Low-energy electron diffraction (LEED) and Auger electron spectroscopy were used to study the reactions of a clean Si{111} surface with Co. Although above surface coverages of about two monolayers the LEED patterns were always 1×1, measurements of the diffracted intensities revealed that there are three 1×1 phases. With increasing initial Co coverage they are called pre-silicide, ${\mathrm{CoSi}}_2$, and post-silicide phases, respectively. The structures of the pre-silicide and of the ${\mathrm{CoSi}}_2$ phase are very similar to one another. The structure of the ${\mathrm{CoSi}}_2${111} surface was determined by LEED intensity analysis to be relaxed bulklike, with first interlayer spacing contracted 20% and second interlayer spacing contracted 5%. The structures of the pre- and the post-silicide phases are unknown.

• Received 4 November 1985

DOI:https://doi.org/10.1103/PhysRevB.33.2900