PHOTOELECTRON CORE-LEVEL SPECTROSCOPY AND SCANNING-TUNNELING-MICROSCOPY STUDY OF THE SULFUR-TREATED GAAS(100) SURFACE

A study of the adsorption of sulfur on the GaAs(100)surface after in s itu thermal desorption of a protective As capping layer is presented. The sulfur flux was generated by the decomposition of silver sulfide i n an UHV compatible electrochemical cell. Use of As-capped samples pro vided a means to study the interaction of sulfur with both the c(2X8) and (4X1) surface reconstructions. Scanning-tunneling-microscopy image s of the sulfur-covered surface indicated the formation of disordered surface layers which display a diffuse (1X1) low-energy-electron-diffr action pattern. This (1X1) phase is attributed to the symmetry of the bulk structure visible through the disordered surface overlayer, cause d by the adsorbed sulfur breaking the surface dimer bonds. Synchrotron radiation core-level photoemission spectra indicate evidence of sulfu r bonding td both gallium and arsenic at room temperature, but that th e relative magnitude of these bonding interactions depends on the Ga/A s ratio of the clean surface. Sulfur 2p photoemission spectra from the annealed surfaces show that sulfur diffuses into the topmost atomic l ayers as well as bonding to the surface. Annealing the sulfur covered surface above 400 degrees C results in the formation of a (2X1) low-en ergy-electron-diffraction pattern with a dimer-row structure clearly v isible in scanning-tunneling-microscopy images; Our results would sugg est that the adsorption of sulfur on the c (2X8) clean surface results in dimer rows consisting of both arsenic and sulfur dimers, while onl y sulfur dimers are observed after adsorption on the (4X1) surface. Th e degree to which the clean surface band bending is altered on these r espective,surfaces appears to be related to the precise chemical compo sition of the dimer rows.