X-Ray absorption spectroscopy under conditions of total external reflection: application to the structural characterisation of the Cu/GaAs(100) interface

The theoretical principles and instrumentation requirements of X-ray absorption spectroscopy under conditions of total external reflection are described with particular reference to their application in the characterisation of solid/solid interfaces. The advantages of combining XANES, EXAFS and reflectivity in a single series of measurements are highlighted through the structural characterisation of oxide layers on Cu and GaAs(100). The data reveal the surface of Cu deposited on float-glass to comprise a macroscopic mix of metallic Cu and structurally disordered oxide. The latter appears to have a local structure close to that observed in Cu2O but with a higher oxygen coordination at the surface. The surface of commercial-grade GaAs(100) has ca. 7–9 A of disordered oxide in which Ga coordinates to oxygen both tetrahedrally and octahedrally whilst As is only found in tetrahedral sites. Strong correlated second-shell cation coordinations around both Ga and As central atoms reveal a single non-stoichiometric surface oxide depleted of As at the surface. 10 and 100 A thick Cu films deposited on GaAs(100) are found to be completely oxidised with the oxide being more disordered and having a higher oxygen coordination than those deposited on float-glass. For 10 A thick films, Cu appears coordinated to Ga and As through the oxygen which decorates the inner surfaces of micro-voids and fissures in the disordered oxide on GaAs.