Interface roughness and the dispersion of confined LO phonons in GaAs/AlAs quantum wells

Abstract

We report the energies of the confined LO phonon modes in the GaAs wells of GaAs/AlAs superlattices with well widths between 10 and 18 monolayers. We observe confined LO-phonon modes up to 12th order. The measurements were performed on sample structures obtained by two different growth techniques [molecular-beam epitaxy (MBE) with growth interruption and migration-enhanced epitaxy (MEE)], which favor the smoothness of the GaAs/AlAs and the AlAs/GaAs interfaces. Measurements on a sample, where the ``normal'' GaAs/AlAs interfaces were grown on purpose without growth interruption, show weaker Raman signals and confined phonon modes up to lower order than observable for samples with growth interruption. The energies of the confined LO phonons as a function of the confinement wave vector lie close to recent neutron results for the LO-phonon dispersion in bulk GaAs. We suggest that previously reported discrepancies are due to stronger interface roughness than in the present samples. We show that the high-order confined phonons are considerably more sensitive to roughness than the low-order confined LO phonons. Therefore the former and not the latter should be used for characterization of interface roughness. For determination of the phonon dispersion, on the other hand, the uncertainties are higher for the higher-order confined modes. We observe discrete confined modes corresponding to sections of the quantum well with thicknesses differing by one monolayer.