ARTICLE IN PRESS
1260
M. Mobarak / Physica B 404 (2009) 1259–1263
specimen (not more than 7 K). The temperature was measured
using two calibrated thermocouples. Silver paste was also used
here as an ohmic contact. The sample with its holder (which
contains an electric heater) was placed in a cylindrical container.
It was closed at both ends to reduce convection currents and also
to be evacuated. This was done to avoid the sample oxidation at
high temperatures or water vapor condensation in low-tempera-
ture measurements. A temperature difference was controlled by
energizing one of the two heaters. The temperature gradient
direction was perpendicular to the c-axis. Two calibrated
thermocouples were used as temperature-sensing elements
between the two ends of the crystal. The measurements of the
temperature and the developed thermovoltage were carried out
simultaneously. For measuring either Hall voltage or thermo-
voltage, a high-sensitivity potentiometer was used (2727 portable
DC potentiometer).
1600
1200
800
400
0
10
20
30
40
2θ (deg.)
50
60
70
80
Fig. 1. The X-ray diffraction intensity as a function of the Bragg angle of GaInSe2
powder.
5. Analysis and discussion of the results
stoichiometric compound corresponding to GaInSe2, with a very
small excess of Ga in agreement with literature data [10]. The
crystal was identified by X-ray powder diffraction measurements
Measurements of the electrical conductivity (s) were carried
out in a direction parallel to the c-axis (i.e. in the plane of the
layers). The measurements cover a temperature range extending
˚
as a single phase with lattice constants a ¼ 8.002 A and c ¼ 6.537
from 120 up to 570 K. Fig. 2 shows electrical conductivity log
s vs.
˚
A. These results are in good agreement with previous X-ray
103/T for a typical GaInSe2 single crystal sample. As shown, in the
investigated temperature range, the logarithm of the conductivity
showed a linear dependence on temperature with two modes of
conduction in addition to the transition region that appeared
between them. Thus in this semiconductor, on the low-tempera-
ture side (extrinsic conduction) and at high-temperature side
(intrinsic conduction), the conductivities are assumed to obey the
following Arrhenius equations respectively:
structure investigations for GaInSe2 [11]. In Fig. 1, the peaks of the
X-ray powder diffractogram as a function of 2y (twice the Bragg
angle) are reported, indicating the single-phase nature of the
produced crystal.
3. Electrical conductivity and Hall effect measurements
Specimens for measurements were prepared by cleaving the
initial single crystals in two mutually perpendicular natural
cleavage planes, and were rectangular shaped with dimensions
6.7 ꢁ2.4 ꢁ1.2 mm3. This sample, which has a length three times
its width (to avoid Hall voltage drop), was used for electrical
conductivity and Hall effect measurements simultaneously.
Electrical load wires of copper were soldered to the sample with
a small point of silver past which was used as an ohmic contact.
The ohmic nature was checked by recording the I–V character-
istics in both forward and reverse bias directions. A pyrex cryostat
was used for adjusting the low temperature (with the aid of liquid
nitrogen) and high temperature (via an electric heater). The
cryostat (which contains the crystal) was evacuated (10ꢀ4 Torr) for
avoiding vapor condensation or oxidation. In this experiment we
used a very sensitive potentiometer (2727 portable DC potenti-
s
s
¼
¼
s
0 expðꢀ
0 expðꢀ
D
Ea=2 KTÞ; 240 KhTi120 K;
Eg=2 KTÞ; 570 KhTi350 K,
s
D
where s0 is a constant,
the energy gap width, T the absolute temperature and K the
Boltzmann constant. From the above relationships the energy gap
DEa the impurity ionization energy, DEg
D
Eg is deduced to be 1.8 eV while
in quite good agreement with previous literature data [10]. The
increase of in the intrinsic part (above 350 K) is regarded as a
result of excitation of the carriers from the valence band to the
conduction band. However, in the extrinsic part (below 240 K),
DEa is 0.079 eV; these results are
s
s
increment is regarded as a result of ionization of impurity atoms.
-2
ometer) and an electromagnet (Oxford
N 177 type) which
generates 6000 G. If J, C and H are the current, the C-axis and
the magnetic field, respectively, then the conditions of the
measurements can be written as (J//C?H). Both the direction of
the current and the magnetic field were reversed, in order to avoid
thermogalvanomagnetic effects. The electrical properties were
determined using the four-point direct-current van der Pauw
method [12].
-2.5
-3
-3.5
-4
4. Thermoelectric power measurements
-4.5
-5
For TEP measurements, we utilized the prepared crystal after
removing it from the ampoule because it had a cylindrical shape.
The investigated sample was adjusted to be 8 mm in diameter and
5 mm in length by a polishing process. The sample was washed by
pure ethanol and hot distilled water and then left to dry without
1
2
3
4
5
6
7
8
103/T(K-1)
heat treatment. The measurement of the TEP (
a) was made by
Fig. 2. Electrical conductivity of GaInSe2 as a function of temperature.
establishing a temperature gradient between the two ends of the