ARTICLE IN PRESS
Physica B 387 (2007) 383–391
www.elsevier.com/locate/physb
Influence of composition on optical and electrical properties
of Ge–Se–In thin films
M.A. Abdel-Rahima,, M.M. Hafiza, M.M. El-Nahassb, A.M. Shamekha
a
Physics Department, Faculty of Science, Assuit University, Assiut, Egypt
Physics Department, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt
b
Received 2 February 2006; received in revised form 23 April 2006; accepted 27 April 2006
Abstract
The optical properties of as-deposited GexSe92xIn8 (x ¼ 10, 12.5, 15 and 20 at%) have been studied. Various optical constants have
been calculated for the studied compositions. The mechanism of the optical absorption follows the rule of non-direct transition. It was
found that, the optical energy gap (Eg) decreased with increasing Ge content. This result can be interpreted on the basis of the chemicalbond approach proposed by Bicermo and Ovshinsky. The values of the refractive index (n) and the extinction coefficient (k) increased
with increasing Ge content. Annealing of Ge12.5Se79.5In8 films at temperature higher than the glass transition temperature was found to
decrease the optical energy gap.
Electrical conductivity was measured in the temperature range (300–450 K) for the studied compositions. The effect of composition on
the activation energy (DE) and the density of localized states at the Fermi level N(EF) were studied. The electrical conductivity
measurements show two types of conduction channels that contribute two conduction mechanisms. On the other hand, the electrical
resistivity and the activation energy were found to decrease with increasing the annealing temperature. Transmission electron microscope
(TEM) investigation indicates the separation of crystalline phase after annealing at the temperatures higher than the glass transition. The
results were discussed on the basis of amorphous crystalline transformations.
r 2006 Elsevier B.V. All rights reserved.
Keywords: Ge–Se–In chalcogenide glass; Optical properties; Electrical properties
1. Introduction
The physical properties of chalcogenide glasses have
attracted much interest due to their wide technical
applications [1]. Besides, the physical properties of
chalcogenide semiconducting glasses are strongly dependent on their compositions [2,3]. Chalcogenide semiconductors have truly emerged as multipurpose materials and
have been used to fabricate technological important
devices: IR detector, electronic and optical switches and
optical recording media [4]. Such wide-ranging applications
are possible due to some unique phenomena like photoinduced structural transformations [1]. In recent years,
efforts are being made to develop chalcogenide-based
Corresponding author. Tel.: +20 882235678; fax: +20 8814311378.
E-mail address: maabdelrahim@yahoo.com (M.A. Abdel-Rahim).
0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2006.04.038
erasable optical storage media. Thermal processes are
known to be important in including crystallization in
chalcogenide glasses. Recently thin films of amorphous
chalcogenide glasses [5] have been studies as materials for
phase-change optical storage.
Crystallization of chalcogenide films is accompanied by a
change in the optical band gap. Separation of different
crystalline phases with thermal annealing has been
observed in ternary glasses. The effect of thermal annealing
is interpreted on the basis of amorphous crystalline
transformation [6–8].
It has been reported that amorphous Se has special
interest because of its device applications such as rectifiers,
photocells, xerography, switching and memories device,
etc. [9]. In the pure state it has disadvantages because of its
short lifetime and low sensitivity. To overcome this
difficulty, several workers [6–9] have used certain additives
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M.A. Abdel-Rahim et al. / Physica B 387 (2007) 383–391
(Ge, Te, Bi, Sb, As, etc.) to make binary alloys with
selenium, which in turn gives high sensitivity, a high
crystallization temperature and smaller aging effects. The
addition of third element (In) expands the glass forming
area and also creates compositional and configurational
disorder in the system [10–12].
On the other hand, the Ge–Se–In system is of special
interest in view of the fact that it forms glasses over a wide
domain of compositions [11]. Therefore, it is a suitable
model system for the investigation of variation of a certain
physical property with composition or with average
coordination number (Z).
Broisova [11] proposed that glass formation in Ge–Se–In
system occurs up to about 15 at% In and 60–90 at% Se, the
rest is germanium. Data on the density and the glass
transition temperature Tg of several glass compositions of
this system have been published recently by Mahadevon
et al. [12]. On the other hand, various workers [10,13–16]
have reported the electrical, optical and structural studies
of Ge–Se–In glasses.
The present work deals with some experimental observations on the effect of composition and heat treatment on
the optical and electrical properties of GexSe92xIn8
(x ¼ 10, 12.5, 15 and 20 at%). Transmission electron
microscopy (TEM) and X-ray diffraction were used to
determine the structural changes of Ge12.5Se79.5In8 films
under different conditions. The effect of thermal annealing
on the optical and electrical properties was analyzed on
Ge12.5Se79.5In8 thin films.
2. Experimental details
The bulk GexSe92xIn8 (x ¼ 10, 12.5, 15 and 20 at%)
chalcogenide glasses were prepared by the melt-quenched
technique. Materials (99.999% pure) were weighted according to their atomic percentages and were sealed in an
evacuated silica tubes, which were heated at 1100 1C for
15 h. The ampoules were frequently rocked at maximum
temperature to make the melt homogeneous. The quenching was performed in ice water.
Thin films were prepared by thermal evaporation at
vacuum of 105 Torr using an Edwards E-306 coating
system. The evaporation rates as well as the film thickness
were controlled by using a quartz crystal monitor FTM5.
The film composition was checked by the energy-dispersive
spectroscopy (EDS) technique and was found to be the
same as that of the starting materials. In order to determine
the absorption coefficient a and the optical constants of the
films as a function of the incident light wavelength, the
transmittance T(l) and reflectance R(l) were recorded at
room temperature using a double-beam spectrophotometer
(Shimadzu UV-2101 combined with PC). The electrical
conductivity measurements were carried out on films with
evaporated gold electrodes using a conventional circuit
involving a Keithly 610C electrometer. The measurements
were made in the dark at 300–450 K temperature range.
3. Results
3.1. Structure
A DSC trace for Ge12.5Se79.5In8 chalcogenide glass in the
temperature range from room temperature to complete
crystallization is presented in Fig. 1. Three characteristic
phenomena are clear in the studied temperature region.
The first one (Tg1 and Tg2) corresponds to glass transition
temperatures, the second one (Tc) corresponds to the onset
crystallization temperature. The last characteristic temperature (Tp) identifies the crystallization temperature. The
appearance of a double glass transition indicates unusual
phase separation occurring in the glass alloy. The phase
separation, leading to double glass transition, may arise
after the glass to be supercooled melt transition or the glass
may be diaphasic to start with itself. The phenomena of
double glass transition has been observed in many glassy
systems [10,17–19].
Further evidence for the structure transformations
taking place in Ge12.5Se79.5In8 composition due to the heat
treatment has been obtained through TEM micrograph
and diffraction patterns. Fig. 2(a) and (b) shows the
electron diffraction patterns and TEM micrographs for
annealed films of thickness 300 Å. Structural transformations were observed after thermal annealing of the asprepared films. The microstructure obtained for Ge12.5Se79.5In8 films annealed at 545 K for 2 h is shown in
Fig. 2(a). A polycrystalline structure consisting of different
crystalline phases with different shape and size are
embedded in amorphous matrix is clear. Some of these
crystallized particles are interconnected and others are
isolated. This is confirmed by the sharp diffraction rings
appeared in the electron diffraction pattern of the
Ge12.5Se79.5In8 after annealing for 2 h at 545 K as shown
in Fig. 2(b). In order to identify the crystalline phases
appeared in DSC and TEM examinations, the X-ray
diffraction patterns for the composition Ge12.5Se79.5In8
Heating rate = 10 k /min
Exo.
Tp
Ge12.5Se79.5In8
Tc
Tg2
Tg1
ΔQ
384
Endo.
350
400
450
500
550
T°K
Fig. 1. DSC trace for powdered Ge12.5Se75.5In8.
600
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385
Fig. 2. TEM images and electron diffraction patterns for Ge12.5Se75.5In8 films annealed at 545 K for 2 h.
Ge-Se
30
40
Se-Se
In2 Se3
50
60
I
Ge Se2
10
20
70
80
2θ
Fig. 3. X-ray diffraction pattern for Ge12.5Se75.5In8 films: (a) as-prepared (b) annealed at 545 K for 2 h.
annealed at 545 K for 2 h are examined. Analysis of the Xray diffraction pattern reveals that the observed crystalline
peaks are due to GeSe2, Ge–Se, Se–Se and In2Se3 as shown
in Fig. 3.
structural randomness of the system and (iii) interband
absorption which determine the optical energy gap.
In the high absorption region (a4104 cm1) the parabolic relation can be applied [21,22],
ðahnÞ ¼ Bðhn E g Þr ,
3.2. Absorption edge
The absorption coefficient (a) was computed from the
experimentally measured values of transmittance T(l) and
reflectance R(l) according to [20] the following relation:
T ¼ ð1 RÞ2 expðadÞ=½1 R2 expð2adÞ,
(1)
where d (cm) is the film thickness. Fig. 4 shows the
dependence of the absorption coefficient a on the incident
photon energy (hn) for as-deposited GexSe92xIn8 (x ¼ 10,
12.5, 15, 20 at%) films. It is observed that the value of the
absorption coefficient a increase with increasing the photon
energy. On the other hand, the absorption coefficient
increases with increasing the Ge content.
In chalcogenide glasses, a typical absorption edge can be
broadly ascribed to either the three processes: (i) the weak
absorption tail which originates from defects and impurities, (ii) urbach tail which is strongly related to the
(2)
where B is a constant that depends on the transition
probability, Eg is the optical energy gap of the material and
r is a number which characterizes the transition process,
having a value 1/2 for the direct allowed transition and value
of 2 for the indirect transition. The present system of
GexSe92xIn8 obeys the rule of non-direct transition [20–23].
Fig. 5 shows the plots of (ahn)1/2 versus hn for
GexSe92xIn8 films. The intercept with x-axis gives the
value of the indirect optical band gap (Eg). The value of the
optical band gap decreases from 1.96 to 1.87 ev as Ge
content increased from 8 to 20 at% as shown in Fig. 6(a).
The optical absorption depends on the short-range order in
the amorphous state and defects associated with it.
In the exponential edge region where ao104 cm1 the
absorption coefficient is governed by the Urbach rule [23]
i.e.,
a ¼ a0 expðhn=E e Þ,
(3)
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13
1.96
12
1.94
E indir (eV)
10
g
ln (α)
11
0.10
0.09
1.92
0.08
1.90
Ge10Se82In8
9
Ee (eV)
386
0.07
1.88
Ge12.5Se79.5In8
Ge15Se77In8
8
1.86
Ge20Se72In8
0.06
10
7
1.8
2.0
2.2
2.4
2.6
2.8
3.0
12
14
16
Ge at. %
18
20
3.2
Fig. 6. The effect of Ge content on the width of localized state tails Ee and
the indirect optical energy gap Eg for GexSe92xIn8 thin films.
hν (eV)
Fig. 4. Plots of the absorption coefficient (a) versus the incident photon
energy (hn) for GexSe92xIn8 system.
1000
The spectral dependence of refractive indexes (n) and
extinction coefficient (k) on the wavelength for as-prepared
GexSe92xIn8 thin films are shown in Fig. 7(a and b).
As shown in Fig. 7(a), n has maximum value nmax at
wavelength lc, which is shifted towards longer wavelength
as the Ge content is increased. On the other hand, the value
of extinction coefficient k decreases with increasing
wavelength. As shown in Fig. 7(a and b) the values of n
and k increased with increasing Ge content.
The dispersion of refractive index n(l) was analyzed
using the concept of the single oscillator and can be
expressed by the Wimple–Didomenico relationship [28,29]
as
Ge10Se82In8
(αhν)1/2 (cm-1 eV)0.5
Ge12.5Se79.5In8
Ge15Se77In8
800
Ge20Se72In8
600
400
200
n2 ¼ 1 þ
0
1.8
2.0
0.5
Fig. 5. Plots of (ahn)
GexSe92xIn8 thin films.
2.2
2.4
hν (eV)
2.6
2.8
3.0
versus photon energy (hn) for as-prepared
where Ee is interpreted as the band tail width of the
localized states in the gap region, hn is the photon energy
and a0 is a constant. Therefore, plotting the dependence of
ln(a) versus hn should give a straight line. The inverse of the
slope gives the band tail width (Ee) of the localized states at
the hand gap. The effect of Ge content of the investigated
films on Ee are shown in Fig. 6(b). It is observed from the
figure that the band tail width Ee gradually increased with
increasing Ge content.
On the other hands, the values of refractive index (n) and
extinction coefficient (k) have been calculated using the
following relations [24–27]:
R ¼ ½ðn 1Þ2 þ k2 =½ðn þ 1Þ2 þ k2 ,
(4)
a ¼ 4pk=l;
(5)
where R is reflectance or reflectivity.
EoEd
,
E 2o E 2
(6)
where E is the photon energy, Eo is the oscillator energy
and Ed is the dispersion energy which measures the average
strength of interband optical transitions. Fig. 8 shows the
relation between (n21)1 and (hn)2 for the investigated
compositions. The values of Ed and Eo have been
determined from the slopes and the intersection of the
straight lines with (n21)1 axis. The variation of Eo and
Ed with Ge concentration of the investigated compositions
are shown in Fig. 9. The values of Ed and Eo decrease by
increasing the Ge content. The decrease of Eo and Ed could
be attributed to the increase in the number of scattering
center due to the dissolving of Ge atoms in the glass film
matrix [30].
For a better understanding of the optical properties of
the investigated films, it is necessary to determine some
optical constants such as high frequency dielectric constant
eL and carrier concentration (N). The relation between the
optical dielectric constant e, wavelength (l) and n is given
by the following equation [30,31]:
2 e
N 2
2
¼ n ¼ L (7)
l,
2
m
pc
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n
M.A. Abdel-Rahim et al. / Physica B 387 (2007) 383–391
387
3.6
Ge10Se82 In8
3.4
Ge12.5Se79.5 In8
3.2
Ge15Se77 In8
3.0
Ge20Se72 In8
2.8
2.6
2.4
2.2
2.0
300
400
500
600
λ (nm)
(a)
700
1.6
900
Ge10Se82 In8
1.4
K
800
Ge12.5Se79.5 In8
1.2
Ge15Se77 In8
1.0
Ge20Se72 In8
0.8
0.6
0.4
0.2
0.0
350
400
450
500
λ (nm)
(b)
550
600
650
Fig. 7. Spectral dependence of (a) the refractive index and (b) the extinction coefficient (k) for as-prepared GexSe92xIn8 films.
Ge10Se82In8
20
Ge12.5Se79.5In8
19
E (eV)
0.22
Ge15Se77In8
0.20
d
Ge20Se72In8
Ed
18
17
15
0.18
2
1/(n -1)
16
o
E (eV)
0.16
0.14
0.12
1.8
2.1
2.4
2
2.7
3.0
2
4.2
4.0
3.8
3.6
3.4
3.2
Eo
10
12
14
16
18
20
Ge %
(hν) (eV)
Fig. 9. Plots of Ed and Eo versus Ge content in GexSe92xIn8 films.
Fig. 8. Plots of (n21)1 versus the photon energy (hn) for as-prepared
GexSe92xIn8 films.
where eL is the lattice dielectric constant, c the velocity of
the light, e the electronic charge, and m the electron
effective mass. From the linear plot of n2 versus l2, the
lattice dielectric constant el and the ratio N=m of the
investigated film can be determined. As shown in Fig. 10,
the value of el increases with increasing Ge content. This
behavior was observed in many chalcogenide glasses
[31–34].
One of the compositions (Ge12.5Se79.5In8) was chosen to
study the effect of heat treatment on the optical energy gap
by heating the as-prepared films at different temperatures
for 2 h. Fig. 11 shows the variation of the absorption
coefficient plotted as (ahn)1/2 with the photon energy (hn) at
different annealing temperatures. The dependence of the
optical energy gap Eg on the annealing temperature is
shown in Fig. 12(a). It is observed that Eg slightly increases
with increasing annealing temperature up to 475 K
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0.095
2.0
9.0
εL
8.8
(a)
1.9
0.090
(b)
8.6
0.085
Eg (eV)
1.8
8.4
0.075
ε
L
8.2
0.080
1.7
1.6
8.0
Ee (eV)
388
0.070
1.5
7.8
Ge12.5Se79.5In8
7.6
0.065
1.4
300
7.4
350
400
450
500
ann. temp. (k)
550
600
7.2
10
12
14
16
Ge at %
18
20
Fig. 12. The effect of annealing temperature on (a) the indirect optical
energy gap (Eg) (b) the width of localized state tails (Ee) for Ge12.5Se79.5In8
films.
Fig. 10. Plots of eL versus the Ge content in as-prepared GexSe92xIn8
films.
-4
Ge10Se82In8
-5
Ge12.5Se79.5In8
-6
1400
-7
as-prepared
458 k
483 k
508 k
563 k
-8
(αhν)1/2 (cm-1eV)1/2
1200
1000
800
ln (σ)
Ge12.5Se79.5In8
Ge15Se77In8
Ge20Se72In8
-9
-10
-11
-12
600
-13
-14
400
-15
2.2
200
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3.0
3.1
3.2
1000/ T (k-1)
0
1.50
1.75
2.00
2.25
2.50
hν (eV)
2.75
3.00
Fig. 11. Plots of (ahn)1/2 versus photon energy (hn) for as-prepared and
annealed Ge12.5Se79.5In8 films.
followed by sharp decrease with increasing annealing
temperature above the glass transition temperature. On
the other band, the effect of annealing temperature on Ee
of the localized states at the band gap are shown in Fig.
12(b). It is observed from the figure that the Ee gradually
decreases with increasing annealing temperature up to
475 K and suddenly increases with increasing annealing
temperature above the glass transition temperature.
3.3. Electrical conduction
The effect of Ge content on the electrical conductivity (s)
of the as-prepared GexSe92xIn8 (10pxp20) films of
thickness 2000 Å deposited onto glass substrate held at
room temperature were studied. Measurements were taken
during heating from 300 up to 450 K. The dark electrical
Fig. 13. Plots of ln(s) versus 1000/T for GexSe92xIn8 films.
conductivity (s) as a function of reciprocal temperature for
as-prepared films is shown in Fig. 13. The electrical results
exhibit two types of conduction channel which contribute
two conduction mechanisms. Below the glass transition
temperature, the electrical conductivity shows a slight
increase with increasing temperature and corresponds to
transport by hopping conduction. On the other hand, at
temperature T4Tg the exponential increase of s with T can
be regarded as a regular band-type conduction in extended
state. The conductivity s at T ¼ 333.5 K and the activation
energy for conduction (DE) calculated from Fig. 13 as a
function of composition are shown in Fig. 14. The two
parameters changed with composition in an exactly
opposite manner, DE decreasing and s increasing with
increasing Ge content. This confirms that the change in s is
connected to a corresponding change in DE. A similar
trend reported earlier [10] of decreasing Tg with decreasing
Se content.
To calculate the density of states at the Fermi level
N(EF) it is assumed that the measured conductivity is the
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M.A. Abdel-Rahim et al. / Physica B 387 (2007) 383–391
sum of two components:
s ¼ shop þ sext ,
(8)
where shop is the contribution of conduction due to
hopping between the nearest localized states and sext is
the contribution of conduction between the extended
states. At ToTg, the contribution occurs via variable
range hopping of the charge carriers in the localized states
near the Fermi level, and is characterized by Mott’s
0.97
2.1 x 10-5
0.96
1.8 x 10-5
1.5 x 10-5
0.95
1.2 x 10-5
0.94
0.93
9.0 x 10
-6
6.0 x 10
-6
σ (Ω−1cm-1)
ΔE (eV)
2.4 x 10-5
3.0 x 10-6
0.92
0.0
10
12
14
16
Ge at.%
18
20
Fig. 14. Variation of the activation energy for conduction (DE) and
electrical conductivity at 333.5 K with Ge content for GexSe92xIn8 films.
-8.8
Ge10Se82In8
Ge12.5Se79.5In8
-9.2
Ge15Se77In8
ln (σT1/2)
-9.6
389
variable-range hopping relation [35]:
pffiffiffiffi
shop ¼ ðs0 = T Þ expððT o =TÞ1=4 Þ
(9)
and T o ¼ 18a3 l=KNðE F Þ,
(10)
where N(EF) is the density of localized states at the Fermi
level, a is the coefficient of exponential decay of the
localized state wave function and is assumed to be
0.124 Å1 [36] and K is Boltzman’s constant. pffiffiffiffi
Fig. 15 shows the linear plots of ln (shop T ) versus
(1/T)1/4 for the investigated compositions. The density of
states at the Fermi levels N(EF) has been calculated for the
investigated compositions and listed in Table 1.
According to Mott [35] and Hill [37] two other hopping
parameters can be calculated, hopping distance R (cm) and
the average hopping energy W (eV), these parameters are
given as
R ¼ ½9=8 pak TN ðE F Þ1=4
(11)
and W ¼ ½3=4pR3 NðE F Þ.
(12)
The calculated values of R and W for the samples under
investigation are also listed in Table 1.
To study the effect of annealing temperature on the
electrical conductivity of Ge12.5Se79.5In8 composition, the
values of ln s vs. 1/T were plotted for as-prepared and
annealed films at different temperature for 2 h as shown in
Fig. 16. The conduction activation energy (DE) and the
corresponding measured electrical conductivity at 385.5 K
are shown in Fig. 17. It seems that the activation energy
(DE) decreased and s increased with increasing the
annealing temperatures.
Ge20Se72In8
4. Discussion
-10.0
-10.4
-10.8
-11.2
0.233 0.234 0.235 0.236 0.237 0.238 0.239 0.240
T -1/4 (k-1/4 )
Fig. 15. Plots of ln(sT0.5) versus T 1/4 for as-prepared GexSe92xIn8
films.
The change of the indirect Eg, n and absorption index k
associated with the present of Ge has a direct effect on the
amount and strength of the possible different bonds
formed in the network structure of the investigated
compositions. According to, the chemical-bond approach
proposed by Bicermo and Ovshinsky [38], which assumed
that (i) the atoms of one type combine more favorably with
atoms of a different type, (ii) bonds are formed in sequence
of decreasing bond energy until all the available valencies
of the atoms are filled and (iii) each constituent atom is
coordinated by (8N) atoms where N is the number of
Table 1
Density of localized states at the Fermi level and hopping parameters for as-deposited GexSe92xIn8
Composition
B (cm eV)0.5
T0 107 (K)
N(Ef) 1018 (cm3 eV1)
R 107 (cm)
At 300 K
W (eV)
At 300 K
Ge10Se82In8
Ge12.5Se79.5In8
Ge15Se77In8
Ge20Se72In8
1054.7
1069.5
1093.3
1202.7
1.6
4.87
5.55
14.18
24.8
8.17
7.17
2.81
4.6
6.08
6.28
7.94
0.098
0.130
0.134
0.170
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390
Ge12.5Se79.5In8
-6
As-prepared
Ann. at 503 k
Ann. at 563 k
-7
-8
ln (σ)
-9
-10
-11
-12
-13
-14
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3.0
3.1
3.2
1000/ T (k-1)
Fig. 16. Plots of ln(s) versus 1000/T for Ge12.5Se79.5In8 as-prepared and
annealed films at different temperatures for 2 h.
0.98
0.94
6.0 x 10-5
5.0 x 10
0.90
-5
4.0 x 10-5
0.88
0.86
3.0 x 10
σ (Ω−1cm-1)
7.0 x 10
0.92
ΔE (ev)
-5
0.96
-5
0.84
0.82
2.0 x 10-5
Ge12.5 Se79.5 In8
0.80
300
350
400
450
500
550
600
Ann.Temp. (k)
Fig. 17. Effect of annealing temperature on the activation energy for
conduction (DE) and electrical conductivity at 385.5 K for Ge12.5Se79.5In8
films.
outer shell electrons and this is equivalent to neglecting the
dangling bonds and the other valence defects.
Keeping the above assumption in mined, the obtained
results can be interpreted on the basis of the prevalent
structural arrangement in these glasses, wherein the parent
glass Se92In8 has two types of chemical bonds Se–In and
Se–Se in all the three dimensional network structures. The
introduction of Ge atoms instead of Se atoms leads to
replacement of Se–Se and Se–In bonds by Ge–Se, Ge–Ge
and Ge–In bonds [10] that leads to a decrease of the optical
band gap and increasing the optical constants n and k with
increasing Ge content.
On the other hand, the decrease of bond energy with
increasing Ge content is responsible for increasing
the material’s electrical conductivity and decreasing the
strength or rigidity of the lattice and consequently the glass
softening temperature [10].
The effect of annealing temperature on the optical
energy gap for the Ge12.5Se79.5In8 films is shown in
Fig. 12(a). In the process of heat treatment at temperatures
up to Tg, the indirect optical energy gap increased with
increase in the annealing temperature. The observed
changes in the optical energy gap have been successfully
analyzed on the basis of the theory proposed by Mott and
Davis [39] for amorphous materials. During thermal
annealing, the unsaturated defects are gradually annealed
out [40] producing larger numbers of saturated bonds. The
reduction in the number of unsaturated defects decreases
the density of the localized states in the band structure and
consequently increases the optical energy gap. This is
confirmed by the observed decreases in the width of the
localized states tails with increase in the annealing
temperature. On the other hand, thermal annealing above
Tg is known to be important in including crystallization in
semiconducting chalcogenide glasses [41–43]. TEM observation indicated amorphous crystalline transformation
after annealing at 545 K for 2 h as shown in Fig. 2. During
annealing at temperatures higher than Tg the indirect
optical energy gap decreases and the width of the localized
states tails increases with increase in the annealing
temperature. This result can be interpreted by assuming
the production of surface dangling bonds around the
crystallites [44] during the process of crystallization. It has
been suggested by many authors [44,45] that nearly ideal
amorphous solids crystallize under heat treatment and that
in the process of crystallization dangling bonds are
produced around the surface of the crystallites. Further
heat treatment causes the crystallites to break down [45]
into smaller crystals thereby increasing the number of
surface dangling bonds. These dangling bonds are responsible for the formation of some types of defects in highly
polycrystalline solids. As the number of dangling bonds
and defects increase with increase in annealing temperature, the concentration of localized states in the band
structure also increases gradually. Hence the heat treatment of the films causes an increase in the energy width of
localized states thereby reducing the optical energy gap.
The increase in the electrical conductivity and the
decrease in the activation energy for conduction after
annealing could be attributed to the amorphous-crystalline
transformations.
5. Conclusions
From the above results and discussion, one can conclude
that the optical absorption measurements for the asdeposited films indicate that the absorption mechanism is
due to the indirect transition. The optical energy gap (Eg)
for the as-deposited GexSe92xIn8 thin films decreases with
increasing Ge content. On the other hand, the values of
refractive index (n) and extinction coefficient (k) increase
with increasing Ge content. Annealing Ge12.5Se79.5In8 films
showed an increase in the energy gap as the annealing
temperature increased up to the glass temperature Tg
followed by sharp decrease on increasing the annealing
temperature above Tg. TEM showed that amorphous
ARTICLE IN PRESS
M.A. Abdel-Rahim et al. / Physica B 387 (2007) 383–391
crystalline transformation occurred after annealing at
temperature higher than Tg.
The electrical results of as-deposited GexSe92xIn8 thin
films show two types of conduction channels which
contribute two conduction mechanisms. The electrical
conductivity (s) at 333.5 K increased and the activation
energy for conduction (DE) decreased with increasing the
Ge content. On the other hand, the electrical resistivity and
the activation energy were found to be decreased on
increasing the annealing temperature.
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