Carrier relaxation in semiconductors with multiple inequivalent valleys

Abstract

The relaxation of an optically generated electron-hole plasma is reviewed for the case of multi-valley scenarios in semiconductors. A suitable model substance is bulk AlxGa1−xAs in the vicinity of the crossover from a direct to an indirect-gap material. The electrons distribute among the inequivalent valleys at the Γ, X and L points of the reciprocal space as a result of efficient intervalley coupling induced by deformation as well as alloy-disorder potentials. Exchange and correlation effects in the plasma lead to a differential narrowing of the direct and indirect gaps according to their individual population. These gap shifts are well described by a selfconsistent multi-valley model for the renormalization effects. A disorder-assisted nucleation of electron-hole droplets is found to occur on a picosecond timescale in indirect-gap AlxGa1−xAs. A comparison is drawn to the case of type-II superlattices with special emphasis on the condensation of quantum-confined electron-hole drops. Finally, the unusual process of stimulated emission related to the indirect gap in AlxGa1−xAs is highlighted. This indirect stimulated emission occurs in the visible spectral range at room temperature and is thus most interesting for laser applications.