Relaxation of photodarkening in SiO-As₂(S,Se)₃ composite layers
Investigated in this work is the reversible photostimulated red absorption edge shift (photodarkening), ∆Eg, of As₂(S,Se)₃ nanoparticles embedded into the SiO matrix. As compared to continuous chalcogenide films, the remarkable ∆Eg increase (up to 4 times) with decreasing of chalcogenide particle si...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
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nasplib_isofts_kiev_ua-123456789-1198612025-02-09T14:58:57Z Relaxation of photodarkening in SiO-As₂(S,Se)₃ composite layers Indutnyi, I.Z. Shepeliavyi, P.E. Indutnyi, V.I. Investigated in this work is the reversible photostimulated red absorption edge shift (photodarkening), ∆Eg, of As₂(S,Se)₃ nanoparticles embedded into the SiO matrix. As compared to continuous chalcogenide films, the remarkable ∆Eg increase (up to 4 times) with decreasing of chalcogenide particle sizes in composite SiO-As₂(S,Se)₃ layers was revealed. The exponential dependence of ∆Eg on storing time at different temperatures has been obtained. An activation energy of the transition of A₂2S₃ nanoparticles structure from a metastable photoexposed state to a ground annealed state is equal to 0.78 ± 0.06 eV. The effects are related to a spatial confinement of a photoexcited carrier diffusion length and an influence of particle sizes on intermediate-range order scale structure relaxation in the chalcogenide nanoparticles. 1999 Article Relaxation of photodarkening in SiO-As₂(S,Se)₃ composite layers / I.Z. Indutnyi, P.E. Shepeliavyi, V.I. Indutnyi // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 2. — С. 59-62. — Бібліогр.: 11 назв. — англ. 1560-8034 PACS 78.66J,F, 81.15, 78.40.P, 73.61.J, 73.50.M https://nasplib.isofts.kiev.ua/handle/123456789/119861 en Semiconductor Physics Quantum Electronics & Optoelectronics application/pdf Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Investigated in this work is the reversible photostimulated red absorption edge shift (photodarkening), ∆Eg, of As₂(S,Se)₃ nanoparticles embedded into the SiO matrix. As compared to continuous chalcogenide films, the remarkable ∆Eg increase (up to 4 times) with decreasing of chalcogenide particle sizes in composite SiO-As₂(S,Se)₃ layers was revealed. The exponential dependence of ∆Eg on storing time at different temperatures has been obtained. An activation energy of the transition of A₂2S₃ nanoparticles structure from a metastable photoexposed state to a ground annealed state is equal to 0.78 ± 0.06 eV. The effects are related to a spatial confinement of a photoexcited carrier diffusion length and an influence of particle sizes on intermediate-range order scale structure relaxation in the chalcogenide nanoparticles. |
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Article |
| author |
Indutnyi, I.Z. Shepeliavyi, P.E. Indutnyi, V.I. |
| spellingShingle |
Indutnyi, I.Z. Shepeliavyi, P.E. Indutnyi, V.I. Relaxation of photodarkening in SiO-As₂(S,Se)₃ composite layers Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Indutnyi, I.Z. Shepeliavyi, P.E. Indutnyi, V.I. |
| author_sort |
Indutnyi, I.Z. |
| title |
Relaxation of photodarkening in SiO-As₂(S,Se)₃ composite layers |
| title_short |
Relaxation of photodarkening in SiO-As₂(S,Se)₃ composite layers |
| title_full |
Relaxation of photodarkening in SiO-As₂(S,Se)₃ composite layers |
| title_fullStr |
Relaxation of photodarkening in SiO-As₂(S,Se)₃ composite layers |
| title_full_unstemmed |
Relaxation of photodarkening in SiO-As₂(S,Se)₃ composite layers |
| title_sort |
relaxation of photodarkening in sio-as₂(s,se)₃ composite layers |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
1999 |
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https://nasplib.isofts.kiev.ua/handle/123456789/119861 |
| citation_txt |
Relaxation of photodarkening in SiO-As₂(S,Se)₃ composite layers / I.Z. Indutnyi, P.E. Shepeliavyi, V.I. Indutnyi // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 2. — С. 59-62. — Бібліогр.: 11 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
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| first_indexed |
2025-11-27T02:19:32Z |
| last_indexed |
2025-11-27T02:19:32Z |
| _version_ |
1849908239690366976 |
| fulltext |
59© 1999, Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
Semiconductor Physics, Quantum Electronics & Optoelectronics. 1999. V. 2, N 2. P. 59-62.
PACS 78.66J,F, 81.15, 78.40.P, 73.61.J, 73.50.M
Relaxation of photodarkening in
SiO-As2(S,Se)3 composite layers
I. Z. Indutnyi, P. E. Shepeliavyi, V. I. Indutnyi*
Institute of Semiconductor Physics of NASU, 45, prospect Nauki , 252028 Kiev, e-mail: indutn@dep14.semicond.kiev.ua
*Institute for Problems of Information Recording, National Academy of Sciences,
ul. Shpaka, 2, Kyiv-113, 252113, Ukraine
Abstract. Investigated in this work is the reversible photostimulated red absorption edge shift
(photodarkening), ∆Eg, of As2(S,Se)3 nanoparticles embedded into the SiO matrix. As compared to
continuous chalcogenide films, the remarkable ∆Eg increase (up to 4 times) with decreasing of
chalcogenide particle sizes in composite SiO-As2(S,Se)3 layers was revealed. The exponential depend-
ence of ∆Eg on storing time at different temperatures has been obtained. An activation energy of the
transition of As2S3 nanoparticles structure from a metastable photoexposed state to a ground annealed
state is equal to 0.78 ± 0.06 eV. The effects are related to a spatial confinement of a photoexcited carrier
diffusion length and an influence of particle sizes on intermediate-range order scale structure relaxation
in the chalcogenide nanoparticles.
Keywords: photodarkening, chalcogenide glasses, nanoparticles, SiO-As2S3 , SiO-As2Se3
Paper received 25.05.99; revised manuscript received 24.06.99; accepted for publication 12.07.99.
1. Introduction
The photoinduced transformation of thin amorphous
chalcogenide layers have attracted considerable attention
owing to possibilities of developing basic principles of their
physical mechanisms and exploring possible technical ap-
plications in optical recording, holography, microelectro-
nics, optoelectronics. The most investigated chalcogenides
are As2S3 and As2Se3 binary glasses. Illumination with the
bandgap light induces structural changes of vacuum-evapo-
rated chalcogenide layers, which results in changes of opti-
cal (shift of absorption edge, ∆Eg), chemical (change of solu-
bility), mechanical and others properties [1-3]. In as-evapo-
rated layers displayed are both irreversible photostimulated
transformations caused by polymerization of breaking bonds
and reversible ones which can be eliminated by thermal an-
nealing just below the glass transition temperature. The an-
nealed films exhibit only reversible photostimulated trans-
formations which accompanied by the reversible red shift
of absorption edge (i.e., photodarkening). A large number
of scientific papers have been devoted to investigation of
the mechanism of this phenomenon, a lot of models have
been proposed [2-8], but there is no complete explanation
of all experimental results as yet.
Recently, the effect of essential enhancement of
photodarkening was revealed in composite SiO-As2S3 lay-
ers, when sizes of As2S3 particles, embedded in SiO matrix,
are comparable with the scale of the medium-range order in
the chalcogenide structure [9]. In this paper the results of
further investigations of photodarkening in the composite
films and peculiarities of ∆Eg relaxation are presented.
2. Experimental procedures
The samples were prepared by thermal vacuum evaporation
and sequential deposition of a large number (from 48 to 130
couples) of ultrathin island-type SiO and As2S3 or As2Se3
layers onto polished silica substrates at room temperature.
The effective thickness of each SiO layer was equal to 5 nm
for all samples, and the effective thicknesses of the
chalcogenide layers were changed for different samples from
2.5 to 0.7 nm in order to obtain a total chalcogenide thick-
ness in multilayer samples from 90 to 130 nm. During SiO
deposition some part of the substrate was screened to pre-
pare a reference film of As2S3 or As2Se3 composition. The
layers thickness being determined «in situ» by a quartz-crys-
tal-oscillator monitoring system was also measured after
deposition by microinterferometer both for the multilayer
(composite) films and the reference ones. The deposited films
were annealed at 450 K in the ambient atmosphere for two
hours. Photodarkening was induced at 77 K by exposure to
a high-pressure Hg lamp (light intensity 50 mW/cm2) for
I. Z. Indutnyi et al.: Relaxation of photodarkening in SiO-As2S(Se)3 composite...
60 SQO, 2(2), 1999
ααααα (104, cm-1)
2
6
a
0
4
6
8
b
hν, eV
Fig. 1. Spectral dependence of absorption coefficient, α, of As2Se3 layer
with thickness 127 nm (a), and multilayers (SiO - As2Se3)90 sample with
effective thickness of each As2Se3 layer in composite of 1.36 nm (b).
- annealed, - exposed samples.
Fig. 2. Magnitude of the light-induced edge shift, ∆Eg , as a function of
chalcogenide layer effective thickness. Dots 2-5 correspond to composite
SiO - As2S3 samples, 2� - SiO - As2Se3 , 1 and 1�- reference As2S3 and
As2Se3 films.
one hour. The subsequent annealing-exposure circles were
carried out in the same conditions. The storing of the sam-
ples at elevated temperatures were carried out in a thermo-
stat with a temperature stability of 1 K.
The composite samples obtained form an effective SiO-
As2(S,Se)3 media with a smaller fraction of chalcogenide.
SiO layers are transparent in the spectral range of As2S3
and As2Se3 interband transitions which enables to study the
photodarkening of chalcogenide nanoparticles in the SiO
matrix by means of measuring their absorption coefficient
(α) in annealed and exposed samples. For calculation of
chalcogenide nanoparticles absorption from photometric
measurements of the composite samples we used total
chalcogenide layers thickness measured on the reference
films. More detail experimental conditions on the sample
preparations and optical measurements are described in [9].
3. Results
Absorption spectra, α vs photon energy, hν, are shown in
Fig. 1 for two cases: (a) the reference As2Se3 layer of
127 nm thickness and (b) the composite (SiO - As2Se3)90
film (90 - number of SiO - As2Se3 layer-couples in the sam-
ple), with effective thickness of each As2Se3 layer in the
composite of 1.36 nm. The open circles are the absorption
after annealing of the films, while the solid circles are the
same ones after illumination. As seen from Fig. 1, both sam-
ples exhibit photodarkening, and photostimulated effect in-
creases in composite samples. In this work the
photostimulated optical gap changes, ∆Eg , were obtained as
the absorption edge shifts at α = 2.5.104 cm-1 level.
Fig. 2 shows the dependence of ∆Eg on effective thick-
ness of As2Se3 and As2S3 layers in composite samples. Dots
denoted by figures 1 and 1� correspond to reference As2Se3
and As2S3 layers . The results show essential increasing of
photodarkening when the chalcogenide layer effective thick-
ness (and dimensions of chalcogenide island in SiO matrix)
decreases. For composite SiO - As2S3 sample with effective
As2S3 layer thicknesses of 0.7 nm ∆Eg increases more than
4 times in comparison with reference «thick» film; for
selenide based composite sample with effective As2Se3 layer
thickness of 1.36 nm the ∆Eg increases approximately by
2.5 times. It should be mentioned that maximum values of
photodarkening obtained so far for chalcogenide films (to
our knowledge) corresponds to ∆Eg = 0.15 eV for As2S3
and 0.66 eV for As2Se3 [2] that are by 2 - 2.5 time less than
maximum ∆Eg revealed in our composite samples. The sec-
ond feature of composit layers is smaller absorptivity of
chalcogenide inclusions than that of continuous reference
films.
Fig. 3 shows, as an example, the dependence of
photostimulated ∆Eg on the time of exposure (kinetics of
photodarkening) of composite (SiO - As2Se3)90 sample. We
can see that after fifteen minutes of exposure the
photostimulated absorption edge shift is saturated. That is
why we used one hour exposure to ensure the saturation of
photostimulated transformation.
The exposed state of chalcogenide is metastable and re-
laxes towards the ground annealed state. Such relaxation
occurs also at smaller temperatures than the glass transition
ones, for example at room temperature, but more slowly. To
investigate the relaxation peculiarities, the kinetics of ∆Eg
decreasing with storing time at different temperatures were
measured. Fig. 4 shows the dependencies of -ln (∆Eg) on the
storing time for the SiO-As2S3 composite samples with ef-
fective As2S3 thicknesses of 0.7 nm. Numbers on the plots
indicate the storing temperatures, the dots are experimental
16
12
8
4
0
8
6
4
2
0
1.6 1.8 2.0 2.2 2.4 2.6
1.6 1.8 2.0 2.2 2.4 2.6
0 1 2 3 4 5 6
5
4
3
2
1
1 �
2 �
120 127
d, nm
∆E
g
, e
V
0.4
0.3
0.2
0.1
0.0
I. Z. Indutnyi et al.: Relaxation of photodarkening in SiO-As2S(Se)3 composite...
61SQO, 2(2), 1999
Fig. 3. Dependence of ∆Eg on the time of exposure. SiO-As2Se3 compos-
ite samples with effective As2Se3 thicknesses of 1.36 nm.
Fig. 4. ∆Eg relaxation at different temperatures: a) - 313 K, b) - 333 K,
c) - 353 K. SiO-As2S3 composite samples with effective As2S3 thick-
nesses of 0.7 nm.
0
5
0
a)
313 K
0
0
b)
333 K
0
5
0
5
c)
353 K
-l
n
(
∆E
g
),
e
V
values, solid lines are linear approximations. The kinetics
of relaxation at all investigated temperatures can be ex-
pressed by the exponential dependence:
∆Eg =A+B.exp(-t/τ), (1)
were A, B are the constants, (A+B=∆Eg
0 is initial value of
∆Eg immediately after exposure), τ is the characteristic time
of relaxation, that decreases with growing the storing tem-
perature. Using the formula (1) for fitting the experimental
dots in Fig. 4 (solid line) we obtained τ values for different
storing temperatures. The dependence of ln(τ) on inverse
temperature is shown in Fig. 5 and has the Arrhenius-type
behavior.
4. Discussion
The reversible photodarkening in As2S3 layers is caused by
two-stage process: photoexcitation of the electron system
of chalcogenide followed by relaxation of the amorphous
structure into the new metastable state. Numerous investi-
gations involving optical (Raman, IR-spectra) measurements
[7], polarized X-ray and other measurements [4-6] have led
to the conclusion that photodarkening are connected with
structural changes in the chalcogenide medium-range or-
der, the appearance of photoinduced homopolar bonds were
also admitted. Most of the researchers considered the elec-
tron stage of photoinduced transformations as including for-
mation of some localized exciton-like state by a photoexcited
electron-hole pair with the respective rearrangement of
bonds. Such a self-trapped exciton (STE) can relax to the
initial state, became metastable as intimate valence-alter-
nation pair, or relax to other bonding configuration, some
of them may contain more homopolar bonds. As suggested
by Fritzsche [2], these recombination-induced bond
rearrangements accumulate during exposure to produce a
more disordered structure on the medium-range scale. The
top of the As2(S,Se)3 valence band is formed by the inter-
actions between the lone-pair p-orbitals of the chalcogens.
As it follows from Fritzsche�s model, the optical gap nar-
rowing is mainly caused by stronger lone-pair in the ex-
posed state relative to the annealed state and to a lesser ex-
tent by local defects - an increase in the concentration of
metastable valence-alternating pairs (VAP), or homopolar
bonds.
The models which include a recombination stage through
STE consistently explain the known temperature depend-
ence of photodarkening [3]: with decreasing temperature the
electrons are localized near the sites of excitation, the prob-
ability of STE formation increases, and the effect is enhanced.
By using such models, we can explain the obtained depend-
ence of ∆Eg on effective layer thickness (which is related to
t (104,s)
2.0
1.5
1.0
0 2 4 6 8 10
∆E
g
, e
V
t, min
0.12
0.08
0.04
0.00
0 10 20 60
-l
n
(
∆E
g
),
e
V
t (104,s)
2.0
1.5
1.0
0.0 0.5 1.0 1.5 2.0
-l
n
(
∆E
g
),
e
V
0.0 0.5 1.0 1.5 2.0 2.5 3.0
t (103,s)
2.5
2.0
1.5
1.0
I. Z. Indutnyi et al.: Relaxation of photodarkening in SiO-As2S(Se)3 composite...
62 SQO, 2(2), 1999
Fig. 5. The temperature dependence of decay time, τ (the same sample as at
Fig. 4). Points are experimental values, solid line is linear approximation.
the chalcogenide particle dimensions). When the size of the
As2(S,Se)3 particles embedded into the dielectric SiO ma-
trix is decreased, the diffusion length of the photoexcited elec-
trons is confined, resulting in an enhancement of the STE for-
mation rate, photostructural transformations and
photodarkening. The increase of ∆Eg may be also related to
the effect of mechanical strain in nanoparticles, but our esti-
mation showed [9] that this effect was negligible.
The decrease of α in composite sample in comparison
with continuous chalcogenide films may be caused by two
reasons. The first one is creation of As - O bonds at SiO-
As2(S,Se)3 interface. Such local regions may be considered
as microinclusions of phase with smaller absorptivity (the
optical gap of As2O3 is 5 eV[10]). The second one is a quan-
tum-dimensional effect that can be revealed in absorption
spectra when the nanoparticle size becomes close to the Bohr
radius of an exciton aB. For As2(S,Se)3 the estimated val-
ues of aB is equal to several angstroms[11].
The peculiarities of photodarkening relaxation result
mainly from the mechanism of appearing the metastable state
with lower optical gap. Exponential kinetics of relaxation
are connected with thermostimulated transitions from the
metastable state (medium order structural changes - twist-
ing of the adjacent As(S,Se)3/2 pyramids about their shared
chalcogene atom, other changes of the spatial arrangement
of neighboring structural units, or local defects - VAP,
homopolar bonds) into the ground state and, as a conse-
quence, with the decrease of their concentration, N(t), when
growing the time of storing. When ∆Eg<< Eg we can sup-
pose with reasonable accuracy that ∆Eg(t) ∝ N(t) [8], and
N(t) = N0exp(-t/τ), where N0 is an initial concentration of
the photoinduced changes immediately after exposure. It re-
sults in exponential dependence of ∆Eg vs time that was re-
vealed in the experiments (see Eq. (1)). The characteristic
time of relaxation, τ, from the metastable to the initial an-
nealed state is described by expression:
1/τ = Ω.exp(-Ea/kT), (2)
where Ω is an oscillation frequency in the metastable state,
Ea is an activation energy of the transition. This expression
reasonably fits the experimental data in Fig. 5, and from
such fitting one can obtain the Ea value. For a composite
SiO - As2S3 with effective chalcogenide layers thicknesses
of 0.7 nm we obtained Ea = 0.78 ± 0.06 eV.
Values of Ea for evaporated As2S3 films have been esti-
mated from the above experimental study of the annealing
kinetics and the dependencies of photodarkening on the light
intensity [8]. It was obtained that Ea varies from site to site,
and the number distribution of the photogenerated defects
having an energy barrier Ea can be approximated with a
gaussian function, exp[-(Ea-E0)2/σ2], where E0 and σ2 de-
note the central barrier height and the variance. For As2S3
films E0 = 1 eV, and σ = 0.5 eV [8]. In our composite sam-
ples Ea variations are less pronounced and central barrier
height is lower than that of continuous layers. This result
can be explained, possibly, by a more homogeneous struc-
ture of chalcogenide nanoparticles embedded into the di-
electric matrix and spatial confinement of structure relaxa-
tion.
References
1. V. M. Lyubin, Photostructural transformations in chalcogenide vitre-
ous semiconductors, Avtometrija (Russia) 4, 18-27(1988).
2. H. Fritzsche, The origin of reversible and irreversible photostructural
changes in chalcogenide glasses, Philos. Magazine B 68(4), 561-
572(1993).
3. S. R. Elliott, A unified model for reversible photostructural effects in
chalcogenide glasses, Journ. Non-Cryst. Solids 81(1-2), 71-98(1986).
4. C. Y. Yang, M. A. Paesler, and D. E. Sayers, Measurement of local
structural configurations associated with reversible photostructural
changes in arsenic trisulfide films, Phys. Rev. B 36(17), 9160-
9167(1987).
5. L. F. Gladden, S. R. Elliott, G. N. Greaves, Photostructural changes in
bulk chalcogenide glasses: an EXAFS study, Journ. Non-Cryst. Sol-
ids 106(1-3), 189-192(1988).
6. J. M. Lee, M. A. Paesler, and D. E. Sayers, Kinetic X-ray absorption
studies and computer structural modeling of photo-darkening in amor-
phous arsenic sulfide, Journ. Non-Cryst. Solids 123(1-3), 295-
309(1990).
7. O. I. Shpotyuk, Coordination defects formation model for reversible
photostructural transformations in amorphous As2S(Se)3, Physics and
applications of non-crystalline semiconductors in optoelectronics,
A. Andriesh and M. Bertolotti, 123-129, Kluwer Academic Publish-
ers, Netherlands, 1997.
8. K. Tanaka, Light intensity dependence of photodarkening in amorphous
As2S3 films, Thin Solid Films 157(1), 35-41(1988).
9. I. Z. Indutnyi, P. E. Shepeljavi, Reversible photodarkening in As2S3
nanolayers, Journ. Non-Cryst. Solids, 227-230, 700-704(1998).
10. W. M. Pontuschka, P. C. Taylor, ESR in X-irradiated As2O3 glass,
Sol. State Commun. 38(7), 573-577(1981).
11. N. F. Mott, F. A. Davis, Electron Processes in Non-Crystalline Materi-
als, Claredon Press, Oxford, 1979.
ln
τ,
s
1000/T, K-1
2.8 3.0 3.2 3.4 3.6
14
12
10
8
|