Influence of Oxygen on the B 1s and La 3d Line Shapes of a Lanthanum Hexaboride (100) Surface

Abstract

X-ray photoelectron spectroscopy was used to investigate the initial stages of adsorption on the (100) surface of a single crystal of lanthanum hexaboride. Numerous previous studies had not resolved the issue of whether oxygen adsorbs at lanthanum sites, boron sites, or both. We find that oxygen adsorption produces new peaks in the La 3d region, whereas the B 1s peak is unaffected. The La 3d3/2 peak at a binding energy of 854.8 eV on the clean surface splits into two components at 855.2 and 852.1 eV after a 10 L O2 exposure at room temperature. Annealing the surface to 600 °C and then to 700 °C produces sharp c(2 × 2) and p(2 × 1) LEED patterns, respectively. The La 3d peaks still show two components for both LEED patterns indicating oxygen bonding to La in both oxygen overlayer structures whereas the B 1s region remains unaffected. Thus while the XPS data clearly reveal oxygen adsorption at La sites, there is no indication of adsorption at boron sites for low O2 exposures. More extensive oxidation at higher temperatures shows formation of both boron and lanthanum oxides.