Intrinsic Electrical-Transport Properties of Single ZnO Nanowires

Single-crystalline zinc oxide (ZnO) nanowires (NWs) were synthesized by the thermal evaporation method. The intrinsic electrical-transport properties of ZnO NWs were studied by carrying out four-probe measurements on individual NWs. The electrodes were made by the standard electron-beam lithography technique. The current-voltage characteristics and the zero-bias resistivities, \textit{$\rho $}(T), were measured over a wide range of temperatures between 0.25 and 300 K. We found that, in many cases, the temperature behavior of \textit{$\rho $} could be well-described by the thermal-activation model involving three activation energies ($E_{1}$, $E_{2}$ and $E_{3})$. Our values of $E_{1}$ (approximately, several tens meV) extracted from the \textit{$\rho $}(T) around 300 K are close to the ionization energies of the major known shallow donors in ZnO. Our values of $E_{2}$ (approximately, several meV) extracted from the \textit{$\rho $}(T) in the intermediate temperature regime might originate from electron activation from the lower impurity band to the upper Hubbard subband ($D^{-}$ band). Finally, at very low temperatures ($<$ 5 K), the measured \textit{$\rho $}(T) indicated diminishing values of $E_{3}$ (approximately, thousandths meV), suggesting essentially metallic behavior.