3.0 Energy(eV) Figure 2 -15 Depicts the surface state exhibited by Y2O2S: Eu3+ before and after electron beam aging. Note the loss of C (28leV) and S (158eV) with concurrent increases in Y (ll0eV) and 0 (519eV) after the phosphor has been exposed to prolonged electron beam excitation [TRO96].
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51 T A BLE I Burn ChararttrisJi(s of P-typc P h osphors Excittd wit/, 10 kV Eltctrons l 1 / C No .or F.lectron Bum Coru11n1 Hi' ion Burn RrJu11ion i n Decay coulombs/an? C x I0lt,cml Constanl, r T ime Ph,"rhnr I P -No. Use Chemical Aclivator Necasary to c~t I Compositio n Reduce These , Arter : Ito J/o Measure. Others I coulombs/cni! I I mcnts I I I ( 'c''' J,1iva1rJ Silialcs I ~ Flying Spot Sanner I (CaO)J.M,0.(SiOih I Ce About 10-1 10 I 60 10-l m _ Flyin1 Spot Sanner CaO.Ali()3.SiO2 Ce 10 I . ' . Sulfur 1l11n111111c,I, "' 1nJ I r2 I (l\cillol\ll~ Zn SCdS I Cu 11.9 0 .140 15 10:1 Cu 1rtiva1cd P4 { Direct View TV, {ZnS Ag (Blind) ZnSCdS/ZnS Al 4 S 0 .370 0 .011 25 112 I P7 Cmde Screens ZnS Ag '9.~ 0 .170 40 130 for O!cll!Mcopcs ZnCdS Cu 10. 0 0.167 JO 150 PII Pholography ZnS Ag 16.6 0 .100 0 .08 0 ,1.1) IO.JS17 ISO :J P14 Radar ZnCdS Cu 16.6 0.100 25 100 P22 Projection Color TV ZnS Aa 20.0 o.ou JO 110 : ZnS Ag.Al 3).3-o .oso 2l 175 ---Oiyscn and Sulfur Pl Scopes Radlr ZniSiO, Mn 104, 0 0 .016 0.0111 0 .0211, l domir11
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Phosphors Prohahility Eflirirnrirs ( % ) Firsl trapprd f H, s (:akulatc Re.port e d Carriers ( X }:',) (eV) ... -... . . ..... ... -. .... ......... ....-------. ... . ___ ,,_ ..... . -. -Z11S:Ag:CI 0 .fili 2'.l 21 liolr. } 18 .. ZnS:Cu: A I O.fi6 21 1 9 2 1 hole 2 18 Y10!S:Eu 0 .66 1 : 1 13 electron 2 u Y1O: S :Th 0 8 1 0 1 9 -24 l!l hole 2 4.1 La101S:E11 (lj 10 11 dcctrnn ) u .. l,al.hS : l b 0.5 1 3 12 hole l 4 1 Yi(\:Eu o.n t t elr.ctron 4 5 8 YVOd'.u 0 .66 8 A elwrou 4 4 0 Zn1Si(\Mn 0 8 9 ll hole 4 5.5 Table 2-2 External radiant efficiencies for a number of phosphors including ZnS:Ag,Cl, ZnS:Cu,Au,Al and Y202S : Eu3+ [OZA90] 52 f.'111 (eV) . ; 2 .Vi fl2 l .9B 2 .29 l ,!lB 2 n 2.()3 2 0 2Jfi
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CHAPTER 3 EXPERIMENTAL APPROACH Introduction Y2O2S: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 m and a range of sizes from 1 to l0m. 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. 53
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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 Eu203 ) [KOT95] The general chemical reaction is as follows: This reaction is assisted by Na2C03 and other fluxes. Depending on the desired Eu content of the product, y can take on values from 0.00 to 0.04.
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During this reaction, S reacts with the Na2CO3 flux which produces polysulfides [PHA91]: 55 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
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phosphor screening industry. Silica (SiO2 ) thoria (ThO2), and phosphate (Po/ ) coatings have been deposited by wet chemical techniques which are described in the literature 56 [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 Y2O2S:Eu3+ from Osram Sylvania was suspended in a solution of 0.lM 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 Coven 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 (Po/ ) coating, a 0.1 M solution of phosphoric acid was prepared. The phosphor
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57 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 m ixture 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 .s.i.Q2 Coating Si02 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.
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58 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 Si02 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 Si02 coating occurs due to the attractive surface potentials associated with the phosphor particle and the Si02 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.
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59 Pulsed Laser Ablation Recently, pulsed laser ablation has been used to coat particles with diameters ranging from 1 to 1000 m [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 Al2O3 Ag on Al2O3 YBa2Cu3O7 on Al2O3 TiO2 on SiO2 and Ag on SiO2 [FIT97]. Pulsed laser ablation was used to produce
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thin metal (Ag) and intermetallic TaSi coatings on the surfaces of the P22R particles. Characterization Techniques Cathodoluminescent and Auger Electron Spectroscopy 60 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 lA and 15A. Data was collected in the dN/dE mode using a 4eV peak-to-peak modulation voltage. Auger peaks from Y (77eVMNN), S (152eVLMM), C (272 eV-KLL), and O (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
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61 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 2x10s Torr. Typical AES operating parameters were 1, 2, or 3 kV primary beam energies, with sample current densities of 200 to 300 A/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
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distribution (intensity as a function of wavelength) were acquired. 62 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 2x10-0 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 (2x10a to 1x10-6 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
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63 pressure of better than 2x10-9 Torr was reached, the oxysulfide phosphors were subjected to electron beam bombardment at pressures from 1x10s 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 A/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 m inute for 24 hours.
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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 ( 2) T' (S ) CoulombLoad(--) =Current ----*--cm ime ec 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
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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 65 The primary beam voltage of the AES was set to 7 kV, and the sample current was set at 0.2 A/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
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66 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.
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67 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 = O 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.
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68 .,.,,_. "l"'. c _~i ~ 9 -...... i Fluidization System fluidized powder 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 nanoparticle coatings of TaSi and Ag on the P22R phosphor particles.
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69 AES/CL Experimental Setup Fixed lenses AES PC 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.
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Vacuum Chamber Isolated Sample Carousel E-beam Picoammeter Figure 3-3 Sample current measurement is 70 90 V 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.
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71 1-,--------------------------------,~--, 0.8 0.6 -;-en = 0.4 C .s::. C, -~ m 0.2 y = 0.CXXJ2x 0.0044 y = 0 .CXXJ2x -0.1575 -0.2--'--------------------------------' /Ja;.V~(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=O 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.
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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 A/cm2 The experiment was conducted in the best vacuum available (typically -1x10s Torr) in the AES spectrometer and Auger peak-to-peak heights (APPH) as well as Auger peak energies for Y, C, Sand O were recorded during electron bombardment. The initial and final CL spectra were recorded along with the total brightness, 72
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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 Pro~erties of Y2Q2S:Eu3+ 73 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 Y202S:Eu3+ was carried out in typical FED operating conditions to obtain a baseline for comparison with other experiments reported in future chapters.
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74 SEM images, as shown in figure 4-1, shows that Y202S: Eu3+ particles were faceted with sizes ranging from 1 to 10 m with an average particle size of 4.Sm. 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 2 e 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. 284A and c=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 Y202S: Eu3+
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75 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 200A/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 5D0 7F2 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 20a/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
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76 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 xintercept 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
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77 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
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conditions to minimize the degradation effects due to ambient. A 2keV primary electron beam with DC current density of 272 A/cm2 was used for the aging studies of the phosphor. 78 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 l.4x10a 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
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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, Sand 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 Oto 5-10 C/cm2 and then remained relatively constant, while the AES signals for carbon and sulfur
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80 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 0, C, and S to the Y AES signal. The vapor pressure of yttrium is extremely low (-10-10 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
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81 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 Oto 22 C/cm2 The S/Y decrease corresponds to the slow increase in CL brightness observed from Oto 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 Schange 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 Oas 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
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82 A measurable shift (-1.5 eV) to higher energies of the Auger peaks of Y and Oas 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 Speaks 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, 0, 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
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83 surface species. The C and Speak 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 Y202S04 may be occurring. If the conversion of Y202S to Y203 or to Y202S04 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
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84 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 Y2O2S: 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 Y2O2S: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 Y2O2SO4 :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 Y2O2S: Eu3 + The presence of Y2O2SO4 : Eu3+ would be surprising since the temperature required for oxidation in air of Y2O2S: Eu3+ to produce Y2O2SO4:Eu3+ is about 750-975C [OZA91]. Since these temperatures are clearly not achieved by electron beam heating, the phase conversion of Y2O2S: Eu3+ to Y2O2SO4 : Eu3+ or
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85 Y2O3:Eu3+ by an electron beam assisted reaction is suggested. Figure 4-13 shows the overall change in brightness as a function of accelerating voltage thrshold voltage data for unaged versus Y2O2S: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 Y2O2SO4: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
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86 non-saturated region, suggests that the Eu3 + residing in the excited volume of the Y2028 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 0.057m. The range of a 640 eV electron is 0.07m. 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 Y2028 particles and is -130A.
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87 Summary Initial experiments in a vacuum of -1x10a Torr have shown that CL degradation may occur by the partial surface conversion of Y2O2S: Eu3+ to Y2O2SO4 : Eu3+ or Y2O3 : 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: Y2O2S + 202 < > Y2O2SO4 This reaction occurs in the presence of oxygen or oxygencontaining 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.
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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 m. Particles are faceted with an average diameter of 4.5 m and a distribution from 1 to 10 m. 88
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0 90 0 .80. 0 ? O 0 6Cl I o 0 J o .~o 0,lO o.a o 0 ,10 I 1 0. G 10.0 0 so. o BO.O 40,0 20. 0 lO.IJ I t ,...._ N 0 0 '-' h I 20. 0 80. 0 I ,...._ N 0 '-' l ,...._ C') 0 0 '-' 40. 0 ,...._ 0 -'-' ,...._ C') 0 '-' ~i 50. 0 ,...._ N -'-' I 89 ,...._ -0 N ,...._ '-' N 0 N '-' lo.. eo. o Y.2029 2+-1424 -r J ... -r-.L, 40,, 0 50. 0 80,0 Figure 4-2 X-Ray powder diffraction spectra for Y202S: Eu3 as a function of 28 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.
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90 0 Sulfur Oxygen Q Yttrium A Figure 4-3 Schematic of the crystal structure of Y202S: Eu3+ 3 rhombehedral uni ts make up the full hexagonal structure for the material. Lattice parameters are shown above with a=3.284A and c=6.509A.
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91 45000 40000 N u. 0 35000 Q 30000 N t 25000 u. LL ,._ Ill 0 I 0 C Q 0 Q) 20000 ll) .... C -t ..... .J 15000 u. u. u -t N u. 0 10000 Q Q 5000 0 -::I" m ("\I ("\I 0 (0 ro (0 0 m ("\I -::I" I'-m .-("\I (T') -::I" L!) -::I" 0 (T') (0 m (T') (0 m ("\I L!) ro L!) L!) L!) L!) (0 (0 (0 I'I'-I'Wavelength (nm) Figure 4-4 Cathodoluminescent intensity as a function of wavelength. The spectra was obtained with a 2kV accelerating voltage and 20A/cm2 current density. The larger visible (red) transitions are indexed. The characteristic narrow line emission is due to the luminescent transition being localized on the Eu3+ ion.
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0 9 0 8 0 7 0 6 I 0 5 Ql C -.c .!:' 0.4 ... a::i 0 3 0 2 0 1 0 0 y = 0 .00023x -0 .20034 Threshold //t Voltage / Slope /'7 1/ 1000 2000 3000 4000 Accelerating Voltage (Volts) Saturation Voltage 5000 92 6000 Figure 4-5 Representative curve of brightness (a.u) as a function of accelerating voltage for Y202S: Eu3+ powder CRT phosphor. The beam current density was set at 20A/cm2 The threshold voltage can be extrapolated from the linear portion of the curve as shown by the dashed line and equation and is found to be 871 volts. The slope of the linear portion of the curve represents relative luminous efficiency, and the saturation voltage (the voltage at which the CL response becomes sub-linear) is shown.
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1 0 9 Initial Charge Drop } o.s / Slow Increase Steady Decrease / : 0 7 '-
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94 4500 1 .02 4000 O xygen : 3500 3000 CL Area 0 98-:-.c _, -~ 2500 Q) 0 .96 : :::c ~tl\~l\llt\~ Ji!: 2000 Sulfur ...
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95 1.4 1 2 In 1 Oxygen/Yttrium 0 ; ~0.8 l: CL Area/1. 5 c..0 6 -c.. CX0 4 Sulfur/Yttrium 0 2 Carbon/Yttrium 0 0 5 10 15 20 25 30 Coulomb Load (Clcm2) -carbon/yttrium -sulfur/yttrium -CL Area (a.u) -oxygen/yttrium Figure 4-8 AES Peak-to-Peak Height Ratios and CL Brightness(CL Area/1.5) a s functions of Coulomb Load. The normalized AES intensities show that C and S leave the surface probe region and the 0/Y remains constant. The decrease of C/Y and S/Y ratios match the increase in CL to approximately 5 C/cm2 Once the C/Y and S/Y ratios stabilize, the CL intensity steadily decreases. Note that CL brightness has been reduced to fit to scale.
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2000 ~ ----------------~ _, 1000 b -1000 "iii C Cl) -2000 Cl) LJ.J
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97 600 500 Oxygen > y = 0 .0845H + 526.47 a) >i 400 en ... a) C LU 300 .:.:= Carbon ('!;I a) y = -0.2072H + 271.93 Q. 200 Cl) Sulfur LU <( y = -0.0392H + 158.17 100 Yttrium y = 0 0639H + 88. 76 0 0 5 10 15 20 25 30 Coulomb Load (O'cm2) Figure 4-10 Examination of AES peak energy shifts during coulornbic aging. AES energy data has been linearly fitted with equations displayed to determine the direction of energy shift for Y,O,C and S. The Y and O shift is slightly positive and amounts to approximately 1.5 eV from beginning to end of the experiment. C and S slopes are negative with sulfur energy shift of -.5 eV. The carbon AES energy cannot be determined with accuracy due to the total loss of the C AES peak at the end of the experiment. The C energy values after the total loss of C (-5 C/cm2 ) is an artifact of the software used to determine the AES peak energies.
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98 0 9 0 8 -:-0 8 -=-_, 0.6 _, 0.7 Final\ 0 4 ;;; 0 6 ;;; C C a) a) E 0 5 0 2 .... C ...J 0 4 ...J u 0 u 0.3 E C C -0.2 L! 0 2 0 1 -0.4 0 -0. 6 500 550 600 650 700 750 Wavelength(nm) Figure 4-11 CL spectral intensity distribution as a function of wavelength before and after 2keV degradation. Small changes in peak heights are visible, but very difficult to discern. Initial spectrum intensities are denoted on the left hand y-axis scale, final intensities on the right hand y-axis.
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99 CL Spectral Intensity 0 9 (\ ~-, 0 8 ..._,,/, -:0 7 632 nm = .. (11;1 i 0 6 Q) 629 nm C.l -~ 0 5 C 617 nm l Q) .. Q) ... C 0.4 ...J Q u 0.3 ...J 626 nm u 0 2 0 1 0 600 605 610 615 620 625 630 635 640 Wavelength(nm) Figure 4-12 CL spectral intensity for the 600 to 640 nm region before and after 2keV degradation has occurred. The spectra are very similar to each other, so in order to understand what changes have occurred, the difference is plotted. Negative variations in the difference graph indicate CL peak height decreases (612, 617, 626 nm), and positive deflections indicate peak increases or appearances (618, 629, and 632nm).
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1 0 8 -=0 6 .., (IO i::l Q) 0 4 C .... .c CS') -~ cc 0 2 0 -0. 2 100 ,,,,,--~ lnitial-Unaged /// 0 Clcm2 ./ ~nal-Aged / 19.9 Clcm2 y = 0 .0002x 0 .1079 / .,,. / / / / / / y = 0 0002x-0 .128 --Y ,e;; 7 //1000 2000 3000 4000 5000 6000 70 0 Accelerating Voltage (eV) Figure 4-13 Threshold voltage experiment for standard Y202S:Eu3+ The unaged B-V curve was taken at 0 C/cm2 and the aged B-V curve was taken at 19.9 C/cm2 Note the increase i n threshold voltage for the degraded sample. Trendlines extrapolated from the linear portion of the curves determine the turn-on voltage. Vth (unaged) =539 volts, Vth (aged) =640 volts.
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CHAPTER 5 EFFECTS OF OXYGEN AMBIENTS ON THE DEGRADATION OF Y2O2S:Eu3+ Introduction In this chapter, the CL degradation of Y2O2S:Eu3+ powder phosphor will be examined in oxygen ambients. The results will be compared with the baseline experiment of chapter 4. A comparison of the results will show that Y2O3:Eu3+ is formed at the surface of the phosphor in the oxygen ambient experiments. In the last chapter, it was shown that CL degradation may be caused by the formation of Y2O3:Eu3+ near the surface and/or surface charging phenomena associated with the chemical changes. The evolution of these layers will be examined by time incremented threshold voltage experiments and CL spectroscopy conducted during an aging 101
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102 study in oxygen and the surface layer formation will be correlated with the CL degradation of Y202S:Eu3+ These experiments will provide insight into the growth of the surface layer and its effects on the luminescent properties of Y202S: Eu3+ Degradation of Y2Q2S: Eu3+ in an Oxygen Ambient As stated in the previous chapter, CL degradation of Y202S :Eu3+ has been found to be sensitive to oxygen and/or oxygen containing gases. The conversion of Y202S: Eu3+ to Y202S04 : Eu3+ was indicated by AES analysis and CL spectroscopy. In order to better understand the gas-surface interaction induced by the electron beam, better control of the gas in the experiments is required. Oxygen of 99.9999% purity was leaked into the AES spectrometer during a CL degradation experiment to compare the degradation rate and surface changes induced by the electron beam in an oxygen ambient versus the baseline experimental data. Electron beam conditions were kept nearly the same (2keV beam energy and 274 A/cm2 current density). The base system pressure was
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103 l.5x10a Torr before being backfilled to 1.1x10-6 Torr with oxygen gas. Figure 5-1 shows the CL brightness decrease as a function of coulomb load for this experiment. The curve is very similar to the baseline CL degradation curve in figure 4-6, in that the brightness instantaneously dropped (not shown in figure 5-1), then recovered -4% over -5C/cm2 and then fell again over the next 15 C/cm2 The most noticeable differences in the two curves (figure 4-6 and figure 5-1) is the much smaller coulomb load to reach maximum brightness in the oxygen ambient (<1 C/cm2 ) and the larger brightness loss (-35%) after 20 C/cm2 These data dramatically demonstrate that the gas constituents and their partial pressures are important factors in determining the rate and extent of degradation of the Y202S:Eu3+ phosphor. Presumably this is because the surface reaction and consequent CL degradation may be rate limited by the amount of reactive gas impinging on the surface and therefore available for reaction. This effect is easily seen by comparing the rates of surface conversion as shown in figure 5-2. C and Sare very quickly removed from the surface region (<1 C/cm2 ) and the O and Y surface concentrations increase steadily throughout
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104 the experiment. A small system fluctuation occurred at -4 C/cm2 and is noted in figure 5-2. Concurrently with the system fluctuation, the AES and CL signals intensities decreased. Both effects are consistent with a small decrease in current density at the sample. Another small decrease was noted at 15 C/cm2 which is also indicative of a decrease in current density. In order to reduce the effects of these changes on the interpretation of the data, AES signals were normalized to the Y Auger peak-to-peak height (APPH). This normalization is shown in figure 5-3 as a function of coulomb load and the discontinuities at 4, 15 and 20 C/cm2 are largely removed from the APPH ratios. It is much easier to interpret the surface changes that have occurred using this type of graph. It can be seen that the initial increase in brightness occurs as the C and Sare lost from the surface (<0.1 C/cm2). This loss, in the baseline experiment, took nearly 5 C/cm2 to complete. Therefore the presence of the oxygen gas increased the rate of C and S removal by about fifty times when compared with the baseline experiment. The 0/Y APPH ratio drops throughout the experiment while the CL brightness steadily decreases.
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105 Figure 5-4 shows the initial (0.1 C/cm2 ) and final (19.5 C/cm2) AES spectra as a function of energy (eV). The C and S AES peak heights decreased throughout the experiment while the Y and O intensities increased. Positive Auger peak energy shifts to higher energies are observed on the Y and O peaks as well as on the secondary emission peak. The C and S Auger peak energies shift to lower energies. Figure 5-5 depicts the AES peak energy shifts as a function of coulomb load for 0, C, Sand Y. Below 1 C/cm2 C and Speak energies shift negatively (approximately leV) while the O and Y peak energies shift slightly positive (-leV larger energy). These trends were also observed in the baseline experiment. Figure 5-6 shows the initial and final CL spectra as a function of wavelength (nm). Both spectra are similar to one another after normalization, so the CL difference is determined from the two spectra and is shown in figure 5-7. The 600-640 nm region has been expanded to better display the differences between the two data sets in figure 5-6. The most notable feature is the large positive deflection in the CL difference curve at 611.9 and the large negative deflections at 617 and 626 nm. The large positive deflection
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106 is indicative of the growth of a luminescent Y203:Eu3+ during the degradation in oxygen. The initial CL spectra does have a small peak at 611.9nm, suggesting a small amount of Y203 : Eu3+ in the unaged powder. This small peak was also present in the baseline experiment. The small amount of Y203 : Eu3+ present in the powder may be a byproduct of the sulfide fusion production process which entails heating yttria ( Y203 ) and europia (Eu203 ) in a sulfur atmosphere to produce yttrium oxysulfide (see Chapter 3). After degradation, smaller positive deflections are found at 616, 618 and 625 nm. Some spectral broadening of the two emission peaks of Y202S:Eu3+ (626 and 617 nm) can be seen by comparing the initial and final CL spectra. This broadening produces positive deflections centered around the main peak. This broadening, may be caused by changes occurring in the local Eu3 + site symmetry producing wider, less intense emission. Unfortunately, the broadening obscures any information about the presence of the Y202S04:Eu3+ which would produce positive deflections at 616 and 618 nm. Threshold voltage data before and after degradation are shown in figure 5-8. The conversion of Y202S:Eu3 + to Y203:Eu3 + has occurred to a sufficient extent to alter the low voltage
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107 (1 kV to 2kV) brightness efficiency of the phosphor similar to the baseline data. The threshold voltages were extrapolated from figure 5-8 and were found to be 765 V and 1290 V, respectively, for the unaged and aged spot. These electron energies correspond to penetration depths of -0.087m and 0.173m, respectively, corresponding to a dead layer thickness is -860A. The dead layer thickness after CL degradation in oxygen is -730A thicker than that found for the baseline experiment. This suggests that the surface conversion and consequent CL degradation occur at a much higher rate in the oxygen ambient. In figure 5-8, a false cross-over in brightness of the unaged and aged phosphors can be seen at approximately 4500 volts, where the relative brightness of the aged phosphor is greater than the unaged phosphor. Each B-V curve in the figure is normalized to its maximum brightness value. This normalization process causes difficulties in interpretation of the data when the maximum brightness for the two threshold experiments does not occur at the same accelerating voltage. The brightness of the degraded spot in actual counts, is -30% lower than the undegraded spot at all
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voltages. Thus the crossover seen on the graph is a false crossover (4500 V) due to the normalization process. 108 It is clear that the presence of an oxygen ambient enhances the surface reactions and CL degradation rate when compared to the baseline ambient data. The increased amount of oxygen at the surface allows for faster reactions and removal rates of C and S from the surface, thus decreasing the coulomb load to maximum brightness. The CL degradation rate appears to depend at least in part on the removal rate of species from the surface. Thus, an increased impingement rate of reactive gas species will result in an increased removal rate. Therefore, the CL degradation rate and total degradation under equal coulomb loads, should be higher in the experiment which have a larger flux of reactive gas available at the surface. A comparison of the CL degradation curves as a function of coulomb load for the baseline and oxygen ambient experiment are shown in figure 5-9. In both curves, an increase to maximum brightness is followed by a steady decrease, but both the rate and extent to which these occur is enhanced in the oxygen ambient. Since the phosphor does not degrade while the electron beam is turned off, the surface reaction must be driven by the ionization of gas
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and/or surface species with subsequent reaction and volatilization. Time Dependent Threshold and CL Degradation Experiment 109 To this point, it has been shown that changes near the surface of the phosphor (within AES analysis limits) are correlated with CL degradation in the early stages of degradation. The threshold voltage studies have shown that this threshold voltage has increased after degradation. Since the data by Kingsley and Prener on ZnS powders [Kin72] show that threshold voltages can be correlated to the thickness of the dead layer, threshold voltage experiments during a CL degradation experiment could be used to study the increases in thickness of the "dead layer" during the CL degradation. A CL degradation experiment was conducted under electron beam and vacuum ambient conditions similar to those used in the oxygen ambient experiment. A 2keV primary beam operated at a current density of 291 uA/cm2 was used to bombard the phosphor in a backfilled oxygen ambient of 1x10-6 Torr. The initial system pressure was l.3x10s Torr.
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110 During the degradation experiment, threshold voltage experiments were performed at time intervals of O, 15, 30, 45, 90, 180, 360, 720 and 1440 minutes. For the current density and spot sized used (0.011 cm2 ) used, these times correlate to 0, 0.26, 0.46, 0.71, 1.42, 2.84, 11.3 and 22.6 C/cm2 loads, respectively. Figure 5-10 shows a plot of the APPH ratios and CL brightness as a function of coulomb load for the experiment. Note that the C/Y APPH ratio is not plotted because no C peak was detected at the beginning of the experiment. It is interesting to note that in the absence of surface C contamination, the CL brightness curve did not exhibit an initial climb to maximum brightness. Only a steady decay of CL brightness is seen. This may be evidence that the initial loss of C is the cause of the increase to maximum brightness seen at the onset of all previous experiments. Small spikes (labeled A-H) in the CL brightness curve occurred at the times when continuous electron bombardment was interrupted to collect the threshold voltage data (Figure 5-15 shows the series of B-V curves taken at the time intervals labeled A-Hin figure 5-10 and will be discussed in greater detail). The spikes are simply the
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111 large initial brightness recorded by the CL spectrometer which quickly decays to a steady state value due to charging or thermal quenching. The O/Y APPH ratio decreases slightly throughout this experiment, as was observed in the previous oxygen ambient experiment (figure 5-3). The S/Y APPH ratio decreases to below the AES detection limit in approximately 2-3 C/cm2 Figure 5-11 shows the AES peak energy shifts which occur throughout the experiment as a function of coulomb load for 0, Sand Y (no C AES signal was detected). The Y and O peak energies increase by approximately leV after a 2-3 C/cm2 load while the Speak energy decreases during the same interval by approximately 2 eV. The data are consistent with that seen in the original oxygen experiment. The decrease at 3C/cm2 followed by an increase at -5C/cm2 in the S AES peak energy is seen from 3-5 C/cm2 was carefully investigated and concluded to be experimental error, since it precisely corresponds with the interval between point E and point F (figure 5-10). Figure 5-12 shows the initial and final CL spectra as a function of wavelength (nm). The initial spectrum was taken prior to the degradation experiment and the final spectrum
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112 was taken at point H shown in figure 5-10. A large increase in the 612 nm Y2O3:Eu3+ peak indicates an increase in the amount of this phase sampled by the 2 keV electron beam. An expanded view of the CL spectra (600-640nm) is shown in figure 5-13. The CL difference (subtraction of initial and final normalized spectra) is shown on the secondary y-axis. Negative shifts (indicating losses in CL intensity) are apparent at 617 and 626 nm due to decreased emission from Y2O2S:Eu3+ The largest positive deflection occurs at 612 nm. This peak is the main emission peak of Y2O3:Eu3+ Small positive deflections are seen at 616 and 618 nm which may correspond with the main emissions of Y2O2SO4:Eu3+ However, similar deflections are seen around the 626 nm peak (625 and 627nm), thus, these peaks are attributed to spectral broadening of the emission peaks of Y2O2S:Eu3+ Figure 5-14 depicts the brightness (a.u) as a function of accelerating voltage (eV) for the an early (point B of figure 5-10) and final (point Hof figure 5-10) threshold voltage experiments. Threshold voltages were extrapolated for the linear regions of these curves volts. The threshold voltages are found for the initial threshold voltage experiment was calculated to be 166 volts and 866 volts for
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113 final threshold experiments. These correspond to dead layer thicknesses of 0.019 m to 0.101 m, respectively. The difference between these thicknesses is found to be 820A. This calculated dead layer thickness is slightly smaller (860A) than that found in the original oxygen ambient experiment. At 20 C/cm2, the total CL degradation is higher than in the first oxygen experiment (-35% compared to 25%). It is possible that the larger initial depth of the dead layer in the first oxygen experiment (0.087m compared to 0.019m) caused the slower CL degradation rate under similar experimental conditions. Figure 5-15 shows the data from the entire series of threshold voltage experiments conducted at different stages of the CL degradation experiment. The slopes of the B-V curves in the linear region are all very similar except for the B-V curve at t=0 seconds, similar slopes indicate no large net efficiency change during the experiment. The threshold voltage increases with increasing coulomb load and is easily visualized in figure 5-16. Linear trend lines were used to obtain the threshold voltage values for each successive experiment. The linear equations which fit the
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114 curves of figure 5-16 are shown in figure 5-17. The threshold values are shown next to the respective equations. Figure 5-18 is a plot of the dead layer voltage and dead layer thickness versus coulomb load. Initially, the threshold voltage is small (166v) and correspondingly, the thickness of the dead layer is thin (0.015m). As the phosphor is bombarded by electrons, the growth of the surface dead layer is fast, having increased to a thickness of 0.08 min the first 5 C/cm2 Above this coulomb load, the growth rate slows dramatically and the dead layer approaches a thickness of 0.09 mat 20 C/cm2 Although this dead layer thickness seems small compared to the average particle size (4 m), its importance becomes apparent in considering the penetration depth of an electron. A 2keV electron has a penetration depth of -0.3 m. Thus a 0.09 m dead layer (though not entirely non-luminescent) is 30% of the usable luminescent volume, presumably converted from Y202S:Eu3+ (an efficient phosphor) to less efficient phosphors (either Y203 : Eu3+ or Y202804 : Eu3+ ) If this layer was completely non-luminescent and the power dissipation and activator concentration (Eu3+) were both
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115 constant as a function of depth, a 30% loss in CL brightness would be expected for a dead layer of this thickness, similar to the value actually observed. Figure 5-19 shows the relationship between the APPH ratios and the dead layer thickness. Summary The sensitivity of CL degradation of Y202S:Eu3 to oxygen ambients has been shown. The data show that surface reactions involving surface C and Sand impinging oxygen molecular gas dissociated to atomic species by the electron beam, reacted quickly and produced volatile compounds such as S02 and CO2 The loss of C and S from the surface with subsequent formation of Y203 produced a dead (low luminescent) surface layer. Time dependant threshold voltage experiments have shown that the growth of this dead layer correlates with the decrease in CL. The dead layer thicknesses are much larger than those found in the baseline experiments where Y202S:Eu3 was exposed to an electron beam in good vacuum. The presence of oxygen gas increased the rate of removal of carbon and sulfur from the surface, and
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116 increased the CL degradation rate. Since the ionization efficiency of gases are a function of the electrons kinetic energy, studies using different accelerating voltages may show different CL degradation rates; this will be reported next.
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117 0 .95 0 9 0 .85 -=-.. ti;! 0.8 System Fluctuations : 0.75 Initial Fast Increase \ CLArea(a.u)for -< 0.7 ..J u 0 .65 Steady Decrease \_ 0 B kfill xygen ac 1 0 6 0 .55 0 5 0 5 1 0 15 20 25 30 Coulomb Load (C/cm2 ) Figure 5-1 CL brightness as a function of coulomb load for the oxygen backfill experiment. System pressure was maintained at 1x10-5 Torr. Note the changes in CL brightness with various system fluctuations (typically current density changes). Also, note the very fast increase to maximum brightness (<1 C/cm2), and the relatively fast decrease in CL for the duration of the experiment as compared to figure 4-6.
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118 8000 1.-------------------~ 7000 6000 Oxygen -;5000 SemFlu~~ CL Area (a.u) 0.9 0 8 0.7 0.6 0 5 0.4 0.3 0 2 0 1 -::c 4000 C. C. ci 3000 },,\lfv~~,,4""1,1l!illr~MJ Sulfur 2000 Yttrium 1000 Carbon o~------------------------'-o 0 5 10 15 20 25 30 Coulomb Load (Ocm2 ) Figure 5-2 Auger Peak-to-Peak Height for 0, C, S and Y as a function of Coulomb Load. Note the system fluctuations (current density changes) cause a decrease in CL brightness as well as a decrease in APPHs. C and Sare quickly removed from the surface at a higher rate than in the complimentary background experiment. Loss of C and S occur before 1 C/cm2. The rate of removal of these species is governed by the availability of oxygen containing gas present at the surface of the phosphor available for reaction.
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.-. 4 ...--------------------------~ 1 2 3.5 3 Oxygen/Yttrium 119 .. : 2.5 0 8 f ._. 0 5 5 10 15 20 Carbon/Yttrium 25 Q) C .. .c 0 6 -~ cc ._. Q) 0 4 0 2 ..,j u 0 5 ...__ _________________ ---1. 0 Coulomb Load (Ocm2 ) Figure 5-3 AES Peak-to-Peak Height ratios normalized to the Yttrium Auger Peak-to-Peak Height. Initial increase in CL brightness corresponds with the removal of non-luminescent surface layer containing C and S. CL Area (brightness) is depicted for comparison. Note that although the AES ratios have stablized after 2 C/cm2, the CL brightness continues to decrease. This indicates that the degradation and chemical modifications may be occurring below the escape depth of the Auger electrons (5-10 atomic layers)
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120 2000 -r----------------------3000 1000 -1000 ::z::: C. -2000 -3000 -4000 I \ Sulfur+ Yttrium Yttrium Secondary Emission Peak / Oxygen 2000 1000 0 --1000 _i ::z::: -2000 < -3000 -4000 -5000 -5000 ~----------l---------1--------....... -6000 0 200 Energy (e\l) 400 600 Figure 5-4 Comparison of Initial and Final AES spectra taken at 0.1 C/cm2 and 19.5 C/cm2 respectively. Note the decrease in C and S APPH at the end of the experiment. Also, note the positive energy shift of the Y and O Auger peaks, as wel l as the positive shift of the secondary emission peak. C and S APPH's shift negatively in energy.
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121 y = 0 .3647x + 515.00 fll) Oxygen Peak Energy $' al ';; 400 =i -Q) C w llO .lli y =-9.114x + 271.06 Carbon Peak Energy (IJ al Q. 20J (I) Sulfur Peak Energy w < y = -0.678 1 X + 159.8 1 1(l) Yttrium Peak Energy y = 0 .137x + 96.95 0 0 5 10 15 20 25 30 Coulomb Load Clcm2 ) F igure 5 5 Auger Peak energy shifts as a function of coulomb load. Note the fast changes (as compared with the baseline experiment) that occur below 1 C/cm2 O and Y shift t o higher energies and C and S shift to lower energies. Equations are linear f its to the data presented.
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1 2 122 ,::, 0 8 0 8 "" 0 6 -0 6 b ; 0 4 C 0 4 Initial 0 2 Q) .... C -0 ..J 0 2 u 0 2 -0. 4 :e 0 C -0. 6 --0.2 -0. 8 500 550 600 650 700 750 Wavelength (nm) Figure 5-6 Initial and Final CL spectrum as a function o f wavelength in nm. Large changes have occurred in the 600 to 640 nm region. The increase in the peak at 611 nm is indicative of Y203:Eu3+ growth.
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123 1 2 612 nm 625 nm 626 nm 617 nm -= 0 8 = 11:1 11:1 -0 6 (I) (.) C C (I) .... .c ... Cl 0 4 i: Cl ..J 0 2 ..J u u 0 0 2 600 605 610 6 1 5 620 625 630 635 640 Wavelength (nm) Figure 5-7 Initial and Final CL spectra as a function of wavelength and the CL difference spectra. Note the large negative deflection at 611. 9 nm. This large difference is indicative of the growth of Y203:Eu3 Spectral broadening can be seen in the peaks at 626 nm and 617 as shown by "wings" on the CL difference spectra around the peaks.
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.-. _, I Q) e op .c Cl .::: a:i 1 2 -r---------------------------~ 124 0 8 0 6 0 4 0 2 0 -0. 2 & y = O .llll33x 0 ./ ,/ / y = 0 .00027x 0.34847 /'~-1/ 500 1000 1500 2000 2500 3000 3500 4000 4500 5 0 Acceleratin g V oltage (eV) Figure 5-8 Threshold voltage experiment for phosphor prior to and after degradation experiment in oxygen gas. Note that the early portions of the curves (lkV-2kV) do not have the same slope, indicating a change in luminous efficiency. The threshold values have been calculated as 765 volts and 1290 volts. The difference in penetration depth is approximately 860A. Note each line is normalized to itself, so the crossover at 4500 volts is not indicative of the aged spot brightness being higher than the un-aged spot.
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125 1.05 ~----------------, 1 -:0 .95 Background "" 0.9 0.85 a.I cl: 0 8 ..1 0.75 u D.7 0.65 \_ Oxygen Backfill 0.6 -t-----+------+------+--+-----+-----1 0 5 10 15 20 25 30 Coulomb Load (Clcm2 ) Figure 5-9 A comparison of the CL degradation rates as a function of coulomb laod for the baseline experiment and the oxygen ambient experiment. Note that both curves show an initial increase in brightness (
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Ill 0 .: 11;1 c:: ::: C. C. 1 2 6 4 2 3.5 1 8 1.6 3 Oxygen/Yttrium 1 4 A-C Stops for Threshold Experimen1s 2 5 1 2 2 1 5 0.8 CL Area 0 6 0 4 0 5 0 2 Sulfur/Yttrium 0 0 0 5 10 15 20 2 5 3 0 Coulomb Load (Clcm2 ) Figure 5-10 A P P H rati o s and C L degradatio n curv e f o r the oxygen amb i ent e xperiment with thresh old v oltage e xperiments d u ring the degradatio n Points m arked A through H a r e the p oints at whic h threshold v oltag e e x periments were c omp leted. N o C/Y ratio is shown since no C was not present on the surface at the beginning of the experiment. Notice also that no initial climb to maximum brightness is exhibited.
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127 600 500 $' Cl) >. 400 Cl .. Cl) C 300 Cl) C. Sulfur Energy Drop ai 200 Cl _, < ~~,Jul~~i)<><,1'11,.11.r'\,W'~~ Sulfur Peak Energy 1111\1"'1-"""' 100 Yttrium Peak Energy 0 0 5 10 15 20 25 30 Coulomb Load (Ocm2 ) Figure 5-11 Auger peak energy shifts for the oxygen ambient experiment with threshold experiments during the degradation. 0 and Y peak energies increase slightly prior to 3 C/cm2 and Speak energy decreases in the same load. Notice the drop in sulfur peak energy between 3 and 5 C/ cm2
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128 1 2 626 nm 0.8 617 nm I 0 8 -=-\I 0 6 0.6 0 4 i;; 611 nm C a, -0 4 0.2 C ...J u 0 0.2 E -0. 2 C 0 -0.4 5 0 550 600 650 700 -0. 6 -0. 2 -0.8 Wavelength(nm) Figure 5-12 Initial and Final CL spectra as a function of wavelength in nm. Initial spectrum was taken prior to the beginning of the degradation experiment (0 C/cm2 ) and the final spectrum was taken at the end of the experiment (-22 C/cm2 ) Notice the large increase in the 611 nm peak, and decreases in the 626 nm and 617 nm peaks.
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-=--Q) C ,.; C') "i: cc ...I u 129 611 nm__. A 617 nm 0 8 1626nm 618 nm 0 6 ; J _.627 nm Q) CJ C \ Q) ... 0.4 -~...._r,/_,,.J 616 iirii '--...,_,625 nm ...I 0 2 u J 0 6 0 605 610 615 620 625 630 635 6 0 -0. 2 Wavelength(nm) F igure 5-1 3 Expanded v i ew of the initial and final CL spectrum as a function of wavelength in nm. CL difference is shown on the secondary y-axis. Notice the loss in intensity (negative deflections on the CL difference curve) for the Y202S:Eu3 peaks, 617 and 626 nm. A positive deflections are seen at 611 nm and 618 nm corresponding to Y203 : Eu3
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130 1 2 ~-------------------------~ 0 8 ,/ Initial Spectrum / 0 6 T=l5 min/ II Final Spectrum -.c Cl ;:: CCI 0 4 I T=1440 min 0 2 J 0 +---==----+-------1-------1-------l-------l------'-------1 0 1000 2000 3000 4000 5000 6000 7000 Accelerating Voltage (eV) Figure 5-14 Initial and Final threshold voltage experiment. Threshold voltages were found for the curves and are 166 volts and 866 volts. These voltages correspond to dead layer thicknesses of 0.019m and O.lOlm, respectively. This suggests a dead layer thickness of s20A.
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131 1 2 ,----------------------------~ = 0 8 0 6 .. .c Cl'l .::: s;i 0 4 0 2 0 Initial 1000 2000 3000 4000 5000 6000 Accelerating Voltage M Figure 5-15 A Series of threshold curves taken during the degradation of Y202S: Eu3 in an oxygen ambient of 1. Ox10 -5 torr. Time intervals 7000 are denoted in the legend. Notice the rightward shift in threshold voltage as the CL degradation experiment continued. No large changes in luminous efficiency are seen in the linear portion of the curves, and no change in saturation voltage is seen.
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132 0 6 -r--------------------------_..../ __ ...-,:: 0.5 = 0 4 t:i 0 3 .... .c Cl c: i:a 0 2 0 1 400 Initial Final 600 800 1000 1200 1400 1600 1800 2000 Accelerating Voltage M Figure 5-16 Expanded view of low voltage region of figure 5-15. Notice the smooth trend in increasing threshold voltage from the initial to final threshold experiment. This indicates continually increasing depth of dead layer through out the CL degradation experiment.
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0.7 0 6 0 5 -=-_, 'i 0 4 Q) C .... ", 0.3 = a::I 0 2 0 1 0 400 133 y = 0 .00031 X -0 .05180 166 V y = 0 .00039x -0 .10896 282 V y = 0 .00040x 0 .16506 414 V y = 0 .00042x -0 .22473 540 V y = 0 .00041 X -0 .27043 661 V y = 0.00039x -0.28628 y = 0 .00036x -0 .29512 y = 0 .00036x -0.30944 600 800 1000 1200 1400 1600 1800 2000 Accelerating Voltage M Figure 5-17 Trendline fits and equations for CL degradation experiment in oxygen ambient. The threshold voltages found from the linear portions of the curves are displayed next to their respective equations. Note the threshold voltage (indicative of dead layer thickness) increases rapidly then slows down. This indicates a fast layer growth (possibly reaction limited) followed by a slow growth (diffusion limited).
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2 Q) =i !:: 0 > 'C 0 .c !I) Q) I,,. .c f,134 1000 0 1 Threshold Voltage .... 900 0 .09 E = -800 -------0 .08 700 0 .07 Q) Dead Layer C all: 600 0 0 6 CJ Thickness .c 500 0 .05 f,-I,,. 400 0.04 Q) >i 300 0 .03 ..J 'C 200 0 .02 Q) 100 0 .01 Q 0 0 0 5 10 15 20 25 30 Coulomb load ( Ocm2 ) Figure 5-18 Threshold voltage and dead laye r thickness (calculated from figure 2-7). Note the fast initial growth rate (<5 C/cm2 ) and the slow rate afterwards. This type of growth is indicative of a reaction limited growth followed by a diffusio n limited growth. The growth may become limited by the movement of S through the dead layer as the CL degradation continues past 5 C/ cm2
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4 3 5 3 (I) 2 5 0 ; i,:i c:: 2 ::c C. C. , "Cl -=-i,:i -: 0 8 ~ l al C 0 6.E Cl CL Area .::: cc ...I 0 4 U Sulfur/Yttrium 0.2 0 5 10 15 20 25 3 0 Coulomb Load (C/cm2 ) Figure 5-19 Correlatio n betwee n AES APPH r atios and dead layer thickness as a f unctio n of coulomb load (C/cm2 ) 0/Y and S/Y APPH ratios decrease and stabilize in 5 C/cm2 Very good correlation between the dead layer thickness, the surface changes and the CL degradation are found. The generally smooth curve found for CL brightness is disprupted by the BV experiments conducted during the degradation. This indicates that the CL degradation is driven by the growth of surface phase.
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CHAPTER 6 BEAM ENERGY DEPENDANCE ON DEGRADATION RATES OF Y202S: EU3+ Introduction In the previous chapters, it has been shown that the CL degradation of Y202S: Eu3+ is dependent on type and partial pressure of gas in the vacuum. It is postulated that impinging molecular gas, like 02 striking the surface of the phosphor is dissassociated by the electron beam. Subsequently, the dissassociated gas molecule reacts with the surface of the phosphor. Initially C and Son the surface are reacted to volatile gas species which is carried away from the surface (C01 2 so1 2). The loss of C and S may initially be rate limited by the amount of reactive gas species available. Once a S depleted region exists near the 136
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137 surface, further S removal should be slowed by S diffusion through the thickening S deficient layer. The diffusion of S may be driven by chemical potential gradients near the surface due to initial S depletion and/or may be driven by electric field enhanced diffusion induced by the presence of the electron beam and the charge induced near the surface. An electron's ability to dissassociate gas is related to its kinetic energy [Eng94], assuming that the ionization cross-section is numerically equal to the disassociation cross-section. Classically, the ionization cross section decreases at energies greater than 200eV due to the reduction in interaction time with a gas molecule [Eng94] As the electron's kinetic energy decreases, its velocity decreases and the amount of time the electron spends in the vicinity of a gas molecule increases. This allows for an increased probability of ionization of the gas molecule. If CL degradation is limited by the amount of dissassociated gas available for reaction at the surface, it may be possible to observe a change in the rate of Sloss from the surface at different accelerating voltages (different dissassociation efficiencies). It is suspected that lowering the accelerating voltage of the electron will
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138 increase the amount of dissassociated gas available, thus increasing the rate of removal of S from the surface. Conversely, an increase in accelerating voltage will decrease the dissassociation efficiency, decreasing the amount of dissassociated gas at the surface, and decreasing the rate of Sloss as seen by AES analysis. This expectation may be simplistic due to other changes associated with lower accelerating voltage. The change in electron beam interaction volume will affect the CL degradation rate. Assuming that the conversion rate (Y2028 to Y203 ) is constant for all electron beam energies, an increase in the CL generation volume will produce a slower rate of CL degradation. Thus the CL degradation rate may be higher for lower accelerating voltages due to a smaller electron beam interaction volume and also due to the higher dissassociation efficiency of gas molecules. The diffusion of Smay be affected by a change in electric field strength near the surface and changes in surface temperature due to beam power, thus the effects of lower accelerating voltages on CL degradation are unclear. Three experiments were conducted in vacuum ambients backfilled with oxygen gas to 1x10-6 Torr. A 1, 2 or 3keV,
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electron beam will be used with nominal current densities between 100 and 400 A/cm2 1 keV Experiment 139 A base pressure of 1.sx10s Torr was achieved initially and oxygen backfilling raised the system pressure to 1.ox10-6 Torr. Due to limitations of the electron gun, a sample current density of 150a/cm2 was the maximum obtainable for the experiment. This limited the total coulomb load attainable in a 24 hour degradation experiment to -16 C/cm2 Figure 6-1 shows the APPH ratios for O/Y, S/Y, and C/Y and the CL brightness as functions of coulomb load (C/cm2). The CL brightness decreased nearly 60% during this experiment. This decrease is larger than that observed in previous 2keV experiments (typically 30-40% decrease in brightness). The O/Y APPH ratio appears to decrease steadily while the S/Y and C/Y APPH ratios decrease rapidly initially and then stabilize after -0.5C/cm2 Note that the S/Y APPH ratio has been shifted up on the y-axis by 1 unit to separate it from the C/Y APPH ratio. Figure 6-2 shows the initial (0.06C/cm2 ) and final AES spectra (14.1C/cm2 ) as a function of energy. Both C and Shave been removed from the
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140 surface and both peaks have decreased below the sensitivity of the AES spectrometer while the O and Y APPHs have increased during the experiment. The AES peak energy shifts of C and Scan only be examined early in the experiment (until -3 C/cm2). These results suggest a near-surface loss of C and Sas has been seen in the experiments reported in Chapter 4 and 5. The near-surface is an oxygen-rich, sdepleted material, most likely Y203 based on the conclusions of Chapter 5. Figure 6-3 shows the AES peak energy shifts (in eV) as a function of coulomb load (C/cm2). The trend line fits show that O and Y exhibit -1.5eV positive shift in energy during the experiment and the C and S energies shift negatively by -leV, again consistent with the results of Chapter 4 and 5. These shifts may be associated with changes in surface charge changes induced by the chemical modifications occurring at the surface. Figure 6-4 shows the CL spectral intensity (a.u) as a function of wavelength (nm) for the initial (0 C/cm2 ) and the final CL spectra (16 C/cm2). Only small changes can be seen on this graph (specifically the increase in emission at 611nm). The expanded spectra and the CL difference curves are shown in figure 6-5 for the 600-640 nm wavelength regime. The largest
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141 change is found at 611 nm, indicating an increase in the emission from Y2O3:Eu3+ Large peaks seen at 616, 618, 625 and 628nm, can be attributed to spectral broadening of the main emission peaks of Y2O2S: Eu3+ at 61 7 and 62 6nm as discussed in Chapter 5. Thi s may be caused by the S removal and densification to Y2O3:Eu3+ Large negative deflections at 626 and 616 nm show the loss in intensity exhibited by the Y2O2S: Eu3+ main emission peaks. 2keV Experiment For this experiment, the vacuum pressure prior to oxygen gas backfilling was 1. 3x10a Torr. After backfilling, the pressure was stable at 1.1x10-6 Torr. The electron beam current density was 265A/cm2 Figure 6-6 shows the APPH ratios for O/Y, C/Y and S/Y as well as the CL brightness as a function of coulomb load on the secondary y-axis. Comparing the AES ratios to the lkev ratios of figure 6-1, the S/Y and C/Y APPH ratios decrease more with coulomb load than the corresponding ratios at lkeV. The O/Y ratio exhibits a fast decrease (<1 C/cm2 ) and recovery (2C/cm2 ) and decreases slowly for the duration of the experiment. The S/Y and C/Y ratios decrease quickly and stabilize after
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142 -3C/cm2 By comparing these decreases with figure 6-7, which shows the initial and final AES spectra as a function of energy, it is clear that Chas dropped below the AES sensitivity after 3 C/cm2 Figure 6-7 shows relatively large amounts of C and Son the surface in comparison to the lkeV experiment. The C peak is very well defined, whereas in figure 6-2 it is only slightly above the noise level. Total CL degradation is estimated to be 45 percent at 15C/cm2 in figure 6-6 which compares to 60% degradation at 15 C/cm2 for the lkeV experiment (see figure 6-1). This indicates that CL degradation is more severe at lower accelerating voltages. This could be due to the smaller beam interaction volume and/or due to the increase in dissassociation efficiency. The initial and final AES spectra shown in figure 6-7 shows that the 0, Y and Speaks all exhibit a small negative energy shift. The peak energy shifts and associated trend lines and equations are shown in figure 6-8 as a function of coulomb load (C/cm2). Unlike previous experiments, which showed positive Y and O peak shifts, Y and O peaks shifts were negative during this experiment. This may be caused by the loss of the relatively good conducting layer of C, which initially was able to reduce surface charging induced by the
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143 electron beam. Once the C is removed, the phosphor would charge and the energy of the AES electrons would be changed accordingly. A decrease in AES peak energy indicates a less negative charge near the surface since all peaks are shifted above the expected AES values initially. Figure 6-9 shows the initial and final CL spectra as a function of wavelength in nm. The spectra are very similar and the differences can only be seen in figure 6-10, where the expanded spectra and CL difference are shown. Peak intensity increases are found at 611, 616, 620, 625 and 630 nm. The 611 nm peak is attributed to the oxide growth. The (616,620) and (625,630) nm peaks may be caused by spectral broadening of the Y2O2S:Eu3+ emission peaks, indicating local symmetry changes around the Eu3+ ions. 3keV Experiment During this experiment, the current density was set at 37lA/cm2 The vacuum pressure was l.8x10s Torr prior to backfilling with oxygen gas at 1x10-6 Torr. Figure 6-11 shows the APPH ratios for O/Y and S/Y. No C was present on the surface at the beginning of the experiment (within AES detection limits). As is typical for oxygen gas backfill
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144 experiments, the O/Y ratio decreases in a non-linear fashion (until -5-10 C/cm2 ) then continues to decrease slowly for the duration of the experiment. The S/Y ratio shows that S leaves the surface by -4 C/cm2 The rate of removal of Sis slower than that observed in the lkeV experiment, but the difference between the 2keV and 3keV rates are more subtle (compare figure 6-1,6-6, and 6-11). The most drastic change, in comparison with the previous two experiments, is seen in the CL brightness plotted as a function of coulomb load. The total CL decrease during the 3keV experiment is only -10%. Comparing the CL degradation at 15C/cm2 the percent losses are 60%, 45% and 5% for lkeV, 2keV and 3keV, respectively. As the accelerating voltage is increased, the excitation volume is increased, and the dissassociation efficiency of the electrons is decreased, thus the CL degradation is slower. Figure 6-12 shows the initial (0.08 C/cm2 ) and final (27C/cm2 ) AES spectra as a function of energy in eV for the 3keV experiment. Notice that the C signal is below the detection limit of the AES spectrometer at the beginning of the experiment. A significant S signal is present at the onset but has fallen below the noise level by 27 C/cm2 Y and O APPHs increased slightly during the experiment. Auger
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145 peak energy shifts in figure 6 -12 are apparent on only the Y, but the peak energies shown in figure 6-13 as a function of coulomb load (C/cm2 ) show that both the O and Y peak energies exhibited positive shifts (to higher energies) while the Speak energy decreased. The shifts in energy are 7.2, -1.0 and 4.6 eV respectively for 0, Sand Y at 20 C/cm2 This is consistent with a negative charge buildup on the surface of the phosphor. The initial and final CL spectra are shown in figure 6-14 as a function of wavelength (nm). Large changes can be discerned without expanding the graph and determining the differences between the two spectra which are shown in figure 6-15. Peaks have increased in intensity at 611 and 630nm (Y2O3:Eu3+ ) and 618 and 627nm. The 616,618 and 625,627nm peaks are attributed to broadening of the main emission peaks of Y2O2S:Eu3+ The 618nm peak could also be the main emission peak of Y2O2SO4:Eu3+ as was seen in Chapter 4, but the similarity to the positive deflections seen on the 627 nm peak coupled with the low luminous efficiency of Y2O2SO4:Eu3+ leads the author to believe that both peaks are caused by spectral broadening induced by local changes occurring at Eu3+ emission sites.
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Peak decreases are noted at the emission wavelengths for Y202S :Eu3+ (616 and 626 nm). Discussion and Comparison of 1.2 an 3 Kev Data 146 The CL degradation rates versus coulomb load (C/cm2 ) for the 1,2 and 3 keV experiments are shown in figure 6-16. The CL degradation rate is higher at lower accelerating voltages. At lower accelerating voltages, the degradation curve takes on an exponential decay (lkeV), whereas at higher accelerating voltages (3keV) the CL degradation appears nearly linear. The larger excitation volume at 3keV seems less affected by the surface conversion of Y202S to Y203 A comparison of the rate of removal of Scan be seen in figure 6-17 as a function of coulomb load. The S/Y ratios are depicted for the 1, 2, and 3KeV experiments. Only the very beginning of each experiment is shown (<2C/cm2 load). As expected, the rate of Sloss is highest for the lkeV experiment. The S/Y ratio has fallen to the noise level by -0.2 C/cm2 whereas the S/Y ratios for the 2keV and 3keV experiment decrease and stabilize at -0.8 C/cm2 It is interesting to see the effect of Con the removal rate of S
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147 in the 2keV experiment. Note that the Sloss rate is slower at 2kev then it is at 3keV. This does not follow the expected pattern for dissassociation efficiency, but upon examining the initial conditions of the 2keV and 3keV surface (figure 6-7,6-12) it is seen that a large amount of C existed on the surface during the 2keV experiment, whereas no C was seen on the 3keV surface. The volatilization reactions for Care similar to those for S, and in the presence of C, the S desorption rate would be lower due to the competing process of C volatilization. This can actually be seen where the initial loss of Sat 2keV is slow (0-0.3 C/cm2), but once the amount of C (as seen in Figure 6-6) has decreased, the rate of Sloss increases (-0.3 C/cm2). Thus the presence of Con the surface effectively slows the S loss rate by acting as a competing process for reaction with dissassociated gas species.
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148 Depth Resolved CL studies of Y 2 Q 3 : Eu3+ Emission Previously, it has been shown that the near surface conversion of Y 202S: Eu3+ to Y 203 : Eu3 causes a spectral redistribution in the CL spectra. In order to substantiate further the existence of the Y203 : Eu3 surface after electron beam aging, depth resolved CL studies were used after the 3keV experiment discussed previously in this chapter. By varying the accelerating voltage of the electron gun, it is possible to excite consecutively greater depths in the phosphor. In order to be sure that the area examined was within the aged spot, the beam was refocused within the aged spot and then CL spectra were taken at energies of 300V to 5000V with the current density set at 176A/cm2 The refocusing of the electron beam alters the CL spectra greatly. Since the beam energy exhibits a gaussian dependance on distance from its center, the highest energy density is found near its center. By narrowing the electron beam, the CL spectra produced comes from the very center of the larger degraded spot. Figure 6-18 shows the change in the wide spot 3keV spectra and the narrow spot 3keV spectra. The narrow spot is dominated by the 611 nm emission of
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149 Y203:Eu3+ This indicates that the conversion occurs at the highest rate where the beam energy density is highest (near the center of the wide spot). The use of the normalized intensity, coupled with the narrowing of the beam accentuates the oxide emission by reducing the amount of Y202S:Eu3+ sampled in the narrow beam. Figures 6-19 through 6 -26 show the evolution of CL spectr a as a function of wavelength (nm) for increasing accelerating voltages. The lower voltage graphs (400,500 and 1000 V) in Figures 6 -19, 6 -20 and 6 -21, respectively) are dominated by the oxide spectrum (main emission 612nm) with only a small contribution from Y202S: Eu3+ ( 62 6nm) This indicates, a nearly complete surface conversion of Y202S: Eu3+ to Y203 : Eu3+ As the accelerating voltage is increased from 1500 to 4000 V (figures 6 -22 to 6-26) the Y202S:Eu3+ emission peaks increased i n intensity at a faster rate than the Y203 : Eu3+ peaks. Thus as the electron beam probes deeper, the relative contributions from Y202S: Eu3+ versus Y203 : Eu3+ i s increasing. This is consistent with a luminescent oxide layer at the surface. It is difficult to quantitate the depth to which the conversion took place because the phosphors efficiencies a r e different ( 8 % for Y203:Eu3+ and 13% for Y202S:Eu3+). Also,
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150 since the Y203 formed can only contain the Eu activator concentration of the original Y202S (4 atomic%), the Y203 efficiency will be lowered further. The maximum CL efficiency is observed with a Eu concentration of -10 atomic % for Y203:Eu3+ [Oza90]. The low voltage spectra do verify the presence of the Y203 : Eu3+ at the near surface and confirms the presence of Y202S: Eu3+ below the formed Y203 : Eu3+ Figure 6 -27 shows the complete evolution of Y203:Eu3 and Y202S: Eu3+ emissions in a single graph. Note the increase in Y202S: Eu3 emission at the higher accelerating voltages (2keV and above). Summary Degradation experiments with Ep=l,2 and 3 keV show that the rate of CL degradation is faster at lower accelerating voltages, but Y203 : Eu3 dominates the CL spectra more at 3 keV. Higher dissassociation rates of gas molecules for lower accelerating voltages increases the amount of available, reactive gas at the surface which rapidly react to form volatile C and S containing products. The sulfur desorption rate is slowed by the high concentrations of Cat the
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151 surface of the phosphor. In the presence of oxygen gas, the conversion of Y202S: Eu3+ to Y202804 : Eu3+ could not be verified, but the presence of Y203 : Eu3+ CL emission peaks indicate that the surface has been chemically altered and a luminescent form of Y203 : Eu3+ is the product of the electron beam stimulated surface reactions. It is possible that the Y202804 : Eu3+ emission intensity is too weak to measure, or that it is a metastable phase in the presence of oxygen gas. Confirmation of the oxide surface has been found by using depth resolved CL studies.
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3 5 ---------------........ 3 2.5 2 -.;= a: 1 5 :J: C. c.. 0.5 0 xyge n/Yttrium Sulfur/Yttrium CL Brightness Carbo nlYttri um 152 0 9 0 8 = 0.7 0 6 I (I) 0 5 0 4 -~ cc Q.3 ...J 0 2 u 0 1 0-----------------0 0 5 10 15 20 25 Coulomb Load (C/cm2 ) Figure 6-1 APPH Ratios and CL Brightness as functions of coulomb load (C/cm2 ) for the lkV accelerating voltage experiment. Note the fast 30 removal of C and S. Also note the total loss in brightness of the lkeV experiment, nearly 60%. The S APPH ratio has been shifted up by 1 unit to separate it from the C/Y APPH ratio.
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153 2000 ...-------------------1000 -:--1000 _, -2000 C. C. < -3000 -4000 -5000 Final Spectrum I Oxygen -6000 ...__ ________________ ___, 30 Energy (eV) 600 Figure 6-2 AES spectra taken at 0.06 C/cm2 and 14.1 C/cm2 for the lkeV accelerating voltage experiment. Note the loss of Sand C during degradation, and the slight increases in O and Y peak heights. A small, positive energy shift can be seen on the oxygen peak.
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154 600 y = 0 .1026x + 520.35 500 Oxygen Peak Energy $' Cl) ->. 400 =i ... Cl) y = -0.3086x + 271.92 C w 300 Carbon Peak Energy Cl) C. 200 =i ... < Sulfur Peak Energy 1 0 0 Yttrium Peak Energy y = 0.0875x + 85.038 0 0 5 10 15 20 25 30 Coulomb Load (Clcm2) Figure 6-3 0, C, S, and Y peak energy shifts during coulomb aging with lkeV primary beam. Note that the Y and O slopes are positive, indicating an increase in peak energy during the experiment, while the Sand C peak energies shift negatively (denoted by negative slope of trendline equation).
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:: b 'iii C Cl) .... C -..J u E C 1 2 ....---------------1 1 0 9 155 -0.7 =: 0 8 0 6 Final Spectrum 0 4 0.2 0 -0 5 b 'iii 0.3 ; 0.1 .... C ..J u -0. 1 C -0.3 u: -0.5 -0. 2 ........_ ________________ -0.7 500 550 600 650 700 750 Wavelength (nm) Figure 6 4 Initial (0 C/cm2)and final CL spectrum (16 C/cm2 ) for the lkeV degradation experiment in oxygen. The only noticeable difference is the slight increase in the 611 nm peak, suggesting the presence of Y203 : Eu3+ at the end of the experiment.
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156 1 2 0 .06 611 616 618 0 .04 -=-=-0 8 . 0.02 --.? 0.6 Q) u C i,, C Q) ... C .J u 0 Q) .. 0 4 628 0.2 ,.-0.02 i5 .J Final 1111 u '~ -0.04 0 -0. 2 -0.06 600 605 610 615 620 625 630 635 640 Wavelength (nm) Figure 6-5 Expanded CL spectra (initial and final) and the CL difference spectra as function of wavelength. A large increase in the 611 nm peak indicates the presence of Y203 : Eu3+ Peaks at 616,618 and 625,628 are deflections associated with spectral broadening of the main emission peaks of Y202S:Eu3+ Large negative deflections at 617 and 626 nm indicate loss in emssion intensity for Y202S: Eu3+ caused by aging during the 1 keV experiment.
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157 3 1 0 9 2 5 0 8-::--=0 xyge n/Yttrium _, _, 0.7 2 --0.6 I 0 CL Brightness ; Q) 1.5 0 5 S c::: .c :::c 0.4 -~ C. 1 cc C. 0.3 ...J Carbon/Yttrium 0 5 Sulfur/Yttrium 0 2 u I 0.1 0 0 0 5 10 15 20 25 30 Coulomb Load (C'cm2 ) Figure 6-6 APPH ratios for O/Y, C/Y, and S/Y as a function of coulomb load (C/cm2 ) for 2 keV, 1. 1x10-6 Torr 02 experiment. Also shown is the CL brightness as a function of coulomb load. The S/Y and C/Y APPH ratios decrease slower than those in the lkeV experiment (see figure 6-1). CL degradation is approximately 45% at 15 C/cm2 whereas, at 1 keV the CL degradation was approximately 60%.
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3000 2000 1000 -=-0 ::c C. c. -1000 <( -2000 -3000 -4000 Yttrium 30 Final Scan Initial Scan Carbon Sulfur+ Yttrium 200 Energy (eV) 400 Oxygen 600 Figure 6-7 Initial and final AES spectra for the 158 2 keV degradation experiment. Notice the large amount of C and S present on the surface prior to aging. It is possible that the large amount of C initially present on the surface protected the phosphor from degradation up to 3 C/cm2 Peak energy shifts are visible on the O and Y peaks.
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600 500 > Cl) ;,: 400 CS) .. Cl) C w 300 Cl) c. 200 (/) U.J cX 100 0 y = -0.0254x + 520.97 y = -0.1843x + 284.23 L.o 1 A h t ~ 1 .. .. y = -1.2274x + 163.75 l o u .. r u . .. V . y = -0.0335x + 91.87 L 0 5 159 Oxygen Peak Energy Carbon Peak Energy Sulfur Peak Energy . '" Yttrium Peak Energy L I I 10 15 20 25 30 Coulomb Load (C/cm2 ) Figure 6 8 AES peak energy shifts exhibited during the 2keV aging experiment. All peaks shift negatively. Thi s i s unlike previous experiment s where the O and Y energies shifted positively.
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-=-. ; C a) .... C ..J u E C 160 1.2 -r-------------------~ 1 2 0 8 -=-0 8 -0 6 0 6 ; 0.4 C a) Final Spectra .... 0 2 C 0.4 ..J u 0 0.2 C -0. 2 t! 0 -0. 4 -0. 2 -0. 6 500 550 600 650 700 750 Wavelength (nm) Figure 6 9 Initial and final CL spectra as a function of wavelength (nm) for the 2keV aging experiment. The spectra are very similar on full scale. Small changes exist near 611nm.
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1 6 1 1.2 0.25 616 625 0 2 0 .15 0 8 626 -=-=I nitial 0 1 .. -(0 0 6 Final 0 .05 Q) CJ 620 C i C Q) .... C ..J u Q) 0 4 0 Q 0 2 -0.05 d -0. 1 0 6 0-0.15 6 0 0 2 -0. 2 Wavelength(nm) Figure 6-10 Initial and final expanded CL spectra and CL difference spectra as a function of wavelength (nm) Decreases on the CL difference spectr a at 616,620 and 625,630 nm are attributed to spectral broadening of the main Y202S: Eu3+ emission peaks which have decreased in intensity (617 and 626 nm). The 611 nm peak denotes the increase in emission from the Y203 : Eu3+ surface phase.
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--I r 162 4 ~-------------.1.2 3.5 3 Oxygen/Yttrium .1 .. I -, 2.5 0 i 2~ . -.0.9 S a: :c 1.5 .... C. CL Brightness .c C, 0.8 C. 1 cX I I ..J Sulfur/Yttrium .. 0.7 u 0 5 10 15 20 25 30 Coulomb Load Clcm2 Figure 6-11 APPH ratios and CL brightness as a function of coulomb load (C/cm2). Note that no C/Y ratio is present. No C was detectable on the surface of the phosphor at the beginning of the experiment. The 0/Y APPH ratio decreases throughout the experiment. The S/Y ratio decreases to a level below AES detection in -4 C/crn2 The removal rate of S from the surface is slower than that found for the lkeV experiment. The CL brightness decrease at 3keV is very small (-10% loss)
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163 3000 4000 2000 Initial Spectra 3000 \ 1000 2000 -=--=-""" .... 1000 """ 0 0 ::c -1000 ::c C. C. C. -1000 c. cX -2000 \ cX --2000 -3000 E -3000 .: C I.L -4000 Yttrium Oxygen -4000 -5000 Final Spectra -5000 -6000 -6000 Energy (e\J) Figure 6-12 Initial and final CL spectra for the 3 keV degradation experiment. Note that no measurable C existed on the surface of the phosphor at the beginning of the experiment. The S APPH h a s f allen belo w the AES d etectio n limit by 27 C/cm2.
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164 600 . 500 y = 0.3654x + 531.79 Oxygen Peak Energy 400 >i C) ... Q) C w (Ii:! w C. 300 -200 y = -0.0858x + 157.27 --Sulfur Peak Energy 100 --y = 0 .2356x + 93.206 Yttrium Peak Energy 0 I I I I I 0 5 10 15 20 25 30 Figure 6-13 Auger peak energy shifts as a function of coulomb load for the 3 keV degradation experiment. 0 and Y peak energies increased by 7 eV and 4.6 eV respectively, at 20 C/cm2 Speak energy shifted slightly negatively as denoted by the trend line equation leV at the end of its measurability (-10 C/cm2 )
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1 2 0 8 _, (w:I i 0 6 'iii C Q) .... C 0.4 ...J u 0 2 0 5 0 -0. 2 165 617 / 630 Final Initial 600 650 700 7 0 Wavelength(nm) Figure 6-14 Initial and final CL Intensity distribution as a function of wavelength (nm) for the 3 keV degradation experiment. Only a small increase is seen at the 611 nm Y203:Eu3+ nm. The 617 and 626 nm em ission is attri b uted to Y202S: Eu3 +
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1 6 6 1 2 0 4 627 618 0.3 611 0 2 ~ 0 8 630 -=-\ _, -0 1 Q) (.) ~ 0 6 C Q) ... Q) .. 0 C 616 ..J Q u 0 4 ..J Final Spectrum'-4 -0. 1 u 0.2 -0. 2 \:::,....0 -0.3 600 605 610 615 620 625 630 635 640 Wavelength(nm) Figure 6-15 Expanded CL spectra for the 3keV degradation experiment. The CL difference is shown a s w ell on the seco n dary y-axis (right). The 611 nm p eak d enotes the C L emission of Y203:Eu3+ The 617nm and 6 2 6nm peaks are Y202S : E u3+ emission peaks that have d ecreased during d e gradation.
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167 3 keV = 0 8 2 keV I 0 6 .. en : cc ...J u 0.4 1 keV 0 2 0 -------------------+-----------. 0 5 10 15 20 25 3 0 Coulomb Load (Gcm2 ) Figure 6-16 Comparison of CL degradation rates for 1, 2, and 3keV accelerating voltage experiments. The The dashed line shows the 15 C/cm2 comparison point. As expected, lower accelerating voltages produce faster d egradation due to decreased excitation volume and increased disassociation efficiency for gases impinging on the surface.
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168 C+S competing 0 9 0.8 0.7 ; 0 6 ::c c. 0.5 C.
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1 2 1 i' 0 8 C _, 0 u 0 6 i;; C 0 4 Q) C ..J u 0 2 0 5 0 -0.2 169 / Initial Wide Beam 611 Final Narrow Beam 600 650 700 Wavelength (nm) Figure 6-18 Comparison of 3keV Wide and narrow beam spectra. The narrow beam samples a small region within the degraded area. The dramatic difference in the two spectra indicate that the Y203 : Eu3+ formed is centered within the larger spot where the energy density of the electron beam is highest.
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1 2 -,------------------1 -:0.8 (ti :; 0.6 'iii C 0 4 a, .... C 0.2 612 626 -0. 2 ...,__ ________________ __J 605 610 615 620 625 630 635 640 645 Wavelength(nm) I-400 V Intensity I 170 Figure 6 -19 400 V CL Intensity spectra as a function of wavelength (nm) after the 3 keV degradation experiment. The spectra is dominated by the Y203:Eu3+ spectra (612 and 630 nm). The 626 peak is the main emission peak of Y202S:Eu3+ indicating only a very small contribution.
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= 'iii C a, .... C 171 1 2 612 1 0 8 0 6 0 4 626 6 3 0 0.2 v 1 0 -0.2 605 610 615 620 625 630 635 640 645 Wavelengt h(nm) 1--500 V CL Intensity I Figure 6-20 500 V CL Intensity as a function of wavelength(nm) after 3keV degradation. The peaks 612 and 63 0 are associated with Y203 : Eu3+ emission. The 62 6 and 61 7 peaks are the main emission peaks of Y202S: Eu3 + Very little Y202S :Eu3+ is found from the surface to thi s depth ( -sooA)
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172 1 2 612 1 -=0 8 (ti 0.6 b ;; 0 4 C Q) 626 ... C 0.2 //\\._~/ 0 6 5 610 615 620 625 630 635 640 6 5 -0.2 Wavelength(nm) 1000 V Intensity I Figure 6-21 1000 V CL Intensity as a function of wavelength (nm) after the 3kev degradation experiment. Only small contributions are apparent from the 617 and 62 6 nm emissions of Y202S: Eu3+ The spectra i s dominated by the 612 nm emission of Y203 : Eu3+
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173 1.2 612 1 -0 8 :: 0 6 t; C Q) ... C ...J u 0 4 626 0 2 0 / f\ _ 6 5 610 615 620 625 630 635 640 6 5 -0. 2 Wavelength(nm) l--1500V Intensit y j Figure 6 -22 1500 V CL Intensity as a functio n o f wavelength in nm for the 3 keV degr a d ation experiment. Notice that the 62 6 nm em ission of Y202S: Eu3+ i s beginning t o increa s e in intensity.
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-=-... llO b '1ii C a, .... C 1 2 -r-----------------612 0 8 0.6 626 0 4 617 0 2 \,_ G 0 6 5 610 6 1 5 620 625 630 635 640 6 5 -0.2 Wavelength(nm) I2000V Intensity I Figure 6-23 2000 V CL Intensity as a function of wavelength (nm) taken after the 3 keV degradation experiment. 174
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1.2 1 612 -0.8 ""' 626 (ti i 0 6 -~ 0.4 617 Q) .... 0.2 \ .f J .....,,. ....,... 0 -0.2 605 610 615 620 625 630 635 640 Wavelength (nm) 3000 V Intensity I Figure 6-24 3000 V CL Intensity Spectra as a function of wavelength (nm) Note that the Y202S: Eu3+ main emission peak is lower in intensity than is the 612 nm peak of Y203:Eu3+ The spectra was taken after the 3keV degradation experiment in oxygen. 175
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176 1.2 612 1 0 8 626 (IO -~ 0.6 11,1 I C a) .... C 617 0 4 0 2 '-.-) \"0 605 6 1 0 615 620 625 630 635 640 645 Wavelength (nm) 4000 V Intensity I Figure 6-25 4000V CL intensity as a function of wavelength (nm) The oxysulfide emission (626nm) i s nearly equal to the oxid e emission (612nm) The spectra was taken after the 3keV d e gradation experiment.
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1 2 -r--------------------~ 0 8 0 6 ti) C Q) 0 4 0 2 612 626 617 '\ __ _/' 0 :t:=:=------4-----+---+------+-----+--____:::::=:ii:::::::==='605 610 615 620 625 630 635 640 Wavelength(nm) 1-5000 V Intensity I 177 Figure 6-26 Normalized CL Intensity as a function of wavelength (nm) for the 3keV degradation experiment. Note that the Y203 : Eu3+ peak ( 612nm) is equal in intensity to the 626 nm peak of Y202S:Eu3+
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0 8 -:0.6 _, -l" 'vi C Q) ..... C 0 4 ..-""-t' ..-ro ..-(j) 0 C.D ..-..-I'-... ..-C.D C.D ..-("\I C.D C.D Wavelength(nm) ..-(j) ""-t' I.[) C.D (T) 0 (T) (T) C.D C.D 626 5000 4000 3000 2000 1500 1000 500 400 Figure 6-27 3-D surface representation of CL intensity as a function of wavelength (nm) and accelerating voltage (V). It is 178 easy to see that the low accelerating voltage regime is dominated by the oxide (Y2O3 :Eu3+ ) spectrum (611nm). Only at higher accelerating voltages does the Y2O2S: Eu3+ intensity (626nm) approach the oxide emmission intensity (-4000V).
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CHAPTER 7 MODELING OF THE CL DEGRADATION OF Y202S: EU3+ Introduction In the preceding chapters, it has been shown that the CL degradation of Y202S: Eu3 is dependent upon many parameters such as electron beam energy, ambient gas pressure and type of gas. Each of these parameters was studied independently, and in each case, was found to be related to the CL degradation. In this chapter, some rudimentary models will be used to better understand the effects these parameters have on the CL degradation. In so doing, the calculations may indicate which of these factors is most important in understanding and semi-quantitatively predicting the degradation phenomena. 179
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180 To better understand the CL degradation process, and knowing that the CL degradation is correlated with the growth of the dead layer during the aging process, modeling of the brightness as a function of depth for the phosphor may prove invaluable in understanding CL degradation. Since the dead layer growth is dependent on the pressure, type of gas and dissassociation efficiency of the gas at particular accelerating voltages, some kinetic modeling of gas-surface interactions is necessary. Using Makhov's law (see Chapter 2), a relationship will be obtained between the brightness versus depth for the phosphor and the power dissipated by the electron beam as a function of depth. The brightness will be modified by a model for the CL efficiency as a function of depth for Y202S:Eu3 after taking into account the activator concentration distribution and concentration quenching phenomena. The model's accuracy will be determined by comparison to CL degradation rates reported in Chapters 4-6. A second model will be used to estimate the amount of dissassociated gas available at the surface of the phosphor for reaction. This model will incorporate the dissassociation efficiency of the gas at a given
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181 accelerating voltage, gas pressure, temperature, and the heat of adsorption of the gas on the surface. By varying the parameters in this model (accelerating voltage, temperature, pressure, and heat of adsorption), a better understanding of the surface kinetics associated with the electron beam stimulated reaction will be attained. Both models together will be used as a guide to consider ways by which the CL degradation of Y202S:Eu3 can be slowed or stopped. Modeling of Brightness as a function of Depth In order to understand the relationship between CL degradation, electron kinetic energy and dead layer thickness, a model relating the brightness of the phosphor as a function of depth to these parameters is required. In Chapter 2, Makhov's Law was discussed which relates the CL brightness as a function of depth to the product of CL efficiency at a given depth and the power dissipated by the electron beam at that depth. Equation 2.6 was expressed as follows:
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182 dB dP =p(x) dx dx (7.1) 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) i s assumed to be constant for all depths. Thus, in order to obtain an expression for dB/dx, functions must be modeled for p(x) and dP/dx. Makhov's Law does not take into account absorption of light, nor does it consider the effects of the diffusion of carriers on CL brightness. CL Efficiency as a function of Depth In most studies, p(x) is assumed to be constant at all depths. In many cases, this is acceptable since the activator concentration is a constant as a function of depth, and all activators are equally efficient emitters. Both of these assumptions are incorrect for the industrially processed Y202S:Eu3 phosphor. Firstly, the activator concentration (Eu3 ) is not a constant as a function of
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183 depth. Instead, it has been observed experimentally to have a high surface concentration and a lower constant concentration after some depth into the particle as reported by Ozawa [Oza91]. Figure 7-1 shows the Eu concentration normalized to the nominal concentration as a function of depth. The Eu distribution as a function of depth was determined by etching the phosphor particles, measuring the weight loss to determine the thickness of the layer removed, then using ICP to determine the amount of Eu in the etched material. The bulk of the particle does have the expected Eu concentration (equal to 1 on the y-axis), but in the near surface region (<0.Sm), a concentration gradient leads to higher surface concentrations. The importance of the high surface concentration is easily understood when it is correlated with the penetration depths of low energy electrons that are of interest in FEDs (.3m-1.Sm). It i s also important to the present study since a 2keV electron beam was used for the AES and CL measurements. The maximum range for these energies is only about lm which means that up to 50% of the excited volume does not have the expected activator concentration. Thus, the assumption that the activator concentration is a constant as a function of depth
PAGE 194
is generally not accurate for the energy regime of this study. 184 a mathematical fit of the normalized Eu concentration as a function of depth has been obtained using the data found in literature (shown in figure 7-1) and can be expressed as follows: Eu (x) = 2 ( 1012) x 1.5672+1 (7.2) where Eu(x) is the normalized Eu concentration at the chosen depth x. With this expression, the fractional Eu concentration can be determined at any depth over the range of the primary electron. Since the Eu3+ concentration is not constant, the fractional distribution of Eu throughout the range of an electron will not be linear. The fractional distribution is the total amount of Eu contained within a fixed depth measured from the surface of the particle, normalized to the total Eu concentration contained within a chosen primary electrons range). This was examined for primary electron energies of 500, 1000, 2000 and 4000 eV and the fractional
PAGE 195
185 Eu concentration as a function of depth for these energies is shown in Figure 7-2. The graph shows that there exists an initial sharp increase in the total fraction of Eu3+ and then tapers off. The fractional amount of Eu3+ contained in this region is larger for lower accelerating voltages. If only the Eu3+ concentration determined the CL brightness emitted by the phosphor, the depths required to reduce the CL brightness by 50% would decrease with decreasing accelerating voltage. For instance, a phosphor with a dead layer of 0.017m on the surface would exhibit about a 50% decrease in CL brightness using a S00eV primary electron beam, whereas the same dead layer would produce only a 12% CL brightness loss at 4keV. Deriving the CL efficiency from the total Eu concentration curve is a rough first approximation. However, the phosphors brightness is known to depend on the amount of Eu3+ in the phosphor. For instance for Y2O2S:Eu3+ the optimum Eu3+ concentration for CL is about 4 atomic %[Oza91]. If the activator concentration is too high, the Eu ions are too close and electron wave overlap which reduces the overall brightness (known as concentration quenching). If the concentration is too low, the CL brightness is linearly
PAGE 196
186 reduced. It can be seen from figure 7-1 that the Eu concentration is higher than the optimum concentration near the surface (where Eu/Euinner=l, the Eu concentration is 4%). This high concentration, coupled with the concentration quenching effect, may actually reduce the near surface CL brightness and lead to a ubuilt-in" dead layer where the brightness output of the phosphor is low due to concentration quenching near the surface. The concentration quenching curve for Y202S:Eu3+ is shown in figure 7-3 [Oza90] where the relative CL brightness is shown as a function of Eu mole fraction. In summary, the CL efficiency for Y202S:Eu3+ typically assumed to be a constant in previous studies, should consider the Eu concentration as a function of depth as modified by the concentration quenching factor. The value of the concentration quenching factor at a given depth can be obtained from the Eu concentration at that depth. Power Dissipation as a function of Depth in Y 2 Q 2S:Eu3+ The second term in Makhov's equation to be consideration is the power dissipated as a function of depth. Starting with the assumptions of chapter 2 (equation
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187 2.1 modified by equation 2.4), the modified Bethe equation proposed by Joy and Luo will be used to obtain the energy dissipated by an electron as a function of depth. From this, the power dissipated as a function of depth will be obtained and is given by: de ds -785pzln( 1.166e). Ae j1 ( 7. 3) The modified Bethe equation describes instantaneous energy loss by an electron as a function of depth. a more detailed look at the Bethe equation and the modified Bethe equation is given in Chapter 2 (equation 2.1 and 2.4). Makhov's law requires an expression for instantaneous power as a function of depth. This can be obtained knowing the electron's kinetic energy at a given depth and by multiplying (ev/A) by the electron velocity (A/s) at that depth (determined by its kinetic energy). Figure 7-4 shows a plot of the power dissipated by an electron as a function of depth for 1-5 keV initial primary electron energies. The power dissipation curves are nearly flat, increasing
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188 slightly towards the end of range and then tailing off to zero. These power calculations assume a single electron, which is inelastically scattered perpendicular to the surface after each scattering event, and that the electron has lost all its energy at the maximum range for a given initial kinetic energy. a better approximation in future work could be obtained using a Monte Carlo simulator. Predictions of the Brightness Model Combining the Eu concentration as a function of depth, concentration quenching (function of Eu concentration) effect, and the power dissipation curves allow the formulation of a model of the brightness emitted as a function of depth for Y202S:Eu3+ The brightness at a given depth in this model is equal to the power dissipated at that depth, multiplied by the Eu concnetration at that depth, multiplied by the concentration quenching factor determined from the Eu concentration. The total brightness can be obtained by integrating over the entire range of the primary electron.
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189 Figure 7-5 shows the brightness (a.u) as a function of depth for primary electron energies from 1 to Skev. Three regions are observed on these curves. a large peak close to the surface (<0.lm), which is attributed to the interplay between the high Eu concentration and concentration quenching curve at high concentrations, a flat mid-section (not observed at 1 or 2 keV) where the Eu concentration and power dissipation curves are essentially constant, and a tail off region at the end of electron range. The curves are very interesting because they suggest that the CL degradation rate should not be similar for different accelerating voltages. The 1 and 2keV curves exhibit no flat mid-section. If a growing surface dead layer grew at a rate versus coulomb load which was independent of VP, the CL degradation curves would look very different for 1 versus 5 keV electrons. Much of the brightness obtained at 1 or 2keV is from near the surface, and the CL degradation rates would be higher because of the smaller volume. The 3keV and higher primary energies have a larger fraction of brightness generated from depths below the initial brightness hump and thus would not be so strongly affected by the consumption of that region by a non-luminescent layer.
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190 In order for a model to be useful, it must approximate real events. Total CL degradations have been calculated by the model using the experimentally measured threshold voltages from previous chapters. The background experiment of chapter 4 showed a 4% loss in CL brightness at 2keV. The threshold voltage changed from 539V to 640V during the experiment. The cumulative loss in brightness as a function of depth has been calculated and is shown in Figure 7 6 for 1,2, and 3kev primary electron energies. By normalizing to the brightness with a 539V dead layer and integrating to the end of range, the% brightness loss can be calculated from the model at 2keV. It is found to be 6%. The model is in very good agreement with the experimentally determined degradation value (-4%). The 1 and 3 keV cumulative brightness fractions are meant for comparison. As expected, the 1 keV degradation would be larger than the 2 keV with the 3 keV loss the smallest. a second experiment, the time dependent threshold voltage experiment of Chapter 5 (see figure 5-17) was used to determine the validity of the model for experiments in oxygen. The experimental threshold voltage values found for the unaged and aged spot were 166V and 866V ,respectively, and the total CL degradation was
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191 about 40%. The theoretical loss calculated using the model, shown in Figure 7-7 for a 2keV beam, is 45%. The theoretical prediction may have overshot by 5% because it does not take into account the fact that the converted surface layer can actually be another luminescent material (Y203:Eu3+). Thus in both cases, the model is accurate for Y2028:Eu3+ CL degradation. Modeling of Gas-Surface Interactions of Y2Q2S:Eu3+ Introduction The results of chapters 4-6 have demonstrated that CL degradation is dependant upon the gas pressure and the type of gas which interacts with the surface of the phosphor. It has been postulated that the CL degradation is initially rate limited by the amount of reactive gas available at the surface, and as the surface concentration of S decreases (and the dead layer thickness increases), the reaction becomes rate-limited by the diffusion of Sand/or O through the dead layer. It may be possible to confirm these postulates by quantifying the amounts of reactive gas
PAGE 202
192 available at the surface. It has also been suggested that the dissassociation efficiency of a gas by an electron is sensitive to the electrons kinetic energy. This also can be examined by a model which relates the gas pressure and kinetic energy of the electron to the amount of dissassociated gas found at the surface. Gas Impingement On the Surface As a f irst step, the determination of the amount of gas striking the surface needs to be examined. The impingement rate for an ideal gas is defined as the number of molecules striking a unit area in one second and is given as follows: ImpingementRate(P,m,T)=3.5(l02 2) [ P ] (mT) s ( 7. 4) where Pis the pressure of the gas of interest in Torr, mis the gram molecular weight of the gas molecul e and Tis the temperature of the gas. In this case, the gas is 02 (m=lS.99 g/mole) and the pressure of the oxygen can be estimated knowing the initial system pressure and the oxygen backfilled pressure. Typically, the system pressure was near 1x10a Torr, and
PAGE 203
193 backfilled oxygen experiments were conducted at 1x10-6 Torr. The difference between these pressures is a good estimate of the amount of oxygen in the system (neglecting the oxygen present in the system at 1x10a Torr). Figure 7-8 shows the calculated impingement rate as a function of partial pressure of 02 for temperatures of 100, 300, 500 and 700K. At a partial pressure of 1x10-6 Torr and 300K, the impingement rate approaches a monolayer of gas per second. At lx10s Torr, the impingement rate is only 1% of a monolayer per second. Also, as the temperature is increased, the impingement rate decreases. Dissassociation Efficiency of 02 by Electron Impact The impingement rates calculated above need to be modified by the dissassociation efficiency of the gas by electron impact to obtain the amount of dissassociated gas available at the surface. Since no CL degradation occurs while the electron beam is off, it is suspected that the surface reaction is only possible when the gas at the surface has been dissassociated by an incoming electron. a curve relating the dissassociation efficiency to the electron kinetic energy [Eng94] has been reported and is
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194 shown in Figure 7-9. The fit to the curve is only valid for electron energies from 200-10,000 eV. The actual curve decreases below 200eV. Note that the units for dissassociation efficiency are given in #ion pairs/cm*mmHg. An ion pair was defined as an dissassociated gas molecule and the electron lost by dissassociation. Also note that the dissassociation efficiency is defined for a centimeter of travel for an electron. The dissassociation efficiencies will be much smaller for these calculations since the travel distance available is much shorter. It will be assumed that the only gas that will be available for reaction at the surface, must be on the surface physisorbed. The curve of figure 7-9 shows a smoothly increasing dissassociation efficiency as the accelerating voltage decreases. As the electron slows down (decreasing kinetic energy), its interaction time (cross-section) with a gas molecule increases, and the dissassociation efficiency increases. There are two other considerations associated with the determination of the number of ions generated by the electron beam. The first is the mean stay time of the molecules at the surface which depends on the temperature and the heat of desorption of the gas and surface. The
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195 average number of molecules found at the surface will vary with the mean stay time which is defined as follows [Hud92] h MeanStayTime ( t0 Hd T) = t0exp ( ~) es kT ( 7. 5) where t0 is defined as the time between escape attempts by a gas molecule (taken as 10-13 seconds), Hdea is the heat of desorption of the gas from the surface in cal/mol, k is Boltzmanns constant and Tis the temperature of the gas in Kelvin. Figure 7-10 shows the mean stay time plotted as a function of temperature for a constant heat of desorption (5kcal/mol). The mean stay time increases with decreasing temperature. Figure 7-11 shows the variation of mean stay time with the heat of desorption. Typical values for 02 weak physisorption to weak chemisorption are from lkcal/mol to 15 kcal/mol, respectively[Hud92]. As the heat of desorption increases indicating stronger bonds between the gas and surface, the mean stay time increases.
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196 Knowing the partial pressure of the gas and the mean stay time (governed by T and Haes), the average number of molecules/cm2 at the surface can be calculated as follows: na=ImpingementRate*MeanStayTime (7.6) where n a is the average number of molecules/cm2 (assuming a unity sticking coefficient). Fig 7-12 shows the average number of 02 molecules available for electron beam dissassociation as a function of partial pressure of 02 Four curves are plotted for Haes values of 1, 5, 10 and 15 kcal/mol. It can be seen that the average number of molecules on the surface approaches the impingement rate at a given pressure with increasing heats of desorption. The ionization efficiency curve show in figure 7-9 suggests that a single electron is capable of ionizing as many as 10 gas molecules at 1 mmHg in a centimeter of travel at 200eV. Figure 7-13 is a plot of the number of electrons striking a 1 cm2 area per second for electron beam currents from 0.1 to l0a. The current densities were calculated using the experimentally determined beam spot size
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197 (0.0llcm2). Typically, the sample current measured for this spot size i s 3.0a. This results in an electron density of -1016 electrons/cm2*second. Even with an H ctes value of 15kcal/mol, at a partial pressure 1x10-6 Torr of 02 there are only -1014 mol/cm2 Figure 7-14 shows the rate of generation at 2keV for Hctes values of 1, 5, 10 and 15 kcal/mol with a sample current of 3.0a. The system may be limited by the total amount of gas available for ionization at the surface. Thus, within the constraints of the model, lower temperatures, higher Hdes values and higher pressures would increase the CL degradation rate by increasing the amount of reactive gas at the surface. The experiments of chapters 4 and 5 made it clear that higher pressures increased the CL degradation rate. In future work, temperature control of the phosphor could be used to verify a change in CL degradation due to its effect on both H ctes and mean stay time for a gas.
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198 Discussion This study has identified several important parameters related to the CL degradation of Y202S:Eu3 powder CRT phosphor. The surface reaction and volatility of Chas been shown to correlate with small amounts of CL degradation (<5%). The effect of ambient pressure and constituents on the surface reaction rate with S has been shown to affect the CL degradation rate of the phosphor, especially in oxygen dominated ambient. Since the CL degradation is dependant on the surface reaction, the CL degradation rate is also affected by the initial surface condition of the phosphor. Surface charge effects observed may also contribute to the total CL degradation. Since the surface conversion and CL degradation are inter-related, the brightness obtained from any given depth in the phosphor becomes extremely important in understanding the CL degradation phenomena. The concentration of Eu3 light emitting centers versus depth is especially important at lower accelerating voltages and shallower penetration depths. This coupled with the concentration quenching effect and the power dissipated as a function of depth in the
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199 material are all important factors in understanding the CL degradation, as shown above. The formation of Y203 : Eu3 in oxygen ambient has been shown to be the main cause of the CL degradation. Since the temperatures achieved under the electron beam are not nearly that required to form Y203 : Eu3 from Y202S: Eu3 (1100 C), the surface reaction must be stimulated by the electron beam. This is consistent with the lack of degradation i n 02 without electron beam bombardment. Oxygen molecules impinging on the surface are dissociated to atomic species by the electron beam, which are very reactive. These atomic species combine with surface C and S forming volatile gas species (SO x and cox ) which may desorb from the surface. The decreasing surface concentration of S relative to the bulk increases the chemical potential and S continues to move towards the surface where it can be desorbed. The diffusion of sulfur may also be enhanced by the presence of strong electric fields near the surface (which would be consistent with the charge shifted AES peaks). The loss of Sin conjunction with the presence of O leads to the formation of the Y203:Eu3 phase near the surface. The presence of the 612nm CL emission peak demonstrates that this phase is
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200 crystalline in nature with active Eu3 occupying yttrium sites in the lattice. The formation of Y203:Eu3 at the surface causes changes in the surface bonding and surface charging. The secondary emission ratio should change since this property is material dependant and from the experiments, an increase in secondary emission was observed after the formation of Y203:Eu3 Surface bonding will change due to the phase growth and this could affect the adsorption of gas and the number of available sites for dissociation. Also, after formation of the first few monolayers of yttria, the rate at which Sand/or Oare transported to or away from the surface may change, which ultimately may limit the rate of degradation. Surface charging is related to the secondary emission ratio and the electrical conductivity of the material. Therefore, changes in surface charging may occur as well. The surface reaction is itself dependant on many system parameters. Since the surface reaction is electron beam stimulated, it depends on the ionization and dissociation of gas species at the surface, the electron beam energy, amount of gas available at the surface and current density must necessarily affect the CL degradation rate.
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201 The modeling in this chapter has assumed that the "dead" layer region of the material contributed no brightness. In truth, some of this region does contribute to the total brightness since it was found to be a luminescent form of Y203:Eu3+ This was briefly examined to determine the overall effect on the total brightness. The assumptions for the revised model were that the initial dead layer (that found prior to degradation) would contribute no brightness. The measured increase after CL degradation was then considered to consist entirely of Y203:Eu3+ The threshold voltages were taken from an oxygen ambient experiment and were 166 and 866 volts. The modified model predicts a 43% loss in CL brightness, whereas the original model predicted a 46% loss. The modified model results are shown in Figure 7-15 as total brightness versus depth. The modified model does approximate the experimental loss slightly better, but the change is negligible (-3%). Basically, since the Y203 : Eu3+ is at best, half as efficient as the Y202S: Eu3+ and the effieciency is further lowered by the low Eu3+ concentration in the dead layer (as far as efficient Y203 : Eu3+ is concerned) the treatement of the dead layer as a zero brightness layer was a good approximation.
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202 Al though the surf ace conversion of Y202S: Eu3+ to Y203 : Eu3+ accounts for most of the CL degradation observed, other factors which were not studied may contribute to the overall degradation. Since the surface conversion requires the removal of S from the lattice with subsequent conversion to Y203:Eu3+ point defects (even in an ionic crystal) may be created and presumably would increase with increasing conversion. If the production of point defects due to the power density of the electron beam or surface temperature was important, increased CL degradation should be observed for higher power densities. This was not observed. Increased optical absorption of the surface of the phosphor has been found for many phosphors, but the formation of Y203 (which has a larger bad gap than Y202S) would not increase the optical absorption. Interface losses and a change in the critical angle of refraction might be expected from the surface conversion and index of refraction difference between the oxide and oxysulfide. However, rarely was a dark spot observed from degradation (only when a surface coating was present), therefore optical absorption was not a significant degradation mechanism.
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203 In conclusion, removal of C led to small changes in CL brightness, while the removal and conversion of Y202S:Eu3+ to Y203:Eu3+ led to CL brightness losses of -50%. The modification of the model to take into account the CL brightness contribution of Y203:Eu3+ has shown that the layer does not contribute substantially to the total brightness of the phosphor after degradation. It is essentially a "deadn layer due to its low luminescent efficiency limited by the low Eu3+ concentration. Summary Two semi-quantitative models have been developed to aid in understanding the CL degradation phenomena. a model of the brightness as a function of depth has been developed specifically for Y202S:Eu3+ The model takes into account the power dissipated by an electron as a function of depth, the Eu3+ concentration as a function of depth, and the concentration quenching curve found in the literature. Good correlation has been found between experimental and predicted CL brightness decreases.
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204 The second model examined the availability of dissassociated gas species at the surface. Using gas kinetic equations, the average number of gas molecules residing on the surface was calculated. The number of dissassociated gas species at the surface was then calculated using the electrons kinetic energy and an ionization efficiency chart found in the literature. The average number of dissassociated gas molecules at the surface is dependant on temperature, heat of desorption and the partial pressure of the gas in question. The model suggests that the surface reaction is limited initially by the low surface concentration of dissassociated gas available for reaction.
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C: 0 :;::::; ca C: Q) (.) C: 0 (.) ::J w "O Q) .!:::! ca E 0 z (Eu/Eu inner) Concentration 3r----.--------;------;----~------------, 2 1 0'----------------'--------~---~--' 1a 17 1-6 Distance from surface (m) Figure 7-1 Normalized Eu concentration as a function of depth from the surface of the particle [OZA91]. Note the increasing concentration towards the surface. 205
PAGE 216
:::, w .... 0 C: 0 u LL 0 .75 0 5 0 .25 0 0 I I I i I I / / I 206 I .,, I / I / I ,,, I / I / I ,,, I / I ,., / I / f / I / / I / I / I / I / I / I / I / / I / / / / / / / / 20 7 41 0 7 6-107 87 10 6 Depth from surface(m) 500eV 1 keV 2 keV 4 keV Figure 7-2 Fractio n o f Eu contained within a given primary electron's penetration range. The fractions were determined for a 500, 1000, 2000 and 4000 eV primary electrons range (fro m left to right). The higher slope s near the surface (near Depth=0) indicate the higher fractional contributions to the total by the Eu surface gradient shown in Figure 7 1
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rn rn Q) C -0) c co Q) > :.:; ro Q) 0::: 207 Concentration Dependence Curve 1,-------,------..-----------------0 8 0 6 0.4 0 2 t I 0 0 1 0 1 Eu Concentration (mole fraction) Figure 7-3 The concentration quenching curve for Y202S: E u3+ [ O ZA91] The curve was fitted by an analytica l exp r ession and is used in conjunction with the Eu concentration as a function of depth to obtain a relative CL efficiency.
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208 Instantaneous Power 0 8 0 6 0.4 ---------1,-.--. 02--------------~---_ _ _ ,---'-;:------____ ..... ---.. -~--. -----....... 0 2 1 keV 2 keV 3 keV 4 keV ---5 keV 0.4 0.6 0.8 1 2 1.4 Range(um) Figure 7-4 Power dissipation as a function of depth for 1-SkeV primary electrons. The power curves are nearly flat, except at the end of range. Curves are for increasing primary energy (1-SkeV) from left to right. A higher initial kinetic energy results in deeper electron penetration depths.
PAGE 219
1/) 1/) Q) C -.c C) 'i:: a) 209 Brightness as a function of Range 1.Sr-__________________ ___:::_ ____ 0 5 0 0 I ( ~:. :-:.:-.. -_-: ::----" --~:...:.-_::-:...,;, ,__------\. \ 0 2 0.4 1keV 2keV 3keV 4keV SkeV \ t 0 6 \ 0 8 1 Range (um) 1 2 1 4 Figure 7-5 The brightness as a functio n o f depth plotted for 1-5keV primary electro n energies. Three regions are noted. A flat, nearly constant mid-section (no t seen on the 1 or 2keV curve), a tail-off in brightnes s a ssociated with the end o f range, a n d a near surface brightness peak. Primary electron energies increase from left to right.
PAGE 220
1 0 9 (/j 0.8 (/j Q) C: 0 7 ... .r. Cl i::: 0 6 O'.l .... 0 0 5 C: 0 :;; 0 4 -t.) co I,. u.. 0.3 0 2 0 1 0 ---+>.< - : . ... -. 0 1 1keV Brightness fraction for DLV 539to DLV640 0 2 0 3 Depth (um) 0.4 ...... ..... -0 5 210 -+ 0 6 1keV aged 2keV 2kev aged 3kev 3kev aged Figure 7-6 Modeling results for 1, 2 and 3kev brightness fraction as a function of depth (m). Total CL degradation can be estimated at the end of range. All curves are for dead layer voltages of 539 and 640 volts. These are the experimental values from Chapter 4. Experimentally measured CL degradation was 4% at 2keV. The calculated CL degradation (above) is -6%. The model is in very good agreement with the experimentally determined values.
PAGE 221
"' "' Q) C: :c C) c Ill 0 C: 0 u u. 0 9 0 8 0 7 0 6 0 5 0 4 0 3 / ,, ,, 0 1 ,, ,, ,, ,, ,, ,, ,, ,, ,, ,, ,, ,, ,, ,, _,{ ,, 211 ,, o~-----~------------------------------' 0 05 0 1 0 15 0 2 0 25 0 3 0 35 0 4 Range (um) 2keV unaged 2keVaged Figure 7-7 Theoretical change in brightness calculated for an increase in Dead Layer Voltage of 166V to 866V. The voltages were taken from Chapter S's Time Dependant Threshold experiment. The observed CL degradation was -40%. The calculated loss in brightness was 44%.
PAGE 222
"' le E u le E u :::::: 0 E 212 1 .1016 ,-------,------' m..........,.p_in_g.;;__e_m_e_n_t--,..-R_a_t_e_o~f _0_2_G_a_s ___ 1 1o15 1 1014 r -----_, I i 1 1o13 -/ , ..,,( ,. --/./ -, /' / ,,,. . -.,, .,, ,,.' .,,,,/', -,,. -J_ I ' I +-+1o12 ,__ _______ __._ _____ .;.__ __ ___,_ ______ ___;___ __, 1 1 o 8 1 1 0 7 1 1 0-6 1 1 o 5 Partial Pressure of 02 100k 300k 500k 700k Figure 7-8 The impingement rate of 02 gas on a surface as a function of the partial pressure of 02 in the system. Curves are shown for temperatures of 100, 300, 500 and 700 K. Lower temperatures increase the impingement rate.
PAGE 223
0) I E E iC E u ........ (/) Si... ro a.. C 0 213 Ionization Efficiency 15r---~-r-r-~~----, 10 5 0 ---------1 1 er 1-1 o4 100 Electron Energy (eV) Figure 7-9 Curve fit of the ionization efficiency data reported in literature [Eng94]. The ionization efficiency decreases with increasing electron kinetic energy.
PAGE 224
214 1-?r-_ ---+ -,l -----~---_,-__ ------en -a., E 191-------,--~------------l >, ro +-' Cl) C ro 1-101----------.:,,,.,.._ _____ ___J 1-11'---------~------____J 200 250 300 350 400 450 500 Temp (k) Figure 7-10 The decrease in mean stay time with increasing temperature (K) for an Hdes value of 5 kcal/mol. The mean stay time decreases with increasing temperature. T0 is assumed to be 1x10-13 sec.
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0 .01 1 103 14 1 10s -1--6 "' -Q) 1 107 E >. 1-10 8 cu .. CJ) 10 9 C cu 1-10-10 1 10-11 1-10-12 11 0 -13 100 215 I I I I I I I I I I I, i 1-1cr 1 104 1-1cr Heat of Desorption ( cal/mol) Figure 7-11 The variation in mean stay time with hea t of desorption f o r T=300K. Typical values for Hctes for 02 physisorption to weak chemisorption are 1 to 15 kcal/mol. Higher Hctes values induce longer mean stay times, and increase the average number of molecules present on the surface at a given temperature and pressure.
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N E (.) :::::: 0 E Q) 0) m I.. Q) > m I.. Q) .c E :::J z 1 10 14 1 1 o 13 1 1 0 12 111 1 1 o 1 O 1 Ht 1 -1c:P 1 1d 1 1 cf 1-1 er 1 10 4 1 1
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-(/) "' N E (.) ...... (/) -Q) -'iii C Q) 0 ..... C Q) .... .... :::J (.) 217 1 .101 7.--_____ c u _rr_ e _nt_ D _en---rs_it y C a l _cu_ l a t i o n ____ 1 : 1 101 6 I -----.i. -rI 1 1015 1 101 4 '-----------'------'f -=-I ----'------'-----'----'----'--'---'~ 0 1 1 1 0 Measured Current (uA) Figure 7-13 Electron density for sample currents from 0 1 to l 0A. The spot size w a s experimentally measured and found to be 0.011 cm2 Note that the number o f electrons i s higher than the number of gas molecule s present on the surface, even at a partial pressure of 1x10-6 Torr and 15kcal/mol heat of desorption.
PAGE 228
"' lC N E (,) ..... "' C 0 i 1 -10 14 1 -10 13 1 1012 1101 1 i----1-1010 1 1 J 1-1d3 1-1cl --1 1a6 1 1cr 1 1 0 4 1-1 er Number of Ions Generated at 2kev/ 3 u A ----! ; I I ---------..' -.. --I ... --! I 218 100 _________. 10,...._ ________ ......__ ___ _.;i ____ _,_ _ ____;__....:,_....._ __ __, 1 0 a 1 0 ? 1 0-----6 1 1 0-5 O xy gen P artial P r essu r e ( to rr) 1kcal Skcal 10kcal 15kcal Figure 7-14 Rate of generation Total of ions for 02 gas at 300 K with a 2keV electron beam and 265A/cm2 sample current. The total number is limited by the number of gas molecules present. As the average number of molecules increases (increasing H~s 1,5,10 and 15 kcal/mol are shown), the rate of atomic species production increases.
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"' "' Q) C +-.s::. 0) .::: CI) ... 0 C 0 :.;::; u lll .. u. 219 0 9 0 8 0 7 0 6 0 5 0 4 0.3 0 2 0 1 0 0 .01 0 1 Depth (um) 2keV --2keVaged Figure 7-15 Model calculations of brightness loss when the dead layer (0 brightness contribution) is replaced with an Y203 : Eu3+ layer of the same thickness. The Y203 : Eu3+ contribution is modified by the concentration quenching curve found in the literature [Oza91]. The threshold voltages used correspond to the oxygen ambient experiment of chapter 5. Threshold values of V initia1=765 volts and V finai=1290 volts. The brightness loss is calculated for a 2keV primary beam. The model predicts -43 % loss in brightness. The experimentally measured loss was -35%. Without the incorporation of the oxide layer, the theoretical loss was 46%. The calculation is closer to the experimentally observed loss, but the contribution from the oxide is small (-3%).
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CHAPTER 8 DEGRADATION RESISTANT COATINGS FOR Y2O2S:Eu3+ Introduction It has been shown that the CL degradation phenomena of Y2O2S:Eu3+ is related to chemical reactions of some phosphor surface species (namely C and S) in the presence of electron beam and is sensitive to the overall pressure and gas species. In particular, oxygen ambients have been shown to enhance the degradation rate of the Y2O2S:Eu3+ phosphor. Modeling of the gas-surface interaction has shown that the surface reactions are limited by the amount of gas available at the surface (see Chapter 7 for details). The model indicates that the average number of gas molecules present on the surface is a strong function of gas pressure, 220
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221 temperature and heat of desorption. Experimentally, the gas pressure and temperature are easy to modify and study, but in a finished flat panel display neither of these will be controllable. Thus, for industrial relevance, the only factor which is available for minimizing the CL degradation rate of the phosphor is the heat of desorption. Since the gas cannot easily be altered, a new surface material could be used to change the heat of desorption of the gas-surface, thereby reducing the number of molecules present on the surface available for volatilization reactions and/or reducing the diffusion of S to the surface. Since O is reacting with the surface S to form volatile reaction products, protective coatings can be applied to the phosphors effectively separating the gas phase from the phosphor surface and hence reducing the rate of CL degradation. a good coating must provide effective protection from degradation and must not reduce the brightness efficiency of the phosphor below acceptable performance levels. Coatings have been chosen based on ease of preparation and availability. No attempts were made to optimize the coatings and only the viability of protective coatings was studied.
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222 Phosphate and silica wet chemistry coatings were studied since they were readily available and easy to produce in the laboratory. Also, TaSi and Ag coatings were deposited by a novel pulsed laser ablation technique, and Al was deposited on a screened phosphor by electron beam evaporation (see Chapter 3 for details). CL degradation experiments on all coated materials mentioned in this chapter were performed at 1x10-6 Torr backfilled with oxygen gas as in Chapter 6. a primary beam energy of 2keV and current densities ranging from 200 to 300a/cm2 were used. In this chapter, the results of the CL degradation experiments for the coated phosphors will be discussed and compared. Comparison of CL Degradation Rates Figure 8-1 shows the CL degradation rates as a function of coulomb load for all the coated materials. a baseline Y202S:Eu3 oxygen experiment with no coating is shown for comparison (marked original). All coated materials exhibited less CL degradation at 20 C/cm2 then the original powder with the exception of the phosphate coated material.
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223 Comparing the phosphate coated powder to the uncoated powder, the initial CL degradation rate (<5 C/cm2 ) is faster, the steady rate loss after 5 C/cm2 is lower than the uncoated material but at 20 C/cm2 the total degradation is approximately equal. By far, the best CL maintenance was demonstrated by the TaSi and Ag coated materials. Both coatings were deposited by pulsed laser ablation. The TaSi coated material degraded only 15% and the Ag coated material degraded only 25% after 20 C/cm2 These were followed by the Si02 wet chemistry coated material which exhibited a 45% loss in brightness and finally the phosphate coated material which degraded 52%. TaSi Coated material The TaSi coating exhibited the smallest amount of CL degradation. Figure 8-2 shows the APPH ratios for 0/Y and S/Y as a function of coulomb load as well as the CL brightness (right hand axis). At -1 C/cm2 the sulfur within the AES detection limit has been reacted and desorbed, correlating with the small CL brightness increase at very low dose. For the duration of the experiment the S/Y ratio increases while the 0/Y ratio drifts down. The S/Y ratio
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224 seen may increase due to the overlap with a Y peak. This is, however, unlikely since the Auger sensitivity of the Y peak is about 5 times smaller than the Speak. The AES signal of Ta and Si were verified separately and are not shown. The Ta AES signal is relatively high in energy and a 4keV primary energy was required to excite it. It was found that the TaSi coating is not continuous and the phosphors constituents(O, Y and S) are always detected. Figure 8-3 shows the initial (0.07 C/cm2 ) and final (15 C/cm2 ) AES survey for the TaSi coated material. No C was present initially on the surface. The main Si AES peaks are at 92 and 107 eV. If Si is present, its signal is masked by the low energy Y peaks. All peaks have shifted to higher energies by the end of the experiment. This indicates a more negative surface charge present. These peak energy shifts are shown in figure 8-4. Using the trend line equations shown, the energy shifts were calculated and found to be 1.2, 1.3 and 1.8 eV for 0, Sand Y, respectively. The initial and final CL spectra as functions of wavelength (nm) are shown in figure 8-5. Immediately apparent is the large increase in the 612 nm Y203:Eu3+ peak. Other changes can be seen in figure 8-6 where the wavelength
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225 scale is smaller and the CL difference curve is shown. Peak increases are found at 612, 618, 628 and 632nm. The 612 and 632 nm peaks are attributed to the oxide formation at the surface. The 618 and 628, along with the peak decreases at 617 and 626 nm are attributed to spectral broadening of the Since the coating is discontinuous, it is possible that the TaSi coating reduced the available number of S for reaction and slowed the degradation process. Surface sites with no coating, reacted with the ambient and formed Y203:Eu3+ TaSi may provide a diffusion barrier limiting the migration of S to the surface. Thus only a fractional amount of the surface would be available for the volatilization and conversion reactions. Unprotected areas would be converted to Y203:Eu3+ After the aging experiment, the presence of Ta on the surface was reverified. Thus the surface coating was still present after degradation and was not volatilized. Ag Coated Y 2 Q 2S: Eu3 + The Ag coated material performed well exhibiting about half the total CL degradation observed in the uncoated baseline experiment at 15 C/cm2 Figure 8-7 shows the APPH
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226 ratios and the CL brightness as functions of coulomb load. Only 0/Y, S/Y and Ag/Y APPH ratios are shown since no C was present on the surface at the beginning of the experiment. The S/Y APPH ratio decreased quickly (-1 C/cm2 ) and became undetectable for the remainder of the experiment. The 0/Y APPH ratio climbed slightly until about 4 C/cm2 while the Ag/Y APPH ratio decreased. During this time, the CL brightness increased slightly (-4%). It is suspected that the Ag migrated along the surface during electron beam aging away from the center of the electron beam. After the experiment, a dark ring was observed with a lightened center region where the electron beam had impacted. As the Ag migrated, the phosphor underneath became more exposed to the electron beam and oxygen ambient and the CL degradation occurred at near normal rates. Figure 8-8 shows the initial (0 C/cm2 ) and final (15 C/cm2 ) AES spectra. Initially, a significant Ag peak is present, but has been nearly removed by 15 C/cm2 The Ag may have volatalized or it moved away from the area sampled by the electron beam. A small Cl peak is shown, which may be the result of a light HCL etch done prior to coating. The S APPH has fallen below the AES spectrometer sensitivity at the end of the experiment. The
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AES peaks exhibit smaller energy shifts then did the TaSi coated material. 227 Figure 8-9 shows the FED's peak energy shifts and trend lines for 0, Ag, Sand Y. The relative shifts are 0.2, 0.3, 1.2 and 0.5 eV, respectively, to higher energies. These shifts are relatively small which may result from the high conductivity of Ag reducing the charge buildup at the surface. Similar to the TaSi CL spectrum, the Ag coated phosphor exhibited a large increase in the 612 nm Y203:Eu3+ peak after a 15 C/cm2 load as shown in figure 8-10. The expanded scale spectra and CL difference curves are shown in figure 8-11, and the appearance of the oxide peak is more apparent. New peaks are found at 612 and 630nm, both associated with the oxide phase. The spectral broadening of the 626 and 617 nm peaks of Y202S:Eu3+ are seen at 616,618 and 625, 627 nm. The Ag coating provides some protection against CL degradation, but migrates away from the electron beam, exposing the phosphor underneath. Another possibility is the formation of a volatile Ag2S or Ag20 species. Comparing the rate of degradation of the Ag coated material to the baseline experiment in figure 8-1, it is seen that the rate
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of degradation after 10 C/cm2 is very similar to the baseline rate of degradation. 228 Figure 8-12 shows the APPH ratios and CL brightness as functions of coulomb load for the silica coated phosphor. No evidence of the silica coating was found by AES or SEM analysis. This phosphor was obtained from Kasei Optronics of Japan. The method used for coating the phosphor is described in Chapter 3. a nano-particle sol of Si02 is slurried with the phosphor particles. The Si02 nano-particles adhere to the phosphor due to coulomb attraction. These types of coatings are typically very thin (
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229 expanded CL spectra and CL difference curves as functions of wavelength from 600 to 640 nm. Spectral broadening of the 626 and 617 nm Y2O2S:Eu3+ peaks is visible at 616,618 and 625, 627 nm. a small Y2O3 :Eu3+ peak is visible at 612nm. The SiO2 coating did not slow the CL degradation enough to stand out from the baseline oxygen experiments. The data examined shows that its surface reactions and CL changes are very similar to the baseline experiments. Phosphate coated L02 S: Eu3+ The wet chemical process used to produce this coating left large nodules on the surface of the particles. Figure 8-15 shows the surface of the phosphate coated particles as seen in an SEM. The CL degradation was very close to the baseline oxygen experiment (as seen in figure 8-1). However, the changes which occurred on the surface of the phosphate coated particles were very different than the baseline experiments. Figure 8-16 shows the APPH ratios for O/Y, S/Y, P/Y and C/Y as well as the CL brightness as functions of coulomb load (C/cm2). C is lost quickly (-2 C/cm2), possibly volatilized by surface oxygen bonded in a phosphate configuration. The O/Y ratio drops over the same load,
PAGE 240
230 supporting this postulate. Once the C is removed, the S/Y ratio begins increasing suggesting the arrival of sulfur to the surface. The O/Y APPH ratio follows the loss of C, and then follows the loss in P until -10 C/cm2 It is possible that initially the C is most readily reacted and volatilized, and once it is depleted phosphate containing species are next to be consumed. As the Pis volatilized, S may replace it at the surface. Probably in a similar configuration, such as Y2O28O4 The initial (0 C/cm2 ) and final (15 C/cm2 ) CL spectra are shown in figure 8-17 and the expanded spectra and CL difference curves are shown in figure 8-18. In figure 8-18, no oxide peak is visible (612 nm), only peaks associated with spectral broadening of the main Y2O2S:Eu3+ peaks are seen. The degradation of the phosphate coated material does not produce the Y2O3:Eu3+ phase observed previously. When Y2O3:Eu3+ is produced at the surface (as is the case for TaSi and Ag coatings) the CL degradation was slower. The combination of the coating limiting S sites for reaction and the early production of Y2O3:Eu3+ in uncoated areas may slow the CL degradation rate.
PAGE 241
231 The phosphate coating does not appear to impede the CL degradation. S readily replaces P, possibly by forming Y202804 The CL brightness loss of the phosphate coated material occurs at a higher rate than does the baseline experiment. This system may not be gas limited because a large amount of oxygen has been placed on the surface on purpose (as phosphate groups). Thus the surface conversion and reaction occurs faster for the phosphate coated sample as seen in figure 8-1. Summary Coatings have been applied to Y202S:Eu3+ by various deposition techniques. These coated material's CL performance has been compared with the baseline oxygen experiments of Chapter 6. It has been shown that a tenacious coating, which does not migrate or volatilize during aging provides the best protection against CL degradation. Of the coatings tested, the TaSi pulsed laser ablation coating demonstrated the best ability to protect the phosphor and to remain on the surface. After 15 C/cm2 the TaSi coated
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232 material had degraded 3 times less then the baseline The formation of Y203 : Eu3+ on the surf ace slows the rate of CL degradation in oxygen as well. It is suspected of providing a diffusion barrier to S. The silica and phosphate coatings did little to slow the degradation rate. Both provide an extra source of oxygen which may have enhanced the surface reactions removing C, Sand P.
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0 9 -:0 8 _, -i 0.7 C .. .c -~ 0.6 .. cc ..J u 0.5 0 4 0.3 0 233 TaSi Ag Si02 Phosphate ngma 5 10 15 20 25 30 Coulomb Load (C/cm2) Figure 8 1 Comparison of CL degradation for the coated phosphors. An uncoated powder is shown for comparison and is marked "Original". The silica and phosphate wet chemistry coatings faired little better than the original powder. The Ag and TaSi coated powders degraded slower than the original powder.
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4 3.5 3 II) 2 5 0 (1:1 er:: 2 ::c C. C. 1 5 <( 0 5 0 0 234 1 2 1 .15 1 1 Oxygen/Yttrium 1 .05 (1:1 t:! Q) 0 .95 S .c CS) 0 9 "i: cc CL Brightness 0 .85 d 0.8 0.75 Sulfur/Yttrium 0.7 5 10 15 20 25 30 Coulomb Load (Ocm2) Figure 8-2 APPH ratios for 0/Y and S/Y as well as CL brightness as functions of coulomb load for the TaSi coated powder. Note the very fast decay of the S/Y ratio, and the stable 0/Y ratio through-out the experiment.
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235 2000 1500 1000 500 0 -500 -No C -1000 -Sulfur ::c: -1500 C. C. -2000 cX Yttrium Oxygen -2500 -3000 -4000 -3500 -5000 ...______ ____________ __-L, -4500 0 Energy (eV) 600 Figure 8-3 Initial and final AES scan for the TaSi coated Y2O2S: Eu3+ taken at 0 07 C/ cm2 and 15 C/cm2 No C was present initially on the surface. Small peak energ y shifts are visible on all thre e peaks.
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236 600 -r--------------------~ 500 $' Q) -400 >i C') ... Q) C 300 w Q) C. tJ) 200 w
PAGE 247
1 2 1 -=0 8 _, ltl -.? 0 6 II) C Q) .... C 0 4 ltl E C 0 2 0 5 0 2 237 1 5 1 -=-_, Final ltl \ 0 5 in C Q) ..., C 0 ltl C u. 0 5 0 600 650 700 7 0 1 Wavelength(nm) Figure 8-5 Initial (0 c / cm2 ) and Final ( 1 5 C/cm2 ) CL Intensity spectra for the TaSi coated material d egrade d at 2 k eV and -1x10-6 T orr 02 Note the large increas e in t h e oxid e emiss i o n pea k at 612 nm.
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1 2 0 8 -_, ti, i 0 6 'i C G) ... 0 4 .: _, u 0.2 0 6 0 -0. 2 238 0 .35 612 627 0.3 CL Difference 4 \ 0.25 616 628 I 0 2 = ti, ._. G) Final 632 0 .15 CJ C i G) ... 0 1 Initial 0 _, 0.05 u 0 630 635 6 0-0.05 -0. 1 Wavelength(nm) Figure 8-6 Enhanced CL spectra and CL difference curves as functions of wavelength(nm) for TaSi coated phosphor. Note the large increase in the 612, 632 nm emission indicating the growth of the oxide phase. Spectral broadening peak pairs are found around the 617 and 627 nm oxysulfide emission peaks at 616, 618 and 626, 628 nm.
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4 3.5 -=3 .. -ui 2 5 0 2 a: ::c 1.5 C. C. 1 ex 0 5 0 Oxygen/Yttrium 239 -=-.. 0 8 i Q) 0 6 S .: C'l 0.4 ..J u 0 2 -t-----t-----+----t-----+-----+-----+ 0 0 5 10 15 20 25 30 Coulomb Load (Clcm2) Figure 8 7 APPH ratios for Ag coated sample. Note the slow loss of Ag and the comparable increase in S. This indicates that the Ag is moving away from the electron beam (possibly due to heat gradients), exposing the phosphor beneath the coating. The CL degradation curve is shown on the right y-axis.
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240 3000 ....-------------------2000 1000 ::c -1 ODO C. C.
PAGE 251
600 500 $' Q) ';; 400 en -Q) ifl 300 (Iii Q) a. 200 (I) LU < 100 0 0 241 Oxygen y = 0 .0122x + 520.62 Silver y = 0 .0235x + 362.89 y = -0.0865x + 169. 2 Sulfur Yttrium y = 0 .0381 X + 103.07 5 10 15 20 25 30 Coulomb Load jC/cm2) Figure 8-9 AES peak energy shifts for the Ag coated phosphor. The peak shifts are small, possibly due to the c onducting nature of the coating.
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-=-_, -I Q) C .... Cl 'i: CCI .J u E C 242 1 2 ~-------------1.5 0 8 0 6 0.4 0.2 0 5 0 -0. 2 -=-_, -I 0 5 Q) C Fina l .... Cl 'i: CCI 0 .J u Initial C -0. 5 u. 600 650 700 7 0 -1 W avelength(nm) Figure 8-10 Initial and final CL Intensity spectra as a function of wavelength (nm). Note the large increase in the oxide peak at -612 nm.
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243 1 2 1 1 612 CL Difference 626 625 0 9 627 0.7 0 8 -=-=-.. 0 5 .. ti::, ti::, -630 0 6 a, Final CJ a, C ..., .c I:) 'i: cc ..J u l Initial C 0.3 a, i.. 0 4 Q ..J u 0 2 0 -1== ==:;f:::::=F==-----==+-------=t---+-..JJ---+--\~=F===t 6 TI'-~ 610 615 620 625 -0. 2 .....___ __________________ ___. -0. 5 Wavelength(nm) Figure 8-1 1 Ag coated oxysulfid e expanded CL and CL difference spectra. Spectral broadening of the oxysulfide main peaks at 626 and 617 nm occurs at 616, 618nm and 62 5 6 2 7 nm. N ote the emissive increases a t 612 and 6 3 0 nm. The 612 nm peak is the oxide peak emission. The 630 nm may be the second strongest emiss i o n of the oxide, but is d ifficult to discern.
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7 6 -=5 _, 0 4 ;:: a: 3 ::c C. C. c:t 2 0 0 244 1 0 9 Oxygen/Yttrium 0 8 -0.7 -0.6 CL Brightness Q) ~~iji.1utkl.J~h'~M\.41\\.t,ht~."'LtiM~~1~~ .. /,w1~~ 0 5 S Sulfur/Yttrium .c 0 4 .g, c:c 0.3 ...J u Carbon/Yttrium 0.2 0 1 0 5 10 1 5 2 0 2 5 30 Wavelength(nm) Figure 8-12 APPH ratios and CL brightness curve as functions of coulomb load (C/cm2)for the silica coated phosphor. No silicon signal could b e distinguished because o f overlap with low energy Y peaks. The curves are very similar to the baseline oxysulfide oxyge n experiments of chapter 6
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245 1 2 ....---------------1 5 0 8 = -=-... 0 6 Final J 0 5 b i;; Ill C Q) .... C E C C Q) .... C 0 4 0 C u. 0 2 Initial l 0 5 0 55 0 600. 0 650. 0 700. 0 75 .0 0 2 1 Wavelength(nm) Figure 8-13 Initia l and Final CL spectra as a function of wavelength (nm) for the Silica coated oxysulfide powder. N o differences can be discerned.
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1 2 0 8 -=-.., 0 6 (I) C Cl) .. C 0 4 E C 0 2 0 0 2 246 0 1 4 626 625 0 1 2 0 1 CL Difference 0 .08 -=-.., \ 627 -616 0 .06 Cl) u 617 C Cl) 612 ... Final 0 0 4 Q 0.02 ..J u Initial 0 605 0 610.0 6 1 5.0 20. 0 625 0 630. 0 635 0 64 -~-02 0 0 4 Wavelength(nm) Figure 8-14 Silica c oated oxysulfide enhanced CL spectra and CL difference spectra. Some spectral broadening i s o bserved o n the oxysulfide main emission peaks ( 617 and 626 nm) and a small oxide peak is visible at 611 nm but not nearly to the extent seen in the TaSi and Ag coated materials.
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'1<-~A-~ 4696A -2B Heavy Phosphate Coated YOS 247 Figure 8-15 SEM of phosphate coated oxysulfide powder. The scale at bottom right is lm. Note the large nodular outcropppings on the particles.
PAGE 258
6 5 4 Ul 0 ; 3 (13 a: ::c Q. 2 Q. ex 0 -1 248 0 9 0 8 0.7 = Oxygen/Yttrium (13 0 6 i Sulfur/Yttrium QI 0 5 S .: Cl 0.4 ..I Phos/Yttrium 0.3 u 0 2 ......... ~ .;..,~~-...,,,_,.,._,......._.,,..,,,...,.,._"'l/ Carbon/Yttrium 5 10 15 20 25 3 0 1 0 Coulomb Load (Clcm2 ) Figure 8-16 APPH ratio s and CL brightness a s functions o f coulomb load f o r the phosphate c oated phosphor. Note that the 0/Y ratio firs t follows the C/Y and then the s lope changes and follows the P/Y ratio. C may be more easily volatilized than the P. Surface oxygen m a y be enhancing the degradation.
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-=--('1:1 'iii C Q) .. C ..J u ('1:1 E C 1 2 -r---------------------~ 0 8 0 6 0 4 0 2 0 5 -0. 2 -=0 5 -('1:1 Final 'iii C Q) .. C _J,._) 0 ..J u ('1:1 C I n i tial u. 0 5 600 650 700 7 0 -1 Wavelength(nm) Figure 8-17 Initial (O C/cm2 ) and final ( 1 5 C/cm2 ) CL spectra for the phosphat e coated oxysulfide powder degraded at 2keV and -1x10-6 Torr 02 249
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250 1 2 .--------------------~ 0 1 618 CL Difference 617 '= 0.8 (,::, 'iii O 6 C (I) .... C d 0 4 (,::, Initial 627 \ 632 Final 0 .05 0 (I) u C -0.05 Q ...J E C 0 2 o ~==::::::~j'.5=31==-=2~::__--+---+---+___: ~::::i-~~-0.15 -0. 1 u 6 0 605 610 615 620 625 630 635 6 0 -0. 2 ~----------------~-0. 2 Wavelength(nm) Figure 8-18 Enhanced CL spectral distribution and Cl difference curves for the phosphate coated powder. Note the absence of the 612 nm oxide emission. Only spectral broadening of the oxysulfide emission peaks at 617 and 626 nm are observed.
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CHAPTER 9 SUMMARY AND CONCLUSIONS The CL degradation of Y202S:Eu3 powder phosphors was studied by Auger electron spectroscopy (AES) and cathodoluminescence (CL) spectroscopy. Electron beams of 1, 2 and 3 keV energies were used to induce cathodoluminescence, as well as generate auger electron signals. DC current densities ranged from 100 to 400 a/cm2 for CL and AES. Vacuum pressures during the CL degradation studies ranged from a base pressure of 1x10s Torr to 1x10-5 Torr of backfilled oxygen gas. CL degradation occurred slowly at 1x10s Torr and the AES and CL data indicated the loss of Sand adventitious contaminant C from the surface of the phosphor. The CL degradation observed only occurred while the electron beam was turned on. The reactions involved O containing gases, 251
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252 the electron beam and the surface C and Sand created a thin non-luminescent layer (-130A). This layer growth, which for modeling purposes was taken to be a continuous layer, decreased the total brightness of the phosphor. The dead layer thickness increase was determined from threshold voltage curves obtained before and after the CL degradation experiment. The increased threshold voltage and CL degradation may have been caused by the conversion of Y202S: Eu3+ to Y202S04 : Eu3+ or due to increased non-radiative recombination near the phosphor surface due to charging. Further experiments examined the effects of gas pressure and type of gas on the rate of surface change and CL degradation rate, and both were much higher in the presence of oxygen gas. a conversion of Y202S: Eu3 + to Y203 : Eu3 occurred in the presence of oxygen. The growth of this layer was studied by time dependant threshold voltage experiments, and was found to correlate well with both the AES changes observed and the CL degradation. The complete layer thickness was calculated to be -600 A from threshold experiments. The growth of the dead layer was initially fast (corresponding to the Sand Closs), then slowed for the remainder of the CL degradation.
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253 The dependance of CL degradation rate on the electrons primary energy was also investigated for 1, 2 and 3 keV energies. It was found that the CL degradation was faster for lower accelerating voltages primarily due to the decrease in excitation volume. It was also shown that the presence of large amounts of Cat the surface affected the S loss rate. The CL brightness loss as a function of the growth in dead layer was modeled using Makhov's Law to predict the total brightness loss of the Y202S:Eu3+. The model assumed that the effects on CL brightness due to carrier diffusion was negligible, and the range predicted by the modified Bethe equation was equal to the excitation volume. The absorption of light by the material was assumed to be negligible. The CL efficiency term in Makhov's equation, was modified using experimental data relating the Eu3+ concentration to depth and the concentration quenching curve (which defines the brightness of a phosphor as a function of the molar concentration of Eu3+ present). Very good correlation was found between all experiments and the brightness loss predictions of the model. The observed CL degradation in the baseline experiments was 4%, the model
PAGE 264
254 predicted a 6% loss. The observed degradation in the oxygen experiments was 40% and the predicted degradation was 45%. a more refined model would take into account that some of the degraded material is converted to Y203:Eu3+ a luminescent solid. The model used in this study considers the entire dead layer non-luminescent. The ionization rate of gas at the surface was modeled using the assumptions of kinetic theory of gases to obtain impingement rates for oxygen gas on the surface of the phosphor. The determination of the number of gas molecules available for ionization at the surface assumed a unity sticking coefficient. a modified ionization efficiency for electrons of varying kinetic energies was used to determine the number average of dissassociated oxygen molecules residing at the surface of the phosphor. This model showed that the CL degradation rate is initially limited by the amount of dissassociated gas available for surface reaction. The amount of electrons available to dissassociate gas near the surface exceeds the amount of gas at the surface for all the experimental conditions (pressure, temperature, heat of desorption) examined in this study. Based on these models,
PAGE 265
protective surface coatings were applied to the phosphor particles in an attempt to slow the CL degradation. 255 Non-uniform coatings of TaSi and Ag were deposited by a pulsed laser ablation technique and were found to reduce the CL degradation by 60% and 40%, respectively. The TaSi coating was still present on the surface after the CL degradation experiment, and may have acted as a diffusion barrier to S, preventing the surface formation of Y203:Eu3+ where it was present on the surface. The TaSi coating was tenacious and was not desorbed or dispersed by the electron beam. The Ag coating delayed the CL degradation. Under the influence of the electron beam, the Ag may have migrated away from the beam spot during bombardment, slowly exposing the Y202S:Eu3+ phosphor. It is also possible that the Ag was volatilized as Ag2S during the experiment. Once the amount of Ag had decreased under the electron beam, CL degradation occurred at rates nearly equal to the uncoated phosphor. Wet chemistry coatings of Si02 and phosphate were applied as well, and were found to not slow the CL degradation rate. The CL degradation rate of the phosphate coating was enhanced initially possibly due to the presence of O on the surface of the phosphor in the coating. The
PAGE 266
256 surface reaction sequence observed by AES was altered (compared to the uncoated powder), but the CL degradation rate was not inhibited. The Si02 coated material exhibited AES surface changes similar to those of the uncoated phosphor and the presence of Si could not be verified by AES. Thus, it was not studied in as much detail as were the other coatings.
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Bec96 Bet53 Bla94 Bli57 Bru92 Der94 Don78 Eve71 LIST OF REFERENCES Bechtel, H., Czarnojan, W., Haase, M., Wadow, D., J.Soc.Inf.Display., ~, 219 (1996) Bethe, H., Ashkin, J., Passage of Radiations through Matter, Experimental Nuclear Physics (Wiley, New York, 1953) Blasse, G., Grabmaier, B., Luminescent Materials (Springer-Verlag, New York, 1994) Bliss, W.H., Juong, C.J., Facsimile Scanning by Cathode Ray Tubes, R.C.A.Rev., 15, 275 (1957) Brundle, C.R., Evan C.A, Wilson, S., Encyclopedia of Material Characterization, (Manning Publications Incorporated, Greenwich, CT, 1992) Derbyshire, K., J.Solid.State.Tecnology. Nov., 1994 Donalato, C., J. Optics, -22_, 19 (1971) Everhart, T.E, Hoff, P.H., J.Appl.Phys., 42, 5837 (1971) 257
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Fit97 Gil56 Han52 Has90 Hay69 Hol97 Hud92 Ito89 Jac95 Joy89 Kad92 Kan71 Kin72 258 Fitzgerald, J., Singh, R.K, Nano-particle Flux Based Dry Coatings, Patent Disclosure, University of Florida, Gainesville, FL 1997 Gilfrich, H.P., Z.Phys., 145, 241 (1956) Hanle, W., Rau, K.H., Z.Phys., 133, 297 (1952) Hase, T., Kano, T., Nakozawa, E., Yamamoto, H., Advances in Electronics and Electron Physics, 79, 273 (1990) Haynes, J.W., U.S Patent# 3,607,371 Holloway, P.,Sebastian, J., Trottier, T., Jones, S., Swart, H., Petersen, R., Degradation Mechanisms and Vacuum Requirements for FED Phosphors, Flat Panel Display Materials II, MRS Proceedings, 424, 1997 Hudson, J.B., Surface Science, (ButterworthHeinemann, Stonehame, MA, 1992) Itoh, S., Kimizuka, T., Tanegawa, T., J.Electrochem.Soc., 136, 1819 (1989) Jacobsen, S., Yang, S., Zhang, F., Summers, C., SID Display Manufacturing Conference, Late News Paper, 1995 Joy, D.C., Luo, S., J.Scanning. 11, 177 (1989) Abdel-Lader, a., Elkholy, M.M., J.Mat.Sci, ll, 2887 (1992) Kanaya, K., Okayama, S., J.Appl.Phys., ~' 46 (1971) Kingsley, J.D., Prener, J.S . J.Appl.Phys., 43, 3073 (1972)
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Kla87 Kom96 Kot95 Lav85 Lev91 Lum78 Mak60 Mar57 Myk76 New76 Nie88 Oza90 Pat94 259 Klaassen, D., de Leeuw, D., J. Luminescence, 37, 21 (1987) Kominami, H., Horikawa, K., Auki, T., Nakamura, T., Nakanishi, Y., Hatanaka, Y., Proc. 15th Int. Display Res. Conf., 867 (1996) Kottaisamy, M., Jaguannathan, R., Rao R.P., Auudaithai, M., Srinivason, L.K., Sunfaram, V., J.Electrochem. Soc., 142, 3205 (1995) Laverne, J.A., Mozumber, a., J.Phys.Chem., 89, 4219 (1985) Levy, F., Meyer, R., International Display Research Conference, San Diego, CA, Oct. (1991) Lumb, M., Luminescence Spectroscopy (Academic Press, London, 1978) Makhov, A.F., Fiz.Tverd.Tela., ~' 2161 (1960) Martin, W., Z.Phys., 147, 583 (1957) Myklebust, R.L., Newbury, D.E., Yakowitz, H., J.Scan.Electron.Mic., 460, 15 (1976) Newbury, D.E., Yakowitz, H., J.Scan.Electron.Mic., ~' 1917 (1988) Nieminen, R.M., J.Scan.Electron.Mic., ~' 1924 (1988) Ozawa, L., Cathodoluminescence (VCH Publishers, New York, NY, 1990) Patel, M., Pu, C.J., Sharpe, L., Langmuir, 11, 2003 (1995)
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Tro96 Tun79 Yac90 Trottier, T.A., Swart, H.C., Jones, S.L., Holloway, P.H., J.Soc.Inf.Display. ~, 354 (1996) Tung, C.J., Ashley, J.C., Ritchie, R.H., J.Surf.Sci., fil, 427 (1979) 261 Yacobi, B.G., Holf, D.B., Cathodoluminescence Microscopy of Inorganic Solids (Plenum Press, New York, 1990)
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BIOGRAPHICAL SKETCH Troy Anthony Trottier was born on April 3rd, 1969, in Westfield, MA. He spent much of his childhood attempting to burn and/or blow up things. At Westfield High School, he graduated with honors. Upon graduation, he enrolled at Western New England College in electrical engineering and spent the next 4 years learning how to burn and/or blow things up electrically. Upon graduating, he got a job. After years of traveling around North America, he realized the job did not allow him to follow his dream (seldom did he get to burn or blow things up). He learned of the UF Department of Materials Science and a man by the name of Holloway who would be sympathetic. After being accepted for PhD research, he was given the job of burning and/or blowing up computers, lab equipment and phosphor particles. He has spent the last 4 years, following his dream, doing just that. 262
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I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Paui:rtl!lf/1::iair Professor of Materials Science and Engineering I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor o Philoso2 Associate Professor of Materials Science and Engineering I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. (j2__(~ Rolf E. Hummel Professor of Materials Science and Engineering I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Rab,/s~Jf Associate Professor of Materials Science and Engineering
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I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Qd_e r c..David B Tanner Professor of Physics This dissertation was submitted to the Graduate Faculty of the College o f Engineering and to the Graduate School and was accepted as partial fulfillment of the requirements for the degree of Doctor of Philosophy. August, 1997 Alin~ Dean, College of Engineering Karen A. Holbrook Dean, Graduate School
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