Influence of g-irradiation on photoluminescence spectra of CdTe:Cl
Photoluminescence properties of high-resistivity CdTe:Cl crystals irradiated with g-rays have been studied. An enhancement of near-edge luminescence intensity is observed after a low dose g-irradiation (D < 10 kGy). For larger doses of g-irradiation both a decrease of the total luminescence inten...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
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| Zitieren: | Influence of g-irradiation on photoluminescence spectra of CdTe:Cl / N.D. Vakhnyak, S.G. Krylyuk, Yu.V. Kryuchenko, I.M. Kupchak // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2002. — Т. 5, № 1. — С. 25-30. — Бібліогр.: 31 назв. — англ. |
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irk-123456789-1195612017-06-08T03:06:31Z Influence of g-irradiation on photoluminescence spectra of CdTe:Cl Vakhnyak, N.D. Krylyuk, S.G. Kryuchenko, Yu.V. Kupchak, I.M. Photoluminescence properties of high-resistivity CdTe:Cl crystals irradiated with g-rays have been studied. An enhancement of near-edge luminescence intensity is observed after a low dose g-irradiation (D < 10 kGy). For larger doses of g-irradiation both a decrease of the total luminescence intensity and the intensity redistribution between the lines of excitons bound to different acceptors occur. For the donor-acceptor recombination with A-centers participation, an increase of the Huang-Rhys factor S is found with D increase. This fact can be explained by decrease of the A-centers concentration. The experimentally determined S values are compared with a calculated S(R) dependence for different distances R between donors and acceptors. 2002 Article Influence of g-irradiation on photoluminescence spectra of CdTe:Cl / N.D. Vakhnyak, S.G. Krylyuk, Yu.V. Kryuchenko, I.M. Kupchak // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2002. — Т. 5, № 1. — С. 25-30. — Бібліогр.: 31 назв. — англ. 1560-8034 PACS: 78.55.Et; 78.55.-m http://dspace.nbuv.gov.ua/handle/123456789/119561 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Photoluminescence properties of high-resistivity CdTe:Cl crystals irradiated with g-rays have been studied. An enhancement of near-edge luminescence intensity is observed after a low dose g-irradiation (D < 10 kGy). For larger doses of g-irradiation both a decrease of the total luminescence intensity and the intensity redistribution between the lines of excitons bound to different acceptors occur. For the donor-acceptor recombination with A-centers participation, an increase of the Huang-Rhys factor S is found with D increase. This fact can be explained by decrease of the A-centers concentration. The experimentally determined S values are compared with a calculated S(R) dependence for different distances R between donors and acceptors. |
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Vakhnyak, N.D. Krylyuk, S.G. Kryuchenko, Yu.V. Kupchak, I.M. |
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Vakhnyak, N.D. Krylyuk, S.G. Kryuchenko, Yu.V. Kupchak, I.M. Influence of g-irradiation on photoluminescence spectra of CdTe:Cl Semiconductor Physics Quantum Electronics & Optoelectronics |
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Vakhnyak, N.D. Krylyuk, S.G. Kryuchenko, Yu.V. Kupchak, I.M. |
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Vakhnyak, N.D. |
| title |
Influence of g-irradiation on photoluminescence spectra of CdTe:Cl |
| title_short |
Influence of g-irradiation on photoluminescence spectra of CdTe:Cl |
| title_full |
Influence of g-irradiation on photoluminescence spectra of CdTe:Cl |
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Influence of g-irradiation on photoluminescence spectra of CdTe:Cl |
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Influence of g-irradiation on photoluminescence spectra of CdTe:Cl |
| title_sort |
influence of g-irradiation on photoluminescence spectra of cdte:cl |
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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2002 |
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| citation_txt |
Influence of g-irradiation on photoluminescence spectra of CdTe:Cl / N.D. Vakhnyak, S.G. Krylyuk, Yu.V. Kryuchenko, I.M. Kupchak // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2002. — Т. 5, № 1. — С. 25-30. — Бібліогр.: 31 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
AT vakhnyaknd influenceofgirradiationonphotoluminescencespectraofcdtecl AT krylyuksg influenceofgirradiationonphotoluminescencespectraofcdtecl AT kryuchenkoyuv influenceofgirradiationonphotoluminescencespectraofcdtecl AT kupchakim influenceofgirradiationonphotoluminescencespectraofcdtecl |
| first_indexed |
2025-07-08T16:10:35Z |
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1837095755986239488 |
| fulltext |
25© 2002, Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
Semiconductor Physics, Quantum Electronics & Optoelectronics. 2002. V. 5, N 1. P. 25-30.
1. Introduction
Due to a high average atomic number and a large
energy gap, cadmium telluride is a very promising
material for production of non-cooled X- and γ-ra-
diation detectors. During the last decade, great ad-
vancements were achieved in growing high-quality
CdTe material and in development of effective meth-
ods for detector signal processing. This promoted a
wider application of CdTe detectors in medicine,
radiology, space studies, etc. (for a recent review,
see, e.g., [1-2] and references therein). For effec-
tive detection of ionizing radiation, CdTe crystals
should meet the following requirements: i) high re-
sistivity to reduce the leakage current and, thus, to
increase the signal/noise ratio and ii) good trans-
port properties, namely high value of mobility-life-
time product (µτ) for electrons and holes, in order
to realize the full charge collection at electrodes
and, hence, to increase the sensitivity and to im-
prove the energy resolution.
Among different dopants used to obtain high-re-
sistivity (ρ>108 Ω⋅cm) CdTe crystals the best results
were obtained by doping with chlorine. Despite the
behavior of Cl impurity in CdTe is not completely
understood yet, chlorine was established to contrib-
ute to the self-compensation phenomena which is
typical for II-VI compounds (see, e.g. [3]). Accord-
ing to the most studied model of self-compensation,
it is thought that the main intrinsic defects in CdTe
is Cd vacancies (VCd). In the process of CdTe:Cl
material growth, chlorine atoms occupy Te sites
(ClTe) and act as shallow donors (EC-0.014 eV). A
part of CdTe donors interacts with VCd creating com-
plexes (VCd-ClTe) (the so-called A-centers, which act
as acceptors with energy level at Ev+0.126 eV) and
complexes (VCd-2ClTe). A-centers, in turn, compen-
sate influence of ClTe donors on CdTe:Cl electrical
properties [4, 5]. It should be noted however that in
some publications [6-8] this mechanism is consid-
ered as insufficient to explain alone such high resis-
tivity of the Cl-doped crystals.
A lot of CdTe:Cl detector material studies have
been performed earlier. Nevertheless, up to now no
complete picture exists on changes in the impurity
and defect system of CdTe crystals induced by dif-
ferent kinds of crystal irradiation. Recent studies by
Cavallini et al. revealed that high doses of gamma, neu-
tron and electron irradiation cause a significant deterio-
ration of CdTe and CdZnTe detector performance up to
complete loss of the detecting ability [9, 10]. Since CdTe
detectors may be exposed to very high fluxes of γ-radia-
tion, it is extremely important to have detailed knowl-
edge on radiation-induced changes which can occur in
the crystal volume. Therefore, it is necessary to study
PACS: 78.55.Et; 78.55.-m
Influence of γγγγγ-irradiation on photoluminescence spectra
of CdTe:Cl
N.D. Vakhnyak1), S.G. Krylyuk2), Yu.V. Kryuchenko2), I.M. Kupchak2)
1) Yu.Fed�kovych Chernivtsi National University, 2 Kotsyubynsky st., 58012 Chernivtsi, Ukraine
2) Institute of Semiconductor Physics, NAS of Ukraine, 45 Prospect Nauki, 03028 Kyiv, Ukraine
Phone/fax: +380 (44) 265 6391, e-mail: div47@isp.kiev.ua
Abstract. Photoluminescence properties of high-resistivity CdTe:Cl crystals irradiated with γ-rays
have been studied. An enhancement of near-edge luminescence intensity is observed after a low dose γ-
irradiation (D ≤ 10 kGy). For larger doses of γ-irradiation both a decrease of the total luminescence
intensity and the intensity redistribution between the lines of excitons bound to different acceptors
occur. For the donor-acceptor recombination with A-centers participation, an increase of the Huang-
Rhys factor S is found with D increase. This fact can be explained by decrease of the A-centers
concentration. The experimentally determined S values are compared with a calculated S(R) dependence
for different distances R between donors and acceptors.
Keywords: cadmium telluride, photoluminescence, γ-irradiation, Huang-Rhys factor, low dose effect,
self-compensation.
Paper received 20.02.02; revised manuscript received 28.02.02; accepted for publication 05.03.02.
26 SQO, 5(1), 2002
N.D. Vakhnyak et al.: Influence of g-irradiation on photoluminence spectra of CdTe:Cl
influence of γ-irradiation on fundamental charac-
teristics of CdTe:Cl material, which determine di-
rectly or indirectly its detector properties.
In this paper, to study influence of γ-radiation
on the impurity and defect system of high-resistiv-
ity CdTe:Cl crystals we have used a method of low-
temperature photoluminescence (PL) spectroscopy.
2. Experimental
CdTe:Cl single crystals with Cl concentration of
about 2⋅1019 cm-3 were grown by the Bridgman
method; the growth technique details were described
elsewhere [11]. The electrical resistivity of the sam-
ples was ρ~1010 Ω⋅cm and the electron mobility was
µe~700 cm2V-1s-1. For a comparison we studied γ-irra-
diation influence on undoped CdTe samples, too,
but here we will focus on the properties of CdTe:Cl.
The PL experiments were performed at 5 K us-
ing for excitation a cw Ar-laser operating in all-
lines mode. The excitation power was about 10 mW/cm2
and kept constant in all experiments. PL signal was
dispersed with a 0.6 m monochromator and detected
by a cooled photomultiplier and a conventional lock-
in technique.
The samples were exposed to γ-ray using a 60Co
γ-cell at a MRX-γ-25 M setup with a dose rate of
95.28 R/s. A predetermined irradiation dose was
delivered to the sample by setting the necessary ex-
posure time. The irradiation doses used were in the
range D = 1 � 1000 kGy. Measurements of the PL
spectra of the samples were performed after each
irradiation session and were followed by further ir-
radiation to reach the next dose. The time period
between the irradiation sessions and the PL experi-
ments were as short as possible to avoid any relaxa-
tion of lattice defects created by γ-rays.
3. Experimental results
In Fig. 1, PL spectra taken at 5 K are plotted for the
unirradiated CdTe:Cl sample and g-irradiated with doses
1 kGy and 10 kGy. In the exciton region of the spectra
four PL lines denoted in the figure as (D0, X), (A0, X), G
and W are resolved. A weak luminescence from the free
excitons is also observed at 1.598 eV. The origin of the
(D0, X) and (A0, X) lines is well established. The (D0, X)
line at 1.593 eV is due to excitons bound to neutral do-
nors ClTe and the (A0, X) line at 1.589 eV results from
annihilation of excitons bound to neutral Cu acceptors.
Fig. 1. PL spectra of a CdTe:Cl sample before g-irradiation and
irradiated with doses of 1 kGy and 10 kGy. The spectra are
normalized to the respective intensity of the (A0, X) line.
At the same time, some discrepancy exists in the litera-
ture in identification of the lines G (1.590 eV) and W
(1.586 eV). In Ref. [12] the G line was ascribed to excitons
bound to A-centers, and in Refs. [13, 14] it was attributed
to excitons bound to acceptors (VCd-2ClTe). The W line
at first was attributed to donor-acceptor (D-A) recombi-
nation [15]. Later the excitonic nature of this line was
established and it was identified as recombination of
excitons bound to acceptors (VCd - ClTe). In any case,
both G and W lines appear due to introduction of the Cl
impurity into the CdTe matrix, namely, due to the crea-
tion of complexes involving ClTe centers.
Irradiation of CdTe:Cl with γ-rays results in a de-
crease of intensity of all exciton lines connected with the
presence of the ClTe centers ((D0, X), G and W lines) as
compared to the (A0, X) line intensity. The (A0, X) be-
comes the most intensive line in the spectrum after sam-
ple irradiation with a dose 10 kGy. At the same time, the
total exciton intensity for the both samples increases for
low exposure doses reaching the maximum at about 10
kGy for CdTe:Cl and 3 kGy for undoped CdTe. A higher
irradiation dose causes a reduction of the intensities down
to the initial value or even lower. A dose dependence of
integral intensity of all exciton line is shown in Fig. 2 for
Cl-doped and undoped CdTe.
Except these lines we also observed: (a) a weak emis-
sion band assigned in the literature to the conduction
band � acceptor transitions and the D-A recombination
N.D. Vakhnyak et al.: Influence of g-irradiation on photoluminence spectra of CdTe:Cl
27SQO, 5(1), 2002
In
te
ns
ity
o
f
ex
ci
to
ni
c
P
L
, a
rb
. u
ni
ts
Fig. 2. Dose dependencies of intensities of the excitonic PL
(right y-axis) for CdTe:Cl (dots) and undoped CdTe (squares)
and intensity of the DAP recombination line normalized to I ex
(left y-axis, triangles). The lines are drawn as a guide to for the eye.
as well as their phonon satellites at 1.53 � 1.58 eV and (b)
a broad band that includes well resolved LO-phonon
replicas in the region 1.35-1.50 eV. Analysis of the last
PL band at 1.35-1.50 eV is the scope of the paper. This
band was assigned by de Nobel to the recombination of
the donor-acceptor pairs (DAP) consisting of A-centers
and ClTe [16]. However, in this spectral range the DAP
recombination with participation of background impuri-
ties Cu, Ag, Au can be also detected [8]. Analysis of the
1.35-1.50 eV emission is complicated for the possible
coexistence and superposition in this spectral region of
PL bands having different nature [8, 17-19]. Therefore,
a great inaccuracy may arise when determining the pa-
rameters of this band, for example the Huang-Rhys fac-
tor value which characterizes electron-phonon coupling.
In particular, very high values of the Huang-Rhys factor
(S≈2.2 or even higher) are reported for the DAP recombi-
nation [19, 20] which exceed substantially the calculated
one (S≈1.4). As a rule, the most short-wavelength line
(1.478 eV) of the band is considered as zero-phonon line
(ZPL) of the DAP luminescence. However, the 1.478 eV
line is not always observed in the PL spectra of the CdTe:Cl
samples and, on the contrary, it may appear in undoped
crystals in which no VCd-ClTe complexes are created at
all. In several papers, it was reported however that the
1.478 eV line (the so-called Y-line) is connected with the
carrier recombination at extended defects, and a weak
electron-phonon coupling for this line was found [17, 18].
In our recent studies on CdTe:Cl crystals covered by a
thin SiO2 film, a very strong enhancement of the 1.478
eV line intensity was observed after the sample irradia-
tion with the second harmonic of an YAG:Nd laser at a
P > 1MW/cm2 [21]. At the same time, intensities of the
other PL lines at lower energy were almost constant or
even decreased. Thus, we can conclude that the 1.35-
1.50 eV PL band is a superposition of two PL lines: the
Y-line and the A-center DAP recombination with zero-
phonon lines at ~1.478 eV and ~1.455 eV, respectively,
and corresponding LO-phonon replicas.
pulse power P>1 MW/cm2 [21]. At the same time,
intensities of the other PL lines at lower energy were al-
most constant or even decreased. Thus, we can conclude
that the 1.35-1.50 eV PL band is a superposition of two
PL lines: the Y-line and the A-center DAP recombination
with zero-phonon lines at ~1.478 eV and ~1.455 eV,
respectively, and corresponding LO-phonon replicas.
The above speculations were taken into account when
analyzing the DAP emission spectra and determining the
Huang-Rhys factor in the dependence of the γ-irradia-
tion dose. An example of fitting the experimental PL band
with two PL lines (Y-line and DAP) having different S
values is shown in Fig. 3(a). The best fits were obtained
by changing S and the full width at half maximum for the
two lines, whereas corresponding energies of the ZPLs
and the LO-phonon energy (21.2 meV) were kept con-
stant for all doses of γ-irradiation. The Huang-Rhys fac-
tor for the Y-line (S Y≈0.6) was found to be much lower
than that for the DAP PL, and it remained almost con-
stant at increasing dose of γ-irradiation. At the same time,
the Huang-Rhys factor for the DAP recombination ex-
hibits an increase from S DAP≈1.63 for unirradiated sam-
ple to S DAP≈1.70 for irradiated sample with a dose 1000
kGy (Fig. 3(b)). This increase is accompanied with a
decrease of the DAP recombination intensity as compared
to the intensity of exciton lines, see Fig. 2.
1,35 1,40 1,45 1,50
Unirradiated 1 10 100 1000
1,64
1,68
1,72
(a) Experiment
Resulting fit
DAP
Y-line
P
L
in
te
n
s
it
y
,
a
rb
.u
n
it
s
Photon energy, eV
(b)
DAP
S
Dose, kGy
Fig. 3. (a) An example of the 1.35-1.50 eV band fitting with two
lines (see text for details). (b) Dose dependence of the Huang-
Rhys factor for the DAP recombination with A-centers partici-
pation.
28 SQO, 5(1), 2002
N.D. Vakhnyak et al.: Influence of g-irradiation on photoluminence spectra of CdTe:Cl
, is the effective Bohr radius for the
electron (hole), ∗
im - the carrier effective mass, ε0 - the
static dielectric constant of mate-
rial, )]1()/[()]/(2[ 2/1** +Γ= iiiii aaN i
i
ννν ν
ν the normali-
zation factor, νi - the so-called quantum defect parameter
defined from the energy relationship )(
2 / AD
R
ii EE=ν .
In the last expression )(ADE is the donor (acceptor)
ground state ionization energy determined experimen-
tally and )2/( 22
0
4
hε∗= i
R
i meE is the binding energy of
the carrier at the corresponding impurity center within
the framework of the hydrogen-like model.
The Huang-Rhys factor determines the probability
!/~ pSeW pS
p
− of radiative transitions at impurity
center with emission of p LO-phonons. It can be calcu-
lated using well-known formula of the Fröhlich continuum
theory
,
where q is the wave vector, V - the crystal volume,
LOωh the energy of LO-phonons,∞ε - the high-frequency
dielectric constant, rq the Fourier-component of charge
density distribution. For the recombination at an isolated
center (donor or acceptor) the carrier charge distribution
is determined by the formula
,
where N
p
is the normalization constant defined from the
condition 10 =iρ . If a D-A pair is involved in the process
of radiative recombination, then Fourier-component of the
common (electron+hole) charge density distribution is
expressed as
(1)
With such charge distribution the value of the Huang-
Rhys factor becomes dependent on the distance R be-
tween the donor and acceptor:
. (2)
For the case of n=1 the final result takes the analyti-
cal form of the hydrogen-like model presented in [28].
The hydrogen-like model has to be completely valid for
the donor centers due to small experimental values of
ClTe ionization energy (~14 meV) and practically exact
observance of the equality D
R
e EE / =1. For a hole at an
acceptor (A-center) situation is not so clear despite the
equality 120≈≈ R
hA EE meV, because i) Bohr radius of
the ground state in the hydrogen-like model obtained with
handbook values of hole effective mass and static dielec-
tric constant is too small (~ 6 Å) for ignoring possible
changes in both effective mass and screening and, as a
result, in theoretical value of binding energy R
hE itself,
and ii) A-center in fact is not a point defect. It has a defi-
nite spatial structure, which for such small radius of lo-
4. Discussion
The near-edge luminescence in semiconductors is
known to be very sensitive to non-radiative losses at deep
recombination centers created by mechanical stresses,
intrinsic defects, impurities, etc. Increasing PL intensity
at low doses of γ-irradiation implies a reduction of the
concentration of non-radiative centers. This phenomena
is known in literature as the �low-dose effect� or the ra-
diation-stimulated ordering of the crystalline structure.
It results in relaxation of thermodynamically
nonequilibrium metastable phases in crystal volume in-
duced by a low dose of penetrative radiation. The low
dose effect is well studied for Si, Ge and III-V compounds.
It was found, in particular, that radiation-stimulated proc-
esses are enhanced in sub-surface region of a semicon-
ductor and in heterostructures [22, 23]. The low-tempera-
ture PL spectroscopy was shown to be a very effective
tool for studying this phenomena [24, 25]. Radiation-
stimulated ordering of sub-surface region was recently
observed in CdTe as well [26].
As it was shown above, exciton PL intensity depends
non-monotonically on the γ-irradiation dose. An increase
of the exciton PL (by ~ 4 times) indicates on �low-dose
effect� manifestation. For the undoped sample, the maxi-
mal increase of the PL intensity occurs at the lower dose
as compared to CdTe:Cl. Therefore, it is reasonable to
suggest that the �low-dose effect� will not be observed in
high purity crystals with a very low concentration of lat-
tice imperfections. On the contrary, the greater concen-
tration of intrinsic defects and impurities the higher irra-
diation dose is required to reach some improvement in
the lattice perfection.
Let us analyze the shape of the PL spectrum at 1.35-
1.50 eV represented by the Y-line and the DAP recombi-
nation band arising due to the A-centers participation.
The effect of electron (hole) interaction with LO-phonons
on radiative recombination processes at impurity centers
in semiconductors was considered theoretically in a
number of works (see, e.g., [27-29]). Theoretical values
of the Huang-Rhys factor for exciton, free electron�ac-
ceptor and DAP recombination processes were calculated
in these works mainly within the framework of the effec-
tive mass approximation and the hydrogen-like model.
In many cases, however, the obtained results do not coin-
cide quantitatively with experimental data, so a further
development of the theory is necessary.
The most general, i.e. valid not only for the hydro-
gen-like shallow centers, but for relatively deep recom-
bination centers too, is the quantum defect approach
developed in [29]. In this approach, highly localized
ground states of carriers at impurity centers are de-
scribed by the envelope wave functions of the type
where i=e for the bound electron state at a donor and i=h
for the bound hole state at an acceptor, r is a radius-vector,
22
0
2 q/||)/1/1)](/(2[ ∑−= ∞ q qρεεωπ LOVeS h
)]/(exp[)( ∗−=Ψ ii arrNr i
ii
νν
νν
rqrq
3)1(2 d)]/(2[exp)exp( ∗− −= ∫ iii arriN i νρ ν
ρ
)/( 2
0
2 ∗∗ = ii mea εh
−〈= )exp(|)()()( hhheeeh iØØ qrrrqρ
〉− )()(|)exp( eehhe ØØi rrqr
×−= ∞ )/1/1()/()( 0
2 εεωπ LOeRS h
[ ]∫
∞
−+×
0
22 d)/()sin(2|||| qqRqRqhqeqhqe ρρρρ
N.D. Vakhnyak et al.: Influence of g-irradiation on photoluminence spectra of CdTe:Cl
29SQO, 5(1), 2002
calization, can influence to some extent on the hole move-
ment. For these reasons the use of hydrogen-like model is
an open question for the case of A-centers despite formal
validity of 1≈hν
The calculated dependence S(R) for the considered D-A
recombination is shown in Fig. 4. In the upper edge of
this figure, several values of DAP concentration are given,
which correspond to the mean distance R on the x-axis.
The decrease of the Huang-Rhys factor with the decrease
of inter-defect distance in the D-A pair is caused by grow-
ing mutual compensation of charge distributions of these
defects and corresponding decrease in the adiabatic po-
tential shifts for excited states in the configurational
Frank-Condon diagram. With the increase of distance R
between A-centers and donors the dependence S(R) ex-
hibits a saturation at S≈1.4.
Fig. 4. Calculated dependencies of the Huang-Rhys factor S
(right y-axis) and dS/dR on the mean distance R between donor
and acceptor. For several values of R, respective DAP concen-
trations N DAP are given on the top of the figure.
Spectral shape of the DAP band with account for LO-
phonon replicas can be calculated using the following
expression [28]:
(3)
where frequency ω0 determines position of the ZPL, Γ is
the line width, summation is taken over all phonon repli-
cas (p=0, 1, 2 �).
Fitting the experimental spectra obtained at different
doping levels and doses of γ-irradiation we gain infor-
mation on variations in the Huang-Rhys factor, mean
distance between recombination centers and DAP con-
centration due to the corresponding treatment. As it was
mentioned above, for a better fitting between theoretical
and experimental spectra in the studied spectral region,
it is necessary to use more than one series described by
Eq. (3) with different position of ZPLs and values of S.
−−
−∑
=
−
2
0
0
2
2
1
exp
!
~)(
Ã
p
e
p
S
I LO
p
S
p ωωω
ωω
For example, SÀ fitting in Fig. 3(a) have been obtained
with two series of lines (3). As a result, the value of the
Huang-Rhys factorobtained from the fitting of the DAP
recombination series with A-centers participation differs
from the calcu-lated one only by ~20 %. In our case, this
difference is much lower as compared to other works where
the influence of the Y-line with ZPL at 1.478 eV was
ignored. This 20 % difference may be a consequence of
above outlined shortcomings of the hydrogen-like model
for A-centers.
An increase in the Huang-Rhys factor values at higher
doses of γ-irradiation can be explained by a decrease of
the DAP concentration. This conjecture is supported by
the dependence of the relationship I DAP/I ex on the γ-
irradiation dose obtained in experiment (see Fig. 2).
About three times decrease in I DAP/I ex is observed with
the dose rise from 1 to 10 kGy which corresponds to analo-
gous decrease in the DAP concentration N DAP.
From the experimentally obtained dependencies
I DAP/I ex(D)and S DAP(D) it is possible to evaluate the
concentration of the A-centers in the studied material
using the dS DAP/dR dependence (dashed line in Fig.4).
It can be done supposing that the real S DAP(R) depend-
ence has the same curvature as the calculated one. In
practice, the following method have been used for such
evaluation. For the sample irradiated with two doses D1
and D2 (D2 > D1) we obtain the difference in the Huang-
Rhys factors DAP
D
DAP
D
DAP SSS 12 −=∆ from the SA(D) de-
pendence and the DAP intensities ratio DAP
D
DAP
D IIk 21 /=
from the I DAP/I ex(D) dependence. The latter corresponds
to the concentration ratio DAP
D
DAP
D NNk 21 /= . Then, for
each value of N DAP that relates to a D-A distance R1
through N DAP=0.5R-3, we found R2 that corresponds to
decreasing DAP
DN 2 by k times. After that we built a ∆S/
∆R(R) curve, where ∆R = R2 � R1. The intersection of
this curve with theoretical dS DAP/dR dependence gives
an approximate value of N DAP in the dose range D1-D2.
From such evaluation we have found the values
N DAP≤2⋅1015ñì-3 in the range of g-irradiation dose 1-10
kGy. This value is in agreement with results reported in
[30, 31], where the upper edge ~6⋅1015cm-3 for VCd con-
centration in CdTe:Cl was determined.
5. Conclusions
We have studied PL properties of CdTe:Cl crystals sub-
jected to γ-irradiation with different doses. The �low-
dose effect� was found in Cl-doped and undoped CdTe
crystals which manifests in an enhancement of the exciton
PL intensity (by ~4 times) for exposure dose D≤10 kGy.
At further dose increase a quenching of the near-edge PL
occurs due to generation of non-radiative recombination
centers.
A non-monotonic dependence of the exciton PL intensity
on D was accompanied by a redistribution of the relative
intensity of the individual lines: PL intensity of excitons
bound to centers which include ClTe ((D0,X), G and W)
30 SQO, 5(1), 2002
N.D. Vakhnyak et al.: Influence of g-irradiation on photoluminence spectra of CdTe:Cl
decreased more rapidly as compared to the (À0, Õ) line
connected with background Cu impurity. Intensity of the
1.35-1.50 eV line also decreased with increasing D which
is explained by a decrease in concentration of the DAP
consisting of ClTe donors and A-center (VCd-ClTe) accep-
tors. This conclusion is supported by observed increase
in the Huang-Rhys factor S DAP for the DAP recombina-
tion at the γ-irradiation dose increasing.
It was shown that the DAP concentration can be esti-
mated using the S DAP(D) dependence and the calculated
dependence dS/dR, where R is the mean distance between
donor and acceptor. The DAP concentration in investigated
CdTe:Cl samples was estimated to be N DAP ≤ 2⋅1015 cm-3.
Acknowledgements
We wish to express our gratitude to O. P. Shakhov (Insti-
tute of Physics, NASU) for γ-irradiation of CdTe sam-
ples and to O. I. Vlasenko and A.Baidulaeva (Institute of
Semiconductor Physics, NASU) for stimulating discus-
sions.
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