Determination of the atomic geometry of the GaAs(001)2×4 surface by dynamical RHEED intensity analysis: The β2(2×4) model

Abstract

Using dynamical reflection high-energy electron diffraction (RHEED) computations, experimental RHEED rocking curves from the MBE-grown As-rich GaAs(001)2×4 surface have been analysed to provide estimates of the surface structural parameters. Ab initio total-energy computations suggest that surfaces grown in such As-rich conditions are likely to exist in the so-called β2(2×4) phase. Therefore, it is assumed that the most appropriate GaAs(001)2×4 surface model with which to analyse the experimental data is that of the β2(2×4) phase. This phase has two As dimers in the surface layer and a third As dimer in the third layer. The experimental rocking curves analysed were measured in the primary beam azimuth, permitting estimates of those β2(2×4) phase surface structural parameters in the plane perpendicular to the primary beam azimuth. The best experiment–theory fit was obtained using a standard non-linear optimisation scheme (the Marquardt algorithm). We find that by using the β2(2×4) surface model we are able to get a better fit to the experimental data than in earlier work in which the data were analysed using the β(2×4) surface model. This supports the assumption that the β2(2×4) surface model is the most appropriate to describe the surface in the As-rich MBE growth conditions used to produce the sample surface upon which the RHEED experiments were performed. Our best-fit β2(2×4) surface structure is predominantly in agreement with that obtained from a surface X-ray diffraction study and from ab initio total-energy calculations. As in these other studies, our best-fit structure shows a large in-plane relaxation of the second layer threefold coordinated Ga atoms and an outward perpendicular displacement of the first layer As dimers. The good agreement in most details between all of these independent structure determinations suggests that we now have a largely consistent picture of the structure of the GaAs(001) β2(2×4) surface.