Layers of GaAs which had been grown onto (001)Si by means of molecular beam epitaxy were investigated by using conventional and high-resolution transmission electron microscopy. It was found that Lomer dislocations predominated (6:1 ratio) at the interfaces when compared with 60° ones. They were not regularly spaced (the variation could be as much as 4nm), and this was tentatively attributed to the presence of other defects. Their atomic structures, as well as that of the interface, were analyzed. By using anisotropic elasticity and extensive image simulation, it was shown that 2 asymmetrical models could be used to explain the experimental images of more than 70% of the analyzed Lomer dislocations. They had a compact structure which was made up of 8 and 6 atom cycles whose cores were displaced towards the largest spacing adjacent to the dislocation core. This contrasted with the perfect Hornstra model, in which the core of the dislocation was found on the {220} medial plane. These compact cores, and the non-existence of more energetic ones (which were known to be stabilized by impurities) indicated that the GaAs/Si interface was clean. On these non-misoriented substrates, steps were randomly distributed and led to large atomically flat interface facets. The steps were found to be di-atomic, and almost no antiphase boundaries were encountered in the sampled area. Moreover, they were found not to be systematic nucleation sites for the Lomer dislocation, as in the case of GaAs layers which were grown on misoriented substrates.

A.Vilà, A.Cornet, J.R.Morante, P.Ruterana, M.Loubradou, R.Bonnet, Y.González, L.González: Philosophical Magazine A, 1995, 71[1], 85-103