Anomalous space-charge limited currents in nanocrystalline ZnO

ZnO nanocrystals of various grain sizes were consolidated by pressureless sintering from ultrafine powders prepared by the gas phase condensation technique. Their microstructures were characterized by approximately equiaxed ZnO crystallites and a considerable amount of randomly distributed pores. Currents in the samples were measured at room temperature as a function of time under various applied DC voltages. When grain sizes were smaller than its Debye length so that the bulk grains were fully depleted, ZnO exhibited an anomalous space-charge limited (SCL) conduction phenomena which do not occur in conventional ZnO ceramics. The SCL conduction had an extremely longer transient time than in other conventional solids.