Structure investigation of the topmost layer of a thin ordered alumina film grown on NiAl(1 1 0) by low temperature scanning tunneling microscopy

A thin Al2O3 layer grown on NiAl(1 1 0) has been studied by scanning tunneling microscopy at about 4 K. The film exhibited monocrystalline ordering compatible with the (1 1 1) face of γ-alumina with a slightly extended surface unit cell. Although the topographical images change substantially with bias voltage we show that the autocorrelation transform of the images clearly reveals the unit cell of the alumina lattice. By using particular tunneling conditions we have identified almost all oxygen atom positions within the unit cell. The parameters applied lead to a considerable interaction between tip and top atoms of the substrate reflected in the image as local perturbations denoted as scratching.