A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfillment of the academic requirements for the degree of Master of Science. Johannesburg, March 2016. / Zinc oxide (ZnO) continues to receive widespread attention due to its excellent optical and electronic properties; it demonstrates the combined characteristics of high transmittance and electrical conductivity. Despite the tremendous drive for its application in optoelectronic devices, the full nature of the point defects and defect complexes have not been characterised comprehensively. In this work, luminescence characteristics of the intrinsic defects in ZnO thin films and the rare-earth ions Eu3+ and Tb3+ in ZnO powders are investigated under blue-laser excitation.
The thin films used in this study were deposited using the radio-frequency magnetron sputtering method over a 2 hour duration under varied power and substrate bias conditions. The powders were synthesized by the chemical bath deposition method with dopant concentrations of 1.0 mol%. Grazing incidence X-ray diffraction (XRD) was used to determine the lattice properties of the samples. Photoluminescence studies were primarily conducted at room temperature (300 K) with the 457.9 nm, 476.5 nm and 488.0 nm laser lines as excitation sources.
For the ZnO thin films, XRD patterns of a hexagonal wurtzite structure with a c/a ratio of about 1.60 and a u-parameter of 0.38 were obtained. Photoluminescence measurements show a broad emission band in the 500.0-900.0 nm range, centred at 656.0 nm. Annealed films yielded relatively more intense photoluminescence spectra than the as-prepared films. The intrinsic point defects and defect level transitions responsible for the broad emission are discussed.
For the ZnO powders, the XRD patterns of the annealed samples of pristine ZnO, ZnO:Eu3+ and ZnO:Tb3+ are similarly consistent with a hexagonal wurtzite ZnO phase. Energy dispersive spectroscopy (EDS) confirmed the presence of the Eu3+ and Tb3+ dopants in the respective ZnO host while scanning electron microscopy (SEM) measurements showed the morphology of the sample powders. Photoluminescence spectra of pelletized samples, obtained in the 460.0-900.0 nm range, exhibit relatively intense Eu3+ and Tb3+ emission bands superimposed on a broad emission background. The RE3+ emission bands are
attributed to the 5D0 → 7FJ (J = 0, 1, 2, 3, 4) and the 5D4 → 7FJ (J = 0, 1, 2, 3, 4, 5, 6) electronic transitions of Eu3+ and Tb3+, respectively, while the background emission is attributed to intrinsic defects. Crystal-field energy levels for the Eu3+ ion and the Tb3+ ion occupying a C3v symmetry site were deduced from fitting Gaussian curves to the RE3+ emission bands. Possible channels for transfer of energy from the intrinsic defects to Eu3+ and Tb3+ are discussed. / LG2017
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/21750 |
| Date | January 2016 |
| Creators | Bhebhe, Nkosiphile Andile |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | Online resource (104 leaves), application/pdf |
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