Photo- and thermally-induced changes in the optical properties of Ge-S-Se amorphous films
The optical transmissions spectra of amorphous Ge-S-Se films of chemical compositions (GeS₂)₅₀(GeSe₂)₅₀ and (GeS₃)₅₀(GeSe₃)₅₀, prepared by thermal evaporation, have been measured over the whole 400 to 800 nm spectral range. It has been ascertained that annealing of the films leads to the absorpti...
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irk-123456789-1178142017-05-27T03:05:01Z Photo- and thermally-induced changes in the optical properties of Ge-S-Se amorphous films Rubish, V.M. Gera, E.V. Durcot, M.O. Pop, M.M. Kostyukevich, S.O. Kudryavtsev, A.A. Mykulanynets-Meshko, O.S. Rigan, M.Yu. The optical transmissions spectra of amorphous Ge-S-Se films of chemical compositions (GeS₂)₅₀(GeSe₂)₅₀ and (GeS₃)₅₀(GeSe₃)₅₀, prepared by thermal evaporation, have been measured over the whole 400 to 800 nm spectral range. It has been ascertained that annealing of the films leads to the absorption edge shift into the short-wave spectral region. The values of pseudo-gap width Eg and film refraction index n have been determined. Changes in optical properties of films are caused by structural transformations taking place in them under laser illumination and annealing. 2013 Article Photo- and thermally-induced changes in the optical properties of Ge-S-Se amorphous films / V.M. Rubish, E.V. Gera, M.O. Durcot, M.M. Pop, S.O. Kostyukevich, A.A. Kudryavtsev, O.S. Mykulanynets-Meshko, M.Yu. Rigan // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2013. — Т. 16, № 4. — С. 349-353. — Бібліогр.: 30 назв. — англ. 1560-8034 PACS 78.66.Jg http://dspace.nbuv.gov.ua/handle/123456789/117814 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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The optical transmissions spectra of amorphous Ge-S-Se films of chemical
compositions (GeS₂)₅₀(GeSe₂)₅₀ and (GeS₃)₅₀(GeSe₃)₅₀, prepared by thermal evaporation,
have been measured over the whole 400 to 800 nm spectral range. It has been ascertained
that annealing of the films leads to the absorption edge shift into the short-wave spectral
region. The values of pseudo-gap width Eg and film refraction index n have been
determined. Changes in optical properties of films are caused by structural
transformations taking place in them under laser illumination and annealing. |
| format |
Article |
| author |
Rubish, V.M. Gera, E.V. Durcot, M.O. Pop, M.M. Kostyukevich, S.O. Kudryavtsev, A.A. Mykulanynets-Meshko, O.S. Rigan, M.Yu. |
| spellingShingle |
Rubish, V.M. Gera, E.V. Durcot, M.O. Pop, M.M. Kostyukevich, S.O. Kudryavtsev, A.A. Mykulanynets-Meshko, O.S. Rigan, M.Yu. Photo- and thermally-induced changes in the optical properties of Ge-S-Se amorphous films Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Rubish, V.M. Gera, E.V. Durcot, M.O. Pop, M.M. Kostyukevich, S.O. Kudryavtsev, A.A. Mykulanynets-Meshko, O.S. Rigan, M.Yu. |
| author_sort |
Rubish, V.M. |
| title |
Photo- and thermally-induced changes in the optical properties of Ge-S-Se amorphous films |
| title_short |
Photo- and thermally-induced changes in the optical properties of Ge-S-Se amorphous films |
| title_full |
Photo- and thermally-induced changes in the optical properties of Ge-S-Se amorphous films |
| title_fullStr |
Photo- and thermally-induced changes in the optical properties of Ge-S-Se amorphous films |
| title_full_unstemmed |
Photo- and thermally-induced changes in the optical properties of Ge-S-Se amorphous films |
| title_sort |
photo- and thermally-induced changes in the optical properties of ge-s-se amorphous films |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2013 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/117814 |
| citation_txt |
Photo- and thermally-induced changes in the optical properties
of Ge-S-Se amorphous films / V.M. Rubish, E.V. Gera, M.O. Durcot, M.M. Pop, S.O. Kostyukevich, A.A. Kudryavtsev, O.S. Mykulanynets-Meshko, M.Yu. Rigan // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2013. — Т. 16, № 4. — С. 349-353. — Бібліогр.: 30 назв. — англ. |
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Semiconductor Physics Quantum Electronics & Optoelectronics |
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Semiconductor Physics, Quantum Electronics & Optoelectronics, 2013. V. 16, N 4. P. 349-353.
© 2013, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
349
PACS 78.66.Jg
Photo- and thermally-induced changes in the optical properties
of Ge-S-Se amorphous films
V.M. Rubish1, E.V. Gera1, M.O. Durcot1, M.M. Pop2, S.O. Kostyukevich3, A.A. Kudryavtsev3,
O.S. Mykulanynets-Meshko1, M.Yu. Rigan1
1Uzhgorod Scientific-Technological Center of the Institute for Information Recording, NAS of Ukraine
4, Zamkovi Skhody str., 88000 Uzhgorod, Ukraine, e-mail:center.uzh@gmail.com
2 Uzhgorod National University, 3, Narodna sq., 88000, Uzhgorod, Ukraine
3 V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine, 41, prospect Nauky, 03028 Kyiv, Ukraine
Abstract. The optical transmissions spectra of amorphous Ge-S-Se films of chemical
compositions (GeS2)50(GeSe2)50 and (GeS3)50(GeSe3)50, prepared by thermal evaporation,
have been measured over the whole 400 to 800 nm spectral range. It has been ascertained
that annealing of the films leads to the absorption edge shift into the short-wave spectral
region. The values of pseudo-gap width Eg and film refraction index n have been
determined. Changes in optical properties of films are caused by structural
transformations taking place in them under laser illumination and annealing.
Keywords: chalcogenide amorphous films, transmission spectra, optical properties,
photo-structural transformations.
Manuscript received 15.07.13; revised version received 07.09.13; accepted for
publication 23.10.13; published online 16.12.13.
1. Introduction
Chalcogenide non-crystalline vitreous and amorphous
materials attract continuously growing interest caused by
wide possibilities of practical applications and as a
unique object for scientific investigations. Due to a wide
range of photo-induced phenomena, these materials are
current and potential candidates for using in
optoelectronics and photonics (inorganic photoresists,
optical sensors, waveguides, recording media, circuits,
gratings and diffraction elements for various
applications) [1-8].
From this viewpoint, deeper studied are amorphous
films of binary and ternary arsenic chalcogenides that
are characterized by lowering the pseudo-gap
(photodarkening of films) under light irradiation [1, 3, 5-
10], which is assumed to originate from photo-induced
structural transformations that can be subsequently
reversed by annealing near the glass transition
temperature.
In recent decades, intensively studied are photo-
induced effects in amorphous films of germanium
chalcogenides [11-18]. First of all, in view of practical
applications, these materials are attractive if accounting
the fact that they do not contain the poisonous arsenic.
Second, in these films under illumination one can
observe both effects of photodarkening and
photobleaching (growth of the pseudogap and optical
transparency). Moreover, while germanium sulfide films
are characterized only by photobleaching [13-15], the
films GexSe100-x demonstrate both effects: photo-
darkening in Ge-dificient (x<20) and photobleaching in
Ge-rich (x>20) samples [11, 12, 16-18].
In this paper, we report the results of investigating
the influence of laser irradiation and annealing on
transmission spectra and optical parameters of GeS2-
GeSe2 and GeS3-GeSe3 films with the germanium
content 25 and 33.3 at.%, respectively. Our choice of
these sections was caused by the following reasons. In
[15] we showed that, at the same conditions of
illumination, changes in optical parameters of GeS2
films are essentially higher than those in Ge2S3 films. For
instance, the change in the refraction index n of GeS2
film is practically 3-fold higher than that in Ge2S3 film.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2013. V. 16, N 4. P. 349-353.
© 2013, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
350
In the system Ge-Se, the highest changes in optical
characteristics (shift of the absorption edge Eg and
refraction index n) were observed by us in the films
GeSe2 and GeSe3 [17]. Thereof, it is for the films of the
above sections one should expect a high level of photo-
induced changes in optical parameters.
2. Experimental
The bulk glasses Ce33.3S33.85Se33.85 (or (GeS2)50(GeSe2)50)
and Ge25S37.5Se37.5 (or (GeS3)50 (GeSe3)50) were prepared
using direct synthesis from the corresponding elements
in evacuated silica ampoules. The mass of every charge
was 20 gram. In the process of synthesis, we used the
step-like increase of temperature up to the maximum one
(1250 K). At the temperatures 700, 950 and 1250 К, the
ampoules were kept for 8, 6 and 4 hours, respectively. In
what followed, temperature was lowered down to the
temperature of melt homogenization (1100…1150 К).
The homogenization time was 48 hours. The melts were
periodically stirred. After synthesis the ampoules were
air-quenched.
Thin films (thickness ~ 1 μm) were obtained by
vacuum evaporation of glasses of corresponding
compositions from quasi-closed effusion cells onto non-
heated glass substrates. A uniform thickness of layers
was provided by planetary rotation of substrates.
Light exposure of films was made using defocused
radiation of a semiconductor laser (λ = 530 nm,
P = 100 mW). Annealing of the films was performed in
argon atmosphere for 1 and 2 h at the temperature
423 K.
Optical transmission spectra of the films were
measured at T = 300 K within the range 400 to 800 nm
by using the method [19] with a monochromator МДР-3.
The spectral resolution was no worse than 10–3 eV.
3. Results and discussion
Fig. 1 and 2 (curves 1) show transmission spectra for the
as-prepared films Ge25S37,5Se37,5 and Ge33.3S33.35Se33.35. It
is obvious that with growing the germanium content in
the composition of films, the absorption edge is shifted
into short-wave spectral range testifying to increase of
the pseudo-forbidden gap width (Eg).
The Eg value can be determined from the Tauc
relation [13]:
h
EhB
h g
2)(
)( , (1)
which is valid within the range of high energies, when
the absorption coefficient has values (h) ≥ 10–4 cm–1.
In (1), h is the photon energy, B – constant that depends
on film material and characterizes the slope of the Tauc
absorption edge. The parameter B is an interesting
parameter, since it can be taken as a measure of disorder.
For example, for GeS2, GeSe2, Ge2Se3 and GeSe3 films
the values of parameter B1/2 is 549 [14], 913 [12], 552
[12], and 827 cm–1/2 eV–1/2 [12], respectively.
Fig. 1. Dependences of transmission spectra inherent to
Ge25S37,5Se37,75 films on the exposure time: 1 – 0; 2 – 1; 3 – 10;
4 – 20 min.
Fig. 2. Dependences of transmission spectra inherent to
Ge33,3S33,85Se33,85 films on the exposure time: 1 – 0; 2 – 1;
3 – 10; 4– 20 min.
The Eg values for Ge25S37.5Se37.5 and
Ge33.3S33.35Se33.35 films were determined by extrapolation
of the dependences 2/1])([ hh ~ f(h) down to
)( h = 0 (Fig. 3) and are 2.276 and 2.301 eV,
respectively. For the films GeS2, GeSe2, GeSe3, the
pseudo-forbidden gap values obtained by us earlier [15,
17] are equal to 2.289 [15], 1.938 [17] and 1.958 eV
[17], respectively. These Eg values differ to some extent
from the pseudo-gap values for the films of the same
compositions but obtained by other authors. The
determined in [12, 13] Eg values for the films GeS2,
GeSe2 and GeSe3 are equal to 2.53 [13], 2.073 [12] and
2.044 eV [12], respectively. In our opinion, this
difference between Eg values is caused by technological
factors. The refraction index of films was determined
using the dependence [20]:
n= [N + (N2 – s2)1/2]1/2 , (2)
where
2
1
2
2
minmax
minmax
s
TT
TT
sN . (3)
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2013. V. 16, N 4. P. 349-353.
© 2013, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
351
Fig. 3. Plot of (αhν)1/2 versus hν for the films Ge25S37,5Se37,75
(a) and Ce33,3S33,85Se33,85 (b) non-exposed (1) and exposed (2,
3) for 1 (2) and 10 min (3).
In these expressions, s is the refraction index of
substrate; Tmax and Tmin are interferential maxima and
minima in transmission spectra within the range of
wavelengths where the dispersion of the refraction index
is absent. The n values for the films Ge25S37,5Se37,5 and
Ge33,3S33,35Se33,35, determined at the wavelength
λ = 700 nm, are equal to 2.222 and 2.403, respectively.
Light exposure of films results in a shift of
transmission spectra to the short-wave range (Figs 1 and
2, curves 2 to 4). It means that one observes
photobleaching of the films. It is indicative of the growth
of the pseudo-gap width. After illumination, the
refraction index is lowered. The dependences of Eg and
n values for the films Ge25S37,5Se37,5 and
Ge33,3S33,35Se33,35 on the exposure time are depicted in
Fig. 4. It is seen that the maximum changes in Eg and n
values take place at low illumination times. The rate of
photo-induced changes in optical parameters is reduced
with increasing the irradiation time. In this case, the
greater shift of the absorption edge at the transmission
level 0.2 (Figs 1 and 2) as well as greater changes of Eg
and n take place in the film with the Ge content 33.3 at.
% (Fig. 4) under the same exposure conditions.
Fig. 4. Dependences of Eg (1) and n (2) on the exposure time
for Ge25S37,5Se37,75 (a) and Ge33,3S33,85Se33,85 (b) films.
Changes in the absorption edge position (i. e., Eg
values) and refraction index are caused by structural
transformations taking place under laser illumination.
Amorphous films of germanium chalcogenides
(Ge-S, Ge-Se, Ge-S-Se, Ge-Sb-S systems) as well as
respective glasses possess a nano-heterogeneous
structure [6, 12, 18, 21-29]. The basic elements of
structural network in glasses and films with the
germanium content above 25% are tetrahedrons
GeSnSe4-n (n = 0 – 4). However, the matrix of
germanium chalcogenide glasses and films contains a
considerable amount of structural fragments with
homopolar bonds Ge-Ge, S-S and Se-Se . In these cases,
germanium chalcogenide films contain a higher
concentration of homopolar bonds [12, 18, 23]. When
illuminating the films, there take place break and re-
switch of Ge-Ge, S-S and Se-Se bonds and formation of
the heteropolar ones Ge-S(Se) [12, 15, 16, 23].
Photostructural transformations can be also promoted by
such structural defects as over-coordinated and under-
coordinated atoms of germanium and chalcogen. These
processes result in the increase of ordering in the local
structure of films.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2013. V. 16, N 4. P. 349-353.
© 2013, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
352
Fig. 5. Transmission spectra of Ge33,3S33,85Se33,85 film
annealed for 1 h at the temperature 423 K versus the exposure
time: 1-0; 2-1; 3-10; 4-20 min.
Fig. 6. Shift of the absorption edge at the transmission level 0.2
(a), changes Eg (b) and n (c) versus the exposure time for as-
prepared (1) and annealed (2, 3) at the temperature 423 K for
1 (2) and 2 h (3).
With the aim to ascertain the influence of
temperature on photo-induced changes in optical
characteristic, we investigated transmission spectra of
the films Ge25S37.5Se37.5 and Ge33.3S33.35Se33.35 annealed
for 1 and 2 h at the temperature 423 K. It should be
noted that this temperature is much lower than the glass-
transition temperature for these films.
Thermal annealing of films results in a shift of
transmission spectra to the short-wave range. It means
that one observes photobleaching of the films. In this
case, Eg value grows, while n is decreased. So, for the
Ge33.3S33.35Se33.35 film annealed for 1 and 2 h, Eg is equal
to 2.397 and 2.410 eV, respectively. The refraction index
values in the same conditions are, respectively, equal to
2.320 and 2.185. In the film Ge25S37.5Se37.5, the degree of
Eg and n changes is considerably lower.
Thermostimulated changes of Ge-S-Se film optical
characteristics are caused by structural transformations.
Like to the case of laser illumination, annealing of the
films results in break and re-switching the homopolar
bonds Ge-Ge, S-S (Se-Se) and formation of structural
units with heteropolar bonds GeS(Se)4/2.
In this case, thermal polymerization of molecular
fragments with homopolar bonds (for example, Ge2Se6,
Ge2S6, Se2, S2) into the structural network of GeSnSen-4
types can be realized both via the defectless mechanism
and with creation of structural charged defects (for
example, Ge3
–, S3
+, S1
– [30]).
Fig. 5 (curves 2 to 4) shows transmission spectra
for Ge33.3S33.35Se33.35 films illuminated after annealing. It
can be seen that the shift of absorption edge (∆E) under
laser illumination (to the short-wave range) is less in
them than that in the as-prepared ones (Fig. 6a). The
pseudo-forbidden gap width (Fig. 6b) and refraction
index ∆n changes (Fig. 6c) become considerably lower,
too. It means that the level of photo-structural
transformations in the annealed films is lower than that
in in the as-prepared ones. The lower level of changes in
optical parameters in the annealed Ge-S-Se films is
conditioned by a rather less number of structural
fragments with homopolar bonds in their matrix after
annealing, which are able to polymerize under laser
irradiation.
4. Conclusions
Laser illumination (λ = 530 nm) of amorphous films in
the system Ge-S-Se leads to the shift in transmission
spectra to the short-wave side of the spectrum
(photobleaching the films takes place). In this case, the
pseudo-forbidden gap width Eg grows, while the
refraction index value n is lowered. The highest changes
in Eg and n were observed in the film (GeS2)50(GeSe2)50.
These changes of film optical characteristics are caused
by photo-structural transformations that are related with
the decreasing content of structural fragments possessing
homopolar bonds in their matrix. Annealed films show
lower level and rate of photo-structural changes in
optical parameters.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2013. V. 16, N 4. P. 349-353.
© 2013, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
353
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293&295, p. 85-91 (2001).
25. P.P. Shtets, V.I. Fedelesh, V.M. Kabatsij et al.,
Structure, dielectric and photoelectric properties of
glasses in Ge-Sb-S system // J. Optoelectron. Adv.
Mater. 3 (4), p. 937-940 (2001).
26. L. Cai, P. Boolchand, Nanoscale phase separation
of GeS2 glass //Phil. Mag. B 82 (15), p. 1649-1657
(2002).
27. P.S. Salmon, Structure of liquidus and glasses in
the Ge-Se binary system // J. Non-cryst. Solids,
353, p. 2959-2974 (2007).
28. Jun Zhang, Haiyan Xiao, Jie Zhang. Compositional
dependence of refractive index and Raman spectra
of Ge(SxSe1-x)2 glasses // J. Optoelectron. Adv.
Mater. 13 (7), p. 848-851 (2011).
29. Han Xuecai, Sun Guangying, Liu Yu et al., Structure
and vibrational modes of Ge-S-Se glasses : Raman
scattering and AB initio calculations // Chalcogenide
Lett. 9 (11), p. 465-474 (2012).
30. O.I. Shpotyuk, R.Ya. Golovchak, A.P. Kovalskiy et
al., On the mechanism of radiation-indused optical
effects in vitreous As2S3-GeS2 // Ukr. J. Phys. Opt.
3 (2), p. 134-143 (2002).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2013. V. 16, N 4. P. 349-353.
PACS 78.66.Jg
Photo- and thermally-induced changes in the optical properties
of Ge-S-Se amorphous films
V.M. Rubish1, E.V. Gera1, M.O. Durcot1, M.M. Pop2, S.O. Kostyukevich3, A.A. Kudryavtsev3,
O.S. Mykulanynets-Meshko1, M.Yu. Rigan1
1Uzhgorod Scientific-Technological Center of the Institute for Information Recording, NAS of Ukraine
4, Zamkovi Skhody str., 88000 Uzhgorod, Ukraine, e-mail:center.uzh@gmail.com
2 Uzhgorod National University, 3, Narodna sq., 88000, Uzhgorod, Ukraine
3 V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine, 41, prospect Nauky, 03028 Kyiv, Ukraine
Abstract. The optical transmissions spectra of amorphous Ge-S-Se films of chemical compositions (GeS2)50(GeSe2)50 and (GeS3)50(GeSe3)50, prepared by thermal evaporation, have been measured over the whole 400 to 800 nm spectral range. It has been ascertained that annealing of the films leads to the absorption edge shift into the short-wave spectral region. The values of pseudo-gap width Eg and film refraction index n have been determined. Changes in optical properties of films are caused by structural transformations taking place in them under laser illumination and annealing.
Keywords: chalcogenide amorphous films, transmission spectra, optical properties, photo-structural transformations.
Manuscript received 15.07.13; revised version received 07.09.13; accepted for publication 23.10.13; published online 16.12.13.
1. Introduction
Chalcogenide non-crystalline vitreous and amorphous materials attract continuously growing interest caused by wide possibilities of practical applications and as a unique object for scientific investigations. Due to a wide range of photo-induced phenomena, these materials are current and potential candidates for using in optoelectronics and photonics (inorganic photoresists, optical sensors, waveguides, recording media, circuits, gratings and diffraction elements for various applications) [1-8].
From this viewpoint, deeper studied are amorphous films of binary and ternary arsenic chalcogenides that are characterized by lowering the pseudo-gap (photodarkening of films) under light irradiation [1, 3, 5-10], which is assumed to originate from photo-induced structural transformations that can be subsequently reversed by annealing near the glass transition temperature.
In recent decades, intensively studied are photo-induced effects in amorphous films of germanium chalcogenides [11-18]. First of all, in view of practical applications, these materials are attractive if accounting the fact that they do not contain the poisonous arsenic. Second, in these films under illumination one can observe both effects of photodarkening and photobleaching (growth of the pseudogap and optical transparency). Moreover, while germanium sulfide films are characterized only by photobleaching [13-15], the films GexSe100-x demonstrate both effects: photodarkening in Ge-dificient (x<20) and photobleaching in Ge-rich (x>20) samples [11, 12, 16-18].
In this paper, we report the results of investigating the influence of laser irradiation and annealing on transmission spectra and optical parameters of GeS2-GeSe2 and GeS3-GeSe3 films with the germanium content 25 and 33.3 at.%, respectively. Our choice of these sections was caused by the following reasons. In [15] we showed that, at the same conditions of illumination, changes in optical parameters of GeS2 films are essentially higher than those in Ge2S3 films. For instance, the change in the refraction index n of GeS2 film is practically 3-fold higher than that in Ge2S3 film. In the system Ge-Se, the highest changes in optical characteristics (shift of the absorption edge Eg and refraction index n) were observed by us in the films GeSe2 and GeSe3 [17]. Thereof, it is for the films of the above sections one should expect a high level of photo-induced changes in optical parameters.
2. Experimental
The bulk glasses Ce33.3S33.85Se33.85 (or (GeS2)50(GeSe2)50) and Ge25S37.5Se37.5 (or (GeS3)50 (GeSe3)50) were prepared using direct synthesis from the corresponding elements in evacuated silica ampoules. The mass of every charge was 20 gram. In the process of synthesis, we used the step-like increase of temperature up to the maximum one (1250 K). At the temperatures 700, 950 and 1250 К, the ampoules were kept for 8, 6 and 4 hours, respectively. In what followed, temperature was lowered down to the temperature of melt homogenization (1100…1150 К). The homogenization time was 48 hours. The melts were periodically stirred. After synthesis the ampoules were air-quenched.
Thin films (thickness ~ 1 μm) were obtained by vacuum evaporation of glasses of corresponding compositions from quasi-closed effusion cells onto non-heated glass substrates. A uniform thickness of layers was provided by planetary rotation of substrates.
Light exposure of films was made using defocused radiation of a semiconductor laser (λ = 530 nm, P = 100 mW). Annealing of the films was performed in argon atmosphere for 1 and 2 h at the temperature 423 K.
Optical transmission spectra of the films were measured at T = 300 K within the range 400 to 800 nm by using the method [19] with a monochromator МДР-3. The spectral resolution was no worse than 10–3 eV.
3. Results and discussion
Fig. 1 and 2 (curves 1) show transmission spectra for the as-prepared films Ge25S37,5Se37,5 and Ge33.3S33.35Se33.35. It is obvious that with growing the germanium content in the composition of films, the absorption edge is shifted into short-wave spectral range testifying to increase of the pseudo-forbidden gap width (Eg).
The Eg value can be determined from the Tauc relation [13]:
n
-
n
=
n
a
h
E
h
B
h
g
2
)
(
)
(
,
(1)
which is valid within the range of high energies, when the absorption coefficient has values ((h() ≥ 10–4 cm–1. In (1), h( is the photon energy, B – constant that depends on film material and characterizes the slope of the Tauc absorption edge. The parameter B is an interesting parameter, since it can be taken as a measure of disorder. For example, for GeS2, GeSe2, Ge2Se3 and GeSe3 films the values of parameter B1/2 is 549 [14], 913 [12], 552 [12], and 827 cm–1/2 eV–1/2 [12], respectively.
Fig. 1. Dependences of transmission spectra inherent to Ge25S37,5Se37,75 films on the exposure time: 1 – 0; 2 – 1; 3 – 10; 4 – 20 min.
Fig. 2. Dependences of transmission spectra inherent to Ge33,3S33,85Se33,85 films on the exposure time: 1 – 0; 2 – 1;
3 – 10; 4– 20 min.
The Eg values for Ge25S37.5Se37.5 and Ge33.3S33.35Se33.35 films were determined by extrapolation of the dependences
2
/
1
]
)
(
[
n
×
n
a
h
h
~ f(h() down to
)
(
n
a
h
= 0 (Fig. 3) and are 2.276 and 2.301 eV, respectively. For the films GeS2, GeSe2, GeSe3, the pseudo-forbidden gap values obtained by us earlier [15, 17] are equal to 2.289 [15], 1.938 [17] and 1.958 eV [17], respectively. These Eg values differ to some extent from the pseudo-gap values for the films of the same compositions but obtained by other authors. The determined in [12, 13] Eg values for the films GeS2, GeSe2 and GeSe3 are equal to 2.53 [13], 2.073 [12] and 2.044 eV [12], respectively. In our opinion, this difference between Eg values is caused by technological factors. The refraction index of films was determined using the dependence [20]:
n= [N + (N2 – s2)1/2]1/2 ,
(2)
where
2
1
2
2
min
max
min
max
+
+
×
-
=
s
T
T
T
T
s
N
.
(3)
Fig. 3. Plot of (αhν)1/2 versus hν for the films Ge25S37,5Se37,75 (a) and Ce33,3S33,85Se33,85 (b) non-exposed (1) and exposed (2, 3) for 1 (2) and 10 min (3).
In these expressions, s is the refraction index of substrate; Tmax and Tmin are interferential maxima and minima in transmission spectra within the range of wavelengths where the dispersion of the refraction index is absent. The n values for the films Ge25S37,5Se37,5 and Ge33,3S33,35Se33,35, determined at the wavelength λ = 700 nm, are equal to 2.222 and 2.403, respectively.
Light exposure of films results in a shift of transmission spectra to the short-wave range (Figs 1 and 2, curves 2 to 4). It means that one observes photobleaching of the films. It is indicative of the growth of the pseudo-gap width. After illumination, the refraction index is lowered. The dependences of Eg and n values for the films Ge25S37,5Se37,5 and Ge33,3S33,35Se33,35 on the exposure time are depicted in Fig. 4. It is seen that the maximum changes in Eg and n values take place at low illumination times. The rate of photo-induced changes in optical parameters is reduced with increasing the irradiation time. In this case, the greater shift of the absorption edge at the transmission level 0.2 (Figs 1 and 2) as well as greater changes of Eg and n take place in the film with the Ge content 33.3 at. % (Fig. 4) under the same exposure conditions.
Fig. 4. Dependences of Eg (1) and n (2) on the exposure time for Ge25S37,5Se37,75 (a) and Ge33,3S33,85Se33,85 (b) films.
Changes in the absorption edge position (i. e., Eg values) and refraction index are caused by structural transformations taking place under laser illumination.
Amorphous films of germanium chalcogenides (Ge-S, Ge-Se, Ge-S-Se, Ge-Sb-S systems) as well as respective glasses possess a nano-heterogeneous structure [6, 12, 18, 21-29]. The basic elements of structural network in glasses and films with the germanium content above 25% are tetrahedrons GeSnSe4-n (n = 0 – 4). However, the matrix of germanium chalcogenide glasses and films contains a considerable amount of structural fragments with homopolar bonds Ge-Ge, S-S and Se-Se . In these cases, germanium chalcogenide films contain a higher concentration of homopolar bonds [12, 18, 23]. When illuminating the films, there take place break and re-switch of Ge-Ge, S-S and Se-Se bonds and formation of the heteropolar ones Ge-S(Se) [12, 15, 16, 23]. Photostructural transformations can be also promoted by such structural defects as over-coordinated and under-coordinated atoms of germanium and chalcogen. These processes result in the increase of ordering in the local structure of films.
Fig. 5. Transmission spectra of Ge33,3S33,85Se33,85 film annealed for 1 h at the temperature 423 K versus the exposure time: 1-0; 2-1; 3-10; 4-20 min.
Fig. 6. Shift of the absorption edge at the transmission level 0.2 (a), changes Eg (b) and n (c) versus the exposure time for as-prepared (1) and annealed (2, 3) at the temperature 423 K for 1 (2) and 2 h (3).
With the aim to ascertain the influence of temperature on photo-induced changes in optical characteristic, we investigated transmission spectra of the films Ge25S37.5Se37.5 and Ge33.3S33.35Se33.35 annealed for 1 and 2 h at the temperature 423 K. It should be noted that this temperature is much lower than the glass-transition temperature for these films.
Thermal annealing of films results in a shift of transmission spectra to the short-wave range. It means that one observes photobleaching of the films. In this case, Eg value grows, while n is decreased. So, for the Ge33.3S33.35Se33.35 film annealed for 1 and 2 h, Eg is equal to 2.397 and 2.410 eV, respectively. The refraction index values in the same conditions are, respectively, equal to 2.320 and 2.185. In the film Ge25S37.5Se37.5, the degree of Eg and n changes is considerably lower.
Thermostimulated changes of Ge-S-Se film optical characteristics are caused by structural transformations. Like to the case of laser illumination, annealing of the films results in break and re-switching the homopolar bonds Ge-Ge, S-S (Se-Se) and formation of structural units with heteropolar bonds GeS(Se)4/2.
In this case, thermal polymerization of molecular fragments with homopolar bonds (for example, Ge2Se6, Ge2S6, Se2, S2) into the structural network of GeSnSen-4 types can be realized both via the defectless mechanism and with creation of structural charged defects (for example, Ge3–, S3+, S1– [30]).
Fig. 5 (curves 2 to 4) shows transmission spectra for Ge33.3S33.35Se33.35 films illuminated after annealing. It can be seen that the shift of absorption edge (∆E) under laser illumination (to the short-wave range) is less in them than that in the as-prepared ones (Fig. 6a). The pseudo-forbidden gap width (Fig. 6b) and refraction index ∆n changes (Fig. 6c) become considerably lower, too. It means that the level of photo-structural transformations in the annealed films is lower than that in in the as-prepared ones. The lower level of changes in optical parameters in the annealed Ge-S-Se films is conditioned by a rather less number of structural fragments with homopolar bonds in their matrix after annealing, which are able to polymerize under laser irradiation.
4. Conclusions
Laser illumination (λ = 530 nm) of amorphous films in the system Ge-S-Se leads to the shift in transmission spectra to the short-wave side of the spectrum (photobleaching the films takes place). In this case, the pseudo-forbidden gap width Eg grows, while the refraction index value n is lowered. The highest changes in Eg and n were observed in the film (GeS2)50(GeSe2)50. These changes of film optical characteristics are caused by photo-structural transformations that are related with the decreasing content of structural fragments possessing homopolar bonds in their matrix. Annealed films show lower level and rate of photo-structural changes in optical parameters.
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26. L. Cai, P. Boolchand, Nanoscale phase separation of GeS2 glass //Phil. Mag. B 82 (15), p. 1649-1657 (2002).
27. P.S. Salmon, Structure of liquidus and glasses in the Ge-Se binary system // J. Non-cryst. Solids, 353, p. 2959-2974 (2007).
28. Jun Zhang, Haiyan Xiao, Jie Zhang. Compositional dependence of refractive index and Raman spectra of Ge(SxSe1-x)2 glasses // J. Optoelectron. Adv. Mater. 13 (7), p. 848-851 (2011).
29. Han Xuecai, Sun Guangying, Liu Yu et al., Structure and vibrational modes of Ge-S-Se glasses : Raman scattering and AB initio calculations // Chalcogenide Lett. 9 (11), p. 465-474 (2012).
30. O.I. Shpotyuk, R.Ya. Golovchak, A.P. Kovalskiy et al., On the mechanism of radiation-indused optical effects in vitreous As2S3-GeS2 // Ukr. J. Phys. Opt. 3 (2), p. 134-143 (2002).
© 2013, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
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