Growth and optical, electrical and mechanical characterization of zinc oxide nanowires

The topic of this dissertation is embedded into the new-born field of inorganic nanowires. The research was focused on zinc oxide (ZnO) nanowires, in particular, which besides posing fundamental questions in physics, promise broad range of applications. ZnO nanowires were grown by chemical-vapour-deposition (CVD) using the vapour-liquid-solid (VLS) mechanism with gold particles as catalysts. Oxygen was introduced in the argon carrier gas and zinc was carboreduced from the ZnO/C mixture. We produced micrometer long ZnO nanowires and their diameter was controlled by the size of the gold catalyst particles. The diameter range of our ZnO nanowires is between 30 to 100nm. After growth, the alloyed particles were found at the top of the nanowires. ZnO nanowires are composed of single crystals of ZnO in the wurtzite phase. Growth direction is defined by their epitaxial relation with the substrate. From photoluminescence (PL) and cathodoluminescence (CL) characterizations, a deep impurity energy level at 2.3eV was found in a band gap of 3.3 eV, which could be related to oxygen off-stoichiometry. PL measurements showed that this energy level does not depend on the oxygen concentration used during the growth. Scanning-Tunnelling-Microscopy-based CL (STM-CL) measurements were made on single ZnO nanowires and showed that this energy level is likely to come from curved nanowires. Electrical measurements were performed on single nanowires, by using dielectrophoresis to place individual ZnO nanowires at a predefined location. E-beam lithography was used to deposit four metallic contacts on top of individual micrometer long ZnO nanowires. In order to avoid oxidation and the Shockley effect, we used chromium nitride (CrN) as electrical contacts. By measuring the temperature dependence on the resistance we extracted the activation energy of 2eV for charge transport, possibly arising from the same impurity level, as measured in PL. In the field-effect-transistor (FET) configuration, by using PMMA as dielectric layer and Cr as gate electrode, we extracted the charge carrier mobility of an individual ZnO nanowire as being 9.5 cm2/(Vs). In order to discover the origin of the energy level in the band gap, we measured the relaxation time of the current, after UV-illumination was switched off, and we found out that the surface states have significant effects. The elastic properties of individual ZnO nanowires were studied as a function of diameter and point defect/dislocation concentration. Using dielectrophoresis, nanowires were placed over a hole of a microfabricated Si3N4 membrane and, from bending measurements using an atomic force microscope (AFM) the Young's modulus was extracted. The diameter dependence of the Young's modulus showed a maximal value for a diameter of 50nm. Annealing under argon atmosphere showed an improvement of the Young's modulus which is explained by the removal of the defects/dislocations left during the growth. For comparison, the elastic properties of ZnS nanotubes produced by atomic layer deposition (ALD) were also measured. A clear dependence of the Young's modulus on the diameter and the wall thickness were found and attributed to the variation of the crystal structure of the nanotubes.


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