Defects in Silicon-Germanium Strained Epitaxial Layers

The energies of one and two-dimensional dislocation arrays lying near a free surface are evaluated directly from the stress fields of single dislocations in a half-space. These results are used to obtain expressions giving the equilibrium spacings of a number of different arrays relieving misfit in a strained epitaxial system. Numerical calculations are performed for the case of edge and 60º dislocations relieving strain in a silicon-germanium layer deposited on a silicon substrate. This method is also used to calculate the energies of various low angle grain boundaries in a half-space. Single-ended dislocation sources are observed using transmission electron microscopy in two short-period Si-Ge superlattices grown on Si(100). Their formation is linked to the development of non-planar layers during the growth of the superlattices. The relaxation of these superlattices takes place at significantly lower temperatures than equivalently strained homogeneous epilayers. Si-Ge short period superlattices deposited on Si(100) are shown to relax through twinning on {111} planes if the deposited layers become grossly non-planar. Twinning is accompanied by the formation of a diamond hexagonal phase. No 60· a/2(110) dislocations relieving misfit are present in the strained layer structure. The nature and origin of a new type of defect in Si₁_ᵪGeᵪ/Si strained layer structures, the "pagoda" defect, is studied using transmission electron microscopy. The defects are found to propagate in a direction determined by the position of the Si source in unrotated substrates, and to have their origin in the role played by SiC particles (left after cleaning the substrate) during the growth process. Pits that form at the SiC particles are preserved during MBE growth and perturb the strained layers, leading to the formation of pagodas.