RESONANT-RAMAN-SCATTERING STUDY OF DISORDER EFFECTS IN ALXGA1-XAS ALLOYS

We use resonant Raman scattering to investigate the frequency difference between the longitudinal optical phonon modes seen in dipole-forbidden and dipole-allowed polarization geometries in AlxGa1-xAs alloys. This splitting is studied as a function of laser photon energy, aluminum mole fraction x, and the indirect versus direct nature of the electronic band gap. Resonant Raman scattering, through the electron-phonon interaction, is a sensitive microscopic probe of the effects of short-range alloy disorder. For x in the indirect-gap regime, the dominant intermediate resonant state is a GAMMA-valley hole plus an X-valley electron that becomes effectively localized because of its short inelastic lifetime. Raman scattering via this state is described by a calculation of the Raman susceptibility that considers the random-alloy potential generated by local concentration fluctuations.