Atomic core structure of Lomer dislocation at GaAs/(001)Si interface

Abstract

GaAs layers grown on (001) Si by ALMBE have been investigated by conventional and high-resolution TEM. At these interfaces Lomer dislocations have been found to predominate (6:1) in comparison to the 60° ones and are not regularly spaced (spacing variation can be as large at 4 nm), owing probably to the presence of other defects. Their atomic structures, as well as that of the interface, have been analysed. Using anisotropic elasticity and extensive image simulation, it is shown that two asymmetric models can be used to explain the experimental images of more than 70% of the analysed Lomer dislocations. They have a compact structure made of eight and six atom cycles whose cores are displaced 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 distributed, and they lead to large atomically flat interface facets. The steps are found to be biatomic and almost no antiphase boundaries were encountered in the sampled area. Moreover, they are not found to be systematically nucleation sites for the Lomer dislocation, such as it has been argued for GaAs layers grown on top of misoriented substrates.