Room-Temperature UHV-Deposited Titanium Monoxide Films on Oxidized Polycrystalline Copper
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Room-Temperature UHV-Deposited Titanium Monoxide Films on Oxidized Polycrystalline Copper V. M. Fuenzalida1, C. R. Grahmann1, C. Herrera1, R. A. Zárate1, C. Avila2, and M.E. Pilleux2 Universidad de Chile Facultad de Ciencias Físicas y Matemáticas 1 Departamento de Física, Av. Blanco Encalada 2008, Santiago 6511226, Chile 2 IDIEM, Plaza Ercilla 883, Santiago 6511226, Chile
ABSTRACT Polycrystalline copper films thicker than 100 nm were evaporated on silicon wafers with their native oxide under ultrahigh vacuum conditions leading to an rms roughness of ~2 nm of the copper film. X-ray photoelectron spectroscopy (XPS) revealed a clean copper surface with only traces of oxygen. The samples were exposed to air, leading to an oxide film consisting of CuO. TiO films were subsequently deposited onto the oxidized copper films from a resistively heated tungsten boat with the substrate at room temperature. The TiO films exhibited good adherence and were amorphous. XPS measurements revealed that the TiO films were contamination-free and that the first layers of TiO reduced the thin native oxide of the copper substrate from Cu(II) into Cu(I) or Cu(0) and transformed the TiO into TiO2 at the interface.
INTRODUCTION The growth of transition metals on single crystal surfaces has been extensively studied, particularly the growth of copper on rutile TiO2 (110).1-5 There is evidence that copper grows on stoichiometric and Ar+ sputtered TiO2 (110) surfaces in three-dimensional clusters and does not react with the substrate.4-6 The inverse problem, that is, the deposition of oxides on metals, has received less attention. In this case metal-oxide interactions can substantially modify the properties of ultrathin oxide films. Wu and Nix showed that TiO2-x films deposited at 600 K on single crystal Cu(100) substrate using chemical vapor deposition exhibited low reactivity with the substrate.7 The oxygen pre-adsorbed on the copper surface is scavenged by the first deposited titanium oxide layers, thus reflecting the fact that the interaction between oxygen and the copper substrate is comparatively weak compared to the affinity between titanium and oxygen. In further experiments the copper single crystal was pre-oxidized by exposure to O2 at 400 K, thus increasing the reactivity between the copper substrate and the titanium oxide. The initial growth appears to give a uniform oxide film (layer by layer or simultaneous multilayer growth) and, above a coverage of 7 monolayers, 3D-crystallite growth begins. The TiO coating is proposed as an electrically conducting protection, since TiO has a metallic conduction behavior whose surface will spontaneously oxidize to TiO2 after exposure to oxygen. The TiO2 layer may act as an anticorrosive coating, as observed in O8.25.1
alloys that passivate in air with titanium entering mainly as TiO2.7 A potential advantage of this system is that TiO can be deposited on substrates at room temperature, thus avoiding stresses due to the difference in thermal expansion coefficients which pervade other protection sc
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