Atomically resolved imaging of a CaF bilayer on Si(111): Subsurface atoms and the image contrast in scanning force microscopy

The CaF bilayer on Si(111) was imaged with atomic resolution scanning force microscopy (SFM). The CaF bilayer has almost exactly the same geometry as the uppermost two atomic layers of bulk CaF2; the positions of the F atoms below the Ca layer, however, are vacant. We discuss the influence of this difference on atomic scale contrast formation in SFM. The SFM images obtained on the CaF bilayer are very similar to results from the literature obtained on bulk CaF2 with positively terminated tips. SFM observations on the CaF bilayer are explained within a model based on the electrostatic and van der Waals forces between tip and sample. The model predicts that the atomic corrugation is independent of the charge located at the position of the F vacancies and thus explains the similarity of SFM images obtained on bulk CaF2 and the CaF bilayer.