Absorption of nonequilibrium acoustic phonons by low-mobility electrons inGaN

We report direct phonon absorption experiments in n-type GaN epitaxial laye rs grown by molecular beam epitaxy. Nonequilibrium phonons with characteris tic 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. T he phonon absorption is studied as a function of heater temperature through the effects of phonon induced changes of the device resistivity. The exper imental results lead us to the conclusion that in low-mobility GaN, the mom entum conservation cutoff for electron-phonon transitions is shifted to hig her energy than predicted by standard theory. (C) 2001 American Institute o f Physics.