Chemical ordering and boundary structure in strained-layer Si-Ge superlattices

The interface structure of strained-layer Si-Ge superlattices on Si(100) is investigated by selected-area electron-diffraction techniques and high-resolution transmission electron microscopy. Three types of strained-layer Si-Ge superlattices characterized by different strain distributions are compared. Diffraction evidence is presented for the existence of boundary layers at the interfaces consisting of chemically ordered domains with a bilayer stacking of Si and Ge along the 〈111〉 directions. This phenomenon is independent of the strain distribution between Si and Ge, i.e., of the choice of the substrate lattice parameter.