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

We have prepared ordered thin films of NiO and CoO in (100) orientation by evaporating Ni (Co) in an O-2 atmosphere onto Ag(100). The films have been analysed by scanning tunnelling microscopy and low-energy electron diffract ion. In the initial stage (coverage up to a few monolayers), growth and str ucture of the grown films drastically depend on the preparation conditions (in particular, on the temperature of the substrate during deposition and p ost-annealing). In this case we also observe strong interactions with the s ubstrate. Ag atoms are partially removed from the substrate terraces and fo rm islands or migrate to step edges. No indications for incorporation in th e oxide thin films are seen. The oxidic features grow on top of the substra te or in the vacancy islands within the first layer of the substrate left b ehind by the removed Ag atoms. At low substrate temperatures (near room tem perature) an essential part of the oxidic features corresponds to a precurs or state rather than to the fully developed (100) oxide film which only dev elops 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 rea ction had taken place or not. The surfaces of the oxide precursor show a ty pical defect structure similar to those found on cleaved NiO(100) (M. R. Ca stell et al., Phys. Rev. B: Condens Matter, 1997, 55, 7859). This feature s hows 'random walk' at room temperature.