In-plane thermoelectric properties of freestanding Si/Ge superlattice structures

Freestanding, thin-film Si/Ge superlattice (SL) structures over a wide range of periods, from ~300 Å to ~10 Å, have been experimentally investigated for their thermoelectric properties in the plane of the SL interfaces. We have observed a several-fold enhancement in the power factor at 300 K in these Si/Ge SL structures, compared to thin-film SiGe and bulk SiGe alloys. The thermoelectric power factor and Hall-effect measurements, with model calculations for effective conduction-band density of states, have been used to understand the mechanism behind the strong enhancement in power factor in these apparently weakly-quantum confined SL structures. We suggest that the Si/Ge SL structure is an example where bandstructure or valley engineering without appreciable quantum-confinement can enhance thermoelectric power factor. AC calorimetry measurements have also been completed to determine the in-plane thermal diffusivity of these Si/Ge SL thin-films. The variation of thermal diffusivity (D) with the SL period appears complex, with reduction in D coming apparently from both short-period and lattice-mismatch effects. We observe and suggest a mechanism for minima in D values for certain intermediate SL periods. We observe that the behavior of the measured thermal diffusivity with SL period appears characteristic of localization-like effects, similar to the well-known localization of electrons and more recently, photons. We also present the first experimental demonstration of a factorial improvement in the three-dimensional figure-of-merit (ZT_3D) of SL structures with respect to comparable bulk alloys, with all the properties measured in the same direction, suggesting a proof-of-concept validation for thin-film SL structures. The implications of the ZT enhancement with SL/Ge SL structures would be significant for a variety of applications