Hot-phonon generation by split-off hole band electrons in Al x Ga 1-x As alloys investigated by picosecond Raman scattering

Abstract

The initial generation of hot LO phonons by the relaxation of hot carriers in GaAs and AlxGa1-xAs 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 AlxGa1-xAs samples with 0 <EQ x <EQ 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 equals 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 1017 cm-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.