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 similar to 300 Angstrom to similar to 10 Angstrom, 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 300K 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 Si/Ge SL structures would be significant for a variety of applications.