Dzhagan V. Raman scattering in the structures with thin SiGe layers and self-induced SiGe nanoislands

Optical and structural properties of the structures with thin SiGe layers and self-induced nanoislands are systematically investigated. Using Raman spectroscopy, the strain distribution in SiGe layers grown by different techniques is studied. The influence of carbon doping and additional treatments on strain relaxation in Si/SiGe heterostructures is analyzed. Using atomic force microscopy, we analyze the dependence of the volume, shape and area density of self-induced SiGe nanoislands on growth temperature and thickness of deposited germanium. The bimodal island size distribution with two equilibrium island shapes is confirmed. It is shown that during the growth Si atoms diffuse into Ge islands; as the result the islands consist of SiGe solid solution. The rate of Si atoms diffusion increases with the temperature of growth. The theoretical analysis of the growth of strained SiGe islands predicts the existence of a limiting height-to-perimeter ratio that determines the shape of the island. The coveringof the array of islands with silicon layer causes the increase of the Si content in the islands but the strain does not decrease. The is that the islands do not have free surface. The growth of Ge islands on SiGe buffer layer results in the formation of highly dense array of monomodal pyramidal islands. The series of low-frequency peaks in the Raman spectra from superlattices with SiGe nanoislands is found to be caused by the islands but not by the wetting layers between the islands.