Elastic stress relaxation in HRTEM specimens of strained semiconductor heterostructures and its influence on the image contrast

HRTEM observations require ultra-thinned specimens and, due to the very small thickness (1 to 30 nm), an elastic stress relaxation may occur near the free surfaces of strained heterostructures. These relaxation phenomena can modify the structural features of the thinned specimens with respect to the bulk materials. Thus, the role of the relaxation has to be taken into account if the structural properties of the bulk heterostructures are deduced from those of the thinned specimens. In this work we investigate the case of lattice-mismatched semiconductor superlattices, thinned along the [011]-crystallographic direction. The theoretical evaluation of strain fields in ultra-thinned HRTEM samples shows a bending of the lattice, and the local lattice spacings are representative of neither the bulk tetragonally distorted material nor the unstressed material. Our results show that these distortions can be large and must be taken into account whenever HRTEM is used to deduce the local chemical composition or the unit cell dimensions in strained semiconductor materials.