Analysis of surface electronic states based on the characteristics of surface interaction forces by atomic force microscopy

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The aim of this study is to establish a new method utilizing noncontact atomic force microscopy (nc-AFM) in ultrahigh vacuum for characterization of surface electronic states ; the electronic states are origins of the interaction force between tip and sample in the nc-AFM. The interaction is mainly composed of van der Waiils force, chemical bonding force and exchange repulsive force, which have different characteristics with respect to a separation between tip and sample. The separation controlled by nc-AFM feedback circuits and applied bias voltage between tip and sample determine the dominant force to produce the image contrast.Samples were Si(111)7x7 and Ge-deposited Si(lll) surfaces. Using a home-made nc-AFM, atom-resolved images of nc-AFM and damping were successfully observed. The damping images showed atomic contrast much stronger than that in nc-AFM images under particular imaging conditions. A model explaining the contrast in damping images caused by a strong attractive interaction was proposed, based on the decrease in averaged velocity of a cantilever under conditions of a constant frequency shift and a constant amplitude of the cantilever.The bias voltage dependence of the contrast was also examined. Although at sample bias voltages less than 1 V the contrast difference between faulted and unfaulted half was less, at sample bias voltages negatively higher than 2 V, corner adatoms on the faulted half were depicted as spiky protrusions, and the damping decreased much over these adatoms. This indicates that the bias voltage change can lead to tip stably approaching much closer to the sample surface and a strong attractive interaction over specified surface atoms as the electron resonance is induced. A slight change in frequency shift caused a faint change in contrast of the spiky.protrusions in nc-AFM and damping images ; the intermixing between Si and Ge on the film growth was possibly evidenced.