Resolution of Kelvin probe force microscopy in ultrahigh vacuum: comparison of experiment and simulation

An ultrahigh vacuum Kelvin probe force microscope (UHV-KPFM) is used to image the work function change of semiconductor surfaces. We measured the potential drop across the pn-junction on a GaP (1 1 0) surface and the potential variation at steps on the GaAs (1 1 0) surface and determined the resolution for different tip–sample distances. A simple parallel plate capacitor model is used to simulate the effect of varying tip–sample distance on the detection of the electrostatic forces between tip and sample. The model is applied to a potential step and a potential line. The results for different tip–sample distances are compared to those of the experiment; despite small deviations this simple model describes the experimental situation reasonably well. From the simulations it is concluded that for operation of KPFM in air a serious limitation in resolution has to be accepted.