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.