Theoretical Study of Electronic Structures of Bi 2 Te 3 /Sb 2 Te 3 Superlattices

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Theoretical Study of Electronic Structures of Bi2Te3 /Sb2Te3 Superlattices Hong Li, Daniel Bilc and S. D. Mahanti Department of Physics and Astronomy, Michigan State University East Lansing, MI 48823 ABSTRACT To understand thermoelectric properties of multiplayer Bi2Te3 /Sb2Te3 superlattices, especially their charge transport properties, electronic structure calculations were carried out using ab-initio gradient corrected density functional theory. The superlattice structures of (1,1) and (1,2) Bi2Te3 /Sb2Te3 multilayers were optimized and their band structures were compared with each other. Different lattice relaxation effects are observed for the two structures. The cross-plane and inplane effective masses for both these systems are found to be comparable, consistent with experimental mobility measurements. INTRODUCTION To improve the efficiency of thermoelectric materials following the ideas of anisotropic charge and energy transport, great efforts have been given to the synthesis of multilayer systems with low-temperature growth techniques. Thermoelectric efficiency of a material is usually characterized by its figure of merit (ZT), given by S 2σ T ZT = , (1) K el + K latt where S is the Seeback coefficient, σ is the electrical conductivity, T is the absolute temperature, Klatt is the lattice thermal conductivity and Kel is the electronic thermal conductivity. According to this relation, a higher ZT value could be obtained by reducing Klatt in low-dimensional systems by introducing phonon scattering, provided one does not deteriorate carrier mobilities. The multilayer Bi2Te3 /Sb2Te3 superlattices (SL) grown by metallorganic chemical vapor deposition were found to be good thermoelectric materials by Venkatasubramanian et al. [1] and an enhanced ZT ~ 2.4 at 300K in p-type Bi2Te3 /Sb2Te3 SL was obtained. For some Bi2Te3 /Sb2Te3 layer arrangements, they observed that the cross-plane carrier mobilities, which are along the direction perpendicular to the multilayers, were comparable to the in-plane mobilities. However, this behavior is quite different from the observation for the bulk Bi2Te3, which shows a very good in-plane mobility, but a rather small cross-plane mobility. In order to understand this unusual behavior, electronic structure calculations for (1,1) and (1,2) Bi2Te3 /Sb2Te3 SL systems have been performed using density functional theory (DFT) and carrier mobilities are estimated from effective mass calculations. A preliminary report of these calculations can be found in Ref. 2. ELECTRONIC STRUCTURE CALCULATIONS Electronic structure calculations were performed based on the density functional theory (DFT) [3], using the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof [4]

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for the exchange and correlation potential. The full-potential Linearized Augmented Plane Wave (LAPW) method [5] was used and our calculations were performed using the WIEN2K package [6]. The muffin-tin radii were taken to be RMT = 2.5 a.u. for all the atomic species. A plane-wave expansion with RKmax

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