A wedge-like defect in hetero-epitaxial Si-22%Ge films on Si substrates was investigated. The defect was found to be a 2-fold  = 9 twin, with the elastic properties of a wedge disclination triangle. On the basis of the defect arrangement, it was deduced that 2 distinct defect morphologies existed. In one of these, a nascent wedge formed which had the properties of a single disclination. In the other, a triangular morphology was established (a so-called mature wedge) which had the long-range elastic properties of an edge dislocation. The overall defect formation process involved the thermally activated nucleation of a nascent wedge, the growth of a stable nucleus, a transition from nascent wedge to mature twin, growth of the mature twin and the eventual trapping of the defect. The first 2 steps could be understood by considering a single defect in an infinite half-space with an homogeneous stress field. The growth of the nascent wedge was a self-similar process, and the transition from nascent wedge to mature twin required the presence of inhomogeneities (twinned wedges, substrate/film interface) or stress concentrations.

P.Müllner, H.Gao, C.S.Ozkan: Philosophical Magazine A, 1997, 75[4], 925-38