STM-induced void formation at the Al2O3/Ni3Al(111) interface

Under ultrahigh vacuum 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 Al2O3 film and a Ni3Al(1 1 1) substrate. This phenomenon is independent of tip polarity. Constant current (I nA) images obtained at +0.1 V bias and +2.0 V 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 similar to 30 Angstrom deep and similar to 500 Angstrom wide, leads to the eventual failure of the oxide overlayer. Density functional theory calculations suggest a reduction-oxidation mechanism: interfacial 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. (C) 2001 Elsevier Science B.V. All rights reserved.