Nanostructuring of thermoelectric materials has been widely employed in the last two decades with the aim to reduce the thermal conductivity k and/or to increase the power factor S2σ, thus enhancing the energy conversion efficiency. Among different nanostructuring strategies, interfaces in superlattices or nanodots in a host matrix have emerged as promising pathways to reduce the thermal conductivity through phonon scattering at multiple length-scales. Single-crystalline SiGe superlattices with and without nanodots are an ideal platform to study the effects of interfaces and nanodots on thermal transport not only because they are simple enough to permit a comparison of theoretical calculations and experiments, but also because the structural parameters, such as the interface spacing length (i.e. period length), nanodot density and matrix concentration can be precisely controlled during epitaxial growth. Despite previous investigations that showed that k can be lowered by shortening the period length and/or by increasing the nanodot areal density of SiGe superlattices, the understanding of the effects produced by interfaces and nanodots on thermal transport is still not satisfactory. Here we present systematic experimental studies on thermal transport through SiGe superlattices, which were grown by molecular beam epitaxy. First, the experiments clearly show that k of Ge/Si nanodot superlattices linearly decreases with decreasing the period length .
The results indicate that phonon scattering at the Ge/Si interfaces is diffusive while phonon transport through Si spacers is quasi-ballistic. Second, we show that Ge-segregation-driven intermixing around the interfaces play a crucial role in reducing k below the alloy limit . Third, we explore the limits of lowering k by decreasing the period length to values down to ~1.5 nm and the role of SiGe nanodots on the reduction of k . Our results reveal a weakening of the interface effect on phonon scattering and imply a lower limit for k when the period length is shorter than 2 nm. The experiments also suggest that SiGe nanodots with ‘‘pyramid’’-shape have an effect comparable to nominally planar wetting layers on the cross-plane thermal transport. Fourth, we elucidate how k changes during structural evolution from superlattices to alloy thin films through post-growth annealing . The experiments reveal that the effect of Ge/Si interfaces on phonon transport can be weakened and eliminated by enhancing Si-Ge intermixing. The presented results are expected to be relevant to applications requiring optimization of thermal transport for heat management and for the development of thermoelectric materials and devices based on superlattice structures.
 G. Pernot, et al., Nature Materials, 9, 491 (2010).