Relaxation of photoexcited electron-hole plasma in quantum wells

A theoretical study of the relaxation of photoexcited electron-hole plasma in undoped GaAs/${\mathrm{Ga}}_{\mathrm{x}}$ ${\mathrm{Al}}_{1\mathrm{-}\mathrm{x}}$As single-quantum-well structures is presented. We confine this study to high carrier concentrations and subpicosecond laser excitations. Both phonon heating and the exchange of energy between electrons and holes via the Coulomb interaction are included in our model. In agreement with recent experiments, we find that carrier cooling is rather insensitive to variations in the well thickness if the sheet density is kept constant. For given layer thickness and times beyond 0.5 ps, however, cooling occurs at a slower rate at higher values of the sheet density. We show that this effect is largely due to a stronger buildup of nonequilibrium optical phonons at higher sheet densities.