Cross-Plane Thermoelectric Properties in Si/Ge Superlattices
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Cross-Plane Thermoelectric Properties in Si/Ge Superlattices Bao Yang*, Jian L. Liu**, Kang L. Wang**, and Gang Chen& * Mechanical and Aerospace Engineering Department, ** Electrical Engineering Department, University of California, Los Angeles, CA 90095. & Mechanical Engineering Department, Massachusetts Institute of Technology, Cambridge, MA 02139. ABSTRACT In this paper, a set of methods is developed to measure the Seebeck coefficient, electrical conductivity, and thermal conductivity in the cross-plane direction of thin films. The method employs microfabricated heaters, voltage and temperature sensors, and phase-lock amplifiers to determine the temperature and Seebeck voltage oscillation in the cross-plane direction of the samples, from which the thermal conductivity and Seebeck coefficient of thin films are determined simultaneously. The cross-plane electrical conductivity is also measured by a modified transmission line model. These methods are applied to Si/Ge superlattices grown by molecular beam epitaxy. INTRODUCTION Thermoelectric effects in semiconductor superlattices (SLs) are attracting considerable interests because their low dimensionality may increase the thermoelectric figure of merit.[1-6] Thermoelectric transports in the in-plane (along the film plane) and cross-plane (perpendicular to the film plane) directions of SLs are highly anisotropic and need to be characterized separately. In the cross-plane direction of SLs, electrons and phonons may experience much stronger interface scattering than that along the in-plane direction. Previous experiments have shown that the cross-plane thermal conductivity is much lower than the in-plane thermal conductivity in the Si/Ge SLs and 1-2 orders of magnitude lower than the values predicted from properties of their bulk constituents using the Fourier theory.[7-9] Furthermore, the selective emission of hot carriers in the thermionic emission process can improve the thermoelectric energy conversion efficiency in the cross-plane direction of SLs.[10-12] Recently, a significant enhancement in ZT (about 2.4) at 300K in the cross-plane direction of Bi2Te3/Sb2Te3 superlattices has been reported.[13] To understand thermoelectric transport in the cross-plane direction, one needs to measure, in addition to the thermal conductivity (k), the Seebeck coefficient (S) and the electrical conductivity (σ) in the same direction since the device performance depends on the figure-ofmerit Z=S2σ/k.[14] In the cross-plane direction of SLs, the thermal conductivity has been studied extensively.[7-9,15,16] There are also limited experimental reports for the cross-plane electrical conductivity of SLs.[17] The cross-plane Seebeck coefficient of thin films such as SLs, however, has never been reported. This is because the Seebeck coefficient, defined as S=-∆V/∆T, requires the simultaneously determination of the voltage and temperature drops across very thin films (~µm). In this paper, we report a method for the simultaneous measurement of the cross-plane Seebeck coefficient and thermal c
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