Photoemission study of the adsorption of O2, CO and H2 on GaAs(110)

Ultraviolet photoemission spectroscopy with hv < 12 eV has been used to study O₂, CO, and H₂ adsorption on the cleaved GaAs(110) face. It was found that O₂ exposures above 10⁵ L(1LM = 10^?6 Torr sec) were required to produce changes in the energy distribution curves. At O₂ exposures of 10⁶ L on p-type and 10⁸ L on n-type an oxide peak is observed in the EDC's located 4 eV below the valence band maximum. On p-type GaAs, O₂ exposures cause the Fermi level at the surface to move up to a point 0.5 eV above the valence band maximum, while on n-type GaAs O₂ exposures do not remove the Fermi level pinning caused by empty surface states on the clean GaAs. CO was found to stick to GaAs, but to desorb over a period of hours, and not to change the surface Fermi level position. H₂ did not affect the EDC's, but atomic H lowered the electron affinity and raised the surface position of the Fermi level on p-type GaAs. A correlation is found in which gases which stick to the GaAs cause an upward movement of the Fermi level at the surface on p-type GaAs, while gases which stick only temporarily do not change the surface position of the Fermi level.