Effect of valence-subband structure on the energy relaxation dynamics of electrons in GaAs quantum wells grown on Si

Different electron energy relaxation dynamics are observed in narrow and wide modulation Be-doped GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As multiple quantum wells grown on Si substrates (GaAs QW's/Si) arising from different valence-subband structure. The combination of built-in biaxial stress and spatial hole confinement results in a reversal of light- and heavy-hole subbands in narrow GaAs QW's/Si structures. For narrow (wide) quantum-well structures of 40 (188) \AA{} at 4.3 K in which there is a two-dimensional light- (heavy-) hole gas, the energy-loss process of photoexcited hot electrons is extremely rapid (slow) due to strong (weak) energy exchange with the cool light (heavy) holes. The nonequilibrium phonon population was found to build up from the relaxation of hot electrons in the 188-\AA{} wells but not in the 40-\AA{} wells.