Bias Dependence of NC-AFM Images and Tunneling Current Variations on Semiconductor Surfaces

Abstract

Imaging by NC-AFM has attracted a great deal of interest, because of the excellent imaging capability and resolution on an atomic scale, as described in this book. However, the exact nature of what the NC-AFM image represents is still the subject of some dispute because of the complexity of the interaction between the condensed matter making up the sample and the tip. For example, the Si adatom contrast of Si(111)7x7 as depicted by the NC-AFM does not appear the same for different groups owing to various adjustable imaging parameters [1–8]. The interaction includes the van der Waals force, the chemical bonding force, the electrostatic force, the exchange repulsive force and so on. Therefore, we need to ask which interaction is dominant in NC-AFM imaging. Although the interaction forces cannot be measured separately, we have found that the dominant force changes depending on the imaging conditions. The bias voltage between the tip and the sample gives rise to a long-range attractive force and changes the imaging conditions. Here we discuss how the NC-AFM images and tunneling current depend on the bias on a Si(111)7x7 surface and a germanium thin film on Si(111).