Interface roughness and homogeneous linewidths in quantum wells and superlattices studied by resonant acoustic-phonon Raman scattering

Acoustic-phonon Raman spectra of GaAs/AlAs superlattices show two characteristic features. Sharp lines originate in crystal-momentum conserving backscattering by folded superlattice phonons. A continuous emission background with superimposed peaks and dips is observed due to disorder-induced scattering from modes of the whole folded acoustic-phonon dispersion. In this case, neither the crystal-momentum component ${\mathit{q}}_{\mathit{z}}$ along ${\mathit{q}}_{\mathrm{\parallel }}$ nor perpendicular to the growth direction is conserved. Even in high-quality samples the amount of disorder is such that both effects appear at the same time. These phenomena allow us to obtain information on growth-related and intrinsic parameters of semiconductor superlattices and multiple quantum wells. Gaps of the folded-phonon dispersion cause intensity anomalies in the background emission. From the ratio of such features at mini-Brillouin-zone boundary gaps, for which only ${\mathit{q}}_{\mathit{z}}$ is not conserved, to those at internal gaps, where both ${\mathit{q}}_{\mathit{z}}$ and ${\mathit{q}}_{\mathrm{\parallel }}$ can take arbitrary values, the degree of interface roughness and the lateral extent of growth islands are deduced. The resonance behavior of the continuous emission and relative changes of wave-vector conserving folded-phonon to background scattering intensities are analyzed in terms of homogeneous and inhomogeneous broadenings of interband critical points. Changes of the homogeneous linewidth with energy and temperature, as well as electron-phonon interaction constants, are determined.