Structural and optical properties of metastable SiGe/Si films with a low germanium concentration

The properties of metastable Si_{1 ? x} Ge _x /Si (10% < x < 16%) layers grown by molecular beam epitaxy on Si(100) substrates have been investigated using atomic force microscopy, X-ray diffraction, and low-temperature luminescence spectroscopy. It has been shown that ring-like aggregates are formed on the surface of layers grown at temperatures of 500–700°C. The size and shape of these aggregates suggest that their formation is associated with the diffusion instability arising due to the existence of a relationship between the surface diffusion, stresses, and the wetting potential during the growth of the epitaxial film. The existence of deviations from the homogeneous germanium distribution in the layer plane has been confirmed by a detailed analysis of the X-ray rocking curves and two-dimensional diffraction patterns. The structures with severe surface disturbances are characterized by an abnormal change in the decay times of the emission lines of bulk silicon, which indicate the presence of local electric and/or strain fields in subsurface regions. The perturbations of the flat crystallization front are suppressed as the growth temperature of layers decreases to 350°C. Despite the absence of a coating layer of silicon, the photoluminescence spectra of the layers themselves depend weakly on their thickness and growth temperature and remain sensitive only to the technological concentration of germanium. A slowly decaying luminescence associated presumably with the localization of excitons near the SiGe-Si interface has been observed in one of the samples grown at a temperature of 700°C and containing a dense array of ring-like aggregates.