Resonant Raman scattering by acoustical phonons in Ge/Si self-assembled quantum dots: Interferences and ordering effects

We report on resonant Raman scattering by acoustical phonons in self-assembled Ge/Si quantum dot (QD) structures. Continuous emission is observed in the low frequency range for a single QD layer. This scattering is attributed to the breakdown of the wave vector conservation law due to the loss of translational invariance. In samples containing a stack of two Ge QD layers separated by a Si space layer, we observed a strong low frequency oscillating signal. We investigated the dependence of this signal on the spacing between the QD layers. Raman spectra were calculated, considering the deformation potential interaction between acoustical phonons and electronic states confined within the QD. These calculations account well for the experimental data and demonstrate that the oscillations are related to interferences between the QD layers. The effects of ordering and QD position correlation between layers on the interference contrast are discussed. It is shown that the experiments presented here provide an interesting means of probing electronic confinement and organization effects in QD structures.