Growth, composition, and structure of ultrathin vanadium films deposited on the SnO2(110) surface

The growth mechanism of vanadium films deposited on the SnO2(110) surface at room temperature and the structure of the phases formed after annealing at 800 K were studied using X-ray photoelectron spectroscopy (XPS), low-energy ion-scattering (LEIS), low-energy electron diffraction (LEED), and X-ray photoelectron diffraction (XPD). The vanadium films were deposited by thermal evaporation in ultrahigh vacuum (UHV) conditions on an oxygen-deficient SnO2 surface prepared by cycles of sputtering and annealing. In the initial stages of vanadium deposition a redox reaction occurs at the metal-oxide interface leading to the formation of vanadium oxide and metallic tin. Upon increasing the amount of deposited vanadium, we observed the growth of islands of metallic vanadium. XPS and LEIS data show that the surface of the vanadium film is partially covered by metallic tin diffusing from the interface. When the deposited vanadium films are heated to 800 K, the reoxidation of metallic tin and the oxidation of all metallic vanadium take place, with no detectable diffusion of vanadium into the SnO2 bulk. The XPD results rule out the formation of a V-Sn mixed oxide. The simulation of the experimental XPD curves by means of single-scattering cluster (SSC) calculations performed for various structural models indicate that the product of the oxidation is an oxide of formula VOx approximate to 2 with a structure close to that of the rutile VO2(110) surface. The vanadium oxide phase is covered by layers of disordered tin oxide.