Crystal plane influence of the EBIC contrast in zinc oxide varistors

Scanning electron microscopy (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 ‘bright and dark’ contrast consistent with symmetrical, opposed, electric fields on either side of a charged grain boundary, the majority of interfaces were found to be electrically asymmetric showing only either bright or dark contrast. In these cases, the application of an external voltage bias across the grain boundary of several 10′s of mV was necessary to restore the symmetrical structure. The orientations of grains on either side of grain boundaries showing each of these contrast types were determined using electron backscattered diffraction (EBSD) analysis and the grain boundary plane orientation was established using depth resolved EBIC. It was found that the asymmetry in the electrical structure is governed by the orientations of the grain boundary planes on either side of the interface, demonstrating some crystallographic control of the electrical character of the barrier structure.