Direct observation of the barrier structure of a praseodymium-doped grain boundary in a zinc oxide bicrystal by charge collection microscopy

A zinc oxide bicrystal, in which the grain boundary was doped with praseodymium oxide, was prepared by hot pressing. The grain boundary showed characteristic varistor breakdown. Scanning electron microscope (SEM) observation revealed that the amount of visible praseodymium oxide along the interface was variable. Charge collection microscopy, using the remote electron beam induced current (REBIC) configuration, was carried out and the grain boundary was found to be electrically active in the praseodymium oxide rich regions, showing contrast consistent with the presence of a double Schottky barrier (DSB). In Pr poor regions the DSB contrast was lost. The observations are consistent with the idea of barrier enhancement with doping in varistors.