Excitation of hypersound due to coupling with space charge waves of millimeter wave range in GaN films

The excitation of hypersonic waves (f = 30 - 200 GHz) due to coupling with amplified space charge waves of the millimeter wave range in n-GaN films is investigated theoretically. Amplification of space charge waves due to the negative differential conductivity in GaN films placed onto a semi-infinite substrate is considered and possible spatial increments are calculated. The amplified space charge waves can excite hypersonic waves in the substrate at the same frequency due to piezoeffect and deformation potential mechanisms. It is demonstrated that the piezoeffect seems effective for resonant excitation of hypersonic waves.