Study of the Au Schottky contact formation on oxygen plasma treated n-type SnO2 (101) thin films

As-grown and oxygen plasma treated n-type tin dioxide (SnO2) (101) thin films were investigated regarding the properties of Au contacts, surface structure, and band bending. The plasma treatment was performed in a conventional oxygen plasma cleaning system with maximum oxygen ion energies of 400 eV. Whereas the as-deposited SnO2 film formed non-Schottky contacts with Au, the oxygen plasma treated films formed Schottky contacts with Au. Capacitance-voltage and differential Hall measurements indicated the introduction of bulk electron traps up to several 100 nm below the surface due to the oxygen plasma treatment. Angle resolved x-ray photoelectron spectroscopy (AR-XPS) revealed a surface accumulation layer on the as-grown film that was absent after the plasma treatment. These measurements further revealed chemical differences between the as-deposited and the plasma treated SnO2 surfaces. All SnO2 films had Sn2+-O bonds. The AR-XPS Sn2+-O signal intensity increased after plasma treatment, indicating that the oxygen plasma damaged the SnO2 surface. Additionally, an O-2 adlayer was formed due to the oxygen plasma treatment. The damaged layer due to oxygen ion bombardment, oxygen adsorption layer, and possibly bulk traps depleted the surface carriers which enabled the formation of a Schottky contact with Au.