Nonlinear current-voltage characteristics with negative resistance observed at ZnO-ZnO single-contacts

A couple of zinc oxide (ZnO) single crystals with single boundaries (ZnO-ZnO single-contacts) are fabricated by the traditional vapor reaction method and their electrical properties are characterized. The ZnO-ZnO single-contacts obtained show nonlinear current-voltage (I-V) characteristics without varistor-forming constituents. Some of the ZnO-ZnO single-contacts show pronounced nonlinear I-V characteristics with negative resistance. The I-V characteristics of the ZnO-ZnO single-contacts are apparently similar to those of ZnO varistors; however, there are marked differences in the electric structure of the boundaries between the ZnO-ZnO single-contacts and ZnO varistors. The capacitance-voltage (C-V) relations of the ZnO-ZnO single-contacts are quite different from that of ZnO varistors and no evidence for the formation of double Schottky barriers at the boundary region are found. A very slow response to current stress is a feature of ZnO-ZnO single-contacts and it is suggested that any thermal processes including Joule heat would modify the carrier transport efficiency through the boundaries.