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Raman spectroscopy, x-ray diffractometry, atomic force microscopy, photoluminescence spectroscopy and reflectance spectroscopy have been used to characterize ZnO thin films grown by pulsed laser deposition as a function of the post-growth annealing temperature. Raman results show enhancement and broadening of certain Raman features which correlate with changes in the widths of the x-ray diffraction peaks for samples with varying grain size in the 50-100 nm range. These data suggest that electric fields, arising from charge trapping at grain boundaries, in conjunction with localised and surface phonon modes, are the cause of the intensity enhancement and asymmetry of the Raman features. Band-edge photoluminescence and reflectance spectra also altered considerably with increases in grain size, showing clearly observable excitonic structure in the reflectance spectra. An analysis using a deformation potential Hamiltonian demonstrates that the experimental exciton energies are not explicable solely in terms of sample strain and give additional evidence for electric fields in the samples due to charge trapping at grain boundaries. This is supported by theoretical estimates of the exciton energy perturbation due to electric fields and also by the behaviour of the green band in the samples. Detailed studies show that reflectance spectra in nanocrystalline ZnO differ substantially from bulk material. Interaction of excitons, damped by strong electric field effects, with photons leads to exciton-polaritons with substantial damping, eliminating the normal Fabry-Perot structure seen in thin films. Good qualitative agreement is achieved between the model and data and the conclusions are also in good agreement with the photoluminescence and Raman data. Finally, high intensity optical pumping data of these samples again shows a dependence on grain size. All samples show evidence of high excitation effects and the sample with the largest grain size displays random lasing at room temperature. All our results indicate the very strong influence of electric fields due to charge trapping at grain boundaries on the optical properties of nanocrystalline ZnO.

The oxygen- and zinc-terminated polar surfaces of ZnO bulk crystals are examined using both optical spectroscopy and surface probe microscopy techniques. An initial study of as-received ZnO wafers purchased from three different companies reveals that there are large optical differences between each grower’s material in terms of bound-exciton and green band luminescence, and free-exciton reflectance resonances. Topographic data obtained using atomic force microscopy (AFM) and x-ray diffraction results suggest that these optical variations are partly due to the different polishing methods applied to the surfaces. The effects of etchant solutions on each polar face, both in terms of surface morphology and optical characteristics are examined. The O-terminated surface reacts strongly to hydrogen ions in acidic solutions and one observes high vertical-to-lateral etch ratios on such surfaces and dramatic increases in surface roughness, with corresponding decreases in bound-exciton luminescence and free-exciton reflectance resonances. The morphology of Zn-terminated surfaces is dominated by lateral etching which has a tendency to reduce roughness and improve luminescence and reflectance characteristics. Differences in the efficacy of cleaning for each polar face is explored through the analysis of AFM force-displacement plots and lateral force microscopy. The data show that the O-terminated face is strongly hydrophilic (as also seen in the etching behaviour) and that fluid layers of significant thickness can form on this face in ambient conditions. In comparison, the fluid layers present on the hydrophobic Zn-terminated surfaces are approximately half as thick or less. Finally the reflectance spectra of polycrystalline thin film and bulk ZnO wafers are modelled using a semi-classical model of the exciton-polariton interaction. A twoband dielectric response function is found to adequately describe the excitonic resonances, and the effects of Fabry-Perot oscillations in the thin film material are examined.

The implantation of radioactive isotopes into semiconductor materials is a powerful technique that enables researchers to correctly assign chemical identities to defects observed by spectroscopic methods. Since each radioactive isotope has a characteristic decay rate, the intensity of spectroscopic features related to those atoms should also change with a corresponding rate, allowing an accurate identification of defect centres. This report shall discuss a number of Hg- and Au-related defects in silicon studied by Photoluminescence Spectroscopy. Radioactive 197Hg decays to stable Au, enabling defects involving both elements to be studied. Data presented here, based on implantations of 197Hg and stable 200Hg, confirms a previously observed Hg-related feature. It contains three Zero Phonon Lines and is due, we believe, to a three-level ground state and double excited states for the Hg atoms. The ‘FeB’ defect centre, previously shown to involve Au, is also studied. Whilst confirmation of the involvement of Au could not be established, an analysis has shown that thermal dissociation of this defect may negatively influence the results of spectroscopic measurements. A defect probably involving Au and Li is also studied, illustrating the techniques necessary to produce it. Again, this defect appears to dissociate upon room temperature storage at a rate similar to the ‘FeB’ defect, suggesting that Au is involved in this process for each. Finally, the results of an examination of Si:Ge Multi-Quantum Well (MQW) structures are presented. These indicate that much of the luminescence observed is related to either bulk- or surface-related dislocations in the Si rather than the quantum structures themselves.