Thermal and photochemical deposition of sulfur on GaAs(100)

The interaction of sulfur with GaAs(100) is of great interest due to its importance in surface passivation. We have investigated the thermal and photochemical deposition of sulfur on GaAs(100) from H2S using synchrotron radiation soft X-ray photoelectron spectroscopy (SXPS). The deposition of S can be achieved by exposing GaAs(100) to H2S at substrate temperatures greater than or equal to 550 K, a result of the irreversible dissociation of H2S on the surface. Both the rate and extent of sulfur deposition can be greatly enhanced by UV laser or white light synchrotron radiation. Heating of sulfur covered GaAs(100) to greater than or equal to 700 K results in a well ordered (2 x 1) surface reconstruction, as indicated by low energy electron diffraction (LEED). This high temperature stabilized surface is characterized by two well defined states of sulfur, both bonded to Ga atoms. One state can be attributed to surface gallium sulfide while the other to sub-surface sulfur which evidently replaces As atoms. This interpretation is supported by the observation that the As/Ga ratio, as determined by the 3p SXPS peak intensities, decreases linearly with increasing sulfur coverage. The implication of this study for improved GaAs surface passivation is addressed.