C-AFM and KPFM approach to investigate the electrical properties of single grain boundaries in ZnO varistor devices

Abstract

Here, two Praseodymium doped ZnO multilayer varistor (MLV) samples with Ag/Pd electrodes were macroscopically and microscopically investigated. Since the varistor effect is a grain boundary effect, the electrical properties of individual ZnO grain boundaries were especial subject of this study. Both samples were cross sectioned for the AFM based investigation. Besides conductive atomic force microscopy (C-AFM), Kelvin Probe Force Microscopy (KPFM) and KPFM with a voltage applied between the varistor electrodes, so called scanning surface potential microscopy (SSPM), have been employed. Both samples were mapped with electron backscatter diffraction (EBSD) after the AFM based investigations. In C-AFM, a high conductivity of the ZnO grains and a high resistivity of the grain boundaries was proven. With SSPM, individual grain boundaries with an asymmetric electrical behavior were identified. These grain boundaries are causing asymmetric current paths and therefore can cause asymmetric device behavior.