Fermi level pinning during oxidation of atomically clean n-InP(110)

Previously a donor defect level at about −0.1 eV below the conduction band minimum has been associated with InP, in part to explain the nonmonotonic movement of the Fermi level within the band gap during the oxidation of n-InP. We present here new soft x-ray photoemission spectroscopy results which associate this behavior with a level in the oxide on the surface of the InP rather than with a native defect such as a P vacancy. Specifically, we have observed that the reversal in Fermi level movement occurs at exactly the same oxygen exposure as the appearance of structure in the In 4d core line indicative of the formation of In oxide (s). Phosphorus oxide is also indicated, as expected, along with elemental P, but the In oxide is particularly relevant because In2O3 has a sufficient density of shallow donor states to make it an n-type semiconductor. These oxide donor states, rather than native defect states, can compensate native acceptor defect states and cause the observed Fermi level movement. The Fermi level changes which are seen during the reduction of the surface In oxide to In metal are also consistent with the removal of the donor states. This interpretation changes strongly the overall picture of extrinsic states near InP–adatom interfaces, as there is no need to postulate the existence of native shallow donor defects to explain the InP–oxide interface.