The structural and electronic properties of a Co monolayer condensed on the (111) surface of epi- taxial ${\mathrm{CoSi}}_2$ on Si(111) are studied by low-energy electron diffraction (LEED), x-ray photoemission spectroscopy (XPS), angle-resolved ultraviolet photoemission spectroscopy (ARUPS), and work-function measurements. A room-temperature Co deposit is disordered, but mild annealing (420 °C, 5 min) results in a sharp (1×1) surface structure. Such a surface can be clearly distinguished from the starting surface previously annealed at 550 °C for 5 min. The work function is 0.2 eV higher, the LEED intensities versus electron energy are quite different, and the ARUPS curves show surface features with two-dimensional (1×1) dispersion behavior. These surface states are attributed to an ordered (1×1) surface layer of Co atoms with four rather than eight Si neighbors for bulk ${\mathrm{CoSi}}_2$ Co atoms. The initial ${\mathrm{CoSi}}_2$(111) surface is recovered by further annealing at 550 °C, which results in Si segregation at the surface. We suggest that both surfaces, Co rich on the one hand and Si rich on the other hand, essentially represent truncations of the ideal ${\mathrm{CoSi}}_2$ crystal exposing Co and Si (111) planes, respectively. The consequence of the present finding is that the previous observation by ARUPS of a two-dimensional structure in the course of epitaxial ${\mathrm{CoSi}}_2$ growth on Si(111) should be interpreted in terms of ${\mathrm{CoSi}}_2$ formation with a Co-rich (111) surface.

• Received 10 June 1985

DOI:https://doi.org/10.1103/PhysRevB.33.4108