Quantum modeling of thermoelectric performance of strained Si∕Ge∕Si superlattices using the nonequilibrium Green’s function method

Abstract

The cross-plane thermoelectric performance of strained Si∕Ge∕Si superlattices is studied from a quantum point of view using the nonequilibrium Green’s function method. Strain causes the germanium well layers to turn into barriers that promote electron tunneling through the barriers. Electron tunneling produces oscillations in the Seebeck coefficient due to shift in subband energies near the Fermi level. Strain-induced energy splitting can increase the power factor by up to four orders of magnitude in germanium-rich substrates. Also, at large doping, strain lowers the subband energies such that thermoelectric performance is independent of layer thickness between 2 and 4nm germanium barrier layers. The results imply that larger barrier layers can be used at high doping without a performance penalty while avoiding problems with interlayer diffusion that are prevalent in films with small thicknesses.