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Copper Wetting of x-Al(2)O(3)(0001): Theory and Experiment

Description: XPS studies have been carried out on sputter deposited copper on a substantially hydroxylated {alpha}-Al{sub 2}O{sub 3}(0001) (sapphire) surface under ultra-high vacuum (UHV) conditions. XPS-derived Cu uptake curves show a sharp change in slope at a coverage of 0.35 monolayer (on a Cu/O atomic basis), indicative of initial layer-by-layer growth. CU(LMM) lineshape data indicate that, prior to the first break in the curve, Cu is oxidized to Cu(I). At higher coverages, metallic CU(0) is. observed. These data agree with first principles theoretical calculations, indicating that the presence of ad-hydroxyl groups greatly enhances the binding of Cu to bulk sapphire surfaces, stabilizing Cu(I) adatoms over two-dimensional metallic islands. In the absence of hydroxylation, calculations indicate significantly weaker Cu binding to the bulk sapphire substrate and non-wetting. Calculations also predict that at Cu coverages above 1/3 monolayer (ML), Cu-Cu interactions predominate, leading to Cu(0) formation. These results are in excellent agreement with experiment. The ability of surface hydroxyl groups to enhance binding to alumina substrates suggests a reason for contradictory experimental results reported in the literature for Cu wetting of alumina.
Date: August 10, 1999
Creator: Bogicevic, A.; Jennison, D.R.; Kelber, J.A.; Niu, Chengyu & Shepherd, K.
Partner: UNT Libraries Government Documents Department

STM-Induced Void Formation at the Al{sub 2}O{sub 3}/Ni{sub 3}Al(111) Interface

Description: Under UHV conditions at 300 K, the applied electric field and/or resulting current from an STM tip creates nanoscale voids at the interface between an epitaxial, 7.0 {angstrom} thick Al{sub 2}O{sub 3} film and a Ni{sub 3}Al(111) substrate. This phenomenon is independent of tip polarity. Constant current (1 nA) images obtained at +0.1 V bias and +2.0 bias voltage (sample positive) reveal that voids are within the metal at the interface and, when small, are capped by the oxide film. Void size increases with time of exposure. The rate of void growth increases with applied bias/field and tunneling current, and increases significantly for field strengths >5 MV/cm, well below the dielectric breakdown threshold of 12 {+-} 1 MV/cm. Slower rates of void growth are, however, observed at lower applied field strengths. Continued growth of voids, to {approximately}30 {angstrom} deep and {approximately}500 {angstrom} wide, leads to the eventual failure of the oxide overlayer. Density Functional Theory calculations suggest a reduction-oxidation (REDOX) mechanism: interracial metal atoms are oxidized via transport into the oxide, while oxide surface Al cations are reduced to admetal species which rapidly diffuse away. This is found to be exothermic in model calculations, regardless of the details of the oxide film structure; thus, the barriers to void formation are kinetic rather than thermodynamic. We discuss our results in terms of mechanisms for the localized pitting corrosion of aluminum, as our results suggest nanovoid formation requires just electric field and current, which are ubiquitous in environmental conditions.
Date: September 21, 2000
Creator: Magtoto, N. P.; Niu, C.; Anzaldura, M.; Kelber, J. A. & Jennison, D. R.
Partner: UNT Libraries Government Documents Department

Cu interactions with {alpha}-Al{sub 2}O{sub 3}(0001): Effects of surface hydroxyl groups vs. dehydroxylation by Ar ion sputtering

Description: XPS studies and first principles calculations compare Cu adsorption on heavily hydroxylated sapphire (0001) with a dehydroxylated surface produced by Ar{sup +} sputtering followed by annealing in O{sub 2}. Annealing a cleaned sapphire sample with an O{sub 2} partial pressure of {approximately}5 x 10{sup {minus}6} Torr removes most contaminants, but leaves a surface with {approximately}0.4ML carbon and {approximately}0.4ML OH. Subsequent light (6 min.) Ar ion sputtering at 1 KeV reduces the carbon to undetectable levels but does not dehydroxylate the surface. Further sputtering at higher Ar ion excitation energies (>2 KeV) partially dehydroxylates the surface, while 5 KeV Ar ion sputtering creates oxygen vacancies in the surface region. Further annealing in O{sub 2} repairs the oxygen vacancies in the top layers but those beneath the surface remain. Deposition of Cu on the hydroxylated surface at 300 K results in a maximum Cu(I) coverage of {approximately}0.35 ML, in agreement with theoretical predictions.
Date: February 8, 2000
Partner: UNT Libraries Government Documents Department