Generation of high-frequency coherent acoustic phonons in superlattices under hopping transport. II. Steady-state phonon population and electric current in generation regime

For electrically biased superlattices exhibiting the effect of phonon instability (see paper I), we analyze the nonlinear problem of high-frequency acoustic phonon generation. We develop a theory treating self-consistently phonon generation and electron transport through the superlattice. We find that the dominant mechanism associated with the steady-state generation regime is electron heating caused by the nonequilibrium phonons. It is shown that under the generation regime the spectral distribution of phonons is extremely narrow, and that the generated power density can be as high as 10⁶ W/m² for terahertz phonons. The electric current is controlled by the nonequilibrium phonons, and is higher by an order of magnitude than that under subthreshold conditions. In addition, we conclude that for the real superlattice structures the threshold condition is likely to be reached for the miniband-edge phonons having zero group velocity and negligible radiation losses.