|
It is well known for bulk semiconductors that amplification (generation) of a phonon mode can be achieved via the Cerenkov effect when the electron drift velocity exceeds the phonon phase velocity. The following three requirements are necessary for practical use of this effect: high electron mobilities, large electron densities, and strong coupling between electrons and phonons. In this report we show that in quantum well heterostructures these requirements can be met and both confined acoustic and confined optical phonon modes can be efficiently generated (amplified) by the drift of two-dimensional carriers. General formulae for the gain coefficient as a function of the acoustic phonon frequency and structure parameters as well as for the confined phonon increment are derived. Taking into account the electron-acoustic-phonon interaction through the deformation potential as well as the piezoelectric interaction, we found that amplification coefficient can reach hundreds of 1/cm for the AlGaAs-based heterostructures and thousands of 1/cm for the SiGe-based heterostructures in the terahertz phonon frequency range. Amplification takes place in a spectrally separated and relatively narrow amplification bands. We show that the optical phonon increment depends critically on the electron drift velocity. Detailed analysis of the optical phonon increment as a function of phonon wavevector, electron-phonon coupling strength, electron temperature and drift velocity indicates that the electron drift in selectively doped AlAs/GaAs/AlAs and GaSb/InSb/GaAs quantum wells can generate coherent confined optical modes. Finally, we discuss nonlinear mechanisms which stabilize the increase of phonon population and lead to the steady state phonon generation.
|
