Characterization of quaternary chalcogenide As-Ge-Te-Si thin films
Investigated in this paper is the effect of replacement of Te by Si on the optical gap and some other physical operation parameters of quaternary chalcogenide As₃₀Ge₁₀Te₆₀₋xSix (where x = 0, 5, 10, 12 and 20 at.%) thin films. Thin films with the thickness 100-200 nm of As₃₀Ge₁₀Te₆₀₋xSix were pre...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2011
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nasplib_isofts_kiev_ua-123456789-1177642025-06-03T16:28:47Z Characterization of quaternary chalcogenide As-Ge-Te-Si thin films Amer, H.H. Elkordy, M. Zien, M. Dahshan, A. Elshamy, R.A. Investigated in this paper is the effect of replacement of Te by Si on the optical gap and some other physical operation parameters of quaternary chalcogenide As₃₀Ge₁₀Te₆₀₋xSix (where x = 0, 5, 10, 12 and 20 at.%) thin films. Thin films with the thickness 100-200 nm of As₃₀Ge₁₀Te₆₀₋xSix were prepared using thermal evaporation of bulk samples. Increasing Si content was found to affect the average heat of atomization, average coordination number, number of constraints and cohesive energy of the As₃₀Ge₁₀Te₆₀₋xSix alloys. Optical absorption is due to allowed non-direct transition, and the energy gap increases with increasing Si content. The chemical bond approach has been applied successfully to interpret the increase in the optical gap with increasing silicon content. The authors would like to thank Dr. A. Abdglel, Solid State Physics Department, National Center for Radiation Research and Technology, Atomic Energy Authority, Cairo, Egypt for his help and support. 2011 Article Characterization of quaternary chalcogenide As-Ge-Te-Si thin films / H.H. Amer, M. Elkordy, M. Zien, A. Dahshan, R.A. Elshamy // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2011. — Т. 14, № 3. — С. 302-307. — Бібліогр.: 34 назв. — англ. 1560-8034 PACS 61.80.-x, 78.66.Jg https://nasplib.isofts.kiev.ua/handle/123456789/117764 en Semiconductor Physics Quantum Electronics & Optoelectronics application/pdf Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
| language |
English |
| description |
Investigated in this paper is the effect of replacement of Te by Si on the optical
gap and some other physical operation parameters of quaternary chalcogenide As₃₀Ge₁₀Te₆₀₋xSix (where x = 0, 5, 10, 12 and 20 at.%) thin films. Thin films with the
thickness 100-200 nm of As₃₀Ge₁₀Te₆₀₋xSix were prepared using thermal evaporation
of bulk samples. Increasing Si content was found to affect the average heat of
atomization, average coordination number, number of constraints and cohesive energy of
the As₃₀Ge₁₀Te₆₀₋xSix alloys. Optical absorption is due to allowed non-direct transition,
and the energy gap increases with increasing Si content. The chemical bond approach has
been applied successfully to interpret the increase in the optical gap with increasing
silicon content. |
| format |
Article |
| author |
Amer, H.H. Elkordy, M. Zien, M. Dahshan, A. Elshamy, R.A. |
| spellingShingle |
Amer, H.H. Elkordy, M. Zien, M. Dahshan, A. Elshamy, R.A. Characterization of quaternary chalcogenide As-Ge-Te-Si thin films Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Amer, H.H. Elkordy, M. Zien, M. Dahshan, A. Elshamy, R.A. |
| author_sort |
Amer, H.H. |
| title |
Characterization of quaternary chalcogenide As-Ge-Te-Si thin films |
| title_short |
Characterization of quaternary chalcogenide As-Ge-Te-Si thin films |
| title_full |
Characterization of quaternary chalcogenide As-Ge-Te-Si thin films |
| title_fullStr |
Characterization of quaternary chalcogenide As-Ge-Te-Si thin films |
| title_full_unstemmed |
Characterization of quaternary chalcogenide As-Ge-Te-Si thin films |
| title_sort |
characterization of quaternary chalcogenide as-ge-te-si thin films |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2011 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/117764 |
| citation_txt |
Characterization of quaternary chalcogenide
As-Ge-Te-Si thin films / H.H. Amer, M. Elkordy, M. Zien, A. Dahshan, R.A. Elshamy // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2011. — Т. 14, № 3. — С. 302-307. — Бібліогр.: 34 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
AT amerhh characterizationofquaternarychalcogenideasgetesithinfilms AT elkordym characterizationofquaternarychalcogenideasgetesithinfilms AT zienm characterizationofquaternarychalcogenideasgetesithinfilms AT dahshana characterizationofquaternarychalcogenideasgetesithinfilms AT elshamyra characterizationofquaternarychalcogenideasgetesithinfilms |
| first_indexed |
2025-11-30T23:47:46Z |
| last_indexed |
2025-11-30T23:47:46Z |
| _version_ |
1850261076868857856 |
| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 3. P. 302-307.
PACS 61.80.-x, 78.66.Jg
Characterization of quaternary chalcogenide
As-Ge-Te-Si thin films
H.H. Amer1, M. Elkordy2, M. Zien2, A. Dahshan3, R.A. Elshamy2*
1Solid State Department, National Center for Radiation Research and Technology,
Nasr City, Cairo, Egypt
2Electronics and Communication Department, Faculty of Electronic Engineering,
Menofia University, Egypt
3Department of Physics, Faculty of Science, Port Said University, Port Said, Egypt
2*E-mail: randa.aly72@yahoo.com
Abstract. Investigated in this paper is the effect of replacement of Te by Si on the optical
gap and some other physical operation parameters of quaternary chalcogenide
(where x = 0, 5, 10, 12 and 20 at.%) thin films. Thin films with the
thickness 100-200 nm of were prepared using thermal evaporation
of bulk samples. Increasing Si content was found to affect the average heat of
atomization, average coordination number, number of constraints and cohesive energy of
the alloys. Optical absorption is due to allowed non-direct transition,
and the energy gap increases with increasing Si content. The chemical bond approach has
been applied successfully to interpret the increase in the optical gap with increasing
silicon content.
xx601030 SiTeGeAs −
xx601030 SiTeGeAs −
xx601030 SiTeGeAs −
Keywords: thin films, optical gap, Si material, radiation effects, cohesive energy.
Manuscript received 09.03.11; accepted for publication 14.09.11; published online 21.09.11.
1. Introduction
Chalcogenide glasses are a recognized group of
inorganic glassy materials which always contain one or
more chalcogen elements S, Se, or Te but not O, in
conjunction with more electropositive elements as As,
Sb, and Bi. Chalcogen glasses are generally less robust,
weakly bonded materials than oxide glasses.
Initially, glasses containing chalcogen elements
were the subject of study owing to their interesting
semiconducting properties, and more recently for their
applications in optical recording [1, 2], technological
applications, like optical imaging or storage media [3]
and in the field of infrared optical transmitting materials,
fiber optics and memory devices [4]. The absence of
long-range order of chalcogenide glassy semiconductors
allows modification of their optical properties to a
specific technological application by continuously
changing their chemical composition [5, 6]. Hence,
studying the dependence of their optical properties on
composition is important to improve technological
application [7, 8]. As chalcogenide glassy
semiconductors, the physical properties of SiTeAs −−
and SiTeGe −− are strongly dependent on
composition, thus composition is especially important in
studying their physical properties. In fact, the chemical
bond approach was very useful in predicting the
semiconductor properties of different compounds and
crystal classes [9]. The present study investigates the
influence of addition of Si (0, 5, 10, 12 and 20 at.%),
which is lower in atomic weight than Te, on the optical
properties of new, amorphous thin
films. In addition, the optical band gap (E
xx601030 SiTeGeAs −
g), average
coordination numbers (Nco) and average heat of
atomization (Hs) of the glasses have
been examined theoretically. The results were
interpreted in terms of the chemical bond approach used
to estimate the cohesive energies of the glasses under
investigation.
xx601030 SiTeGeAs −
2. Experimental details
Different compositions of bulk
(where x = 0, 5, 10, 12, 20 at.%) chalcogenide glasses
were prepared from high-purity (99.999%) As, Ge, Te
xx601030 SiTeGeAs −
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
302
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 3. P. 302-307.
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
Table 1. Values of optical band gap, density, coordination number, heat of atomization (Hs), bond energy and
electronegativities of As, Ge, Te, Si used for calculations.
As Ge Te Si
and Si by using the melt-quench technique. Appropriate
proportions of the raw materials were weighed and
sealed into silica ampoules under vacuum of ≈ ,
which were then heated gradually up to a temperature of
1125 K within 1 h and kept constant for 8 h. Throughout
the heating process, the ampoules were regularly shaken
to ensure homogeneity and then quenched in ice-cold
water to avoid crystallization.
Energy gap (eV) 1.15 0.95 0.65 1.65
Density (g/cm3) 4.7 5 6 2
Coordination number 3 4 2 4
Hs (kcal/g atom) 69 90 46 108
Bond energy (kcal/mol) 32.1 37.6 33 42.2
Electronegativity 2.18 2.01 2.1 1.8
Pa10 4−
Thin films of were prepared
by thermal evaporation of the bulk samples. The thermal
evaporation process was performed inside the coating
system Edward 306E at the pressure close to .
During the deposition process (at normal incidence),
substrates were suitably rotated to obtain films of
uniform thickness. The film thickness was controlled
within the range 100-200 nm using a quartz crystal
thickness monitor Edward FTM5. The elemental
compositions of the investigated specimens were
checked using the energy dispersive X-ray spectroscopy
(Link Analytical Edx). Deviations in the elemental
composition of the evaporated thin films from the initial
bulk specimens did not exceed 1.0 at.%. The amorphous
state of the films was checked using an X-ray
diffractometer (Philips type 1710 with Cu as a target and
Ni as a filter; λ = 1.5418 Ǻ). Absence of crystalline
peaks confirmed the glassy state of the prepared
samples. The double beam spectrophotometer Shimadzu
2101 was used to measure reflectance and
transmittance for the prepared films in the spectral
wavelength range 200 to 1100 nm.
xx601030 SiTeGeAs −
Pa10 4−
VISUV −
3. Simulation results and discussion
Ioffe and Regel [10] suggested that the bonding
character in the nearest neighbor region, which is
described by the coordination number (the degree of
cross linking), characterizes the electronic properties of
the semiconducting materials. It is well known that the
coordination number of covalently bonded atoms in
glass is given by the so-called rule, where N is the
number of outer-shell electrons [11]. The numbers of the
nearest neighbor atoms for As, Ge, Te and Si are
calculated and listed in Table 1.
N−8
The average coordination number is defined simply
as the atom-averaged covalent coordination of the
constituents [12]. In the quaternary compounds
AαBBβCγDλ the averaged coordination number is
generalized as:
( ) ( ) ( ) ( )
λ+γ+β+α
λ+γ+β+α
=
DNCNBNAN
N cocococo
co . (1)
For our compound the average coordination
number is given by the following 2nd relation [13]
Nco = 2xTe + 3xAs + 4xGe + 4xSi. (2)
The degree of cross linking has a profound effect
on the thermal and mechanical properties of
chalcogenide glasses, because increasing the cross
linking makes the atoms become more tightly
bound [14].
The determination of Nco allows estimation of the
number of constraints (Ns). This parameter is closely
related to the glass-transition temperature and associated
properties. For a material with the coordination number
Nco, Ns can be expressed as the sum of the radial and
angular valence force constraints [15]:
( 32
2
−+= co
co
s NNN ) . (3)
The calculated values of Nco and Ns for the
system are given in Table 2. The
parameter r, which determines the deviation of
stoichiometry and is expressed by the ratio of the
covalent bonding possibilities of chalcogen atoms to that
of non-chalcogen atoms, was calculated using the
following relation [16, 17]:
xx601030 SiTeGeAs −
( ) ( )
( ) ( ) ( )SiGe10As30
Te60
cococo
co
xNNN
Nx
r
++
−
= . (4)
The calculated values of r for
system are also given in Table 2.
xx601030 SiTeGeAs −
According to Pauling [18], the heat of atomization
( )BAH s − at standard temperature and pressure of a
binary semiconductor formed from atoms A and B is the
sum of the heats of formation ΔH and the average of the
heats of atomization and corresponding to the
average non-polar bond energy of the two atoms
[19, 20]:
A
sH B
sH
303
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 3. P. 302-307.
Table 2. Some of physical parameters as a function of the Si content for As30Ge10Te60–xSix (where x = 0, 5, 10, 12,
20 at.%) specimens.
Composition
Nco
Ns
R
Hs
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
atomg
kcal
CE
⎟
⎠
⎞
⎜
⎝
⎛
atom
eV
Eg, th
(eV)
Hs/Nco
601030 TeGeAs 2.5 3.25 0.923 57.3 3.624 0.83 22.92
5551030 SiTeGeAs 2.6 3.5 0.733 60.4 3.848 0.88 23.23
10501030 SiTeGeAs 2.7 3.75 0.588 63.5 4.071 0.93 23.52
12481030 SiTeGeAs 2.74 3.85 0.539 64.74 4.161 0.95 23.63
20401030 SiTeGeAs 2.9 4.25 0.381 69.7 4.517 1.03 24.03
Table 3. Bond energy, probabilities and relative probabilities for formation of various bonds in As – Ge – Si – Te
glasses, taking the probability of Si – Si bonds as unity.
Bond Bond energy
(kcal/mol)
Probability Relative probability
(at T = 298.15 K)
Si–Si 42.20 1.03×1031 1
Ge–Si 41.16 1.77×1030 0.17
As–Si 41.13 1.69×1030 0.16
Te–Si 40.02 2.58×1029 0.02
Ge–Ge 37.60 4.30×1027 4.16×10–4
As–Ge 35.61 1.48×1026 1.43×10–5
Ge–Te 35.46 1.15×1026 1.11×10–5
Te–Te 33.00 1.79×1024 1.73×10–7
As–Te 32.70 1.08×1024 1.04×10–7
As–As 32.10 3.90×1023 3.78×10–8
( ) ( )B
s
A
ss HHHBAH ++Δ=−
2
1 . (5)
ΔH is proportional to the square of the difference
between the electronegativities χA and χB of the two
atoms:
( 2
BAH χ−χ∝Δ ) . (6)
ΔH that is strongly correlated with the difference in
the ionicities of different atoms is small compared to the
cohesive energy, because the electronegativities of the
constituent elements such as As, Te, Si are very similar.
In most cases, the heat of formation of chalcogenide
glasses is unknown. In the few materials for which it is
known, the heat of formation ΔH is about 10% of the
heat of atomization and, therefore, can be neglected.
To extend the idea to ternary and higher order
semiconductor compounds, the average heat of
atomization is defined for a compound AαB
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
BβCγDλ as
[21, 22]:
λ+γ+β+α
λ+γ+β+α
=
D
s
C
s
B
s
A
s
s
HHHH
N . (7)
The values of Hs for alloys
obtained using the values of H
xx601030 SiTeGeAs −
s of As, Ge, Te and Sb are
given in Table 2. As shown in this table, the values of Hs
increase with increasing Si content. To correlate Hs with
Eg in non-crystalline solids, it is reasonable to use the
average coordination number instead of the isostructure
of crystalline semiconductors.
It was found that the variation in the theoretical
values of the energy gap with composition in
quaternary alloys can be described [23] by the following
simple relation:
thgE ,
( ) ( ) BgAgABg EYEYYE −+= 1 , (8)
where Y is the volume fraction of element. For
quaternary alloys:
( ) ( ) ( )
( ) ( ) ,
,
DdECcE
BbEAaEABCDE
gg
ggthg
++
++=
(9)
where a, b, c, and d are the volume fractions of the
elements A, B, C and D, respectively. Eg(A), Eg(B),
Eg(C), and Eg(D) are the corresponding optical gaps.
Conversion from a volume fraction to atomic percentage
is made using the atomic weights and densities [24]
tabulated in Table 1. The calculations of , based on
the above equation for the alloys,
are given in Table 2, which reveals that the addition of
Si leads to a change in the considered properties. The
increase in Si leads to increase in and N
thgE ,
xx60 SiTe −1030GeAs
thgE , co. The
various bond energies of the expected bonds in the
system are listed in Table 3. By increasing the Si
content, the average bond strength of the compound
decreases, and hence Eg will decrease.
304
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 3. P. 302-307.
Fig. 1. X-ray diffraction patterns of the amorphous
(where x = 0, 5, 10, 12, 20 at.%) thin
films.
xx601030 SiTeGeAs −
To emphasize the relationship between Eg and the
average bond strength more clearly, Eg is compared with
Hs/Nco which is the average single-bond energy in the
alloy. One observes that Eg, as well as Hs/Nco increase
with increasing Si content, which suggests that one of
the main factors determining Eg is the average single
bond in the alloy [25]. Fig. 1 shows the X-ray diffraction
patterns for thin films. The absence
of diffraction lines in the X-ray patterns indicates that
the films have amorphous structures. Transmission
spectra corresponding to the amorphous
thin films before and after
irradiation of 1 and 15 Mrad are plotted in Figs 2, 3, 4
and show a clear ultraviolet shift of the interference-free
region with increasing Si content.
xx601030 SiTeGeAs −
xx601030 SiTeGeAs −
The values of the absorption coefficient α for the
studied films were calculated from transmittance T and
reflectance R using the equation:
( )
T
R
d
21ln1 −
=α , (10)
Fig. 2. Transmission spectra for (where
x = 0, 5, 10, 12, 20 at.%) thin films before irradiation.
xx601030 SiTeGeAs −
where d is the thickness of the film. According to Tauc’s
relation [26, 27] for the allowed non-direct transition, the
photon energy dependence of the absorption coefficient
can be described by:
( ) ( )gEhBh −= ννα 21 , (11)
where B is a parameter that depends on the transition
probability. Figs 5, 6 and 7 are a typical best fit of
(αhν)1/2 versus photon energy hν for
thin films before and after the radiation exposures 1 and
15 Mrad. The intercepts of the straight lines with the
photon energy axis give the values of the optical band
gap. The variation in E
xx601030 SiTeGeAs −
g as a function of Si content
before and after the radiation exposures 1 and 15 Mrad
are shown in Fig. 8. It is clear that Eg increases with
increasing the Si content of the investigated films. Fig. 9
shows the density of amorphous
thin film, and it is clear that density decreases with
increasing the Si content.
xx601030 SiTeGeAs −
The possible bond distribution at various
compositions may be considered using the chemically
ordered network (CON) model [28]. This model assumes
that: a) atoms combine more favourably with atoms of
different kinds than with the same and b) bonds are
formed in the sequence of bond energies. The bond
energies ( )BAD − for heteronuclear bonds have been
calculated by using the empirical relation [29]:
Fig. 3. Transmission spectra for (where
x = 0, 5, 10, 12, 20 at.%) thin films after the radiation exposure
1 Mrad.
xx601030 SiTeGeAs −
Fig. 4. Transmission spectra for (where
x = 0, 5, 10, 12, 20 at.%) thin films after the radiation exposure
15 Mrad.
xx601030 SiTeGeAs −
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
305
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 3. P. 302-307.
( ) ( ) ( )[ ]
( ) ,30 2
21
BA
BBDAADBAD
χ−χ+
+−⋅−=−
(12)
proposed by Pauling [30], where ( AAD
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
)− and
are the energies of the homonuclear bonds (in
units of kcal/mol) [31]: χ
( BBD − )
A and χB are the
electronegatives for the involved atoms [32].
B
At elevated temperatures, energy bonding effects
can influence the film composition. This is because more
energy is available to the atoms striking a hot substrate,
and they can adjust themselves after striking to form
more favorable (i.e. a higher energy) bonds. Thus, for
the present material, more silicon might be incorporated
into the films at higher temperatures, because it is
possible for silicon to form relatively strong bonds with
other constituents. This hypothesis is supported by the
contents of Table 3, which lists the relative order of the
bond energies of the ten possible bonds in the
system. The hetero-atom single-bond
energies were calculated from the average of the homo-
atom single-bond energies for silicon, tellurium, arsenic
and germanium, with addition of ionic contribution
proportional to the square of the electronegatives
difference between the elements.
GeAsTeSi −−−
Fig. 5. Best fit of (αhν)1/2 versus photon energy hν for
(where x = 0, 5, 10, 12, 20 at.%) thin
films before irradiation.
xx601030 SiTeGeAs −
Fig. 6. Best fit of (αhν)1/2 versus photon energy hν for
(where x = 0, 5, 10, 12, 20 at.%) thin
films after the radiation exposure 1 Mrad.
xx601030 SiTeGeAs −
Fig. 7. Best fit of (αhν)1/2 versus photon energy hν for
(where x = 0, 5, 10, 12, 20 at.%) thin films
after the radiation exposure 15 Mrad.
xx601030 SiTeGeAs −
Fig. 8. Variation in the optical band gap Eg as a function of Si
content for (where x = 0, 5, 10, 12,
20 at.%).
xx601030 SiTeGeAs −
0 5 10 15 20
0
1
2
3
4
5
6
D
en
si
ty
(g
/c
m
3 )
Si content %
Fig. 9. Density dependence on the Si content for
(where x = 0, 5, 10, 12, 20 at.%) glasses. xx601030 SiTeGeAs −
It can be seen from Table 3 that silicon has a better
chance of sticking to the growing film at elevated
temperatures, as it can form strong bonds with tellurium,
the major constituent. However, the bonds are
relatively weak so that a deficiency of arsenic might be
expected on energetic grounds.
TeAs −
Knowing the bond energies, we can estimate the
cohesive energy (CE), i.e. the stabilized energy of an
infinitely large cluster of the material per atom, by
summing the bond energies over all the bonds expected
306
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 3. P. 302-307.
in the system under test the CE of the prepared samples
is evaluated from the following equation [33]
∑ ⎟
⎠
⎞
⎜
⎝
⎛=
100
iiDCCE , (13)
where Ci and Di are the numbers of the expected
chemical bonds and the energy of each corresponding
bond, respectively. The calculated values of CE for all
compositions are summarized in Table 2. CE increases
with increasing the Si content. Increasing the Si content
leads to an increase in the average molecular weight,
which increases the rigidity (strength) of the system.
This approach explains the behaviour in terms of
the cohesive energy. It allows determination of the
number of possible bonds and their type (heteropolar and
homopolar). The energies of various possible bonds in
the system are given in Table 3.
Depending on the bond energy D, the relative
probability of its formation was calculated [34] using the
probability function exp(D/kT) and listed in Table 3.
Bonds, such as , , , and
TeSiGeAs −−−
SiSi − GeGe − TeTe − AsAs−
bonds exist with high priority in the TeSiGeAs −−−
system.
4. Conclusions
Optical data indicated that the allowed, non-direct gap is
responsible for photon absorption in
thin films. Increasing the Si content at the expense of Te
atoms increases the optical gap of these films. The
values for heat of atomization, coordination number,
number of constraints and cohesive energy of
are dependent on glass composition.
The increase in Si content leads to increase in thgE , ,
H
xx601030 SiTeGeAs −
xx601030 SiTeGeAs −
s/ and N
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
Nco co. Cohesive energy increases with
increasing the Si content. The chemical-bond approach
can be successfully applied to interpret the increase in
the optical gap with increasing the Si content.
Acknowledgement
The authors would like to thank Dr. A. Abdglel, Solid
State Physics Department, National Center for Radiation
Research and Technology, Atomic Energy Authority,
Cairo, Egypt for his help and support.
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