Thermal conductivity of GaAs/Ge nanostructures

Abstract

Superlattices are of great interest as platform materials for thermoelectric technology that are capable of directly converting low-grade heat energy into useful electrical power. In this work, the thermal conductivities of GaAs/Ge superlattice nanostructures were investigated systematically in relation to their morphologies and interfaces. Thermal conductivities were measured using ultrafast time-domain thermoreflectance and were found to decrease with increasing interface densities, consistent with past understanding of microscopic phonon transport in the particle regime. The lowest thermal conductivities were observed in (GaAs)0.77(Ge2)0.23 alloys, and transmission electron microscopy study reveals phase separation in the alloys. These alloys can be interpreted as fine nanostructures, with length scales comparable to the periods of very thin superlattices. Electrical transport measurements along the film plane direction showed no significant reduction in electrical properties attributable to the interfaces between GaAs and Ge. Our experimental findings help gain fundamental insight into nanoscale thermal transport in superlattices and are also useful for future improvement of thermoelectric performance using nanostructures.