Monte Carlo simulation of intersubband relaxation in wide, uniformly doped GaAs/Al_xGa_1-xAs quantum wells

Using an ensemble Monte Carlo simulation, we have investigated intersubband relaxation of photoexcited electrons in GaAs/Al_xGa_1-xAs quantum wells having a subband separation smaller than the polar optical phonon energy. Intra- and intersubband scattering through polar optical phonons, acoustic phonons, ionized impurities, and electron-electron scattering are included in the simulation. A comparison is made to recent time-resolved pump and probe experiments performed on uniformly doped samples in which good agreement between theory and experiment is obtained. Our results show that the intersubband decay of electrons from the first excited subband into the ground subband is limited by ionized impurity scattering during the photoexcitation process. Polar optical phonon emission also contributes considerably to the electron decay and occurs from the thermal tail of the heated distribution function in both subbands. Intersubband scattering by intercarrier interaction plays a lesser role for the decay. The heating of the distribution functions is due to ionized impurity intersubband scattering and electron-electron intrasubband scattering, which convert potential energy of an electron into kinetic energy. These mechanisms drive both subbands rapidly towards a single quasiequilibrium distribution with a common electron temperature and chemical potential after the pulse is over. The cooling rate of this distribution function, which governs the intersubband decay, depends initially on the energy relaxation through polar optical phonons, whereas at much longer times acoustic phonon scattering predominates. Thus, the intersubband decay of electrons is nonexponential.