Potential of Si-based superlattice thermoelectric materials for integration with Si microelectronics

Freestanding, thin-film, Si/Ge superlattice (SL) structures over a wide range of periods, from ~300 A to ~10 A, 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. 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 conductivity (k) with the SL period appears complex, with reduction in k coming apparently from both short-period and lattice-mismatch effects. Finally, we will present the first experimental demonstration of a factorial improvement in the three-dimensional figure-of-merit (ZT_3D) of Si/Ge SL structures with respect to comparable bulk SiGe 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 Si-based microelectronic cooling