Nanostructured Telluride Films on Macroporous Silicon for High Efficiency Thermoelectric Devices
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Nanostructured Telluride Films on Macroporous Silicon for High Efficiency Thermoelectric Devices Hans D. Robinson1, Ofer Sneh2 and Vladimir Kochergin3 1
Department of Physics, Virginia Tech, Blacksburg, VA 24061 Sundew Technologies, Broomfield, CO 80020 3 MicroXact, Inc., Blacksburg, VA 24060 2
ABSTRACT We propose using macroporous silicon as an ultra-high aspect ratio scaffolding for epitaxially grown thermoelectric materials, so that thin films can be shaped into materials thick enough for practical devices. The self-limiting nature of atomic layer deposition (ALD) makes it an ideal growth technique for this substrate, as uniform thickness can be obtained at all points inside the macroporous structure, and we demonstrate successful deposition of antimony telluride on pore walls using ALD. Extension of this work to telluride superlattices should enable fabrication of thermoelectric devices with figures of merit (ZT) in excess of 2. Characterization of the thermoelectric and other properties of ALD grown telluride on silicon is ongoing. INTRODUCTION Current research on thermoelectrics spans a very broad range of materials and structures, but the strategies for obtaining higher device efficiencies are by and large the same, and consist of structuring the material at the nanoscale to optimize two distinct quantities. First, the power factor S2σ is optimized by creating a maximally asymmetric density of states near the Fermi level. Second, the lattice thermal conductivity κL is minimized by increasing phonon scattering in such a way that electron mobility is minimally affected. One needs to have very precise control of material structure in order to successfully meet these requirements. It is therefore no surprise that the highest thermoelectric (TE) figures of merit have been seen in structures fabricated by epitaxial growth techniques, since they provide superior control over film structure. Unfortunately, practical TE devices need to be at least tens if not hundreds of µm thick, while epitaxial growth beyond a few microns is too costly and time consuming to be practical. For this reason, many workers have turned to nanostructured bulk materials for TE applications. These efforts are starting to pay off [1], but the nature of these materials intrinsically provides less control over structure than do epitaxial growth methods, and it remains to be shown that it will be possible to optimize their performance to the same extent. To overcome these limitations, we propose using macroporous silicon (MPSi) as a substrate for nanostructured epitaxial films with very high TE figures of merit. MPSi is made from [100] Si wafers through an electrochemical etching process performed under highly controlled conditions. Although the process is in principle non-lithographic, structures with very high regularity can be fabricated if the substrate is first patterned with lithographically defined etch pits. Properly executed, this can result in a uniform array of square pores that are several microns in width, up to 500 µm deep, and wi
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