Superstructures and coincidences of a thin oxide film on a metallic substrate: A STM study

An alumina thin film, grown at 1000 K on a Ni3Al(111) substrate, is investigated using low-temperature scanning tunneling microscopy. Depending on the (positive) voltage applied to the sample, two superstructures are imaged. Using tunneling spectroscopy, this effect is traced back to a lateral variation of the local density of states within the apparent unit cell of the oxide thin film. In the vicinity of the conduction-band edge, the density of empty states for tunneling electrons is locally enhanced over characteristic points of the unit cell. A coincidence lattice between film and metallic substrate, with a unit cell of same size and symmetry, is proposed as the origin of this modulation of the oxide electronic structure.