THE AS SI(111) SURFACE STUDIED BY ANGLE SCANNED LOW-ENERGY PHOTOELECTRON DIFFRACTION/

Photoelectron diffraction experiments from the As/Si(111)-(1 X 1) surf ace are reported in which the surface and bulk components of the Si 2p core-level were measured as functions of azimuthal angle. The Si 2p s urface component shows large intensity variations of up to a factor of 4. Comparisons to fully convergent multiple scattering calculations s how a high sensitivity to surface atomic geometry and indicate that th e As overlayer is relaxed toward the vacuum by 0.2 Angstrom compared t o the bulk Si lattice spacing. The measurements also show sensitivity to the vertical position of the underlying Si layer with a best fit fo und for an unrelaxed Si layer. A single scattering treatment predicts rather well the large diffraction effects observed for the Si 2p surfa ce peak and indicates that they are due primarily to large angle scatt ering from the three nearest neighbor As atoms. The strong intensity v ariations observed show that diffraction effects can introduce large e rrors into surface or adsorbate coverage determinations based on core- level intensities and the single parameter electron escape depth model if these effects are not specifically taken into consideration.