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

Experimental reflection high-energy electron-diffraction (R.HEED) rock ing curves in the [110BAR] azimuth from the molecular-beam-epitaxy (MB E) grown As-rich GaAs(001)2x4 surface have been analyzed by fitting ro cking curves computed using elastic multiple-scattering (dynamical) th eory. The surface is assumed to be composed of unit cells having the ' 'missing row'' structure in which the 2 x periodicity arises from symm etrically dimerized As atoms and the 4 x periodicity from a regular ar ray of missing dimers, such that the As surface coverage is three-quar ters of a monolayer. The surface model permits relaxation in both the surface layer (As) and the second layer (Ga). The best fit to the expe rimental data occurs for a surface unit-cell structure in which the tr iplets of surface As dimers are ''rumpled'' and the second layer (Ga) atoms relax in both the in-plane and perpendicular directions. The eff ect upon the fit of tilts and twists of the As dimers has been studied by imposing small displacements on the dimerized surface As atoms awa y from the best-fit configuration. We find evidence to suggest that th e As dimers are symmetrical to within possible tilt angles of approxim ately 1-degrees and/or twist angles of approximately 1-degrees.