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

The growth, thickness, composition and structure of chromium oxide thin films 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 nm. The film is hydrated with a water content of 10-20%, which decreases upon 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 stage. These nuclei grow predominantly laterally and coalesce to fully cover the 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 parallel to the Cr[001] direction are observed after annealing in UHV. They correspond to narrow segments of mixed and close-packed planes of O atoms and ions 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 formation 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 transport 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 accumulated at the metal/oxide film interface. The film crystallizes in epitaxy with the substrate in the following orientation: alpha-Cr2O3(0001)parallel to Cr(110) and alpha-Cr2O3[21 (3) over bar 0]parallel to Cr[001]. The STM measurements of the unit cell of the alpha-Cr2O3(0001) surface are consistent with a termination by a cation plane and show three tunneling sites assigned to the various possible locations of the Cr3+ ions at room temperature due to surface diffusion. (C) 2000 Elsevier Science B.V. All rights reserved.