Thermal conductivity of Si-Ge quantum dot superlattices

摘要

Quantum dot superlattices (QDSLs) have been proposed for thermoelectric applications as a means of increasing thermal conductivity, σ, and reducing the lattice thermal conductivity, κ_l, to increase the dimensionless thermoelectric figure of merit, ZT. To fully exploit the thermoelectric potential of Si-Ge quantum dot superlattices (QDSLs), we performed a thorough study of the structural interplay of QDSLs with κl using Green-Kubo theory and molecular dynamics. It was found that the resulting κl has less dependence on the arrangement of the dots than to dot size and spacing. In fact, regardless of arrangement or concentration, QDSLs show a minimum κl at a dot diameter of 1.4-1.6nm and can reach values as low as 0.8-1.0WmK~(-1), increasing ZT by orders of magnitude over bulk Si and Ge. The drastic reduction of thermal conductivity in such a crystalline system is shown to be the result of both the stress caused by the dots as well as the quality of the Si-Ge interface.