Optical and AFM studies of ZnO: excitonic properties, surface morphology and etching effects
Fryar, James (2005) Optical and AFM studies of ZnO: excitonic properties, surface morphology and etching effects. PhD thesis, Dublin City University.
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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.
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