Influence of the supercell structure on the folded acoustical Raman line intensities in superlattices.

Abstract

We compare forward and backward Raman scattering results on folded acoustical phonons in GaAs-AlAs superlattices with a detailed theoretical analysis of their dispersion properties and light scattering activity. By forward scattering, which involves phonons with a vanishing wave vector, we first get evidence of zone-center gaps, in quantitative agreement with the elastic model predictions. We also check the zone-center selection rules and conclusively prove the assignment of the light scattering on folded acoustical phonons to a modulated photoelastic (Brillouin) process. In backscattering experiments, one creates phonons with a finite wave-vector and the zone-center selection rules are relaxed. We quantitatively describe this phenomenon, and demonstrate that the backscattering intensities directly reflect the coupling between folded branches and the related zone-boundary gap magnitude. An excellent agreement between measured and calculated intensities is obtained. Finally we emphasize the great sensitivity of the gaps and intensities, contrary to the backscattering frequency shifts, to the supercell inner structure. This greatly enhances the interest of Raman scattering as a tool for characterizing periodic structures.