Characterization of single grain boundaries in a Bi-doped ZnO varistor using a focused ion beam system

This report presents a new method for electrically characterizing single gr ain boundary junctions of ZnO varistors. For the first time to our knowledg e, a focused ion beam system was used to deposit tungsten microelectrodes o n two adjacent ZnO grains. Gallium ions irradiated onto the ZnO surface low ered the resistivity of the ZnO, and therefore highly nonlinear current-vol tage (I-V) characteristics of single grain boundaries could be obtained aft er etching the gallium-irradiated ZnO surface with HClO4 solution. Breakdow n voltages in the I-V characteristics were observed in the range from 2.6V to 3.2V 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 bounda ry with the breakdown voltage of 3.2 V contained precipitates and a thin, a morphous Bi-segregated layer about 1 nm thick. On the other hand, the bound ary 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 vari stors are strongly dependent upon the microstructure of the grain boundary.