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Dielectric breakdown of 7-Å-thick Al2O3 (111) films grown on Ni3Al(111) under ultrahigh vacuum conditions is induced by increasing the bias voltage on the scanning tunneling microscopy tip under constant current feedback. Breakdown is marked by the precipitous retreat of the tip from the surface, and the formation of an elevated feature in the scanning tunneling microscopy image, typically greater than 5 nm high and ∼100 nm in diameter. Constant height measurements performed at tip/sample distances of 1 nm or less yield no tip/substrate physical interaction, indicating that such features do not result from mass transport. Consistent with this, current/voltage measurements within the affected regions indicate linear behavior, in contrast to a band gap of 1.5 eV observed at unaffected regions of the oxide surface. A threshold electric field value of 11±1 MV cm−1 is required to induce breakdown, in good agreement with extrapolated values from capacitance measurements on thicker oxides.

We report the first STM observations of the thermally induced dewetting of an iron oxide scale from an Fe(111) surface. In addition, we report the influence of the presence of S at the metal/oxide interface on substrate topography. Room-temperature oxidation of S-free and (1 × 1)-S-covered Fe(111) surfaces at oxygen partial pressures of 1 × 10-7 to 5 × 10-7 Torr resulted in the growth of oxide islands, with the only difference being the formation of larger islands in the latter case. Line shape analyses of the Fe(MVV) peak indicated a similar growth mechanism in both cases, with the formation of Fe3O4 initially and Fe2O3 at higher exposures of O2. Flash annealing of the oxide formed on the S-free and (1 × 1)-S-covered surfaces in ultrahigh vacuum (UHV) resulted in oxide dewetting, leaving larger oxide islands separated by S-free or S-covered Fe regions, respectively. The presence of S at the metal/oxide interface causes oxide dewetting to occur at lower temperatures than those on a S-free surface. In the case of the (1 × 1)-S-covered phase, flash annealing to ∼720 K induces enhanced sulfur segregation apart from dewetting. When the S/Fe Auger intensity ratio is sufficiently large prior to oxidation (>1.3), flash annealing causes dewetting of the oxide scale, and the final S coverage induces the (2√3 × 1)R30° faceting transformation. However, if the ratio is lower than ∼1.3, the faceting transition is not observed upon annealing, although oxide dewetting and S segregation are noted. In other words, oxide dewetting is “decoupled” from faceting. The additional sulfur segregation to the metal/oxide interface at comparatively lower temperatures is an unexpected occurrence and occurs only if some interfacial sulfur is already present.

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