Preparation of well-ordered, oxygen-rich SnO2(110) surfaces via oxygen plasma treatment

Abstract

The problems involved in preparing well-defined metal oxide surfaces are well known. We are investigating low power rf oxygen plasma treatment as a method for producing metal oxide surfaces of controlled stoichiometry and structure. The effects of exposing a 300 K oxygen deficient SnO2(110)−4×1 surface to an oxygen plasma (0.1 Torr) have been examined using low-energy electron diffraction (LEED), x-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), ion scattering spectroscopy (ISS), and surface conductivity measurements. Results from the plasma-exposed surface have been compared to those obtained from a thermally oxidized (1 Torr O2, 700 K) SnO2(110) surface. LEED observations indicate that low energy ion bombardment processes associated with plasma exposure do not significantly damage (disorder) the surface. In addition, band gap states associated with oxygen vacancies are eliminated as shown by UPS. This is in accord with XPS data that show an increase in the surface O/Sn ratio. The elimination of oxygen vacancies, which act as donors in tin oxide, causes a drop in surface conductivity of two orders of magnitude and a band bending of approximately 0.5 eV compared to the sputter-annealed surface. The comparatively higher ISS O/Sn ratios as well as the larger band bending suggest that a more oxygen-rich tin oxide surface is obtained by room temperature plasma exposure than by high temperature oxidation at considerably higher O2 pressures.