Quantum-interference transport through surface layers of indium-doped ZnO nanowires

We have fabricated indium-doped ZnO (IZO) nanowires (NWs) and carried out four-probe electrical-transport measurements on two individual NWs with geometric diameters of ≈70 and ≈90 nm in a wide temperature T interval of 1-70 K. The NWs reveal overall charge conduction behavior characteristic of disordered metals. In addition to the T dependence of resistance R, we have measured the magnetoresistance (MR) in magnetic fields applied either perpendicular or parallel to the NW axis. Our R(T) and MR data in different T intervals are consistent with the theoretical predictions of the one- (1D), two- (2D) or three-dimensional (3D) weak-localization (WL) and the electron-electron interaction (EEI) effects. In particular, a few dimensionality crossovers in the two effects are observed. These crossover phenomena are consistent with the model of a 'core-shell-like structure' in individual IZO NWs, where an outer shell of thickness t (≃15-17 nm) is responsible for the quantum-interference transport. In the WL effect, as the electron dephasing length L_ρ gradually decreases with increasing T from the lowest measurement temperatures, a 1D-to-2D dimensionality crossover takes place around a characteristic temperature where L_ρ approximately equals d, an effective NW diameter which is slightly smaller than the geometric diameter. As T further increases, a 2D-to-3D dimensionality crossover occurs around another characteristic temperature where L_ρ approximately equals t (<d). In the EEI effect, a 2D-to-3D dimensionality crossover takes place when the thermal diffusion length L_T progressively decreases with increasing T and approaches t. However, a crossover to the 1D EEI effect is not seen because L_T < d even at T = 1 K in our IZO NWs. Furthermore, we explain the various inelastic electron scattering processes which govern L_ρ. This work demonstrates the complex and rich nature of the charge conduction properties of group-III metal-doped ZnO NWs. This work also strongly indicates that the surface-related conduction processes are essential to doped semiconductor nanostructures.