Chemical ordering and boundary structure in crystalline SiGe superlattices

Abstract

Strained-layer superlattices of crystalline silicon and germanium, grown by molecular beam epitaxy on Si(100) have been studied by high resolution transmission electron microscopy (200 and 300 kV) and selected area electron diffraction techniques, to investigate the atomic boundary structure and to obtain information on chemical order in this material. Two types of specimens were investigated: 1. (1)specimens with asymmetrically strained superlattices (cell parameter of germanium totally adapted to that of silicon), with germanium layers having a thickness of two to six monolayers and a superlattice period of 4–6 nm, grown on a silicon buffer at 460–470°C; 2. (2)specimens with symmetrically strained superlattices, with germanium layers having a thickness of three to five monolayers and a lattice period smaller than 1.5 nm, grown at 400–420°C on an Si x Ge 1− x buffer with the same composition as the superlattice. The transmission electron micrographs show the persistence of pseudomorphic growth of the superlattice for both types of specimens. The layer boundaries are sharp in the atomic size range, the interface being defined to within one monolayer of silicon or germanium. Selected area electron diffraction patterns contain weak additional reflections which prove the existence of domains causing a doubling of the periodicity in the [111] direction. This doubling is explained by chemical ordering of germanium and silicon atoms in the germanium layers, in analogy with similar effects in Si x Ge 1− x alloys. An additional strong streaking observed parallel to the direction of growth is found to be a result of inhomogeneities in the lattice period and of lattice distortions. The two additional diffraction effects are observed in both types of specimens.