Electrical and Thermoelectrical Properties of Sb2Te3 Prepared by the Metal-Organic Chemical Vapor Deposition Technique

The V2VI3 binary compounds such as Bi2Te3, Sb2Te3 and their alloys are narrow band±gap semiconductors with a high thermoelectric ®gure of merit Z ˆ So=k, where S is the Seebeck coef®cient, o the electrical conductivity and k the thermal conductivity. These semiconductors have been extensively studied in recent years because of their promising applications especially for thermoelectrical devices [1, 2] and thermal [3] and humidity [4] sensors using the Seebeck and Peltier effects, respectively. Giani et al. [5] have grown Bi2Te3 on pyrex substrates by using the metal-organic chemical vapor deposition (MOCVD) technique in a horizontal quartz reactor. They have also studied in some detail the electrical and thermoelectrical properties of Bi2Te3. Venkatasubramanian et al. [6] have studied the MOCVD growth of Bi2Te3 and Sb2Te3 on a GaAs substrate and used their superlattice structures for thermoelectrical applications. Dauscher et al. [7] have elaborated Bi2Te3 thin ®lms by using pulsed laser deposition (PLD) and have shown that a congruent transfer of stoichiometry occurs from the target to the substrate over several cm and that a good crystallinity is achieved. Magri et al. [8] have investigated the properties of electrodeposited bismuth telluride ®lms and have shown that the ®lm composition depends on the electrolyte composition and the current density. Our studies were carried out in an attempt to make a detailed analysis of the behavior of Sb2Te3 thin ®lms regarding the effect of R ˆ VI=V ratio on electrical and thermoelectrical properties. Sb2Te3 thin ®lms were grown using the MOCVD technique in a horizontal quartz reactor. Triethylantimony (TESb) and diethyltellerium (DETe) were used as antimony and tellurium sources, respectively. To avoid the possibility of any premature decomposition, TESb and DETe sources were both maintained at 20 8C. Hydrogen was used as the carrier gas with a ow rate equal to 3 slm to obtain good results. This is due to a better cracking ef®ciency for a ow rate of 3 slm found by Giani et al. [5]. The substrate temperature was ®xed at 450 8C during the deposition process and controlled by a thermocouple in direct contact with the substrate holder. The VI=V ratio (RVI=V ˆ DETe partial pressure=TESb partial pressure) varied between 1 and 13. In addition, during the deposition of Sb2Te3, the partial pressure of the group V element (Sb) was kept constant and equal to 1 3 10 atm. A Philips X-ray diffractometer, using monochromatic CuKa radiation (e ˆ 1:54051 E A), was employed to obtain diffraction patterns from ®lms deposited on a Pyrex substrate. A wide range of e (from 58 to 308) was scanned so that all possible diffraction peaks could be detected. Surface morphology was examined by scanning electron microscopy (SEM). The composition of the deposited layers was measured using an energy dispersive X-ray (EDX) microanalyzer. To measure Seebeck coef®cients, heat was applied to the sample, which was placed between two small perfectly parallel brass cylinders. The temperature difference between these two cylinders was measured using thermocouples and a sensitive Keithley digital thermometer. The potential difference was obtained at the position of the two thermocouples using a sensitive digital multimeter. The Van Der Pauw technique was used at 300 K to evaluate the sample resistivity, its carrier's mobility and its carrier's concentration. The X-ray diffraction (XRD) pattern was compared with ASTM charts and showed that the deposited layers grew in (0 0 0 l)H and exhibited a polycrystalline phase characterized by the (1 0 1 5)H peak (Fig. 1). The same peak was observed by Mandouh [9] on vacuum-deposited Sb2Te3 thin ®lms and disappeared upon annealing at 473 K. The surface morphology and crystallinity of the deposited thin ®lms on the amorphous substrate were found to depend strongly on the VI=V ratio, and its aspect seemed quite different. The SEM micrograph shown in Fig. 2 is of one of the Sb2Te3 layers deposited at 450 8C with the VI=V ratio ˆ 7. It was observed that the shape of the crystallites was hexagonal but not symmetrical, which is in good agreement with the X-ray data and the fact that the crystallites are randomly oriented with respect to the amorphous substrate. EDX analysis showed that the binary compound was always stoichiometric. Opposite results were obtained on Bridgman Sb2Te3 [10], where progressive loss of Te occurred and where the sample composition contained excess Sb atoms,