Bias dependence of Si(111) 7 X 7 images observed by noncontact atomic force microscopy

Noncontact atomic force microscopy (nc-AFM) imaging of a Si(111)7 X 7 surface has been done in order to examine the bias dependence of the contrast of Si adatoms. While the atomic corrugation depends upon the tip states, the contrast is found to be inverted by increasing the bias voltage at greater frequency shifts. Then, the term of the repulsive force between a Si adatom and a Si atom at the tip apex can play an important role in depicting the topography of the sample surface with atomic resolution. The difference in contrast between faulted and unfaulted halves and peculiar profiles near steps are also presented. (C) 2000 Elsevier Science B.V. All rights reserved.