  Superlattices are promising low-dimensional nanomaterials for thermoelectric
technology that is capable of directly converting low-grade heat energy to
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 our understanding of
microscopic phonon transport in the particle regime. Lower thermal
conductivities were observed in (GaAs)0.77(Ge2)0.23 alloys; 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. Our experimental findings help gain
fundamental insight into nanoscale thermal transport in superlattices and are
also useful for future improvement of thermoelectric performance using
superlattice nanostructures.
