Role of geometry and surface roughness in reducing phonon mean free path and lattice thermal conductivity of modulated nanowires

Abstract

Phonon transport simulations were performed to investigate the effects of geometry and surface roughness on the thermal conductivity of modulated nanowires. The finite volume method and ray tracing method were utilized to determine the phonon mean free path. The simulation results showed that the mean free path of the nanowire can be reduced by fabricating intricate structures under a constant ratio of surface area to volume. Then the specularity of the parts of the surfaces of the modulated nanowires was tuned. The simulations revealed that the impact of the specularity on the mean free path depends on the location of the surface whose specularity was tuned; in particular, the mean free path was sensitive to the specularity of the surfaces of neck parts but insensitive to that of other parts. Finally, the thermal conductivity was evaluated by using the simulated mean free path and other phonon transport properties obtained from first-principle calculations. It was found that optimization of the geometry and surface roughness can reduce sufficiently large thermal conductivities; this effect is beneficial in some semiconductor devices.