Characterization of Single Grain Boundaries in a Bi-Doped ZnO Varistor Using a Focused Ion Beam System

Abstract

This report presents a new method for electrically characterizing single grain boundary junctions of ZnO varistors. For the first time to our knowledge, a focused ion beam system was used to deposit tungsten microelectrodes on two adjacent ZnO grains. Gallium ions irradiated onto the ZnO surface lowered the resistivity of the ZnO, and therefore highly nonlinear current-voltage (I–V) characteristics of single grain boundaries could be obtained after etching the gallium-irradiated ZnO surface with HClO4 solution. Breakdown voltages in the I–V characteristics were observed in the range from 2.6 V to 3.2 V within the same ZnO varistor. The grain boundary with the breakdown voltage of 3.2 V was structurally different from that of 2.6 V. The boundary with the breakdown voltage of 3.2 V contained precipitates and a thin, amorphous Bi-segregated layer about 1 nm thick. On the other hand, the boundary with the voltage of 2.6 V contained no precipitate but rather, a thin, amorphous Bi-segregated layer about 0.5 nm thick. It was suggested that the electrical characteristics of grain boundary junctions within the ZnO varistors are strongly dependent upon the microstructure of the grain boundary.