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 can be reproduced by single scattering cluster calculations for the ideal truncated bulk-like Si(111) structure. Yet, the open diamond structure 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 cluster (about 20 angstrom in radius) in the simulations. The remarkably rich structure observed in the profiles can be related to the presence of two inequivalent emitters and to the large number of scatterers (about 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 scattering peaks associated with nearest neighbours and in the apparent absence 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 surfaces. More important changes are observed for (1 x 1)-Er where a monolayer of a strong scatterer (Er), that reacts with the Si top layer to form a two-dimensional silicide. is deposited on the surface.