This document summarizes research on the electrical behavior of chalcogenide glass thin films with varying composition and thickness. Key findings include: 1) The electrical activation energy and conductivity decreases as the thickness and selenium content of Ge-As-Se-Te films increases. 2) The density of the films increases with higher selenium content and thickness. 3) The pre-exponential factor and activation energy of the films follow a compensation law relationship. 4) Calculations of properties like coordination number, heat of atomization, and cohesive energy help explain trends in the electrical behavior with composition.

1. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print),
ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 59-65 © IAEME
59
EFFECT OF COMPOSITION AND THICKNESS ON THE ELECTRICAL
BEHAVIOR OF Ge – As – Se - Te CHALCOGENIDE GLASSES
H. H. Amer* and K. E. Ghareeb**
*Solid State Department, National Center for Radiation and Technology (NCRT), Atomic Energy
Authority (AEA) - Cairo – Egypt
**Metallurgy Department, Nuclear Research Center, AEA - Cairo – Egypt
ABSTRACT
Thin films of thickness in the range of 500 – 3000 A° of the composition Ge20As30 SeXTe(50-
X)(with x = 0 and 40) were prepared by thermal evaporation. D.C. conductivity was reported for the
investigated films. It was found that the activation energy decreases by increasing the thickness of the
film. It was observed that the increase of Se was followed by increase in the glass transition
temperature. Also, it is noticed that increasing Se content affected the average heat of atomization and
cohesive energy (C.E.) of the composition and the energy gap decreases by increasing Se content.
Keywords: Amorphous, Chalcogenide, Conductivity, Activation Energy, Cohesive Energy.
INTRODUCTION
In Chalcogenide glasses, there are different mechanisms which can be observed in different
temperature regions. The electrical conductivity (σ) in these glasses can be written (1)
in three terms
arise from three different conduction mechanisms as:
σ = σo exp (-∆E/KBT) + σ1 exp (-E1/KBT) + σ2 exp (-E2/KBT) (1)
The first term describes the high temperature region where the dominant mechanism is the
band conduction through the extended states. In this term the constant σo varies from 10-5
to 10-9
-1
cm-1
and depends on the composition, where ∆E is the activation energy, KB is Boltzman’s
constant and T is the absolute temperature. The second region represents the hopping conduction via
localized states. Here the conduction arises from tunneling through unoccupied levels of the nearest
neighboring centers. The value of σ1 is approximately 102
-104
less than σo, partly because of the
smaller density of localized states and their low mobilities. The third region represents the hopping
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conduction near Fermi level. This third contribution to conductivity in an amorphous semiconductor
is analogous to impurity conduction in heavily doped semiconductors. In this case the conductivity is
given by the third term on the R.H.S. of equation (1)(2)
. In this paper the results of D.C. conductivity
for thin film samples of amorphous Ge20As30SeXTe(50-X) (with x = 0 and 40) are presented and
discussed in a frame of chemical bonds involved. In addition, the average coordination number (NCO),
the average heat of atomization (Hs), the constraints (Ns) and the cohesive energy (CE) of the
Ge20As30SeXTe(50-X) glasses have been examined theoretically.
EXPERIMENTAL WORK
1-Preparation of Bulk Amorphous Ge20As30 SeXTe(50-X) (with x = 0 and 40)
Two glasses of bulk amorphous of the system Ge20As30 SeXTe50-X (where x = 0 and 40) were
prepared. These glasses were prepared from Ge, As, Se and Te elements with purity 99.999%. These
glasses are reactive at high temperature with oxygen. Therefore, synthesis was accomplished in
evacuated clean silica tubes. The tubes were washed with distilled water, and then dried in a furnace
whose temperature was about 100o
C. For each composition, the proper amounts of materials were
weighed using an electrical balance type with accuracy ±10-4
gm.
The weighted materials were introduced in the cleaned silica tubes and then evacuated to
about 10-6
torr and sealed. The sealed tubes were placed inside the furnace and the temperature of the
furnace was raised gradually up to 850o
C within one hour and kept constant for 8 hours. Moreover,
shaking of the constituent materials inside the tube in the furnace was necessary for realizing the
homogeneity of the composition. After synthesis, the tubes were quenched in liquid nitrogen. The
glassy ingots could be obtain by drastic quenching. Then, the material was removed from the tubes
and kept in dry atmosphere. The ingot materials were identified as glass due to their bright features.
The proper ingots were confirmed to be completely amorphous using X-ray diffraction and
differential thermal analysis. Homogeneity of prepared samples was proved by determination of
density of different parts (3)
.
2- Preparation of Thin Film Samples
Thin films of the selected compositions were prepared by thermal evaporation technique.
Edward 306E coating unit was used for thin film deposition. The vacuum system consists mainly of a
rotary pump, diffusion pump, penning bridge for measuring vacuum, high A.C. current source and the
bell jar. A specially designed silica boat was used for evaporation instead of using metallic boats to
obtain highly homogeneous uniform films. The silica boat was used instead of metallic boat to omit
the probability of contamination. The heating of the silica was achieved by spiral tungsten wire. The
silica boat had to be cleaned every time before evaporation. This was accomplished by using
hydrochloric acid, then washing several times with boiled distilled water and finally it was dried in a
furnace, whose temperature was about 100o
C. The thin films were checked by x-ray diffraction. The
obtained data reveal no sharp peaks.
3- Density Determination
The density of the considered samples was determined using the method of hydrostatic weight
using toluene. A single crystal of germanium was used as a reference material for determining the
toluene density. The latter has been determined from the formula:
Ge
air
tolueneair
toluene dx
W
WW
d '
''
−
= (2)
Where, W` is the weight of single Ge crystal and dGe is the density of Ge.
dGe = 5.32 gm/cm3

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dtoluene = 5.15 gm/cm3
Then, the sample density was calculated from the formula
toluene
tolueneair
air
sample dx
WW
W
d
−
= (3)
Where W is the weight of the sample (4)
.
Devices used in different measurements are as follows
1- X-ray Philips diffractometer was used to investigate and characterize the structure of the
samples.
2- A micro-Data apparatus, Shimadzu DT-30 model was used for the measurements of DTA.
3- Double-Beam Jasco V-530 UV/V is spectrophotometer. It was used for optical measurements.
4- Edward 306E coating unit was used for thin film deposition.
5- Thickness monitor MAXTEK Model TM-200.
RESULTS AND DISCUSSION
Figure (1) shows that the density increases by increasing the Se content.
4.7
4.8
4.9
5
5.1
5.2
5.3
5.4
5.5
5.6
5.7
0 500 1000 1500 2000 2500 3000 3500
Thickness (t Ao
)
Density(gm/cm3
)
X = 0
X= 40
Fig.(1): Dependence of density on thickness in the system Ge20As30 SeXTe(50-X)
(with x = 0 and 40)
-Effect of thickness on electrical conductivity
The dependence of ln (σ) on 1/T for the investigated films with different thicknesses with
x = 0 and x = 40 are shown in Figures (2) and (3), respectively. All samples follow a common pattern,
where two regions of conductivity are observed: the first is for high temperature range which is for

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62
high described by the first term of equation (1). In this region, band like conduction through extended
states is the dominant mechanism.
-33
-31
-29
-27
-25
-23
-21
-19
-17
1 2 3 4 5 6 7 8 9 10 11 12 13
1000/T (K
-1
)lnσσσσ(ΩΩΩΩ
-1
cm
-1
)
X = 0, t = 500A°
X = 0, t =1000A°
X = 0, t = 2000A°
X = 0, t = 3000A°
Fig.(2): Variation of (ln σ)versus reciprocal absolute temperature for films of (Ge20As30SeXTe50-
X with x = 0) system at different thickness (500A°, 1000A°, 2000A° and 3000A°)
-35
-33
-31
-29
-27
-25
-23
-21
-19
-17
-15
2 3 4 5 6 7 8 9 10 11
1000/T (K
-1
)
lnσσσσ(ΩΩΩΩ
-1
cm-1
)
X = 40, t = 500A°
X = 40, t = 1000A°
X = 40, t = 2000 A°
X = 40, t = 3000A°
Fig. (3): Variation of (ln σ) versus reciprocal absolute temperature for films of (Ge20
As30SeXTe50-X with x = 40) system at different thickness (500A°, 1000A°, 2000A° and 3000A°)

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63
Values of σo and σRT (conductivity of room temperature) for different films were estimated
and listed in Table (1).
Table (1): Compositional and thickness dependence of the electrical characteristics quantities
for the thin film glasses in the system Ge20As30SeXTe50-X) with x = 0 and 40
Comp. Thickness
(t Ao
)
Tg(o
C) Density
(gm/cm3
)
∆E.(eV) σRT
( -1
cm-1
)
Ln σRT σo
( -1
cm-1
) E1(eV)
X = 0
500
1000
2000
3000
245 4.81
5.22
5.31
5.53
0.58
0.35
0.28
0.18
3.86x10-12
1.12x10-11
3.8x10-11
2.4x10-11
-26.28
-25.2
-23.99
-23.9
26.579
25.05
24.12
23.98
1.16
0.69
0.56
0.35
X = 40
500
1000
2000
3000
258 4.9
5.3
5.35
5.6
0.35
0.25
0.16
0.13
4.59x10-13
1.36x10-10
1.7x10-10
3.01x10-9
-28.4
-22.7
-22.5
-21.92
28.2
22.81
22.57
21.97
0.71
0.499
0.322
0.27
To check the validity of compensation law, the pre-exponential factor σo against the activation
energy ∆E is shown in fig.(4). The observed dependence indicates that Ge -As -Se –Te obeys the
compensation law(5,6)
. Such behavior was also observed by Majied(7)
. It was observed that the
electrical activation energy decreases by increasing both Se content and the thickness of the thin film
for the composition Ge20As30SeXTe(50-X) with x = 0 and 40.
23
24
25
26
27
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
∆∆∆∆E(e.v)
σσσσo(ΩΩΩΩ
-1
cm
-1
)
0
5
10
15
20
25
30
σσσσοοοο(ΩΩΩΩ
-1
cm
-1
)
X = 0
X = 40
Fig.(4): Variation of σo as a function of activation energy for the composition
(Ge20 As30SeXTe50-X) with X = 0 , 40 at different thickness (500A° , 1000A° , 2000A°, 3000A°)

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64
- Loffe and Regel (8)
have suggested that the bonding character in the nearest neighbor region,
which is the coordination number, characterizes the electronic properties of the semiconducting
materials. The coordination number obeys the so- called 8-N rule, where N is the valence of an atom;
the number of nearest-neighbor atoms for Ge, As, Se and Te are listed in table (1).The average
coordination number in the quaternary compounds Aα Bβ Cγ Dλ is as:
NCO = [α NCO(A)+β NCO(B)+ γ NCO(C)+λ NCO(D)] / [α + β+ γ+ λ] (4)
Where: α, β, γ and λ are the valences of the elements of compound.
The determination of NCO allows the estimation of the number of constraints (Ns). This
parameter is closely related to the glass-transition temperature and associated properties. For a
material with coordination number NCO and Ns can be expressed as the sum of the radial and angular
valance force constraints(9)
.
Ns = NCO/2 + (2 NCO-3) (5)
To extend the idea to ternary and higher order semiconductor compounds, the average heat of
atomization is defined for a compound Aα BβCγ Dλ as(10,11)
:
Hs = [α Hs(A)+β Hs(B)+ γ Hs(C)+λ Hs(D)] / [α + β+ γ+ λ] (6)
Table (2) shows values of the coordination number NCO, the heat of atomization (HS) and the
bond energy of Ge, As, Se and Te, used for calculations (12)
.
Table (2): Values of the coordination number NCO, the heat of atomization (HS) and the bond
energy of Ge, As, Se and Te, used for calculations
Physical characteristics Ge As Se Te
Coordination no. 4 3 2 2
Hs(Kcal/mol) 90 69 54.17 46
Bond energy(Kcal/mol) 37.6 32.1 78.87 33
Knowing the bond energies, we can estimate the cohesive energy (CE), i.e. the stabilization
energies of an infinitely large cluster of the materials per atom, by summing the bond energies over
all the bonds expected in the system under test. The CE of the prepared samples is evaluated from the
following equation (13)
:
CE = Σ (CiDi/100) (7)
Where Ci and Di are the number of the expected chemical bonds and the energy of each
corresponding bond. The calculated values of NCO, Ns, HS, Hs/ NCO and CE for (Ge20 As30SeXTe50-X)
system are given in table(3).

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ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 59-65 © IAEME
65
Table (3): The calculated values of NCO, Ns, HS, Hs/ NCO and CE for
(Ge20 As30SeXTe50-X with x = 0, 40) system
Composition NCO Ns HS(Kcal/mol) Hs/ NCO CE(eV/atom)
Ge20As30Te50 2.7 3.75 61.7 22.85 3.99
Ge20As30Se40Te10 2.7 3.75 64.97 24.06 5.584
CONCLUSION
1 - The electrical conductivity (σo) decreases with increasing both Se content and the thickness of
the thin film for the (Ge20 As30SeXTe50-X with x = 0, 40) composition.
2 - The electrical activation energy (∆E) decreases with increasing in both Se content and the
thickness of the thin film for the (Ge20 As30SeXTe50-X with x = 0, 40) composition.
3 - The values of energy gap (E1) decrease with increasing the thickness of the thin film for the
(Ge20 As30SeXTe50-X with x = 0, 40) composition.
4 - The values of coordination number (NCO), number of constraints (Ns), heat of atomization
(Hs) and cohesive energy (CE) of the (Ge20As30SeXTe50-X with x = 0, 40 system is dependent
on the glass composition. The increase in Se content leads to increase in E1, NCO, Hs, Hs/ NCO
and CE.
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