Subsurface localization of charge carriers in Si/SiO$_2$/Si$_x$Ge$_{1-x}$ nanostructures

Analysis of the capacitance-voltage $(C-V)$ characteristics reveals an elevated concentration of charge carriers under the surface of a silicon substrate due to the formation of elastically stressed regions induced in the substrate by Si$_x$Ge$_{1-x}$ nanoislands grown on the preliminarily oxidized Si surface. The $C$–$V$ characteristics exhibit charge density peaks at a depth from 700 to 1000 nm for Si/SiO$_2$/Si$_x$Ge$_{1-x}$ structures with various thicknesses of the SiO$_2$ layer. The results of theoretical calculations of the electron density distribution in the bulk of the silicon substrate correspond on the whole to the $C$–$V$ characteristics. The state of the interfaces in the structures with different thicknesses of the oxide layer, which determines the effects of surface and interfacial recombination as well as charge carrier scattering, is studied by analyzing the kinetics of decay of a photo-emf signal and the photo-emf distribution over the surface of the structure. The results can be used in the development of various devices based on SiGe with inclusions of oxide layers.