Crystallographic control of the barrier structure in zinc oxide varistors

Scanning electron microscope (SEM) based remote electron beam induced current (REBIC) microscopy has been used to investigate the electrical characteristics of individual grain boundaries in a zinc oxide based varistor. Although some grain boundaries showed the expected ‘bright and dark’ contrast consistent with symmetrical, opposed, electric fields on either side of a charged grain boundary, the majority of interfaces are electrically asymmetric showing only either bright or dark contrast. In these cases, the application of an external voltage bias to the grain boundary of several ten’s of millivolts is necessary to restore a symmetrical barrier structure. The orientations of grains on either side of each grain boundary were determined using electron backscattered pattern (EBSP) analysis and the grain boundary plane orientation was calculated by observing the lateral shift of the grain boundary EBIC contrast peak with SEM beam voltage and, hence, penetration depth. It was found that the electrical asymmetry is governed by the orientations of the grain boundary planes on either side of the interface, demonstrating some crystallographic control of the barrier structure.