Chemical bonding and structure of the sulfur treated GaAs(111)B surface

We have investigated the atomic structure and chemical bonding present at sulfur exposed GaAs(111)B-(2×2) surfaces using both scanning tunneling microscopy (STM) and synchrotron radiation core-level photoemission. Exposure of the (2×2) surface to a molecular beam of sulfur leads to the appearance of a (1×1) low-energy electron diffraction pattern which becomes increasingly well defined as the sample is annealed. However, at no stage of the annealing process does the surface display an ordered (1×1) ideal termination. Both the photoemission data and STM images show that a large proportion of the As trimer units of the clean (2×2) surface remain after sulfur exposure and annealing to 450 °C with strong evidence of sulfur substituting for As in atomic layers below the surface. The effect of these reactions is to increase the surface band-bending from that of the clean (2×2) surface.