EFFECTS OF THE ELASTIC STRESS-RELAXATION ON THE HRTEM IMAGE-CONTRAST OF STRAINED HETEROSTRUCTURES

In this work, the effects of the elastic relaxation of compositional s tresses caused by the finite size of transmission electron microscopy (TEM) specimens on the image contrast of high-resolution transmission electron microscopy (HRTEM) micrographs of strained heterostructures m ade by cubic materials are investigated. The reduced spatial dimension s, owing to the thinning process of strained heterostructures, cause m odification of the atomic positions in the thinned specimens with resp ect to the bulk ones. This deformation is a function not only of the s pecimen thickness but also of the thinning crystallographic direction. The results show that the strains of an elastically relaxed structure can vary by 15% as a function of the thinning direction ([100] or [01 1]). The bending of the atomic columns caused by the elastic relaxatio n phenomena in HRTEM specimens of strained semiconductor materials can cause a strong background-intensity variation in the HRTEM images. Th is effect is a function of the structure of the investigated materials , indicating that information on the background intensity variation, o wing to the non-uniform lattice distortion of an elastically relaxed h eterostructure made by cubic materials, is contained in the {200} beam s. Thus, the influence of the elastic relaxation cannot be neglected w henever HRTEM is used to deduce the local chemical composition or the local unit cell in strained cubic materials.