A twinned wedge in a si-ge epitaxial film: Twofold Σ=9 twinning

A wedge-like defect in a heteroepitaxial Si-22 % Ge film on a Si substrate has been investigated. The defect is found to be a twofold Σ = 9 twin with elastic properties of a wedge disclination triangle. From the defect arrangement it follows that two distinct defect morphologies exist: in the first stage, a nascent wedge forms which has the properties of a single disclination; in the second stage, the triangle morphology is established (called a mature wedge) with the long- range elastic properties of an edge dislocation. The formation involves thermally activated nucleation of a nascent wedge, growth of the stable nucleus, transition from the nascent wedge to the mature twin, growth of the mature twin and eventual trapping of the defect. The processes (i) and (ii) can be understood by considering a single defect in an infinite half-space with a homogeneous stress field only. The growth of the nascent wedge is a self-similar process, and thus the transition (iii) requires the presence of in homogeneities such as other defects (e.g. twinned wedges or the substrate-film interface) or stress concentrations.