The As/Si(111) surface studied by angle scanned low energy photoelectron diffraction

Photoelectron diffraction experiments from the As/Si(111)-(1 X 1) surface 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 surface component shows large intensity variations of up to a factor of 4. Comparisons to fully convergent multiple scattering calculations show a high sensitivity to surface atomic geometry and indicate that the As overlayer is relaxed toward the vacuum by 0.2 Angstrom compared to the bulk Si lattice spacing. The measurements also show sensitivity to the vertical position of the underlying Si layer with a best fit found for an unrelaxed Si layer. A single scattering treatment predicts rather well the large diffraction effects observed for the Si 2p surface peak and indicates that they are due primarily to large angle scattering from the three nearest neighbor As atoms. The strong intensity variations observed show that diffraction effects can introduce large errors 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.