Determination of the atomic geometry of the GaAs(001)2x4 surface by dynamical RHEED intensity analysis: the beta 2(2x4) model

Using dynamical reflection high-energy electron diffraction (RHEED) computa tions, experimental RHEED rocking curves from the MBE-grown As-rich GaAs(00 1)2 x 4 surface have been analysed to provide estimates of the surface stru ctural parameters. Ab initio total-energy computations suggest that surface s grown in such As-rich conditions are likely to exist in the so-called bet a 2(2 x 4) phase. Therefore, it is assumed that the most appropriate GaAs(0 01)2 x 4 surface model with which to analyse the experimental data is that of the beta 2(2 x 4) phase. This phase has two As dimers in the surface lay er and a third As dimer in the third layer. The experimental rocking curves analysed were measured in the primary beam azimuth, permitting estimates o f those beta 2(2 x 4) phase surface structural parameters in the plane perp endicular to the primary beam azimuth. The best experiment-theory fit was o btained using a standard non-linear optimisation scheme (the Marquardt algo rithm). We find that by using the beta 2(2 x 4) surface model we are able t o get a better fit to the experimental data than in earlier work in which t he data were analysed using the beta(2 x 4) surface model. This supports th e assumption that the beta 2(2 x 4) surface model is the most appropriate t o describe the surface in the As-rich MBE growth conditions used to produce the sample surface upon which the RHEED experiments were performed. Our be st-fit beta 2(2 x 4) surface structure is predominantly in agreement with t hat obtained from a surface X-ray diffraction study and from ab initio tota l-energy calculations. As in these other studies, our best-fit structure sh ows a large in-plane relaxation of the second layer threefold coordinated G a atoms and an outward perpendicular displacement of the first layer As dim ers. The good agreement in most details between all of these independent st ructure determinations suggests that we now have a largely consistent pictu re of the structure of the GaAs(001)beta 2(2 x 4) surface. (C) 1998 Elsevie r Science B.V. All rights reserved.