Scanning tunnelling microscopy on the growth and structure of NiO(100) and CoO(100) thin films

Abstract

We have prepared ordered thin films of NiO and CoO in (100) orientation by evaporating Ni (Co) in an O₂ atmosphere onto Ag(100). The films have been analysed by scanning tunnelling microscopy and low-energy electron diffraction. In the initial stage (coverage up to a few monolayers), growth and structure of the grown films drastically depend on the preparation conditions (in particular, on the temperature of the substrate during deposition and post-annealing). In this case we also observe strong interactions with the substrate. Ag atoms are partially removed from the substrate terraces and form islands or migrate to step edges. No indications for incorporation in the oxide thin films are seen. The oxidic features grow on top of the substrate or in the vacancy islands within the first layer of the substrate left behind by the removed Ag atoms. At low substrate temperatures (near room temperature) an essential part of the oxidic features corresponds to a precursor state rather than to the fully developed (100) oxide film which only develops after post-annealing to higher temperatures (typically around 500 K). I/U characteristics and the sample bias dependency of the contrast of the islands grown have been utilised for identification of whether an oxide reaction had taken place or not. The surfaces of the oxide precursor show a typical defect structure similar to those found on cleaved NiO(100) (M. R. Castell etal., Phys. Rev. B:Condens. Matter, 1997, 55, 7859). This feature shows ‘random walk’ at room temperature.