MULTIPLE-SCATTERING EVALUATION OF RHEED INTENSITIES FROM THE GAAS(001)2X4 SURFACE - EVIDENCE FOR SUBSURFACE RELAXATION

Experimental reflection high-energy electron-diffraction (R.HEED) rocking curves in the [110BAR] azimuth from the molecular-beam-epitaxy (MBE) grown As-rich GaAs(001)2x4 surface have been analyzed by fitting rocking curves computed using elastic multiple-scattering (dynamical) theory. The surface is assumed to be composed of unit cells having the ''missing row'' structure in which the 2 x periodicity arises from symmetrically dimerized As atoms and the 4 x periodicity from a regular array of missing dimers, such that the As surface coverage is three-quarters of a monolayer. The surface model permits relaxation in both the surface layer (As) and the second layer (Ga). The best fit to the experimental data occurs for a surface unit-cell structure in which the triplets of surface As dimers are ''rumpled'' and the second layer (Ga) atoms relax in both the in-plane and perpendicular directions. The effect upon the fit of tilts and twists of the As dimers has been studied by imposing small displacements on the dimerized surface As atoms away from the best-fit configuration. We find evidence to suggest that the As dimers are symmetrical to within possible tilt angles of approximately 1-degrees and/or twist angles of approximately 1-degrees.