Thermal conductivity of Si–Ge quantum dot superlattices

Abstract

Quantum dot superlattices (QDSLs) have been proposed forthermoelectric applications as a means of increasing thermal conductivity,σ, and reducing the lattice thermal conductivity,κl, 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 withκl using Green–Kubo theory and molecular dynamics. It was found that the resultingκl 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 minimumκl at a dot diameter of 1.4–1.6 nm and can reach values as low as 0.8–1.0 W mK−1, increasingZT by orders of magnitude over bulk Si and Ge. The drastic reduction of thermal conductivityin such a crystalline system is shown to be the result of both the stress caused by the dotsas well as the quality of the Si–Ge interface.