Generation and amplification of sub-THz coherent acoustic phonons under the drift of two-dimensional electrons

This paper addresses the Cerenkov emission of high-frequency confined acous tic phonons by drifting electrons in a quantum well. We have found that the electron drift can cause strong phonon amplification (generation). The spe ctra of the confined modes are calculated and their confinement properties are analyzed. The spectra consist of a set of branches, and for each branch ; the confinement effect increases considerably when the phonon wave vector increases. We have studied the coupling between electrons and confined mod es and proved that the coupling is a nonmonotonous function of the wave vec tor for each of the phonon branches. We have obtained a general formula for the gain coefficient as a function of the phonon frequency and the structu re parameters. For each of the branches, the amplification takes place in a spectrally separated and quite narrow amplification band in the high-frequ ency range. For the example of p-doped Si/SiGe/Si heterostructures it is sh own that the amplification coefficients of the order of hundreds of cm(-1) can be achieved in the sub-THz frequency range.