Electronic and atomic structure of the Cu/Si(111) 'quasi-5 x 5' overlayer

We have investigated the atomic and electronic structure of the incommensurate 'quasi-5 × 5′ surface obtained by annealing about 1 ML of Cu on Si(111). Si2p core level photoemission reveals the presence of only one chemically non-equivalent site in the overlayer. Valence band ultraviolet photoelectron diffraction (UPD) in the Cu3d region shows a threefold symmetry of the surface electronic states not observed before. X-ray photoelectron diffraction (XPD) measured on the Cu2p level gives an almost sixfold pattern and indicates a Cu terminated surface, in agreement with Auger electron diffraction measurements of Chambers and coworkers [1]. Simulations of the Cu2p XPD pattern were performed in the single-scattering-cluster spherical-wave approximation, and a good agreement with the data was reached for a Cu2Si model proposed by Zegenhagen and coworkers [2], which is also consistent with the threefold features observed in UPD. A structural refinement was performed for the vertical positions of the Si and the two Cu atoms within this two-dimensional silicide layer.