OXYGEN-ADSORPTION ON GA-RICH GAAS(100)

The adsorption of oxygen on c(8 X 2)-reconstructed GaAs(100) is invest igated with He-diffraction, thermal desorption spectroscopy (TDS) and Auger electron spectroscopy (AES). Adsorption from a seeded nozzle bea m (E(O-2)greater than or equal to 260 meV) and from an ambient gas are considered. Both molecular and dissociative adsorption are observed. The two adsorption behaviors could be clearly distinguished in the noz zle beam adsorption experiments: no structural changes are observed du ring molecular adsorption, whereas dissociative adsorption is found to be correlated to disordering of the surface. In the ambient gas adsor ption experiments, however, some disordering of the surface appears to occur also in situations in which molecular adsorption dominates. The observations are explained with a mechanism in which surface atoms ar e replaced by oxygen molecules. The energy necessary for this process may originate from thermal activation, from incident molecules (excita tions) or from some other source (photons, electrons). The defects are thought to be responsible for the dissociation. The difference betwee n adsorption from the beam and from the ambient gas seems to be due to differences in the O-2-coverage which are obtained in the two situati ons.