Grain size-dependent electrical properties of nanocrystalline ZnO

Undoped and Bi-doped nanocrystalline ZnO was prepared by the inert gas condensation method. Samples of different grain sizes were obtained by annealing treatments at various temperatures. The dc and ac electrical conductivity of undoped and Bi-doped nanocrystalline ZnO was investigated as a function of grain size. In dc measurements, nanocrystalline ZnO of small grain size (<20 nm) exhibited time dependent currents similar to those found in other dielectric solids and which are characteristic for space charge limited conduction. However, unusually long transient times of up to 300 s were observed. Similar samples of larger grain sizes (>40 nm) did not exhibit time dependent conduction but exhibited ohmic behavior. This difference in dc conductivity is discussed in terms of the grain size dependence of the average trap density. A small amount of Bi dopant had no measurable effect on the electrical behavior in the small grain size range but resulted in higher specimen resistance in the large grain size range. ac electrical properties were characterized by impedance spectroscopy. The impedance spectra of undoped and Bi-doped samples of small grain sizes were rather similar and exhibited two semicircles. In contrast, the impedance spectra of samples with larger grain sizes were single, depressed circles with much higher impedances for the Bi-doped sample, in agreement with the dc measurements. A consistent assignment of the contributions due to bulk conductivity, porosity, and grain boundary effects to the overall impedance is presented. (C) 1999 American Institute of Physics. [S0021-8979(99)05711-4].