Investigation On The Synthesis, Optical And Electrical Transport Properties Of ZnO Nanowires

ZnO, as a wide direct band gap semiconductor (Eg=3.37eV) with a large exciton binding energy (60meV), has attracted considerable attention in recent years due to its excellent optoelectronic properties. Up to now, abundant ZnO nanostructures have been fabricated, such as nanowires, nanotubes, nanobelts, nanotetrapods, and so on. Among them, the one dimensional ZnO nanostructures, especially the well-aligned ZnO nanowire arrays, are important because they possess high surface to volume ratio and provide a direct path for charge transport, which are potential in the application of various kinds of high-efficiency devices like light-emitting diodes, dye-sensitized solar cells, field emission devices, etc. However, the exact growth mechanism of ZnO nanowires grown by thermal evaporation method is still under debate by far. Besides, the influence of intrinsic defects on the optical and electrical transport properties of ZnO nanostructures and the mechanism behind it are still not clear yet.To address the above issues, in this thesis we fabricated ZnO nanowire arrays by thermal evaporation and hydrothermal methods, and made a detailed investigation on the growth mechanism, PL, EL, as well as electrical transport properties of ZnO nanowires. We revealed the influence of intrinsic defects on the optical and electrical transport properties of ZnO nanostructures. The significant results achieved in this work are listed as follows:(1) ZnO nanowire arrays are fabricated on bare glass substrate via a noncatalytic thermal evaporation method. The PL measurements reveal that the emission peaks located at405and616nm are related to the Zn vacancy defect. Detailed experiments show that the oxygen flux plays a very important role in the morphology evolution of the as-grown products, which change from nanoflakes to nanowires and finally to nanonails with the increase of oxygen flow rate. The competition between the axial growth and the radial growth results in the morphology evolution. Finally, a two-stage vapor-solid (VS) growth model is proposed to interpret the growth behavior of the ZnO nanowires.(2) The effect of metal coating on the PL properties of ZnO nanowire arrays grown by thermal evaporation has been investigated in detail. The Zn coating induces remarkable enhancement of the ultraviolet and green emissions of the nanowires, while the deposition of Ag leads to a notable decrement of them. A model considering the type of contacts formed between metals and ZnO is proposed to interpret the change of the PL spectra. Also, this model is strongly supported by the PL variation of the nanowires after coating with other kinds of metals.(3) The room-temperature PL spectra of hydrothermal grown ZnO film and nanowires coated with Al are investigated, which exhibit much less UV emission enhancement ratio as against that of nanowires fabricated by thermal evaporation method. A model is suggested to interpret the experimental results considering the influence of the defect on the contact property between metal and ZnO, which is further evidenced by the weak PL enhancement ratio of thermal evaporation grown ZnO nanowires with H2O2treatment.(4) Electrically pumped random lasing (RL) has been realized in ZnO-based metal-insulator-semiconductor (MIS) devices. It is demonstrated that RL originates from the confining and recurrent scattering of light in the random cavities within the porous insulating layer, which are formed due to the glow discharge. The glow discharge also induces the observed negative differential resistance effect following the normal I-V characteristics.(5) A simple microelectrode preparation method is utilized to make single ZnO nanowire-base field-effect transistors (FET), and their electrical transport properties are measured by the double microprobes equipped in a SEM. We have investigated the influence of annealing atmospheres on the electrical properties of ZnO nanowires, such as carrier concentration, carrier mobility, and resistivity, and discussed the origin of carriers in the intrinsic ZnO.Based on the above systematic research, we have achieved a deeper understanding of the growth mechanism of ZnO nanowires, effect of metal-coating on their PL properties, and their EL mechanism. Besides, primary work about the influence of intrinsic defects on the optical and electrical transport properties of ZnO nanowires has been accomplished. In the future, a better understanding of the influence of defects on the physical properties of ZnO nanostructures is required in order to facilitate their practical applications.