The first steps in the oxidation of GaAs(110) are examined through shifts in the As and Ga $3d$ levels as a function of oxygen exposure. We observe a large chemical shift in the As $3d$ levels (about 4 eV) while the Ga $3d$ levels were shifted by less than 1 eV. This gives direct evidence for oxygen bonding to arsenic at the GaAs(110) surface, and illustrates the change in chemical character which can take place in atoms at the surfaces of compounds.

• Received 25 August 1975

DOI:https://doi.org/10.1103/PhysRevLett.35.1356