Scanning tunneling microscopy on the atomic and electronic structure of CoO thin films on Ag(100)

CoO thin films have been prepared by deposition of cobalt in an O2 atmosphere on Ag(100). At elevated substrate temperature (470 K) the films grow in a layer-like manner and introduce rearrangements of the substrate surface. For identification of the oxidic films, characterization of their electronic structure and to optimize the imaging conditions for atomically resolved results, the dependence of scanning tunneling microscopy (STM) images on the sample-bias voltage has been investigated. The CoO-related islands show a characteristic dependence of their step height when referred to the underlying silver substrate. For small sample bias the CoO islands appear as slightly depressed structures, while at higher sample bias the same islands are measured as protrusions. The dependence of the step height can be explained qualitatively by contributions of the tunneling current into or from oxide states, and correlates very well with the width of the optical bandgap. Contrast changes of the films have also been observed as a function of the tip state.