We have used medium-energy ion scattering and scanning tunneling microscopy to determine the geometric structure of the As/Si(111)-(1×1) surface. It has been proposed on the basis of core-level and valence-band photoemission experiments that this surface has a bulklike truncation with the outer silicon monolayer replaced by arsenic. Ion scattering experiments indicate that both ordered and disordered (1×1) phases exist at near-monolayer arsenic coverage, with the relative abundance of each phase dependent on deposition conditions. Tunneling images of the surface confirm the existence of bulk terminated (1×1) regions intermingled with chaotic areas. Finally, examination of the surfaces with angle-resolved ultraviolet photoemission spectroscopy showed little sensitivity to the presence of disorder.

• Received 7 December 1987

DOI:https://doi.org/10.1103/PhysRevB.37.10756