ATOMIC CORE STRUCTURE OF LOMER DISLOCATION AT GAAS (001)SI INTERFACE/

GaAs layers grown on (001) Si by ALMBE have been investigated by conve ntional and high-resolution TEM. At these interfaces Lomer dislocation s have been found to predominate (6:1) in comparison to the 60-degrees ones and are not regularly spaced (spacing variation can be as large at 4 nm), owing probably to the presence of other defects. Their atomi c structures, as well as that of the interface, have been analysed. Us ing anisotropic elasticity and extensive image simulation, it is shown that two asymmetric models can be used to explain the experimental im ages of more than 70% of the analysed Lomer dislocations. They have a compact structure made of eight and six atom cycles whose cores are di splaced towards the largest spacing adjacent to the dislocation core. This is in contrast with the perfect Hornstra model, in which the core of the dislocation is found on the {220} medial plane. These compact cores and the non-existence of more energetic ones which are known to be stabilized by impurities indicate that the ALMBE GaAs/Si interface is clean. On these non-misoriented substrates steps are randomly distr ibuted, and they lead to large atomically flat interface facets. The s teps are found to be biatomic and almost no antiphase boundaries were encountered in the sampled area. Moreover, they are not found to be sy stematically nucleation sites for the Lomer dislocation, such as it ha s been argued for GaAs layers grown on top of misoriented substrates.