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The optical properties of zinc oxide crystals and thin films prepared by different methods are investigated. Single crystal zinc oxide samples prepared by melt and hydrothermal growth techniques were obtained. The influence of polarity and growth method on the optical properties were studied and correlated with their electronic properties. Thin films prepared by molecular beam epitaxy (MBE) and sputtering were studied and the influence of growth conditions and post growth treatment on the optical properties of the films was investigated. The photo-luminescence (PL) of bulk zinc oxide was examined at high resolution. Line widths of less than 0.1 meV were observed. More than a dozen different transitions in the near band edge region (NBE 360-380 nm) were noted, several of which displayed a separation of <0.5 meV which goes some way to illustrating the complexity of the system. Attempts were made, with some success, to reconcile the two main competing identification systems of the NBE transitions and explanations for some of the discrepancies are provided. The controversial deep level transitions in the visible part of the spectrum are fit with 3 Gaussians and their identities discussed with relation to the available literature. The presence of copper impurities was detected in annealed films and a model to explain their behaviour under annealing conditions is hypothesised. Films grown by MBE here at the University of Canterbury are shown to have PL line widths of as little as 2.2 meV, the ratio of active oxygen species in the growth chamber during deposition is shown to effect the optical quality of the films. It is shown that annealing can improve the optical quality of the films and various other methods of influencing the films properties are discussed. Reactive, magnetron, direct current sputtering is shown to be the optimal method of growth for maximising both optical and piezo-electric properties. Optimum annealing temperatures were found at 900 and 1100 ℃ with a local minimum at 1000 ℃. X-ray diffraction, atomic force and scanning electron microscopy measurements in addition to optical PL measurements show the influence of annealing on the polycrystalline sputtered ZnO films. Films grown on glass, silicon, sapphire and quartz were shown to display similar behaviour under annealing conditions. It was found that zinc oxide based devices were liable to be chemically unstable at temperatures above 1100 ℃. The piezo electric properties of the films were examined and attempts were made to prepare a zinc oxide film optimised for both optical quality and piezoelectric properties for possible future applications of a hybrid opto-mechanical coupled devices.

The temperature dependence of the band structure of ZnO has been studied on epitaxial films and bulk crystals with the methods of temperature dependent photoluminescence, photoconductivity, reflectivity and transmission spectroscopy. A major question investigated was the intriguing detail that could be resolved in band edge photoconductivity spectra of both high quality ZnO bulk crystals as well as epitaxial films. The connection of these spectral details in photoconductivity to the excitonic band structure of ZnO was made by comparison to the other spectroscopic methods which have a better understood relation to the semiconductor band structure. Photoluminescence spectroscopy enabled us to get a direct and reliable feedback about the energy fine structure of emitting levels in ZnO. Comparison of the emitting levels of epitaxial films with the emitting levels of high quality bulk material allowed the identification of dominating defect structures and impurities in the epitaxial films. The investigation of the effect of annealing on these emission lines finally allowed us to get a better understanding of the effects of annealing on the crystal and electric structure of epitaxially grown heterostructural films and allowed the determination of the optimum temperature range to be used for improved crystal quality. It has been investigated if temperature dependent reflectivity can serve as a simple tool for the examination of the temperature dependence of the band structure of ZnO. The appeal of reflectivity is its enhanced sensitivity only to free excitonic transitions. This proved a valuable simplification compared to the methods of photoluminescence and photoconductivity: Photoluminescence is limited by phonon-broadening of the multitude of emission levels in the band gap region of ZnO, and photoconductivity has a multitude of processes that are potentially contributing to its spectra, making the identification of their relation to the band structure less reliable. Therefore the applicability of reflectivity for the deduction of the temperature dependence of the band structure has been investigated, by measuring the temperature dependence of the energy positions of the characteristic reflectivity features, with particular focus on the effect of phonon broadening and interaction of close lying resonator levels. The investigation of the temperature dependence of photoconductive centres was enabled through the resulting possibility of directly relating the purely excitonic reflectivity spectra to the complex features in photoconductivity. The temperature dependent evolution of the spectra obtained by photoconductivity then revealed that there are at least two types of photoconductive processes that have to be distinguished: features in photoconductivity that are directly related to the band structure proved to be distinguishable from slow defect related processes in terms of their response speed. For the samples of bulk ZnO as well as epitaxial films, the peaks in photoconductivity only had a meaningful position in regard to the band structure for the cases of spectra that are dominated by fast processes. The spectra dominated by slow processes showed a meaningful temperature dependence of respective dips in the spectra. The strong response of fast photoconductive levels in bulk ZnO allowed us to directly observe the A- and B-free excitons by photoconductivity. Additional fine structure could be observed that is likely to be related to the narrow photo emission lines of neutral as well as ionized donor bound excitons and the upper polariton branch of the A-free exciton. These findings agree with the temperature dependence of related Anti-Stokes phonon replica levels that allow a first estimate of the activation energies of the zero-phonon lines. The energy and temperature dependent lateral transport properties of ZnO are expected to be of importance in ZnO device technology

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This thesis present photoluminescence spectroscopy and X-ray diffraction analyses of four different cadmium zinc telluride samples with different quality and features and similar zinc molar concentration of 10%. Photoluminescence spectroscopy of the samples let us obtain several physical parameters of the samples which are indicators of quality, composition, structure, and impurity levels of the samples. The band gap energy of the samples obtained from the photoluminescence spectra at low temperatures helped us to estimate zinc molar concentration of the samples. Temperature dependence of band gap energy in these samples has been analyzed and exciton-LO phonon interactions in the samples has been analyzed. From temperature dependence of full width at half maximum of the photoluminescence peak several parameters including concentration of impurity centers and inhomogeneity of the samples are determined and compared in order to check the quality of the samples. Thermal quenching of the photoluminescence peak has been analyzed and the processes which are associated with each parameter are determined and discussed. X-ray diffraction analyses of the sample for the location and width of the peaks have been analyzed and several characteristics of the samples including quality, lattice constant and zinc molar concentration of the samples are determined and compared. The parameters that are obtained from these analyses are compared with the ones from the photoluminescence spectra and showed a good agreement between the results of these two non-destructive characterization techniques. / Graduate / 0605 / 0544 / 0794

This thesis present photoluminescence spectroscopy and X-ray diffraction analyses of four different cadmium zinc telluride samples with different quality and features and similar zinc molar concentration of 10%. Photoluminescence spectroscopy of the samples let us obtain several physical parameters of the samples which are indicators of quality, composition, structure, and impurity levels of the samples. The band gap energy of the samples obtained from the photoluminescence spectra at low temperatures helped us to estimate zinc molar concentration of the samples. Temperature dependence of band gap energy in these samples has been analyzed and exciton-LO phonon interactions in the samples has been analyzed. From temperature dependence of full width at half maximum of the photoluminescence peak several parameters including concentration of impurity centers and inhomogeneity of the samples are determined and compared in order to check the quality of the samples. Thermal quenching of the photoluminescence peak has been analyzed and the processes which are associated with each parameter are determined and discussed. X-ray diffraction analyses of the sample for the location and width of the peaks have been analyzed and several characteristics of the samples including quality, lattice constant and zinc molar concentration of the samples are determined and compared. The parameters that are obtained from these analyses are compared with the ones from the photoluminescence spectra and showed a good agreement between the results of these two non-destructive characterization techniques. / Graduate / 0605 / 0544 / 0794

This thesis consists of a study of several thin film semiconductor structures of practical technological use either presently or in the near future. The first system studied is an ultra thin film single crystal gallium arsenide layer. The absorption spectra of these layers are measured and transitions at both the F- point and L-point of the Brillouin Zone are observed, the latter are not normally measurable in thicker layers. The observed shift in the F-point absorption edge is attributed to contributions from the Franz-Keldysh Effect and the Moss-Burstein Effect. The temperature dependence of the L-point energy gap is measured and compared with previous data. The next system investigated is an n-type porous silicon layer coated with p-type polyaniline. Both photoluminescence and electroluminescence spectra and the electrical characteristics have been measured for this system. The interface between the two layers is found to be a rectifying junction consistent with a potential barrier formed at the interface. In forward bias, it is possible to generate electroluminescence in the visible and near infra red regions. The final structure studied is a thin film cadmium sulphide-cadmium telluride solar cell structure. The cells are found to have a low efficiency of around 1% as grown, but a process of treatment with cadmium chloride and annealing in air improves this by a factor of approximately ten. Photoluminescence measurements on the back surface of the cadmium telluride revealed three major emission bands at 1.59 eV, 1.55 eV and 1.45 eV. By varying temperature and incident laser power, attempts at assigning the bands to specific impurity centres in the cadmium telluride is made Using a novel bevelling etch technique to prepare samples, depth dependent measurement of the photoluminescence is possible. This reveals that the major changes associated with the improvement in efficiencies occurs at the interface between the CdS and the CdTe.

The optical properties of zinc oxide crystals and thin films prepared by different methods are investigated. Single crystal zinc oxide samples prepared by melt and hydrothermal growth techniques were obtained. The influence of polarity and growth method on the optical properties were studied and correlated with their electronic properties. Thin films prepared by molecular beam epitaxy (MBE) and sputtering were studied and the influence of growth conditions and post growth treatment on the optical properties of the films was investigated. The photo-luminescence (PL) of bulk zinc oxide was examined at high resolution. Line widths of less than 0.1 meV were observed. More than a dozen different transitions in the near band edge region (NBE 360-380 nm) were noted, several of which displayed a separation of <0.5 meV which goes some way to illustrating the complexity of the system. Attempts were made, with some success, to reconcile the two main competing identification systems of the NBE transitions and explanations for some of the discrepancies are provided. The controversial deep level transitions in the visible part of the spectrum are fit with 3 Gaussians and their identities discussed with relation to the available literature. The presence of copper impurities was detected in annealed films and a model to explain their behaviour under annealing conditions is hypothesised. Films grown by MBE here at the University of Canterbury are shown to have PL line widths of as little as 2.2 meV, the ratio of active oxygen species in the growth chamber during deposition is shown to effect the optical quality of the films. It is shown that annealing can improve the optical quality of the films and various other methods of influencing the films properties are discussed. Reactive, magnetron, direct current sputtering is shown to be the optimal method of growth for maximising both optical and piezo-electric properties. Optimum annealing temperatures were found at 900 and 1100 ℃ with a local minimum at 1000 ℃. X-ray diffraction, atomic force and scanning electron microscopy measurements in addition to optical PL measurements show the influence of annealing on the polycrystalline sputtered ZnO films. Films grown on glass, silicon, sapphire and quartz were shown to display similar behaviour under annealing conditions. It was found that zinc oxide based devices were liable to be chemically unstable at temperatures above 1100 ℃. The piezo electric properties of the films were examined and attempts were made to prepare a zinc oxide film optimised for both optical quality and piezoelectric properties for possible future applications of a hybrid opto-mechanical coupled devices.

Thesis (Ph. D.)--University of Missouri-Columbia, 2007. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on November 21, 2007) Vita. Includes bibliographical references.