Electron transport processes in individual metal oxide nanowires

• Main Authors: Chiu, Shao-Pin, 邱劭斌
• Other Authors: Lin, Juhn-Jong
• Format: Others
• Language: en_US
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/04892551881800509188

博士 === 國立交通大學 === 物理研究所 === 98 === The intrinsic electrical transport properties of individual nanowires (NWs), including ITO, ZnO, and (In,Pb)-doped ZnO NWs, are studied in this thesis. These single-crystalline NWs were synthesized by either the thermal evaporation method or the laser-assisted chemical vapor deposition (CVD) method. Four-probe Ti/Au or Cr/Au electrodes were fabricated by the electron-beam lithography technique. The resistances between 300 and 0.25 K and the magnetoresistances (MRs) between 70 and 0.25 K of these NWs have been systematically studied. The temperature dependent resistivities, ??T), of four ITO NWs with diameters of 110 to 220 nm and lengths of a few ?慆 long have been measured. The results indicate that the as-grown ITO NWs are metallic, but disordered. The overall temperature behavior between 300 and 1.5 K can be described by the Bloch–Grüneisen law plus a low-temperature correction due to the scattering of electrons off dynamic point defects. This observation suggests the existence of numerous dynamic point defects in as-grown ITO NWs. For the nominally undoped ZnO NWs with diameters of 90 to 200 nm, the temperature behavior of ??T) between 300 and 5 K reveals that the electrical-transport mechanisms are due to a combination of the thermal activation conduction and the nearest-neighbor hopping conduction processes. Three distinct activation and hopping contributions with discrete characteristic activation energies are observed. Above about 100 K, the charge transport mechanism is dominated by the thermal activation of electrons from the Fermi level, ?? to the conduction band. Between approximately 20 and 100 K, the charge transport mechanism is due to the activation of electrons from ? to the upper impurity (D−) band. Between approximately 5 and 20 K, the charge transport mechanism arises from the nearest-neighbor hopping conduction within the lower impurity (D) band. Such unique electrical conduction behaviors can be explained in terms of the intricate material properties (in particular, the presence of moderately high concentrations of n-type defects accompanied with a slight self-compensation) in natively doped ZnO NWs. In one heavily doped NW, a surface-related conduction process manifesting the two-dimensional attributes of quantum-interference transport phenomena is observed. The carrier concentrations in our NWs have been estimated, and they were found to lie close to the critical concentration for the Mott metal–insulator transition. The indium- and lead-doped ZnO NWs with diameters of 70 to 90 nm showed behavior of degenerate Fermi gas of their resistivities, ??T). We have measured the MRs of several doped ZnO NWs between 0.25 and 70 K in magnetic fields with directions both perpendicular and parallel to the wire axes. Our quantitative analysis showed that we have to utilize the weak-localization (WL) effects of different dimensionalities to explain the MRs in different ranges of temperature. Otherwise, the MRs can not be satisfactorily described. A characteristic length, named the effective wire width, a, extracted from one-dimensional (1D) WL effect has been introduced. From the perpendicular and parallel MRs, another characteristic length, named the effective film thicknesses, t, was extracted under the framework of the two-dimensional (2D) WL effect. Hence, a core-shell-like structure inside individual nanowires is suggested. Within this model, as the electron phase-coherent length, L?? decreases with increasing temperature, a 1D-to-2D dimensional crossover of the WL effect occurs around the characteristic temperature where L??n~ a, and also a 2D-to-3D dimensional crossover occurs around another characteristic temperature where L??n~ t. The exponent of temperature, p, of the electron dephasing rate, ???1, has been determined. The result suggests that the dephasing mechanisms could be due to the electron-electron (e-e) scattering with large energy transfer or the electron-phonon (e-ph) scattering with reduced phonon dimensionality. In addition, the core-shell-like structure has been verified from the temperature behaviors of low-temperature resistivities in a moderately high magnetic field, which demonstrated the dominating electron-electron interaction (EEI) effect. A dimensional crossover of EEI was also observed under the condition that the thermal diffusion length, LT, became close to the shell thickness t. In a lead-doped ZnO NW, the nonlinearity of the I-V curves around zero-bias is attributed to the 2D property of relatively small shell thickness and the electron motion across the core-shell interface.