Electron–longitudinal-optical-phonon coupling and intersubband scattering in modulation-doped quantum wells under high-electric-field transport conditions

The electron–longitudinal-optical-phonon coupling in GaAs/${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{Al}}_{1\mathrm{-}\mathit{x}}$As modulation-doped quantum wells is studied under the conditions of steady-state high-field transport from theoretical and experimental points of view. The calculations are based on the Lei-Ting approach of high-field transport in a two-subband system. A common hot-electron temperature is assumed for both subbands but the Fermi levels are allowed to split off because of the weakness of the intersubband Coulomb interaction against the impurity and LO-phonon intersubband scatterings. The agreement with experiments requires a phonon lifetime of about 5 ps and a nonequilibrium hot-phonon population drifting with the electron gas. Moreover, we show that the temperature versus power-loss diagrams do not fully characterize the electron–LO-phonon coupling since this coupling increases with the drift velocity through the Doppler shift of the LO-phonon frequency. Finally, for a vanishing electric field, the low-temperature mobility is shown theoretically to be mainly reduced at the onset of the second subband by intrasubband scattering. It is argued that the high-mobility electrons lying at the Fermi level of the ground subband are efficiently scattered within their subband by the low-mobility carriers of the upper subband through the electron-electron Coulomb interaction.