Single-crystalline zinc oxide nanowires with diameters of 90 to 200nm were synthesized by using the thermal evaporation method. Four-probe Ti/Au electrodes were made by using the standard electron-beam lithography technique, and the intrinsic resistivities, ρ(T), of individual nanowires were measured from 300 to 0.25K. The temperature behaviour of ρ(T) between 300 and 5K revealed that the intrinsic electrical-transport mechanisms through individual ZnO nanowires were due to a combination of thermal activation conduction and nearest-neighbour hopping conduction processes. Three distinct activation and hopping contributions, with discrete characteristic activation energies, were observed. Above about 100K, the charge transport mechanism was dominated by the thermal activation of electrons from the Fermi level, μ, into the conduction band. Between about 20 and 100K, the charge transport mechanism was due to activation of electrons from μ to the upper impurity (D-) band. Between about 5 and 20K, the charge transport mechanism arose from nearest-neighbour hopping conduction within the lower impurity (D) band. Such unique behaviours could be explained in terms of the intricate material properties (in particular, the presence of moderately high concentrations of n-type defects accompanied by slight self-compensation) in natively-doped ZnO nanowires. In one heavily-doped nanowire, a surface-related conduction process exhibiting the two-dimensional attributes of quantum-interference transport phenomena was observed. The carrier concentrations in the nanowires were deduced to lie close to the critical concentration for the Mott metal–insulator transition.

Electrical Conduction Mechanisms in Natively Doped ZnO Nanowires. Chiu, S.P., Lin, Y.H., Lin, J.J.: Nanotechnology, 2009, 20[1], 015203