Characterization and properties of transition metal oxide films

This project focused on the investigation of the morphology and structural and magnetic properties of NiO thin films annealed at different temperatures ranging from 100°C to 600°C in air and another transition metal oxide ZnO thin film doped with magnesium and cobalt respectively. In this study, NiO samples with disordered structures were grown by RF magnetron sputtering while MOCVD was employed to fabricate ZnMgO/ZnO and ZnMgO/Sapphire heterostructures with different barrier layer thickness and laser molecular beam epitaxy was applied to grow Co-doped ZnO thin films under different oxygen partial pressure. For the NiO nanostructures, it was observed that a better crystal quality was obtained with an increasing annealing temperature in comparison with the sample without annealing process. The occurrence of recrystallization stage in a high temperature (600 °C) annealing process would result in the aggregation of smaller crystallites to form a bigger grain or cluster on the surface of the films and increase the order state in crystal structure. The strongest magnetization at both 5K and 300K was ascribed to the nanostructured NiO with no annealing treatment and the smallest grain size. In the ZnMgO/ZnO and ZnMgO/Sapphire study, it has been found that ZnMgO thin films grown on the native ZnO single-crystal substrate have a better crystal quality than the films deposited on the sapphire substrate. While there is no manifest relationship between the thickness of ZnMgO layers and the thin films structural properties. Based on the SQUID data for magnetization analysis, the largest magnetization at 5K was detected in the ZnMgO/ZnO with thickness of 72nm. The Co-doped ZnO epitaxial film grown at a high oxygen partial pressure (10-3 torr) shows a better crystal quality than the samples deposited at a lower oxygen partial pressure. Magnetic measurements demonstrate that the films grown at three different oxygen partial pressure show room temperature ferromagnetism which increases with decreasing oxygen partial pressure. It was also observed that the ferromagnetic fraction in the Co-doped ZnO samples prepared under an oxygen partial pressure of 10-3, 10-5 and 10-7 torr is 30%, 46% and 61% respectively, suggesting that dopants clusters are not the main source of room temperature ferromagnetism