The excitation of hypersonic waves (f = 30-200 GHz) due to coupling with amplified space charge waves of millimeter wave range in GaN films is investigated theoretically. An 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. Both diffusion-drift equations for volume electron concentration and also an approximation of two-dimensional electron gas were used jointly with the Poisson equation for the electric field. 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 very effective for resonant excitation of hypersonic waves in the case of a full mechanic contact of GaN film and a non-piezoelectic substrate.
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