Photoemission studies of surface and interface states on III–V compounds

Abstract

The GSCH model for surface states on (110) GaAs proposed by Gregory et al. appears (based on a wide range of results) to be well established and to apply to all faces of all III–V semiconductors. In this model, the filled and empty surface states are associated with and localized on the column V and III atoms, respectively, and a band gap (greater than 1 eV) separates the filled and empty surface states. For example, detailed studies of GaAs and InP locate the bottom of the empty states at 0.7 and 0.25 eV, respectively, below the conduction band minimum (CBM), whereas in GaSb it lies above the CBM. The filled states in all three cases are located well below the valence band maximum (VBM). In the context of the GSCH model, the details of the first steps in oxidation and formation of Schottky barrier with Cs have been examined experimentally. It is concluded from studies of Ga and As chemical shifts by Pianetta et al. that, in the first step of oxidation, the oxygen gains electrons from As but not the Ga surface atoms. The formation of a Schottky barrier with Cs has been studied from very low coverage to completion of the first Cs layer (at which coverage good correlation is found with the literature for thick metallic layers). For GaAs and InP, good correlation is found between the final Fermi-level pinning position and the bottom of the intrinsic empty surface states. For GaSb, where the empty intrinsic surface state do not lie in the band gap, no such correlation is found.