Polarized Raman spectroscopy of multilayer Ge∕Si(001) quantum dot heterostructures

Polarized Raman spectroscopy in backscattering geometry has been applied here for the investigation of $\mathrm{Ge}∕\mathrm{Si}\left(001\right)$ quantum dot multilayer structures (with the number of layers ranging from 1 to 21) grown by the Stranski-Krastanov technique. The characteristic Raman spectra of the dots have been obtained by taking the difference between the Raman spectra of the dot sample and the reference $\mathrm{Si}$ substrate, taken under the same excitation/scattering conditions. We found that the Raman spectra of $\mathrm{Ge}∕\mathrm{Si}$ dots obtained in such a manner are strongly polarized, in particular, for the $\mathrm{Ge}\text{-}\mathrm{Ge}$ (at $\sim 295{\mathrm{cm}}^{-1}$ ) and $\mathrm{Si}\text{-}\mathrm{Ge}$ (at $\sim 413{\mathrm{cm}}^{-1}$ ) vibrational modes. The dependence of peak intensity and peak position of the $\mathrm{Ge}\text{-}\mathrm{Ge}$ and $\mathrm{Ge}$ Raman bands versus the number of dot layers has been analyzed. It was found that studied quantum dot (QD) systems possess prominent anisotropic intermixing. This results in the $\mathrm{Si}$ content in the dots being high and this increases with the number of QD layers. At the same time, the increase of the number of layers was followed by a reduction in the compressive stress within the dots.