Phase formation and phase transformations in Bi-Te films with nanoscale thickness
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IC STRUCTURE AND NONELECTRONIC PROPERTIES OF SEMICONDUCTORS
Phase Formation and Phase Transformations in Bi–Te Films with Nanoscale Thickness K. M. Akhmedov Institute of Physics, National Academy of Sciences of Azerbaijan, Baku, 371143 Azerbaijan e-mail: [email protected] Submitted November 6, 2007; accepted for publication December 12, 2007
Abstract—The processes of phase formation are studied in a binary Bi–Te system using the kinematic electron diffraction technique. It is established that, in the case of both simultaneous and layer-by-layer deposition of bismuth and tellurium and irrespective of the order of their deposition, phases with compositions Bi2Te3 and BiTe are formed at the condensation plane in the amorphous and crystalline state, respectively. The amorphous Bi2Te3 phase is stable at room temperature and crystallizes at a temperature of 423 K. It is shown that ordering of the phase BiTe is not a consequence of atomic order of the structure; rather, it is caused by the real structure of the object (by blocks). PACS numbers: 73.61.Jc, 71.23.Cq DOI: 10.1134/S1063782608090017
1. INTRODUCTION The results obtained in a number of studies [1–5] concerned with phase equilibria in Bi–Te systems are not consistent with each other; the observed discrepancies are mainly connected with the existence region of a specific phase and to the type of phase diagram. Microscopic and X-ray studies and also studies of electrical resistance, thermopower, magnetic susceptibility, Hall effect, and hardness were used to establish that the phase with composition Bi2Te3 exists with a wide range of homogeneity. The boundaries of homogeneity of Bi2Te3 are [3] at 36–40 and 53–55 wt % (at 48–52 and 65–67 at %) of Te. A cubic phase BiTe of the NaCl type with a = 0.647 nm and the space group symmetry B1 was observed in Bi–Te alloys in addition to solid solutions formed by one of the components (Bi) in Bi2Te3. According to [1, 4], all alloys in the Bi–Te system are alloys with variable composition; in this context, the issues of the growth and study of thin-film samples representing microstructural objects with valuable physical properties that are unattainable in bulk samples require study of this system in the course of heat treatment and are caused by scientific interest in the increasing applications of compounds of this system as detectors of infrared radiation, thermal generators, power sources for self-contained systems of cosmic techniques, and implanted cardiostimulators [5–8]. In publications [9–12] concerned with thin layers, formation of films of separate compounds in the Bi–Te system with various orientation of crystallites was mainly considered. It was shown [10, 11] that films with a thickness of 30 nm formed on various substrates preliminarily heated to 473 K were textured. As the film thickness and the substrate temperature were increased,
the texture state became disturbed. In the case of Bi2Te3 condensation on NaCl substrates at room temperature, fine-grain polycrystalline structures were formed [12]. Laser d
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