Raman scattering interferences as a probe of vertical coherence in multilayers of carbon-induced Ge quantum dots

We have probed the effect of disorder in the vertical alignment of Ge quantum dots of a multilayered structure by means of Raman scattering. Despite of using a thin Si spacer of 20 nm between dot layers, the coherent piling up of Ge dots one on top of the other is fully suppressed by the deposition of a submonolayer of carbon prior to the dot growth in each layer. For stacks with perfect dot correlation in the growth direction, the interaction of acoustic phonons with the ensemble of electronic states confined to the dots gives rise to well defined Raman interferences. The interference contrast almost vanishes when carbon is introduced on the dot-nucleation surfaces. Instead, a strong and decreasing background is observed at small Raman shifts. These drastic changes in the Raman spectra of dot multilayers with and without C are very well reproduced by simulations based on the interference model of Cazayous et al.