Dynamic force microscopy across steps on the Si(111)-(7 x 7) surface

Force microscopy in atomic resolution with an oscillating tip has been performed across monatomic steps of the Si(111)-(7 x 7) surface using the tunnelling current or frequency shift as the feedback parameter. The contrast of simultaneously recorded images in both feedback modes is discussed. A significant difference between tip-sample interactions on the upper and lower terrace close to a step is analyzed in detail by means of Kelvin-type measurements. No contact potential variation across the step is found. A simple model for the force contrast is suggested which takes into account the different effective interaction areas or volumes on the upper and the lower terrace.