Stress-induced optical anisotropies measured by modulated reflectance

In the last few years, the understanding of information delivered by reflectance difference/anisotropy spectroscopy (RAS) has grown considerably. However, a full understanding of this optical technique is not yet achieved because surface, interface and bulk effects are present particularly where heteroepitaxial systems are concerned. This is especially true for the case of resonances at the bulk critical points of the dielectric function, which either resemble the dielectric function or its derivative. Previous RAS experiments on zincblende and diamond structure semiconductors found optical anisotropies in the vicinity of the E-0, E-1 and E-1 + Delta(1) critical points. In this review, the origin of these structures is discussed and it is shown that anisotropic in-plane strain in the epilayer or bulk induces resonances at these critical points. This in-plane strain is either caused by the boundaries of the epilayer system (i.e. the surface or the interface) or by symmetry breaking via a surface electric field or a preferred orientation of dislocations. These findings are best supported by applying additionally photoreflectance difference spectroscopy (PRD), where the difference between a spectrum taken with linearly polarized light and with unpolarized light is measured. In contrast to RAS, PRD spectroscopy is specific to the symmetry breakdown occurring due to band bending via the surface or interface electric field and stress.