Structural Dynamics of Al₂O₃/NiAl(110) during Film Growth in NO₂

While continuum descriptions of oxide film growth are well established, the local structural dynamics during oxide growth are largely unexplored. Here, we investigate this using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) for the example of alumina film growth on NiAl(110) following NO₂ exposure. To maintain a well-defined system, we have adopted a cyclic growth approach of NO₂ adsorption and annealing. NO₂ adsorption at 693 K results in the formation of a vacancy island pattern in the NiAl(110) substrate, which is filled with AlO_x by diffusion of O through the alumina film. The patches of AlO_x coalesce to form smooth terraces upon annealing to 1200 K. By repeated cycling, we have grown films of up to 0.9 nm thick. While peak shifts in the XPS spectra indicate that the film maintains its insulating character upon thickening, our STM data show that there is a finite density of states within the band gap. The thickening of the alumina film is accompanied by the formation of trenches in the surface, which we interpret to be the result of film stress relief.