Resonance Raman scattering in GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As superlattices: Impurity-induced Fröhlich-interaction scattering

We report measurements of Raman scattering from LO phonons in resonance with quasi-two-dimensional excitons in a 104-Å GaAs–125-Å ${\mathrm{Al}}_{0.25}$ ${\mathrm{Ga}}_{0.75}$As superlattice. Incoming and outgoing resonances are observed at discrete excitons formed from first and second conduction and valence subbands. As already pointed out by Zucker et al., the resonant Raman profile shows a stronger outgoing resonance as compared with the incoming one. We present a quantitative explanation of the observed asymmetry of both resonance channels by invoking the impurity-induced intraband Fröhlich scattering mechanism, an effect also observed in bulk semiconductors. This analysis differs from that of Zucker et al., which was based on the details of the quantized quasi-two-dimensional excitons. We believe that the latter mechanism may explain resonance asymmetry for a special range of superlattice parameters, while the one proposed here should be more general, extending all the way to the two-dimensional case.