Absorption of nonequilibrium acoustic phonons by low-mobility electrons in GaN

We report direct phonon absorption experiments in n-type GaN epitaxial layers grown by molecular beam epitaxy. Nonequilibrium phonons with characteristic energies up to 15 meV (3.5 THz) are injected from a constantan heater. They propagate ballistically through the sapphire substrate, reach the GaN layer, and are absorbed by the degenerate three-dimensional electron gas. The phonon absorption is studied as a function of heater temperature through the effects of phonon induced changes of the device resistivity. The experimental results lead us to the conclusion that in low-mobility GaN, the momentum conservation cutoff for electron-phonon transitions is shifted to higher energy than predicted by standard theory.