Role of nanostructures in reducing thermal conductivity below alloy limit in crystalline solids

Atomic substitution in alloys can efficiently scatter phonons, thereby significantly reducing the thermal conductivity in crystalline solids to the "alloy limit". It has been difficult to beat the alloy limit without creating defects, dislocations, and voids, which also reduce electrical conductivity, making it ineffective for increasing the material's thermoelectric figure of merit. Using In_0.53Ga_0.47As containing epitaxially embedded ErAs nanoislands a few nm in size, we demonstrate thermal conductivity reduction by almost a factor of two below the alloy limit, and corresponding increase in thermoelectric figure of merit by more than a factor of two. A theoretical model suggests that while point defects in alloys efficiently scatter short wavelength phonons, the ErAs nanoislands provides additional scattering mechanism for the mid to long wavelength phonon - the combination reduces the thermal conductivity below the alloy limit.