Hot-electron relaxation in quantum-well structures: Multisubband occupation

We have considered the effect of multisubband occupation on the cooling rate of quasi-two-dimensional hot electrons in a semiconductor heterojunction. The electron energy relaxation is controlled by the emission and reabsorption of nonequilibrium longitudinal optical phonons. The dynamics of the nonequilibrium lattice excitations is governed by a set of kinetic equations for the occupation number of phonon wave packets that was derived recently by us. Only the situation where the first two electronic subbands are occupied is considered in detail here. We find that inter- and intrasubband scattering increase the electron cooling rate. The size of the effect is comparable to that obtained when the phonons are in thermal equilibrium.