Fabrication and Characterization of Nanostructured Thermoelectric Materials and Devices

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Fabrication and Characterization of Nanostructured Thermoelectric Materials and Devices Brian L. Geist1, Madrakhim Zaynetdinov1, Kirby Myers2, K. Zhang3, X. Chen3, A.D. Ramalingom Pillai3, Helmut Baumgart3, Hans D. Robinson2 and Vladimir Kochergin1 1 MicroXact, Inc., Blacksburg, VA 24060-6376, U.S.A. 2 Department of Physics, Virginia Tech, Blacksburg, VA 24061-0435, U.S.A. 3 Applied Research Center, Old Dominion University, Newport News, VA 23606, U.S.A. ABSTRACT We present results of modeling and experimental characterization of thermoelectric (TE) materials built on new fabrication principles, involving the coating of three-dimensionally structured quantum well super-lattice substrates with PbTe/PbSe. A new system for wafer-scale electrochemical deposition of such structures was specifically developed and will be described in this paper. Scanning electron microscopy (SEM) was used to measure film thickness and electron diffraction spectroscopy (EDS) was used to determine film material concentration. By adjusting deposition parameters, we were able to build stoichiometric PbSe, PbTe and stacked PbSe/PbTe super-lattice films on planar and pre-structured surfaces. The films were thermoelectrically modelled using COMSOL and then characterized using an infrared Seebeck effect measurement system which measured surface heating of the film while measuring the voltage associated with the temperature gradient. We report advances in the design and fabrication of TE materials which improve cost-effectiveness and TE efficiency. INTRODUCTION Thermoelectric (TE) materials have already been developed for niche applications where their high costs are not an impediment to their use. They excel in applications such as converting waste heat to electric or for remote sensing where solid state power generation can greatly extend the service life of devices. [1] The automotive industry [2] and solid state Peltier Effect coolers such as those commercially available from Coleman® are two prominent applications where TE materials are used. However, the application of TE materials to products is limited by rather poor conversion efficiencies of commercial-off-the-shelf materials. The high figure of merit materials were demonstrated in quantum well superlattice (QWS) or quantum dot superlattice (QDS) samples, however, due to slow and expensive fabrication methods such as Molecular Beam Epitaxy (MBE) or chemical vapor deposition (CVD), usable for 100’s of nm thicknesses only, these materials are yet to find practical applications, since typically 100’s of μm thick materials are needed. Here we present a material made on completely new fabrication principles: it is comprised of three-dimensional “wells” of PbTe/PbSe [3,4] fabricated by a conformal coating of macroporous silicon (MPSi) pore walls with the Atomic Layer Deposition (ALD) technique described in [5] as well as the conformal coating of MPSi pore walls with the eALD technique. The PbTe/PbSe QWSs (and QDS, formed during QWS deposition due to a lattice mismatch,) exhibit high ZT values