Surface-bonding geometry of (2 x 1)S/Ge(001) by the normal-emission angle-resolved photoemission extended-fine-structure technique.

Abstract

The surface structure of (2\ifmmode\times\else\texttimes\fi{}1)S/Ge(001) was determined using the angle-resolved photoemission extended-fine-structure technique in the normal-emission direction. By comparison of the experimental data with curved-wave, multiple-scattering calculations, quantitative information about the local adsorption geometry was obtained. In particular, adsorption in a twofold bridge site, with a S-Ge bond length of 2.36\ifmmode\pm\else\textpm\fi{}0.05 A\r{}, was found. The twofold S bridge appears most likely to occur between two partially intact symmetric Ge-Ge dimers, with the Ge dimer laterally displaced by 0.10\ifmmode\pm\else\textpm\fi{}0.05 A\r{} from the bulk position. This result therefore provides evidence for S bonding to strong dangling bonds in the original dimers of the clean Ge(001) surface. There is, however, no evidence of significant surface contraction or expansion in the substrate layers, in contrast to the (2\ifmmode\times\else\texttimes\fi{}2)S/Ge(111) case.