Study On The Fabrication And Photoluminescence Of Ga-doped ZnO Nanostructures

As an important semiconductor material, ZnO nano-structured materials,especially one-dimensional nanostructures have aroused great interest on study inrecent years. ZnO has distinct physical and chemical properties, which makes ZnOnanostructures is not only suitable for basic research, but also has many potentialapplications, such as the production of optoelectronic devices. Research on synthesisof ZnO nanostructures and device applications has continued to progress, but somebasic properties of ZnO materials is still in doubt, such as the origin problem of thevisible emissions in ZnO nanostructures. It is generally known that the visibleemissions in ZnO nanostructures are related to a variety of defects inside the material,but how to link visible emissions at different wavelengths with a specific defect hasbeen controversial, most notably is the origin problem of the green emission. Dopingis an effective way to regulate the electrical and optical properties of ZnOnanostructures, and the origin mechanism of the visible emissions is expected to beanswered combined with the analysis of the feature of defects distributions in thesesamples. Generally, there are four kinds of doping in ZnO nanostructures: n-typedoping, p-type doping, doping with rare-earth elements and doping with transitionmetal elements. Both rare-earth elements doping and transition elements doping canbe used to regulate the luminescence properties of ZnO nanostructures. Herein, wechose Ga-doping to regulate the luminescence properties of ZnO nanostructures basedon the experimental conditions of our lab. It is found that the surface morphology isoften altered obviously by Ga-doping, and the surface of samples can be damagedwith high doping concentration. A readily diffusion doping method is developed toavoid the change of surface morphology for the highly doped ZnO nanostructures.The main contents and conclusions of this thesis are as following:A series of pure and Ga-doped ZnO nanostructure samples with different dopingconcentration are synthesized via CVD method based on the carbon thermal reduction,in which the doping concentration of samples can be adjusted by simply controllingthe experimental conditions. The morphology, structure, composition andluminescence properties are studied carefully using FESEM, XRD, HRTEM, XPS andPL. The results indicate that the effective doping can be achieved in a large range ofdoping concentration, while the secondary phases only occurs in the samples when the doping concentration reaches much high level.By controlling the oxygen flow conditions in the process of sample growth, thecontrollable fabricaiton for pure ZnO nanorod and nanonail arrays can be achieved.After Ga is introduced into the CVD oven as the doping source, the morphology ofsamples will change regularly as the molar ratio of Ga to ZnO increases while Galocated at the non-effective doping position. However, when the Ga source is movedto an effective doping position, the morphology of samples will change greatly, fromZnO nanorods to cone and tree-like ZnO nanostructures. As a contrast, it is found thatthe relatively high Ga doped ZnO nanorods, with the same morphology and withoutany secondary phases, can be obtained by annealing the as-grown pure ZnO nanorodarrays in the Ga atmosphere.The visible emission (peaked at~500nm) always exists in various undoped ZnOnanostructures. However, the intensity of visible emission of samples (or the intensityratio of visible to near-UV emission) decrease apparently and even tend to disappearwith the Ga-doping concentration. After analyzing the O1s peaks in the XPS spectraof pure and doped samples with three sub-peaks, it is found that the relative intensityratio of the vacancy oxygen to the lattice oxygen also decrease with the Ga-dopingconcentration. Furthermore, the intensity ratio of visible to near-UV peak and therelative intensity of vacancy oxygen peaks have almost the same relationship ofvariation as the Ga-doping concentration increases. This result presents strongevidence that the vacancy oxygen is the origin for the visible emission in ZnOnanostructures, and Ga-doping can reduce the oxygen vacancy defects andconsequently inhibit the visible emission in the sample. Our finding has greatsignificance for more understanding the optical properties of ZnO nanostructures andsupplies a new strategy to regulate the optical properties of ZnO nanostructures bydoping.