The crystals of AgInSe₂ were grown by Bridgman technique. According to this technique, the samples have been prepared by the direct melting of the starting materials (Ag, In and Se) in quartz ampoule which was sealed under vacuum of about 10⁻⁴ Torr. The silica ampoule and its charge were mounted in the first zone of a threezone tube furnace. The temperature in the first zone was higher than the melting point, and then the temperature was kept about 24 h for complete melting and mixing of the starting materials. The temperature of the middle zone of the furnace was 1073 K corresponding to the crystallization temperature of AgInSe₂ as reported in the phase diagram [11] . When the ampoule and the melt were gradually entered the third zone, solidification occurred since the temperature was adjusted to be less than the melting point.

Samples of rectangular form of 7.4 × 2.8 × 0.8 mm³ dimensions were used for performing the electrical conductivity and Hall coefficient measurements. In this way, the length of the sample was adjusted to be nearly 3 times its width to avoid Hall voltage drop [12] . A pyrex cryostat was used for adjusting the low temperature and high temperature [13] . The cryostat, which contains the crystal, was evacuated (10⁻⁴ Torr) to avoid water vapour condensation or crystal oxidation. In this experiment, we used a very sensitive potentiometer (UJ33E mark) and an electromagnet (Oxford N 177 type) which generates 5000 G.

For thermoelectric power measurements, the investigated sample was adjusted to be 5 mm in diameter and 10 mm in length by polishing processes. The thermoelectric power was measured by using a pressure contact sample holder with a heater and a heat sink to obtain a temperature difference between the opposed surfaces of the sample (≈3˚C). Also, an evacuated calorimeter (10⁻³ Torr) was used to protect the sample from oxidation and water vapour condensation at high and low temperatures, respectively. Simultaneous measurements of temperature and thermovoltage were carried out to increase the accuracy of the measurements.

Figure 1 depicts the X-ray diffractogram for the prepared AgInSe₂ crystals. The powder XRD analysis confirms the synthesis of tetragonal AgInSe₂ with prominent peaks from (112), (103), (200), (105), (220), (204) (3121), (116), (400), (322), (316) and (424) planes. The lattice parameters a and c of grown crystals AgInSe₂ have been calculated by plotting the lattice parameters, a_hkl and c_hkl (calculated from the Bragg’s Law of the main peaks) against an extrapolation function:

Figure 2 shows the relation between the lattice a and F(q). The estimated values of lattice parameters of AgInSe₂ tetragonal phases are a = 6.0938 Å and c = 11.7775 Å [14] . The Crystallite size (D) of the obtained AgInSe₂ crystals was 70 nm according to the Debye-Scherrer’s formula from the full width at half-maximum (FWHM) b of the peaks expressed in radians [15]

where l is 1.540598 Å for CuKα and q is the Bragg angle.

Figure 3 shows the temperature dependence of electrical conductivity s for AgInSe₂ crystals. In the low tem-

perature range (138 - 273 K), representing the extrinsic region, s increases slowly with temperature as a result of the transition of the carriers from the impurity level to the conduction band. The activation energy DE_d was found to be 0.06 eV as computed in this range. In the same curve, one can notice that the transition region lies between 273 and 330 K. Above 330 K, the intrinsic conduction region begins where s increases. This reveals that both electrons and holes contribute to the conduction at this high temperature range. The temperature dependence in the intrinsic conduction follows the relation:

where σ_o is the pre-exponential factor and DE_g is the energy gap. From Figure 3, DE_g was calculated from the slope of the curve, and found to be 1.24 eV, which is closd to the value that was published before [16] . Meanwhile, the room temperature conductivity was 0.48 W⁻¹cm⁻¹.

Figure 4 shows the relation between Hall coefficient R_H and 10³/T. This curve is divided into two regions: The first part of low temperatures, R_H begins to increase as the temperature increases until 330 K. This region indicated the extrinsic region. The second region above 330 K, R_H decreases as the temperature increases. This region indicated the intrinsic region. The Hall coefficient at room temperature is evaluated as 1054 cm³/C.

Figure 5 shows the temperature dependence on the Hall mobility μ_H (where). From this curve, we can distinguish two regions. In the first region of low temperatures (T < 330 K), μ_H increases with temperature following the relation μαT^2.6. Such behavior is the characteristic of a scattering mechanism of the charge carriers with ionized impurities. While the second region of the high temperatures (T > 330 K), mobility decreases with increasing temperature according to a power relation, where n = 10. This may lead to the assumption that the carrier scattering mechanism is due to the interaction between charge carriers and phonons which is dominant. At room temperature, Hall mobility equals 510 cm²/Vs. Figure 6 shows the carrier concentration as a function of temperature. It is observed from the curve that the temperature increases monotonically with concentration. Within the intrinsic region of conduction, the following relation can be applied to describe the temperature dependence on the charge carrier concentration n_i:

The calculated energy gap from this relation equals 1.24 eV which is in agreement with that previously obtained from the conductivity work. Finally, the charge carriers concentration at room temperature amounts 1.5 × 10¹⁶ cm⁻³ for the AgInSe₂ crystals.

The thermoelectric power (TEP) measurements of AgInSe₂ were carried out as a complementary part to the electrical conductivity and Hall effect. The relation between thermoelectric power α and the temperature is depicted in Figure 7. From this figure, we can see that α increases with temperature up to its maximum value (709 μV/K ) at a temperature 325 K. Above 325 K, α decreases until about 488 K. The growth of α with T (between 220 and 325 K) is due to the thermal activation of the charge carriers. At T = 325 K (the maximum value of α) the intrinsic conduction appears. The decrease of a above 325 K is due to the compensation process which takes place in this range of temperature. TEP of the AgInSe₂ sample has a negative sign over the entire considered temperature range. The value of thermoelectric power at room temperature is 146 αV/K [17] . The behavior of thermoelectric power with temperature in the intrinsic region can be described by the following equation [18]

where b is the ratio of electron and hole mobility, DE_g is the energy gap, K is the Boltzman constant and, are the effective masses of electrons and holes, respectively. Taking into consideration the value of eV(as obtained from the electrical conductivity measurements in the same range of T), the ratio of the electron and hole mobility was calculated from the slope of the line in the high temperature range of Figure 7 and was found to be 1.71. By considering the value of μ_p = 510 cm²/Vs which was obtained from the Hall measurements data, the value of μ_p was estimated to be 235 cm²/Vs. Meanwhile, the ratio was also calculated from the intercept of the curve with α-axis and was found to be 0.01.

Another important equation was employed in the extrinsic region [19] , is as follows:

According to this equation the relation between α and lnT was drawn. Then from the intercept of the line (in the impurity region) with the α-axis, we got = 1 × 10⁻³¹ kg. Taking into account the ratio previously obtained from Figure 7, we evaluated as 1 × 10⁻²⁹ kg. The value of the relaxation time for electrons was 3.18 × 10⁻¹⁰ sec. It was calculated according to the equation while for holes it is 1.88 × 10⁻⁸ s. Furthermore, the diffusion constants for electrons and holes were calculated and found to be D_n = 13.19 cm²/s and D_p = 7.45 cm²/s, respectively (where. The diffusion length L_n and L_p were also calculated and found to be 6.47 × 10⁻⁵ cm and 3.74 × 10⁻⁴ cm for electrons and holes, respectively (where).

Figure 8 shows the dependence of thermoelectric power on the charge carriers concentration n which follows the equation [20]

where A is a constant depending on the scattering mechanisms.

The similar behavior in Figure 7 and Figure 8 suggests that the variation of α may be to the variation of carrier concentration with temperature.