X-RAY PHOTOELECTRON DIFFRACTION FROM SI(111) - SHORT VERSUS LONGER RANGE STRUCTURAL SENSITIVITY

We have obtained polar X-ray photoelectron diffraction (XPD) profiles with a high angular resolution of +/- 1-degrees for Si2p from Si(111) with (7 x 7), (1 x 1)-H and (1 x 1)-Er surface structures. The data ca n be reproduced by single scattering cluster calculations for the idea l truncated bulk-like Si(111) structure. Yet, the open diamond structu re and relatively weak scattering amplitude of Si as well as the high degree of structural order in Si bulk require the use of a large clust er (about 20 angstrom in radius) in the simulations. The remarkably ri ch structure observed in the profiles can be related to the presence o f two inequivalent emitters and to the large number of scatterers (abo ut 300) that contribute substantially to the photoelectron wave. This indicates a longer range sensitivity of XPD than is usually believed. The related interferences result in a complex shape of the forward sca ttering peaks associated with nearest neighbours and in the apparent a bsence of the one related to [311] atomic rows. Finally, the effect of the deviations from bulk geometry in the near surface region is found to be detectable but quite small for the (7 x 7) and (1 x 1)-H surfac es. More important changes are observed for (1 x 1)-Er where a monolay er of a strong scatterer (Er), that reacts with the Si top layer to fo rm a two-dimensional silicide. is deposited on the surface.