Synthesis and Characteriazation of ZnO Films by Magnetron Sputtering and Direct Oxidation
|School||Harbin Institute of Technology|
|Course||Materials Science and Engineering|
|Keywords||ZnO film magnetron sputtering direct oxidation crystallinity photoluminescence morphology|
Zinc oxide is an II–VI compound semiconductor with wurtzite structure and direct band gap. The band gap of ZnO is 3.37eV and the exciton binding energy is 60 meV. ZnO films have many potential applications, such as light emitting diodes, ultraviolet light detectors, piezoelectric parts, surface and bulk acoustic wave devices (SAW), transparent conducting films/electrodes,solar cells, gas sensors. In recent years,the wide band gap compound semiconductors have attracted a great deal of attention because of their commercial interests. Much work has been devoted to the research and development of ZnO films both from scientists and engineers.Two methods were adopted to grow the ZnO thin films in this thesis. One is the reactive radio frequency magnetron sputtering deposition, where the ZnO films were prepared by sputtering the Zn target in O2 atmosphere. Processing variables investigated include substrate temperature, ratio of gas-flow rates, sputtering power, and working pressure. The other one is the direct oxidation, where the ZnO films were prepared by directly heating the sputtering deposited Zn films in air. The process variables investigated include the oxidation temperature and oxidation time. The growth processes of the ZnO films were established based on the systematic investigation of the process parameters and the structural features and properties were characterized.Structure of the as-grown thin films was characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM). The light emission properties were measured by photoluminescence (PL).It is indicated that the growth of the ZnO films were strongly dependent on the substrate temperature during sputtering. With the increasing substrate temperature, the crystallinity was improved accompanying the increase of grain size. The peak intensity related to the defects decreases with the increase of the growth temperature due to the decrease in defect density. The influence of flow rate ratio, sputtering power, and working pressure is not as strong as that of growth temperature. A tentative explanation on the luminescence mechanism for the present samples was proposed according to the experimental results and the open literature.Comparing with the films prepared by reactive sputtering the films prepared by direct oxidation show better quality. The surface morphology of zinc films is maintained in the sample oxidized at lower temperature. The grain size begins to grow up at higher oxidation temperature. All the samples are polycrystalline without obvious orientation. Strong photoluminescence peaks originating from near band gap transition were observed for the samples prepared by direct oxidation. The near band edge emission becomes stronger with the increasing oxidation time and temperature with the sample oxidized at lower temperature. The defect emission shows the inverse rule. In the samples oxidized in two stages at different temperature, the nanowires of about 100 nm in diameter and 1μm in length grow.