Semiconductor nanofilms as thermal phonon polarizers: competing effects of scattering selection rules and boundary mode conversion

Abstract

Phonon scattering selection rules are known to control heat flow through bulk solids. Here we show that these selection rules also modulate heat flow through nanoscale semiconductor films, although through a previously-unexplored mechanism. Using first-principles calculations, we expose a competition between these selection rules and phonon mode conversion at boundaries of nanoscale films, that drives mode-polarized heat currents at cryogenic temperatures ($\le$ 100 K). This polarizing effect is stronger in materials like indium phosphide, where selection rules based on large velocity differences between phonon branches amplifies the longitudinal acoustic (LA) phonon contribution to thermal conductivity by restricting their intrinsic scattering events, while boundary mode conversion in nanoscale films suppresses it by depopulating the LA phonons. The resulting transverse-polarized non-equilibrium phonons will enable symmetry-selective engineering of phonon coupling to electrons, strains and defects in nanoscale films, that is difficult to achieve in bulk solids.