L. Decaro et al., ELASTIC STRESS-RELAXATION IN HRTEM SPECIMENS OF STRAINED SEMICONDUCTOR HETEROSTRUCTURES AND ITS INFLUENCE ON THE IMAGE-CONTRAST, Microscopy microanalysis microstructures, 6(5-6), 1995, pp. 465-472
HRTEM observations require ultra-thinned specimens and, due to the ver
y 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 t
his work we investigate the case of lattice-mismatched semiconductor s
uperlattices, 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 re
presentative of neither the bulk tetragonally distorted material nor t
he 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 straine
d semiconductor materials.