Hot-phonon generation by split-off hole band electrons in AlxGa1-xAs alloys investigated by picosecond Raman scattering

摘要

Abstract: The initial generation of hot LO phonons by the relaxation of hot carriers in GaAs and Al$-x$/Ga$-1$MIN@x$/As alloy semiconductors is studied. Within the initial 2 ps of photoexcitation, only those electrons originating from the split-off hole bands are found to generate a significant number of $Gamma@-valley hot phonons when photon energies of 2.33 eV are used. A picosecond Raman scattering technique is used to determine the hot phonon occupation number in a series of MBE grown Al$-x$/Ga$-1$MIN@x$/As samples with 0 $LSEQ x $LSEQ 0.39. The Stokes and anti-Stokes lines were measured for both GaAs-like and AlAs-like LO phonon modes to determine their occupation numbers. We observe a rapid decrease in the phonon occupation numbers as the aluminum concentration increases beyond x $EQ 0.2. This rapid decrease is explained by considering only those electrons photoexcited from the split-off hole band. Almost all of the electrons originating from the heavy- and light-hole bands are shown to quickly transfer and remain in the X and L valleys without generating significant numbers of hot LO phonons during the initial 2 ps and at a carrier density of 10$+17$/ cm$+$MIN@3$/. A model based upon the instantaneous thermalization of hot electrons photoexcited from the split-off hole bands is used to fit our data. We have obtained very good agreement between experiment and theory. This work provides a clear understanding to the relaxation of $Gamma valley hot electrons by the generation of hot phonons on subpicosecond and picosecond time scales, which has long standing implications to previous time resolved Raman experiments.!25