Novel strain-induced defect in thin molecular-beam epitaxy layers

We have studied the morphology of thin epitaxial Ge films on Si(001). By terminating the surface with a monolayer of As during growth, Ge is forced to grow layer by layer, instead of the preferred mode, which is layer by layer for three monolayers, followed by islanding. In layer-by-layer growth, there are no nucleation sites for misfit dislocations to accommodate the 4% lattice mismatch. Instead, we observe by high-resolution transmission electron microscopy a novel strain relief defect combining two Σ9 boundaries and a twin. An estimate of the defect energy compares favorably with the energy of equivalent misfit dislocations.