The part that was played by coherent islands, in the strain relaxation of Si/SiGe multi-layers, was studied by means of transmission electron microscopy and transmission electron spectroscopic imaging. The samples were grown by means of molecular beam epitaxy, and differed with regard to the number of periods, the period thickness, and the Si/Ge layer thickness ratio. The formation of defect-free coherent islands in strained SiGe layers was the cause of a wavelike morphology of the interfaces. It was shown that coherent islands acted as preferential sticking sites for Ge adatoms, and produced regions of higher Ge concentration. The higher Ge concentrations, which corresponded to a greater lattice spacing at the top of the coherent islands, depleted the strained layers. The corresponding accumulation of strain energy led to the formation of so-called conical defects. Within the latter, the elastic energy was relieved by the nucleation of dislocations.

E.Carlino, L.Tapfer, H.Von Känel: Applied Physics Letters, 1996, 69[17], 2546-8