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 stresses 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 made by cubic materials are investigated. The reduced spatial dimensions, owing to the thinning process of strained heterostructures, cause modification of the atomic positions in the thinned specimens with respect to the bulk ones. This deformation is a function not only of the specimen 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 [011]). The bending of the atomic columns caused by the elastic relaxation phenomena in HRTEM specimens of strained semiconductor materials can cause a strong background-intensity variation in the HRTEM images. This effect is a function of the structure of the investigated materials, indicating that information on the background intensity variation, owing to the non-uniform lattice distortion of an elastically relaxed heterostructure made by cubic materials, is contained in the {200} beams. Thus, the influence of the elastic relaxation cannot be neglected whenever HRTEM is used to deduce the local chemical composition or the local unit cell in strained cubic materials.