Contrast of Atomic-Resolution Images from a Noncontact Ultrahigh-Vacuum Atomic Force Microscope

Contrast variations of atomic-resolution images were investigated on an InP(110) surface using an ultrahigh-vacuum atomic force microscope (UHV-AFM) in the noncontact mode. The contrast of the atomic-scale AFM image suddenly changed during scanning, which seems to be due to the positional change of the atoms on the tip apex. We observed atomic-scale point defects. These phenomena seem to occur only in monoatomic tip-sample interaction. We also observed an atomic-scale dark area which seems to be due to the convolution of the atomically flat tip and point defects.