  Zinc oxide (ZnO) is the most important material for the fabrication of modern
varistors (variable resistors). It is known that the highly nonlinear
current-voltage relationship of ZnO varistors is due to effects taking place at
the grain boundaries. For an accurate investigation of the mechanism of this
process, techniques are required that allow a direct observation of local
electric fields in varistor samples. The objective of this study is to show
that an atomic force microscope in a set-up as an electrostatic force
microscope is capable of the in-situ observation and measurement of an applied
electric field in samples of ZnO thin films. The technique of Surface Potential
Imaging was used to investigate laterally applied electric fields in this type
of sample for the first time. The local change of the electric field across the
samples was monitored and quantified. It was observed here that the morphology
of the surface is convoluted into the surface potential images and the
magnitude of this effect was quantified by taking surface potential images
without an applied electric field. For the given measurement conditions, a
height difference of 80 nm in the topography image resulted in a voltage
difference of roughly 66 mV in the surface potential image. A simple model was
provided that attributes this observation to the surroundings of the surface
atom closest to the imaging tip.
