Resonant Raman scattering by acoustic phonons in self-assembled quantum-dot multilayers: From a few layers to superlattices

Abstract

We report on resonant Raman scattering (RS) by acoustic phonons in self-assembled Ge/Si quantum-dot (QD) multilayers. In previous studies the observation of doublet features in the low-frequency Raman spectra was attributed to superlattice effects, i.e., Brillouin-zone folding, despite the often small number of QD layers. We propose a model which accounts for the low-frequency resonant RS, whatever the number of QD layers, i.e., from a few layers to superlattices. It is shown that the features in the low-frequency spectra (peak frequencies and intensities, doublet splittings and intensity ratios) can all be consistently understood within the resonant RS interference model. RS interferences occur when acoustic phonons interact with an ensemble of localized electronic states. Calculations and experiments were carried out in order to investigate how the RS depends on the number of QD layers and on the multilayer location with respect to the surface. Indeed, spectra are shown to depend on these finite-size effects: Reliable assignments cannot be made when the analysis is restricted to the peak frequencies. RS intensities have been calculated in order to identify the relevant scattering mechanism.