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Thermoelectric converters based on silicon nanostructures offer exciting opportunities for higher efficiency, lower cost, ease of manufacturing, and integration into circuits. This paper considers phonon transport in a broad range of nanostructured materials made from Si, Ge, and their alloys. Our model based on the phonon Boltzmann transport equation captures the lattice thermal transport in silicon–germanium (SiGe) nanostructures, including thin films, superlattices (SLs), and nanocomposites. In nanocomposites, the model captures the grain structure using a Voronoi tessellation to mimic the grains and their size distribution. Our results show thermal conductivity in SiGe nanostructures below their bulk counterparts and reaching almost to the amorphous limit of thermal conductivity. We also demonstrate that thermal transport in SiGe nanostructures is tuneable by sample size (thin films), period thickness (SLs), and grain size (nanocomposites) through boundary scattering. Our results are relevant to the design of nanostructured SiGe alloys for thermoelectric applications.

I. INTRODUCTION

The current energy crisis makes alternative sources of energy very attractive, and research in these areas is a necessity. One of the most ubiquitous and abundant sources of energy is the heat supplied by the sun and dissipated by virtually every energy-consuming process. It is estimated that nearly 15 terawatts of energy is lost annually in the US alone through waste heat emitted to the environment.1 Heat and charge transport in materials are coupled processes, and this coupling leads to thermoelectric (TE) effects: a temperature gradient leads to a voltage drop and vice versa. Thermoelectric effects are important for probing elementary excitations in materials and have practical applications to refrigeration and power generation. Efficient thermoelectric devices are the key to our ability to harvest thermal energy from virtually any source, ranging from waste heat in industrial processes and engines to solar energy from parts of the solar spectrum, which cannot be efficiently captured by photovoltaics. The global need for sustainable energy coupled with the recent advances in thermoelectrics inspires a continued excitement in this field. As thermoelectric generators are solid-state devices with no moving parts, they are silent, reliable, and scalable, making them ideal for small, distributed power generation. Silicon-based thermoelectrics also have potential for on-chip cooling of Contributing Editor: Harald Böttner a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.202 J. Mater. Res., Vol. 30, No. 17, Sep 14, 2015

local hot spots in dense nanoelectronic-integrated circuits where continued scaling of complementary metal oxide semiconductor (CMOS) circuits necessitates improvement in on-chip cooling techniques. However, both waste heat recovery and on-chip thermal management require materials with both low cost and high-thermoelectric conversion efficiency to be competitive with other tec