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

This paper addresses the Čerenkov emission of high-frequency confined acoustic phonons by drifting electrons in a quantum well. We have found that the electron drift can cause strong phonon amplification (generation). The spectra 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 modes and proved that the coupling is a nonmonotonous function of the wave vector 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 structure parameters. For each of the branches, the amplification takes place in a spectrally separated and quite narrow amplification band in the high-frequency range. For the example of p-doped Si/SiGe/Si heterostructures it is shown that the amplification coefficients of the order of hundreds of cm^-1 can be achieved in the sub-THz frequency range.