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

Force microscopy in atomic resolution with an oscillating tip has been perf ormed across monatomic steps of the Si(111)-(7 x 7) surface using the tunne lling current or frequency shift as the feedback parameter. The contrast of simultaneously recorded images in both feedback modes is discussed. A sign ificant difference between tip-sample interactions on the upper and lower t errace close to a step is analyzed in detail by means of Kelvin-type measur ements. No contact potential variation across the step is found. A simple m odel for the force contrast is suggested which takes into account the diffe rent effective interaction areas or volumes on the upper and the lower terr ace. (C) 2000 Elsevier Science B.V. All rights reserved.