1. Phw. Chrm. Sulrds Vol. 51. No. 7. PP. 793404,
oozt-3697/w
1990
0
Pnnted in Great Bntmn.
TRAPPED
13.00 + 0.00
1990 Pergamon Press
plc
HOLES IN SILVER HALIDES
J. P. SP~~NHOWERand A. P. MARCHETTI
Corporate Research Laboratories and Photographic Research Laboratories, Eastman Kodak Company,
Rochester, NY 14650, U.S.A.
Abstract-The
properties of holes in silver halides are reviewed with emphasis on trapped holes. The
chemical and electronic structure of the self-trapped hole in A&l has been well documented. In contrast,
the nature of the intrinsic hole trap in AgBr is still speculative and could benefit from more
experimentation. A variety of trapped-hole species, induced by doping of the silver halides, have been
identified. Magnetic resonance methods have been the most successful techniques for elucidating the
structure of these defects. Little is known about holes trapped in AgF and AgI or in many of the mixed
halide crystals.
Keywords: Silver halides, point defects, hole trans.
_ impurities, magnetic resonance, luminescence, excitons,
OtiMR.
1. INTRODUCTION
The intent of this review is to describe the properties
of holes in silver halides. The emphasis will be on the
properties of trapped holes, although there will be
some discussion of the free hole species. A review by
Galvin of holes in silver halides was published in 1972
and was written, to a large extent, from a photographic point of view [l]. A review of electron spin
resonance investigations of AgCl and AgBr by John
and Hoyer contains information on both trapped
electrons and holes [2]. An excellent source of
information on the properties of silver halides is a
Landolt-Bornstein
compilation [3], and a review of
the silver halide photographic process by Hamilton
contains a wealth of physical data and solid state
information related to photography [4]. The present
review places emphasis on the physical properties of
holes. Of primary importance will be the structure of
the trapped-hole species even though it is still speculative in certain cases. This review will primarily treat
data for holes in AgBr and AgCl as there is
little information available on AgF and AgI. Data on
mixed halide systems will be included where
available.
The properties of the free hole state are discussed
briefly in the second section and holes trapped by
Coulomb interactions are outlined in Section 3. Holes
trapped at centers which undergo a valence change
are detailed in Section 4, which is divided into four
areas: self-trapping, transition metal, chalcogenide,
and rare earth ions. Finally, Section 5 contains a
summary and outlines research areas that could
benefit from further investigation.
A number of physical methods have been used to
study holes in silver halides. An often-used technique
is optical spectroscopy which is usually performed at
low temperature. Optical spectroscopic techniques
include absorption, emission, and excitation measurements as well as lifetime or time evolution studies.
The best technique to provide information about the
geometric structure of the center in question is electron spin resonance (ESR) spectroscopy. A variant of
ESR spectroscopy which can provide information
about several ionic shells surrounding a center is
electron nuclear double resonance (ENDOR) spectroscopy. Optically detected magnetic resonance
(ODMR) spectroscopy combines both ESR and optical spectroscopic methods to provide information
about excited states, often with extremely high sensitivity. A complex variant of ODMR, which provides
excited state information about neighboring ions, is
optically detected electron nuclear double resonance
(ODENDOR) spectroscopy [5]. These techniques are
some of the ones most often used, but other methods,
such as conductivity measurements and some of
the more complex double resonance methods, also
provide information about hole centers.
The silver halides, with the exception of one crystal
modification of AgI, all have cubic crystal structures.
Crystal structure data and relevant lattice properties
are given in Table 1.
2. FREE HOLES
The lowest energy band-to-band
absorptions in
silver chloride and silver bromide are phonon-assisted
indirect transitions in contrast to most alkali halides
where the transition is direct (6, 71. Absorption of
light just above the band-gap energy produces an
electron in the conduction band whose minimum is at
the zone center and a hole in the valence band whose
maximum is at an L-point. Thus free electrons and
shallowly trapped electrons whose wavefunctions are
made up largely of conduction band functions are
expected to be isotropic in nature. A free electron at
the bottom of the band will have a single effective
mass and g-value. In contrast, free holes near the
L-point and shallowly trapped holes (uide infra)
whose wavefunctions are largely valence band functions are expected to show anisotropic behavior. A
free hole will have several effective masses and several
J.P.
794
and A.P.
SFWNHOWER
MARCHETn
Table 1. The space groups, lattice constants ((I and c, for hexagonal AgI), low frequency
dielectric constants (6). and electron and hole mobilities 01) determined for the silver halides
(after Ref. 3)
Space
group
Halide
AgF
A&l
AgBr
A&B )
kid(~)
Fm3m
Fm3m
Fm3m
P6,mc
F43m
a(c) (A)
c(O)
4.936
10.3(143
K)
5.55023
9.55(2
K)
5.71475
IO&(2 K)
4.580(7.494)
6.473
7.90(100
K)
g-values. Unlike the alkali halides, significant p-d
state mixing occurs in the silver halides [6, 71. The
available
data on electron and hole masses and
g-values are given in Table 2.
There are two modifications of AgI that exist at
room temperature. The cubic form has a zinc-blende
(y-phase) structure while the hexagonal form has a
wurtzite (a phase) structure. The lowest energy bandto-band transitions in both /? and ‘J AgI are allowed
[8.9]. Excitation at the band-gap energy produces
both electrons and holes at the zone center for both
crystal modifications. The anisotropic nature of the
wurtzite crystal structure will be reflected in g-values
and effective masses (Table 2).
3. COULOMB TRAPS
One can distinguish a variety of centers at which a
hole (or an electron) can bind. Two possible classifications of centers are: those that bind a carrier by
Coulomb forces only and those that bind a carrier
through a valence change, indicative of more deeply
bound carriers. It seems probable that almost all
centers that bind a carrier undergo some sort of
lattice relaxation in addition to the initial form of
binding. Centers that bind a carrier by a valence
change might be electrostatically attractive, neutral
or, in some cases, repulsive to the carrier that is to be
bound. For centers that bind carriers by Coulomb
interactions alone it is necessary to ascertain whether
the carrier is bound weakly, the binding energy is
given by the effective mass approximation (EMA), or
if it is strongly bound [IO, 1I]. In the case of weak
binding in high dielectric materials the binding energy
(R) is approximated by
R = (m,lm,)R,I~(O)~,
(1)
where R, is the Rydberg constant of a hydrogen
atom. and ~(0) is the low frequency dielectric
constant.
p,(cm’(Vs)-‘)
p.(cm*(Vs)-‘)
-
-
4 x lO'(4K)
5x 1@(2K)
-
3 x l&4 K)
-
-
-
In the case where centers are attractive to electrons,
the effective mass approximation yields reasonable
values for the binding energy (35 and 65meV for
AgBr and AgCI, respectively) although it appears to
overestimate the binding energies in the silver halides
by about 20% [4]. The weak binding occurs because
of the high dielectric constant and the small effective
mass for electrons. The situation for holes is less well
understood. In the effective mass approximation, the
binding energy is “moderated” by the background
dielectric constant. In actual fact the carrier, when it
is close to the binding center, does not experience the
full dielectric constant. If the effective mass is sufficiently large or the dielectric constant sufficiently
small, this polaron will collapse to a “deep” center,
This will occur with rather small changes in effective
mass and dielectric constant as has been outlined in
a recent variational calculation on the shallow-deep
instability of impurity levels in semiconductors [l2].
In work that calculated the energy of shallow donor
states in the silver halides [l3], the effective dielectric
constant has been determined as a function of distance from the binding center, and the half-way point
or “cut-off” distance is estimated to be 0.8 A or
about I.5 a.u. Using this cut-off distance of 1.5 a.u. in
the variational calculations as the critical radius for
the start of the full dielectric shielding leads to the
prediction that for a longitudinal hole effective mass
of I .7 m, (see Table 2) there will be no shallow EMA
states in AgBr. For the perpendicular hole effective
mass of 0.79 m, the prediction of the same calculation
is ambiguous but these Coulomb centers also do not
appear to bind holes shallowly. In the situation where
holes may be attracted by a fractional charge, such as
a half-jog in an edge dislocation, it would seem that
these would be shallow or EMA states [l4].
The hole masses are unknown in AgCl because of
the tendency of the hole to self-trap. The dielectric
constant is, however, smaller than AgBr, and if the
hole masses were similar, then there should be no
EMA hole states in AgCl either.
Table 2. The g-values and polaron effective masses for electrons and holes
in the silver halides (m,= free electron mass) (after Ref. 3)
Halide
AgCl
AgBr
&I(P)
&kc1ran
1.87
I .49
0.06,I .05
&ok
2.46(11)
-
mpi’ScXmn
0.431
0.2865
-
m@Lr
1.71(/1)co.79(1)
-
Trapped holes in silver halides
The situation in AgBr, _ .~I,r could be reversed if the
center which binds holes is a silver ion vacancy (ride
infra). It has been shown that iodide ions are found
preferentially in the first shell of silver ion vacancies
[ 15, 161.If this arrangement increases the local dielectric constant, then the perpendicular effective mass
holes may be bound in EMA states. This is highly
speculative at this point.
Holes are rapidly self-trapped in AgCl and
there are been no observations of holes trapped at
Coulomb centers. The converse appears to be true in
AgBr, where there is evidence from the ODMR
spectrum that trapped holes participate in donoracceptor recombination [17, 18). This is most apparent in CdZC- and Pbz+-doped AgBr where only a
donor and an acceptor resonance are observed and
the emission exhibits the characteristic shift to longer
wavelengths with time after pulsed excitation [19].
This trapped hole species has a g-value of 2.08 and
has an adiabatic trap depth of about 300 mV (181.
The intrinsic hole traps in AgBr were postulated,
early on, to be lattice silver ions [20]. Infrared absorption spectra have been obtained on trapped hole
species, and these were attributed to holes trapped at
iodide centers in AgBr and to holes trapped at silver
ion vacancies in Cd’+-doped AgBr [21]. The delayed
luminescence decay in AgBr was attributed to
donor-acceptor pair recombination, again with the
hole trap being a silver ion vacancy [22]. More recent
work has suggested that the holes in AgBr are
trapped at silver ion vacancies, impurity iodide ions
and iodide pairs (231. To date there has been no
definite proof that holes are bound by the Coulomb
attraction of a silver ion vacancy, but doping experiments and the assignment of the major resonances in
the ODMR of AgBr provide strong circumstantial
evidence that silver ion vacancies are Coulomb traps
for holes in AgBr (17, 191.
The source of confusion until recently in AgBr was
the absence of a shift in the emission peak after
pulsed excitation and two major unidentified resonances in the ODMR spectrum. The usual situation
with pair recombination, as shown in Fig. 1, is to
have a donor which binds an electron in a large orbit
Distance -
Fig. I. A flat band diagram showing the donor and acceptor
levels along with the pair recombination transition.
795
(usually several lattice spacings). This donor-bound
electron undergoes radiative recombination by tunneling to the acceptor-bound hole. In most cases
there is a Coulomb interaction between the donor
and the acceptor and this interaction gives rise to a
distance-dependent term in the expression for the pair
emission energy given below:
&,A = Em,-(ED + EA) + e’lu,
where EDA is the donor-acceptor emission energy,
Egapis the band-gap energy, ED and EA are the donor
and acceptor binding energies, respectively, and the
last term is the Coulomb interaction between the pair.
Since the closer pairs recombine faster than the
distant pairs, the emission wavelength shifts to the
red with time after pulsed excitation. The expectation
in AgBr, because of the emission characteristics, was
an acceptor which had no Coulomb interaction with
the donor, which was thought to have a net positive
charge. Thus the acceptor could not have been a
silver ion vacancy. The identification of the two
major ODMR resonances as intermediate case excitons [24,25], that is, donor-acceptor pairs which are
closer than the Bohr radius of the donor, provided
an explanation for the absence of a peak shift after
pulsed excitation in AgBr [19]. This identification
further strengthened the assignment of the g = 2.08
ODMR resonance as an acceptor transition and
provided the connection between the spectra of pure
AgBr and those doped with Cd’+ and Pb’+. The
ODMR spectra of the doped systems exhibit only two
resonances (g = 2.08 and g = 1.49) which are identified as acceptor and donor transitions, respectively.
These doped systems also exhibit the characteristic
wavelength shift after pulsed excitation. Here the
acceptors are expected to be silver ion vacancies
which will have a Coulomb interaction with the
donors. In AgBr this interaction is masked by the
large proportion of the emission that comes from
close pairs which are intermediate case excitons.
The absorption spectra associated with the trapped
hole species have been used to estimate the trap depth
or the position of the center within the band-gap.
This was often done by using the energy of the
absorption maximum. This technique seriously overestimates the energy associated with the center. This
is particularly true for systems such as the silver
halides in which the electron-phonon
coupling is
large. The feature of the spectrum that is properly
associated with the adiabatic excitation energy of the
center is the O-O band. In systems with large electron-phonon
interactions this feature is often not
observed but it is known to occur at the onset of the
absorption band. Thus, a better procedure would be
to extrapolate the leading edge of the absorption
back to zero. This method is not perfect but it
should give a much closer estimate of the energy. A
similar procedure should be used for emission spectra. The emission from the iodide bound exciton [26]
796
J. P. SWONHOWER and A. P.
illustrates the point. If the energy of the emission
peak (495 nm = 2.504 eV) is associated with this center, then the energy below the band edge would be
180 meV. When the energy of the O-O band, which in
this case can be observed, is used, the energy below
the band edge is determined to be 43 meV. The
estimate from extrapolation of the leading edge is
68 meV, a much better estimate than the peak of the
emission. The infrared (IR) absorption associated
with the trapped hole in AgBr can be interpreted
using this extrapolation procedure. This yields a
value of about 250 meV for the trap depth instead of
the larger 380meV associated with the peak of the
absorption [21,22].
The IR absorptions assigned to trapped holes in
AgBr are broad and structureless. They do not
provide any structural information about the nature
of the trapped hole species. The ODMR transition at
g = 2.08 has been tentatively assigned as a trapped
hole resonance that takes part in donor-acceptor pair
recombination; but this resonance does not exhibit
any anisotropy or hyperfine structure that would
define the structure of the trapped hole species. The
information available on the trapped hole center in
AgBr suggests that the hole is trapped at a silver ion
vacancy, although trapping at a very low level impurity is not ruled out. The microscopic picture that is
expected for this center is one in which the spin
density of the hole is spread among six nearest
bromine ions of the silver ion vacancy with lesser
amounts of density on neighboring silver and bromide ions. A simple structure for the trapped hole
center is proposed in Fig. 2. This center is the
complement of the F center in alkali halides [27].
Definitive proof that the g = 2.08 resonance observed
in the ODMR spectrum of AgBr and AgBr doped
with Cd(2 +) or Pb(2 +) is due to holes trapped at
silver ion vacancies might be obtained from ODENDOR experiments in which this center is probed and
the hyperfine contributions
of the near neighbors
Fig. 2. A proposed structure for the acceptor trapped hole
in AgBr where the acceptor is a silver ion vacancy. This
structure is very speculative.
MARCHEITI
determined and used to identify the structure of the
center.
A similar situation is found in AgBr,_,I,
where
x = 0.03. Here the ODMR spectrum of the emission
associated with the bromoiodide phase consists of
two resonances. One has been identified as an electron (donor) resonance at g = 1.46; the other resonance at g = 1.67 is tentatively assigned to a hole
trapped at a silver ion vacancy which contains one or
more iodide ions in the first shell [28-301. Here again
the emission shows characteristics of donor-acceptor
recombination,
and this acceptor has the correct
charge to cause the emission wavelength to shift with
time after pulsed excitation. The reason for the rather
large difference in g-value in comparison to AgBr is
not understood. It may be related to the fact that
these trapped hole centers are shallow or EMA states
as determined by energetic considerations [31].
Preliminary experiments on the time evolution of
the broad emission band from AgI crystals which
exhibited exciton lines indicative of the existence of
both room temperature phases (/I and y) found a shift
to longer wavelengths with time after pulsed excitation [32]. This can be interpreted as donor-acceptor
pair recombination. If pair recombination is indeed
occurring, then one expects that shallow donorbound electrons are recombining with acceptor
trapped holes. The nature of the trapped hole state is
unknown.
4. VALEXCE TRAPS
A variety of species, either intrinsic to or doped
into the silver halide lattice, undergo a valence state
change upon trapping of a carrier. The number of
such trapped states that are deep-hole trap states is
relatively small. The change of valence of the impurity is indicative of a high degree of carrier localization; the hole is relatively tightly bound in the
potential of the impurity ion. This high degree of
localization in the real space of the crystal implies a
corresponding high degree of delocalization in kspace. As a consequence of this and the high degree
of phonon coupling in the silver halides, the optical
transitions characteristic of the impurity state are
typically very broad. Very little structural information can be obtained from analysis of the optical
spectra.
Magnetic resonance methods have been the techniques of choice to explore the nature of deep-trapping states in the silver halides. Deep trapping implies
that some modest degree of thermal stability exists
for the trap state. Although low temperature methods
are required to assure maximum sensitivity of the
ESR or related technique, frequently the temperature
range over which these species can be studied includes
room temperature as an upper limit. Thus, impurity
states which are important in practical applications of
the silver halides can be addressed as well as other 10~
temperature
photochemically-produced
intermediate
Trapped holes in silver halides
797
Table 3. The g-values, hyperfine and superhypertine constants of the STHs in silver bromochloride
crystals; hyperfine and superhyperfine constants are in milliteslas (after Refs 37, 38 and 50-S)
Complex
(Ag’&)‘(AgBrCls)‘-
(A@, Cl,)‘- (sym)
(AgBr, Cl, )‘-(asy)
(AgBr,Cl,)‘(AgBr,)
gn
2.146
2.024
2.003
2.112
2.108
2.078
g,
2.037
2.133
2.114
2.044
2.044
2.065
AH(A‘l
3.1
2.1
1.9
2.9
2.9
?
[33]. For instance, experiments of this type
can be used to identify trapped-hole structures as
a function of temperature
and wavelength of
exposure.
A variety of transition metal and chalcogenide ions
has been identified as deep-hole traps in the silver
halides. In many cases a number of different structures exist for a particular ion; these differences are
brought about by the diversity of vacancy-impurity
complexes which are possible [34]t, as well as the
potential for aggregation of the impurity ions; because of the enhanced spin-spin interactions in impurity aggregates, and the subsequent broadening of
states
ESR transitions,
magnetic resonance methods have
made more conclusive studies of isolated hole trap-
ping states. These studies are reviewed in depth in the
paragraphs that follow.
(A) Self-trapping
The many similarities in the properties of AgCl and
AgBr disappear in a discussion of the trapped hole
states in the two materials. In contrast to AgBr where
holes are thought to be trapped at silver ion vacancies, the low temperature photophysics of AgCl is
dominated by the self-trapped hole (STH). However,
the relatively small value of the hole drift and Hall
mobilities in AgBr has been interpreted by Toyozawa
and Sumi [35] and Kanzaki [36] as indicating
metastable hole self-trapping at room temperature.
The STH in AgCl was first identified by HGhne and
Stasiw [37,38] on the basis of the similarity of the
ESR spectra observed in exposed samples which had
been doped with Ag,S and Ag,Se to the spectrum
observed in X-ray irradiated Ag+-doped KC1 and
NaCl [39]. The ESR data on the self-trapped hole in
AgCl are given in Table 3. The hole release from Cu*+
in AgCl by blue light irradiation and the IR-induced
decay of the self-trapped hole have been studied in
detail by ESR spectroscopy [4]. An absorption band
observed in Cu*+-doped AgCl was assigned to the
self-trapped hole absorption, and transient absorptions in the IR region of the spectrum were likewise
assigned to self-trapped holes in both AgCl and
AgCl, _,Br,[41,20-221. These absorptions all peaked
at a wavelength of I.1 pm or about 1.2 eV. The
self-trapped hole was thought to participate in pair
t There are a number of examples of this phenomenon
made available by the study of deep electron trapping
centers in the silver halides [34].
QUA,,
&I,
&I,
&w
g,rw
2.6
3.0
2.9
1.9
2.0
?
0
2.1
?
?
?
-
2.6
0
?
?
?
-
14.6
12.4
0
0
0
14.2
12.3
10.8
recombination that was responsible for the bluegreen luminescence of AgCl [41,42].
The energy barrier to go from the free hole state to
the self-trapping state has been measured and found
to be 1.8 meV, while the binding energy of the
self-trapped hole was estimated to be about 0.1 eV
[43-45]. Recent theoretical studies of self-trapping of
excitons has led to a suggestion that the classical
method of estimating the barrier to self-trapping may
be incorrect [46,47]. This work points to a possible
need for a reexamination of the methods used to
derive the height of the barrier in AgCI. A reinterpretation of the energetics of the decay of the self-trapped
hole has shown an excellent fit to Waite’s equation,
that is, to diffusion-limited kinetics [48]. This investigation also produced data that indicate that a small
population of self-trapped holes decay at higher
temperatures, which in turn indicates variation in the
trap depths and structure about the centers. The work
of Kao has suggested that the high temperature
self-trapped hole species are self-trapped holes
with compensating vacancies in the first and second
shells [49].
The data available on the self-trapped hole in AgCl
have been interpreted to give a picture of this center.
It consists of a silver ion in an octahedral complex
of six chloride ions which has formally trapped a
hole on the silver ion. The complex has undergone
a Jahn-Teller distortion in which two opposing or
para-situated chloride ions move away from the
central silver ion [39]. The structure of this complex
is shown in Fig. 3.
A series of ESR investigations were performed
on mixed crystals of AgCI, __I Br,. These studies
Fig. 3. The structure of the self-trapped hole in AgCl (after
Ref. 39).
J. P. SPOONHOWER
and A. P. MARCHETTI
798
examined self-trapped holes which contained bromide
ions in the first shell, along with Jahn-Teller effects,
reo~entation effects and growth processes [SO-551.
The spin resonance data on these various complex
self-trapped hole species are given in Table 3. An
interesting change occurs between the all-chloride
complex and the complexes which contain one or
more bromide ions. The ground state electronic
con~guration was found to be Ix? -y*} in all the
Ag ++
Abs
chloride complexes while the complexes with one or
\
two symmetrically positioned bromide ions have a
13,-- r”) configuration. The asymmetric complex
Conf. Coordinate
and the one with four bromides in the xy-plane have
ground states which are a mixture of the two conFig. 5. A configuration coordinate diagram showing the
figurations but the all-bromide complex returns to the
Ag’+ and Ag* potential curves and the rest&ant shifted
absorption spectrum.
1X2- y’) con~guration.
The self-trapped hole was recorded in ODMR
spectra of AgCl [56,57]. This species was later idenexcitation were a bound-free transition, then the O-O
tified [S&-60]. The self-trapped hole and shallowlyenergy, or zero phonon onset, would be equivalent to
trapped electrons undergo donor-acceptor
pair
the trap depth and the position of the trap above the
recombination which contributes to the blue-green
valence band. Using the extrapolation procedure, a
500 nm) luminescence from AgCl[l8]. The species
value of about 700 meV is obtained for the O-O
observed in the ODMR spectrum has g-values and
energy. This is much larger than the 400 meV obfine-structure and hyperfine constants which are
tained for the acceptor binding energy from an
identical, within experimental error, to those obextrapolation of the emission spectra and an analysis
served by ESR methods [61]. The self-trapped hole
of these data in terms of the donor-acceptor model.
resonance is enhanced in ODMR spectra by the
This discrepancy may be explained by a large elecaddition of part per million amounts of known
tron-phonon
coupling or, equivalently, a displaceelectron trapping dopants such as Ir’+, Mn3+, Ni’+
ment in the potential energy curves for the
and Rh”‘. This is probably due to competition which
self-trapped hole state and a lattice silver ion, as
overwhelms exciton formation and other electronshown in Fig. 5. Here the large difference between the
trapping processes and enhances pair recombination
ground and excited state potential energy curves may
[62,63]. This enhanced STH spectrum is observed
be responsible for the overestimation of the O-O
in an ODMR study of Rh’+-doped AgCi which is energy by extrapolation. The energy estimated from
shown in Fig. 4. Self-trapped hole species have
the analysis of the emission as pair recombination
been observed in the ODMR spectra of AgCl, _.TBr, may suffer the same problem but it would be, again,
samples (64-661.
an overestimate of the energy. Thus it is concluded
The IR absorption spectra that have been obtained
that the relaxed, self-trapped hole lies 400 meV or less
on AgCl (vide supru) can be used to estimate the
above the valence band. The data from the ESR
adiabatic excitation energy of the trapped hole. If this
studies of the decay of the self-trapped holes yiefded
a trap depth equal to or greater than 100meV.
I
I
8
,
I
1.2
1.1
Magnetic Field (lJ
Fig. 4. The ODMR spectrum of Rh”+-doped AgCl (approx.
8 mopm) at 1.8 K with the magnetic field parallel to the
(IOdj direction. The lower spectrum was bbtained with
microwave modulation at 512 Hz. The upper spectrum was
obtained with a magnetic field modulation of 1.8 mf at
204 Hz. The microwave frequency was 32.68 GHz (after
Ref. 61).
(B) Transition metal ions
The case of CuI+,‘+ doped into AgCl has received
a relatively large amount of attention. This has
occurred because in the Cu?+ state, the copper ion
can be photo-ionized to produce Cu’+, thus providing a source of holes independent of photo-produced
electrons. This, in turn, allows high concentrations of
the AgCI:- STH to be observed. Optical excitation
can, however, reverse the valence state change; in
crystals doped with Cu*, holes are deeply trapped at
the copper ion, leading to the formation of the Cut+
species. The optical properties of these centers were
extensively studied in a series of papers by Moser
et al. [67-69]. These optical results were correlated
with ESR studies in an effort to relate the rather
featureless absorption bands with the details provided by the magnetic resonance data. Exposure
within these bands has been shown to produce
Trapped holes in silver halides
electron photoconductivity
[70], suggesting that the
excited states of these impurities are close to or
resonant with conduction band states. Earlier ESR
experimentation
was successful in elucidating the
details of the structure for the defect [71]. As mentioned previously, a variety of centers were found for
the copper ion in the AgCl lattice. These differ in the
number and arrangement of compensating silver ion
vacancies which were complexed to the Cu*+ ion.
Analysis of the spectra showed that the Cu2+ substitutes for the Ag+ on the cation sublattice. Nearby
bound vacancies produce axial distortions of the
complex resulting in three inequivalent Cu’+ sites
(sites A, B, and C). The degree of complexing is
believed to be temperature dependent; at higher
temperatures (above 200 K) motion of the bound
vacancies was suggested by the broadening of the
spectral features. More recently, rich hyperfine structure was observed and analyzed for the [loo] Cu2+
center [72] in AgCI. Optical emission has also been
observed which is believed to involve the participation of the hole trapped at the Cu+ site [73]. There
appear to be no analogous studies of copper doping
in either AgBr, AgI, or in the other mixed halide
systems.
Iron centers in both AgCl and AgBr have been
shown to function as deep hole traps; again, ESR was
used to identify the trapped hole species [74]. Upon
halogenation of the crystals, signals characteristic of
an Fe’+ center were observed. No ESR of the ferrous
state was seen. Optical [75] and Mossbauer [76-791
spectral methods indicate that the Fe*+ occupies a
substitutional position. In both hosts, evidence was
produced for the Fe’+ occupation of an interstitial
position. This is the only documented case of a deep
interstitial defect in the silver halides. In the AgCl
case, more conclusive evidence was given for the
association of four equivalent silver ion vacancies,
producing a tetrahedral complex, as shown in Fig. 6.
Fig. 6. The structure of the interstitial Fe’+ impurity’in
AgCl (after Refs 80 and 81).
199
In the original ESR study, evidence was given for
the vacancy association based upon ESR and conductivity data. Additionally, ENDOR spectroscopy
confirmed this particular structure [80,81].
Subsequent work showed that the same complex,
as well as a trigonally distorted version of the same
species, could be produced by irradiation [82]. This
particular effort has broad significance for it serves
to demonstrate the interplay of electronic and ionic
events that frequently occurs in radiation studies of
ionic materials. In AgCI, optical and Miissbauer data
[75-791 were interpreted in terms of a model which
placed the Fe?+ on a Ag+ site with Ag+ vacancy,
most probably in a next-nearest neighbor [200] position. At temperatures below 168 K there is no evidence from ESR for Fe’+ formation. The authors
suggest that at these temperatures thermal processes
have insufficient probability; the (Fe’+-Ag&,) + hole
complex acts as a recombination
center because
motion of the vacancy is precluded and the center
is therefore attractive to electrons. At higher temperatures (i.e. above 178 K), the tetrahedral (FeCI,)complex forms. Hole trapping is followed by the
motion of the Fe’+ to the interstitial site and loss of
the neighboring silver ions. At temperatures between
168 and 178 K, the so-called B site is formed. This
trigonal complex is believed to be the tetrahedral
(FeCI,)- complex with one of the neighboring Ag+
sites occupied. More recently, Laredo et al. [83] have
investigated the role that competitive electron and
hole trapping plays in determining the extent of hole
trapping at Fe’+ centers. In contrast, these authors
found Fe:,&, formation above 60 K which switched to
Fe:&,,,, formation above 120 K.
For AgBr the formation and decay of the (FeBr,)complex occur at lower temperatures. This difference
is characteristic of the smaller activation energy for
the diffusion of Ag+ ions and the Fe’+ through the
AgBr lattice.
A number of the transition metal ions can function
either as hole traps or electron traps; the resulting
behavior depends on the number and distribution of
associated Ag+ ion vacancies. Eachus et al. studied
the photochemistry of palladium-doped AgBr [84]
and AgCl [85]. Pd’+ enters the lattice either with no
vacancy compensation or forms a single vacancy
(neutral) complex defect. The uncompensated site
traps electrons while the singly vacancy-compensated
site acts as a hole trap. In AgBr, the Pd3+ ion was
most readily detected at 70 K, where the Pd+ signal
(formed as a result of electron trapping) was significantly broadened. Tetragonal distortion of the complex was observed; one or two associated cation
vacancies on a [loo] axis were suggested as an explanation of this effect. For the AgCl case, a similar
complex was observed to trap holes. For temperatures above 80 K, the complex ((PdCl,)4- (Ag&)] was
shown to behave as a hole trap. Interestingly, at lower
temperatures the complex apparently functions as an
electron trap, suggesting that self-trapping by Ag+
800
J. P. SW~NHOWR
ions precludes Pd’+ hole trapping. The thermal prop
erties of this and the other complex defects in this
complicated system were investigated. An analogous
complex defect [(PtCi,)4- (Ag,+,)] in platinum-doped
AgCl was also identified to have a significant hole
trapping cross-section [86].
(C) Chalcogen ides
The chalcogenide ions O?-, S*-, Se2-, and TeZ- in
AgBr and in AgCl extend the optical absorption of
the host material out to well beyond the band-gap
energy. In the case of Te?- doping, the absorption
extends beyond 650 nm in the AgBr case [87]. The
photochemical response of these materials is similarly
extended. The ground states of these centers function
as effective hole trapping states; the excited states are
close to or resonant with the conduction band states,
allowing for efficient photoelectron excitation. Parallel studies of the optical properties of similarly-doped
AgI and the various mixed crystals are lacking at this
point in time. The structure of these impurities cannot
be inferred from the optical studies; again ESR has
been employed to elucidate the details. A variety of
centers is produced; the details are complicated because either Se:- or Te:- defect centers are produced
in crystals doped with Ag,Se or Ag,Te, respectively.
Originally Ebert [88], in investigations of polycrystalline AgCl and AgBr materials, and Busse and
Hennig [89], in single crystal AgBr studies, interpreted their results in terms of monovalent impurity
anions located at substitutional positions. More detailed ESR investigations [90,91] in AgBr showed
that dimer centers were produced upon exposure.
This center is shown in Fig. 7. This structure is
isoelectronic to the V, center observed in the alkali
halides; it consists of a Se:- (or Te:-) molecule with
a [ 1IO] oriented axis. The hole is shared equally by the
two impurity atoms. A pair of defect centers was
identified depending upon the exposure temperature
[92]. They differ in the vacancy compensation. Exposure at 120 K with green light (approx. 500nm)
Fig. 7. The selenium dimeric impurity center in AgBr (after
Refs 90 and 91; note that an uncompensated defect is
shown).
and A. P.
MARCHEITI
produces the trapped hole center, a Se:- defect
center. Note that the center forms not by trapping a
valence band hole, but by electron photoionization;
the photoreaction
hv
Se:- -
Se:- + e -
(3)
describes the mechanism whereby a hole is produced
at the impurity. This so-called T center is aligned
along a lattice [ 1IO] direction and compensated by a
bromide vacancy along the pseudo-molecular axis. It
is believed that an interstitial Ag+ ion is adjacent to
the Se:- center in a [ll l] direction. Warming the
crystal to 170 K or irradiation at 170 K, elicits the
M-center spectrum. The M center is identical to the
T center except for the absence of the compensating
bromide vacancy. Analogous centers were identified
in AgCl [92]. Te:- centers form upon exposure with
525 nm light in Ag,Te-doped AgBr at 170 K. This
species is stable up to 250 K; no analog to the T
center was found when the irradiation temperature
was below 170 K. Unlike the Ag?Se-doped AgBr
case, no information is available on the compensation
scheme for the Te:- defect center. Apparently,
AgCl : Te has not been investigated. Sulfide-doped
AgCl and AgBr have also been investigated by ESR
techniques but with less success than for either the
Ag,Se- or Ag,Te-doped silver halides. Growth of
crystals with isolated impurity centers is exceedingly
difficult. Owing to the low solubility of the sulfide ion,
there is a tendency for the S- ions to aggregate.
Coupled with this is the fact that the g-tensor anisotropy for the paramagnetic sulfide centers is
smaller than for the analogous selenium or tellurium
centers. Very little structural information was made
available. Hiihne and Stasiw [90] failed to identify the
structure details of any one center even though they
were able to produce a number of such sulfide centers
with exposure. Mononuclear chalcogen ions can be
produced by exposing samples which have been
double-doped. In a series of papers [93-951 Schwarz
reported on the effects of silver halide doping with
cadmium-selenium
pairs, cadmium-tellurium
pairs,
copper-selenium
or -tellurium pairs, and coppersulfide pairs. In AgCl crystals co-doped with Cu and
Se or S, an ESR spectrum characteristic of the Cu”
ion is observed. However, unlike the singly-doped
case, the octahedral chlorine environment of the CL?+
ion is distorted by the presence of the Se’- or S’- ion.
For the selenium-doped AgCI, evidence for a strong
interaction with a single “Se nucleus is available from
the superhyperfine structure in the spectrum. The
structure for this particular center is given in Fig. 8,
where a [loo] orientation for the defect complex is
shown. The evidence for this particular structure is
less convincing for the case AgCI:Cu, S because
crystals doped with sulfur enriched in the 33Sisotope
were not examined. Sonoike et al. [96] later extended
this work to studies of AgCl crystals doublydoped with silver sulfide and mercurous or mercuric
Trapped holes in silver halides
801
point out that the experimental g-values do not
provide a good match either to the expected values
for an O- center or to an 0; center, based upon
parallel studies of these defects in the alkali halides.
This structural problem has yet to be resolved.
Fig. 8. The structure of the Cu2+/Se2- center in doubledoped AgBr (after Ref. 93).
chloride. It is worth noting that relatively high levels
of the mercury salts were added to the melt, and that
in addition the crystals were rapidly quenched during
the growth, presumably to preclude aggregation of
the sulfide ions. Photoproduction of the signals only
begins above 200 K; the production was thermally
activated as well; because the activation energy was
close to that of the silver interstitial formation and
migration, the authors postulated that the initial
diamagnetic sulfur center was a AgS-/Ag+ complex.
The effect of the mercury was to stabilize the sulfide
ESR signals; there was no indication of incorporation
of the mercury ion into the sulfide complex.
The incornoration of selenium or tellurium with
cadmium into AgCl crystals leads to somewhat more
straightforward results [94]. Blue light irradiation at
170 K leads to the production of an orthorhombic
center with a Se- ion strongly bound to the two
adjacent Ag+ ions; bonding to a single Cd+ occurs
in the orthogonal direction. Analogous results were
obtained for the tellurium/cadmium
doubly-doped
case.
AgBr:Se crystals co-doped with Cd or Cu were
also scrutinized [95]. A variety of centers were detected by ESR. At irradiation temperatures of I40 K
(with 520nm light) an axial center (labelled A)
with [ lOO] symmetry in AgBr:Se, Cd is observed.
With high irradiation temperatures (250 K), a second
defect can be identified in the same crystal. The
so-called B center again has axial symmetry along a
(100) direction; its g-tensor has different values from
the A center. A full analysis of the complex observed
in AgBr:Se, Cu was not given (g-values were not
reported), but hyperfine data led to the conclusion
that the complex defect produced by light exposure in
this material consisted of a Se’- substituting for Brwith a neighboring Cu’ in an interstitial position.
ESR in oxygen-doped AgBr crystals has also been
reported [97]. The spectrum possesses axial symmetry
with a [loo] direction as the symmetry axis. Identification of the exact nature of the oxygen bonding has
not been accomplished for this material. The authors
(D) Rare earth ions
Certain rare earths doped into the silver halides
have been investigated; because of the tendency of
these ions to be incorporated as trivalent species, and
the necessity of charge compensation by Ag+ vacancies, rather complicated impurity-vacancy complexes
are found. Although interesting in their own right,
there is, unfortunately, little practical consequence to
motivate these investigations because with a rare
exception, the rare earths are photochemically inert
[98]. The exception to the rule is the case of europium
in AgCl [99], where the product of irradiation has
been identified as a hole trap. In a non-oxidizing
atmosphere, the europium enters the lattice in the
divalent state. Depending on the temperature, either
a single vacancy (T = 93 K) or no vacancies (at room
temperature) are associated with the Et?+. The single
vacancy is associated with the europium in a nearest
cation position; a spectrum with orthorhombic symmetry is observed. Exposure with blue light at room
temperature causes a reduction in the number of Eu2+
ions; hole trapping by the Eu*+ (the uncompensated
site only) and conversion to the trivalent species
occurs. The photoreaction is reversible, however.
Irradiation with red light absorbed by the silver metal
colloid band causes the release of electrons which are
subsequently re-trapped at the Eu3+ sites. Optical
studies of the rare earth ions are lacking. Unlike the
transition metal dopants, a study of the rare earth
doped silver halides ought to yield a great deal of
structural information.
Table 4 contains a summary of the data provided
by ESR investigations of the various dopant ions in
the silver halides which undergo a valence state
change as a result of hole trapping.
5. CONCLUSIONS
This review has sought to present the status of our
present knowledge of the physical properties of holetrapping states in the silver halides. After a brief
overview of the band and transport properties of
these materials, hole trapping by Coulomb and
deeper potentials was considered; because of the
extension of the trapped-hole wavefunction over
many lattice sites in the case of shallow trapping by
the Coulomb potential, the details of the geometry
and bonding of the central impurity are still lacking
for a great many systems. Perturbative methods
should be applied to unravel the structure of this
class of defects; the application of uniaxial stress
coupled with either far-infrared or ODMR techniques is appropriate, as it has been quite successful in the study of similar shallow defect states in
802
J. P.
SFQ~NHOWER
and A. P. MAKHETI-I
semiconductors and ionic solids [lOO, 1011.The recent
identification of an intermediate case exciton in AgBr
provides an explanation for the paradoxical recombination behavior in this material. The non-exponential
luminescence kinetics, observation of separate donor
and acceptor resonances in ODMR, and the lack of
a luminescence wavelength shift with time after
pulsed excitation have all been reconciled.
The deeply-trapped hole states in the silver halides
have been the subject of more detailed studies.
Table 4. The g-values and hyperfme constants for dopant ions which undergo a valence state change in silver
halides as a result of hole trapping
Defect
AgCl :Cur+ (A)
A&l : Cu?+ (B)
AgCl:Ct?+ (C)
AgCl:(FeCI,)-(tetr.)
AgCI : (FeCI,)- (trig.)
AgBr : (FeBr,)-
AgCl : Pt
A@: Pd
AgBr : Pd
A&l : Se(T)
AgCI:Se(M)
AgBr : Se(T)
AgBr : Se(M)
AgBr:Te
AgBr:S
AgBr : Se
AgBr:S
AgBr:Se
AgBr:Te
A&l : S
AgCI : Se
AgCl : Se. Cu
AgCl : S, Cu
AgCl : Se, Cd
AgCl :Te, Cd
A&l: S, Cd
AgBr : Se, Cd(A)
AgBr : Se. Cd(B)
AgCI:S, Hg
AgBr:O
AgCl : Eu(93 K)
g-value
g, = 2.00 f 0.02
g, = 2.28 f 0.02
g, = 2.26 + 0.02
g, = 2.07 k 0.02
g, = 2.30 & 0.02
g, = 2.07 + 0.02
g = 2.0158 +O.OOl
g = 2.006
D= -(81.3)x
IO-‘cm-’
F = - (6.3 + 1.0) x IO-‘cm-’
g = 2.045 f 0.005
g, = 2.369
g, = 2.012
g, = 2.184
g, = 2.056
g, = 2.152
g, = 2.101 f 0.001
g! = 2.078 + 0.001
gz = 2.023 f 0.001
g,V= 2.100 f 0.001
gy = 2.086 f 0.001
g, = 2.013 rt 0.001
g,V= 2.143 i 0.001
gy = 2.057 f 0.002
g, = 2.017 f 0.001
g, = 2. I I6 f 0.002
gJ = 2.086 + 0.003
g: = 2.005 f 0.002
g, = 2.248 + 0.002
g! = 2.062 f 0.003
g, = 1.887 f 0.001
g = 2.015 ~0.005
g = 2.08 f 0.01
g = 2.017 f 0.005
g = 2.12 *0.015
g = 2.21 f 0.03
g = 2.014 + 0.005
g = 2.082 f 0.015
g, = 2.02 f 0.01
g, = 2.14 f 0.01
g,, = 2.015 & 0.005
g, = 2.15 f 0.005
g, = 2.240 + 0.005
g! = 2.041 f 0.003
g; = I .907 rf:0.003
g, = 2.35 f 0.01
6’ 2.21 f 0.01
g, = 1.80 f 0.01
g, = 1.925 f 0.005
g, = 1.950 * 0.005
g, = 2.26 + 0.003
g, = I.91 * 0.03
gt = 2. I8 & 0.005
g, = 2.065 f 0.005
g, = I .999 f 0.003
g, = 2.03 I f 0.003
g, = I.881 f 0.005
g, = I .793 + 0.00
g = 1.9945 f 0.0015
Additional parameters
Ref.
71
71
71
a = (75.2 f 1.0) x IO-‘cm-’
a = (187 f 1.0) x IO-‘cm-’
a = (288 & 5) x IO-‘cm-’
(I; = (35.6 + 0.03) x 10-4cm-’
01 = (6.9 i 0.03) x 10W4cm-’
a: = (40 + 5) x 10-4cm-’
a;. = (40 + 5) x Joe4 cm-’
(I: = (169 k 0.5) x 10-4cm-’
82.74
82
82,74
86
85
84
92
92
92
92,90
90
Linewidth
Linewidth
Linewidth
Linewidth
Linewidth
Linewidth
Linewidth
=
=
=
=
=
=
=
9.0 f 0.5 mT
10.0 + 2.0 mT
7.0 f 0.5 mT
7.0 &-1.0 mT
10.7 + I.5 mT
4.6 f 0.7 mT
6.0 + I.0 mT
89
89
88
88
88
88
88
93
93
94
94
94
95
95
96
q = 6.5 + 0.5 mT
cI = 4.5 it: 0.5 mT
a’J’ = -(31.l kO.3) x 10-4cm-’
cIJ3 = - (13.7 + 0.3) x IO-acm-’
97
99
Trapped holes in silver halides
Although ESR data exist for many of these isolated
impurities, our knowledge of more complex impurity
aggregates is still inadequate. In addition, problems
with specific isolated impurities still remain. In particular, our understanding of the AgBr:S case is quite
poor, perhaps, in part, because of the difficulty in
preparing quality single crystals for study. The case
of oxygen in the silver halides merits attention. The
structure for this presumably ubiquitous impurity
remains an enigma. Again, the double resonance
methods ENDOR and its optical analog ODENDOR
could quite conceivably shed some light on this
problem.
The structure of the self-trapped hole has been
detailed by magnetic resonance methods. ODENDOR might be fruitfully applied to the study of this
species in an effort to provide details concerning the
environment for the site. Does the geometry of the
self-trapped hole result from a “bare” Jahn-Teller
distortion, or do secondary interactions with an as yet
unidentified impurity play an important role?
Although optical studies of hole traps have not
provided much detailed knowledge of the impurity-vacancy complex structure in the silver halides
for the transition metal and chalcogenide ions, optical investigations of the rare earths ought to provide
such an opportunity. Europium in AgCl has been
shown to be photoactive and because a wealth of
optical data on analogous centers exists for the alkali
halides, these results should be readily interpretable.
If the constraint of being photoactive is relaxed, or if
new photoactive rare earths are identified, a wide
class of materials becomes available for such studies.
In general, little data exist for trapping species
(both electron or hole traps) outside AgCl and AgBr;
studies of these entities in AgF and AgI are essentially
non-existent. Investigations of trapping states in
mixed halide crystals are less rare, although the full
spectrum of hole trapping states studied in the “pure”
silver halides has yet to be examined.
Ackno&dgemenrs-The authors would like to thank R. S.
Eachus, J. F. Hamilton, L. M. Slifkin and M. S. Burberry
for reading this manuscript and for many helpful suggestions, changes and additions.
803
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