Brittle-plastic relaxation of misfit stresses in the Si(001)/Si_{1 − x} Ge _x system

Abstract

It is found that stressed luminescent samples of Si(001)/Si_{1 − x} Ge _x /Si with a germanium concentration of up to 16% contain microcracks. In contrast to ordinary cracks, microcracks are characterized by partial cracks of the substrate that are not perpendicular to the plane of the plate and are detected by specially designed X-ray analysis techniques. In a 60-nm-thick layer of SiGe with a germanium concentration of 5%, individual microcracks arise in samples near the edges of the same type, and the traces of these microcracks coincide with the ripple profiles of the corrugated growth surface. As the germanium concentration increases, first, the number of microcracks near the edges of a sample increases, while the corrugated growth relief nearly completely disappears, and then microcracks appear even in the central region of the sample, thus leading to the vanishing of the curvature of the structure in this region without visible signs of plastic relaxation. At the same time, even a thin (20 nm) layer of SiGe exhibits a layered structure, and diffuse scattering near the peak of the SiGe layer increases, which points to the presence of fragments of the layer that are misoriented by ±0.015°; the intensity of the diffuse scattering may amount to 0.5% of the layer intensity. A spatial analysis of the luminescence of samples with microcracks shows that the emergence of microcracks hardly affects the peak position and the half-width of the emission line of the SiGe layer. At the same time, the intensity of exciton emission from both the SiGe layer and the bulk of silicon is significantly (several times) changed when passing to the regions with microcracks. All the phenomena observed can be accounted for under the assumption that, between the stages of the loss of the plane crystallization front and the development of plastic relaxation of misfit stresses, there exists an earlier unknown stage of growth in which the concentration of nonequilibrium vacancies increases by four to five orders of magnitude and then the vacancies condensate into micropores.