VACUUM ULTRAVIOLET PHOTOELECTRON-SPECTROSCOPY STUDY OF THE INTERACTION OF MOLECULAR SULFUR WITH THE GAAS(100) SURFACE

The surface chemistry resulting from the interaction of molecular sulfur with the clean GaAs(100) surface has been studied using surface-sensitive synchrotron radiation photoemission spectroscopy. Clean GaAs(100) surfaces were obtained by the thermal decapping of a protective As overlayer deposited on a molecular-beam-epitaxy-grown GaAs epilayer. Molecular sulfur S2 was generated by the electrochemical decomposition of Ag2S in ultrahigh vacuum. The intensity of the chemically shifted components on both As and Ga 3d core levels has been found to critically depend on the As-to-Ga ratio in the topmost atomic layer of the clean decapped surface. This As-to-Ga surface ratio can be selectively altered by annealing the surface at different temperatures during the decapping procedure. Experiments have been carried out on both the Ga- and As-rich surfaces and it has been found that the magnitude of the chemical shifts can also appear to depend on the precise surface composition. For the exposure times investigated in this study, the sulfur treatment results in the GaAs surface being terminated in a monolayer of sulfur bonded to both Ga and As surface atoms.