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 Ge/Si(001) 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 Si substrate, taken under the same excitation/scattering conditions. We found that the Raman spectra of Ge/Si dots obtained in such a manner are strongly polarized, in particular, for the Ge-Ge (at ∼295 cm^-1) and Si-Ge (at ∼413 cm^-1) vibrational modes. The dependence of peak intensity and peak position of the Ge-Ge and 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 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.