XPS, LEED and STM study of thin oxide films formed on Cr(110)

The growth, thickness, composition and structure of chromium oxide thin fil ms formed by exposing Cr(110) single-crystal surfaces to gaseous oxygen at 300 and 625 K have been investigated by XPS, LEED and STM measurements. The oxide films formed at the two temperatures are significantly different. At 300 K, a granular and non-crystalline oxide is formed, which grows with a constant similar to Cr2O3 stoichiometry up to a limiting thickness of 0.9 n m. The film is hydrated with a water content of 10-20%, which decreases upo n annealing. Nuclei of oxide with a lateral dimension of similar to 0.7 nm and a height of similar to 0.2 nm have been observed in the nucleation stag e. These nuclei grow predominantly laterally and coalesce to fully cover th e substrate surface prior to the thickening stage. At 625 K, a first stage of oxygen adsorption is observed in which stripes 1.5-2.3 nm wide and paral lel to the Cr[001] direction are observed after annealing in UHV. They corr espond to narrow segments of mixed and close-packed planes of O atoms and i ons having a geometry and orientation similar to those of the anions planes in the oxide crystals. Rows of adatoms, possibly Cr3+ ions of oxide nuclei , are observed above the stripes. Thickening at 625 K leads to the formatio n of a non-crystalline oxide, which grows up to a limiting thickness of 4.6 nm. The presence of Cr3+ vacancies related to a significant cation transpo rt through the oxide film in this temperature regime is detected. After UHV annealing at 825 or 925 K, the film is anhydrous. The Cr3+ vacancies are a ccumulated at the metal/oxide film interface. The film crystallizes in epit axy with the substrate in the following orientation: alpha-Cr2O3(0001)paral lel to Cr(110) and alpha-Cr2O3[21 (3) over bar 0]parallel to Cr[001]. The S TM measurements of the unit cell of the alpha-Cr2O3(0001) surface are consi stent with a termination by a cation plane and show three tunneling sites a ssigned to the various possible locations of the Cr3+ ions at room temperat ure due to surface diffusion. (C) 2000 Elsevier Science B.V. All rights res erved.