| Summary: | 碩士 === 國立臺南大學 === 材料科學系碩士班 === 98 === There are two main areas of concerns in this thesis. One is the study of optical characteristics of bulk ZnO with different thicknesses of Al2O3 surface passivation layers. The other is the study of optical characteristics of aluminum-doped ZnO thin film.
Micro-Raman spectra, photoreflectance (PR) spectra, and photoluminescence (PL) spectra are utilized in analyzing the capping layer effect on bulk ZnO substrate. The results of micro-Raman spectra show that the growth of Al2O3 surface passivation layers do not affect the crystal quality of bulk ZnO. The surface/interface sensitive PR spectra indicate that the energy band bending takes place in the interface of Al2O3/ ZnO. With the increase of thickness of Al2O3, the interfacial energy band bending becomes large. The interfacial energy band bending is from the formation of interfacial density of states. Interfacial density of states can be related to the existence of defects, impurities, etc. in the interface region. The interfacial defects and impurities act as the trapping centers of carriers which lead to the decrease of exciton recombination energies observed in PR spectra. PL spectra show that the light emissions of the samples of ZnO bulk with different capping thicknesses of Al2O3 are all from ZnO bulk. The relatively weaker emission intensity of PL is owing to the stronger light scattering in the thicker Al2O3 layer.
The second area of concern in this study is optical characteristics of aluminum-doped ZnO thin film. Micro-Raman spectra, photoreflectance (PR) spectra, and photoluminescence (PL) spectra are used in optical analyses of aluminum doping effects. The increase of E2(high) shifts in micro-Raman spectra can be related to the larger strain induced when the aluminum doping concentration is increased. As the sample is doped with aluminum, the formations of aluminum segregations and ZnO grains are in the interfaces of ZnO and Al2O3 after post thermal annealing at temperature 900oC. That is, the existences of interfacial band tail states extend into the energy bandgap and give the light emission of PL in the energy range from 2.8 to 3.0 eV. With the increasing of the doping concentrations of aluminum, the overall bandgap narrowing leads to the red shift of exciton transition energy in OR spectra. Besides, the interfacial carrier confinement effect increases with the blue shift of PL peak emission energy when aluminum doping concentration is increased. The more the aluminum doping concentration, the more the formation of interfacial strains, defects, and impurities exist. Therefore, the integrated PL intensity of defect band is larger than exciton transition with the increase of doping concentration. The domination mechanisms of PL emissions in this study can also show that the bound excitons play the main emission role with the doping concentration of 2% and 4% while the domination mechanisms of PL emissions are free excitons with the doping concentration of 6% and 8%.
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