published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / abstract / Physics / Master / Master of Philosophy

Zinc oxide (ZnO) is a promising material for ultra-violet optoelectronics applications due to its direct band gap and large exciton binding energy. Defect in semiconductor plays an important role in determining the optical and electrical properties. It is thus crucial to understand the defects‟ performance for realizing the device fabrication. Green luminescence (GL) having the peak at 2.4-2.5 eV is a defect related emission band commonly found in the luminescence spectra of many of the ZnO materials. Despite of the effort devoted for several decades, its origin and emission mechanism remain controversial. In this thesis, the origin of the GL emitted from the ZnO films grown by pulsed laser deposition (PLD) is studied using a comprehensive spectroscopic approach, including the Hall effect measurement, photoluminescence (PL), Raman spectroscopy, positron annihilation spectroscopy (PAS), and secondary ion mass spectroscopy (SIMS). ZnO thin films are grown by PLD method with the growth parameters (namely the substrate temperature and oxygen pressure during the growth) systemically varied. Annealing studies in argon atmosphere reveal the correlation between the free electron concentration and the hydrogen concentration in the samples. Two oxygen deficient defect related Raman modes are also identified and they anneal out after annealing at high temperature. We have investigated the introduction the GL systematically grown by different growth parameters, undergone different post-growth annealing treatment, and different methods of growth. Two kinds of GL’s are identified. The first kind of GLs has peak at 2.47 eV without the fine structure, and the other has the peak at 2.45 eV having the fine structure of separation of 0.07 eV. The GL with the fine structure is originated from the surficial region of the ZnO film. The GL without the fine structure is introduced after the annealing 900℃ irrespective of the initial growth conditions. PAS results show a strong correlation between the thermal introductions of a kind of Zn-vacancy and the GL without the fine structure. Moreover, a donor-acceptor-pair (DAP) emission is induced in the low temperature PL spectrum after the same annealing temperature of 900℃. The GL and the DAP emissions are thus associated with the involvement of the VZn. Furthermore by comparing the photon energies of the GL and DAP with the previous first principle calculated results, the GL is ascribed to the conduction band to the (-/2-) acceptor level of VZn, and the DAP involves the (0/-) acceptor level of VZn The presence of the conduction band to the (0/-) level transition is compatible with the results of the photoluminescence excitation (PLE) study. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Normal human skin fibroblast cells were used to study the effect of zinc supplementation of the media on cell growth and the competitive binding activity of low density lipoprotein (LDL). Cells were grown in the media containing Dulbecco's Modified-Eagle Medium (DMEM), 5% (v/v) fetal calf serum (FCS), and various levels of zinc. Cell counts and protein determination revealed that there was no stimulatory effect of zinc on the growth of cells, showing a flat growth curve with up to 6 μg/ml zinc supplementation. However, zinc supplementation of greater than 6 μg/ml to the medium appeared to be toxic to the cell and thereby prevented growth. When zinc was removed from the medium using Epoxy-activated Sepharose 6B coupled with iminodiacetate, zinc concentration in the medium was markedly reduced to 0.045 μg/ml from 0.210 μg/ml. The cell growth study using this zinc depleted medium exhibited a growth curve similar to that obtained from the earlier study, suggesting that 0.045 μg/ml of zinc in the control medium was still sufficient to support normal cell growth. For the LDL binding study, cells were grown in the media with various levels of zinc supplementation for 7 days and the competitive binding activity of LDL was determined. When cells were grown in the zinc removed medium with 1.5 μg/ml zinc supplementation, the maximum amounts of ¹²⁵I-LDL bound and internalized in the cells were observed; however, higher levels of zinc supplementation to the growth medium caused decreased ¹²⁵I-LDL binding to the cell receptors. These results suggest that zinc may be involved in the binding of LDL to the receptors. / Graduation date: 1983

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