Doping and characterisation of ZnO nanowires and crystals
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ZnO is a wide bandgap semiconductor with a direct band gap of 3.37 eV and an exciton binding energy of 60 meV at room temperature. Due to their large band gap, high exciton binding energy and the ease of forming versatile low-dimensional nanostructures, ZnO nanowires have been widely studied for applications in optoelectronic devices. The lack of a reliable method for p-type doping and for controlling the n-type compensation limited to native defects in ZnO has hindered the development of ZnO-based devices. Group V elements, in particular nitrogen that has an ionic radius similar to that of oxygen, is widely believed to be a promising candidate for realising p-type doping in ZnO. In contrast, hydrogen, a common impurity in ZnO, can act as a shallow donor in ZnO. This project primarily aims to investigate the properties and behaviours of these two important impurities plus the native defects in both bulk and nanowire ZnO. In this first part of the project, arrays of ZnO nanowires were fabricated using gold (Au) as catalyst. New insights into controlling nanowire merging phenomena were demonstrated in the growth of ZnO nanowires using monodispersed Au colloidal nanoparticles as catalysts. Both nanowire diameter and wire distribution density were found to be strongly dependent on the density of Au catalytic nanoparticles. Structural analysis and spectral cathodoluminescence imaging of the c-plane nanowire cross-sections revealed that thin isolated nanowires growing from the Au nanoparticles began to merge and coalesce with neighbouring nanowires to form larger nanowires when their separation is inferior to a certain threshold distance. The distribution of nanowire diameters and their green emission were found to be strongly dependent on the density of the Au nanoparticles. The merging phenomenon was attributed to electrostatic interactions between polar nanowire tips during growth and well-described by a cantilever bending model. The grown nanowires were subsequently doped with nitrogen by plasma annealing at 300°C. The chemical states of nitrogen dopants in ZnO nanowires and the optical properties of doped ZnO were studied by complementary chemical and optical techniques. It is found that nitrogen exists in multiple states: NO, NZn and loosely bound N2 molecule. The work establishes a direct link between a donor-acceptor pair (DAP) emission at 3.232 eV and the concentration of loosely bound N2. These results indicate that N2 at Zn site is a potential candidate for producing a shallow acceptor state in N-doped ZnO. Results are also reported on the electronic properties and kinetic behaviour of hydrogen dopants in bulk ZnO crystals. Hydrogen was found to be at the bond-centred site by forming O-H bonds after hydrogen plasma annealing. Hydrogen shallow donor and hydrogen bound to basal plane stacking faults (BSFs) are observed in the low-temperature high-resolution CL measurements of H-doped ZnO single crystals. Under the electron beam irradiation, hydrogen donors bound to BSFs dissociate from these defect sites and migrate to the periphery of the electron interaction volume. The hydrogen donors are confirmed to be in the HBC, // configuration by means of XANES measurements.
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