Enhancement of the Thermoelectric Figure of Merit in Gated Bismuth Telluride Nanowires
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Enhancement of the Thermoelectric Figure of Merit in Gated Bismuth Telluride Nanowires Igor Bejenari1, Valeriu Kantser2 and Alexander A. Balandin1 1 Department of Electrical Engineering and Materials Science and Engineering Program, University of California – Riverside, Riverside, California 92521 U.S.A. 2 Institute of Electronic Engineering and Industrial Technologies, Academy of Sciences of Moldova, Kisinev, MD 2028 Moldova ABSTRACT We theoretically studied how the electric filed effect can modify thermoelectric properties of intrinsic bismuth telluride nanowires, which are grown along [110] direction. The electronic structure and wave functions were calculated by solving the self-consistent system of the Schrodinger and Poisson equations by means of both the Thomas-Fermi approximation and the spectral element method. The thermoelectric parameters were determined using a constant relaxation-time approximation. The external electric field can increase the Seebeck coefficient of a nanowire with 7 - 15 nm lateral dimensions by nearly a factor of two, and enhance the figure of merit by an order of magnitude. INTRODUCTION Bismuth telluride and its solid solutions (Bi2-xSbxTe3, Bi2Te3-ySe) are one of the best known thermoelectric materials for the modern commercial application. These materials possess notable properties like a high anisotropic multi-valley Fermi surface, a small value of the thermal conductivity, and an optimal value of a carrier concentration at room temperature. Because the electron dimensional confinement and the enhancement of the photon boundary scattering control the electron transport, bismuth telluride based superlattices and quantum wires have advanced thermoelectric properties when compared to bulk bismuth telluride. For example, the figure of merit of the Bi2Te3/Sb2Te3 superlattices achieves its maximum value of 2.4 at room temperature [1]. Because of a larger confinement effect, for thermoelectric applications the bismuth telluride nanowires are better than the superlattices. The bismuth telluride nanowires can be obtained by electrochemical deposition of the material in the nanopores of anodized alumina membranes, by Taylor-Ulitovsky technique, and by high pressure injection of the melt into capillaries [2, 3]. Electric field effect (EFE) is a powerful tool to control electrical properties of low dimensional structures. Experimental and theoretical studies showed that the EFE can significantly improve thermoelectric properties of the Bi nanowires and PbTe films [4-6]. While dimensional confinement of electrons leads to a modification of their density of states, the Fermi level can be changed due to the EFE control of the electron (hole) concentration in the nanowire. The EFE on the nanowire thermoelectric properties is also a result of the energy spectrum modification and the local dependence of the electron distribution function. Also, the applied electric field causes additional quantum confinement. The EFE on the nanowire thermoelectric properties is also due to the energy s
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