Characterization and properties of cobalt and europium co-doped ZnO nanostructures fabricated by CVD and K，Na , Sm doped ZnO nanostructures fabricated by hydrothermal method

摘要

Spin-based devices are the next generation devices to replace current semiconductor devices. For realizing the functions of spin-based devices, new materials have to be developed. Diluted magnetic semiconductor (DMS) is one of the promising materials for spintronics devices, especially DMS based on oxide semiconductor such as ZnO after the predication by Dietl and his colleagues. After the prediction, many researches on the ZnO based diluted magnetic semiconductors have been performed. Room temperature ferromagnetism has been widely reported. However, the mechanism is not clear. Secondary phases and magnetic clusters have been claimed to be the origin of ferromagnetism. For example, Co doped ZnO films only show room temperature ferromagnetism when it is prepared under an oxygen poor environment, which makes it possible to produce magnetic Co clusters. Therefore, intrinsic ferromagnetism is doubtful. However, it is widely accepted that Co doped ZnO quantum dot has intrinsic ferromagnetism. Hence, the ferromagnetism in nanostructured diluted magnetic semiconductor is of interest. In this thesis, two methods CVD and hydrothermal approaches have been used for the fabrication of ZnO based nanorods doped with magnetic element and nonmagnetic element. For CVD method, Eu and Co are codoped into ZnO. It shows that Co doped ZnO alone easily induces Co magnetic clusters, which is confirmed by XRD analysis. However, if a certain amount of Eu is codoped into ZnO, the Co doping can be stabilized. For the hydrothermal method, Na, K and Sm have been used as dopant to incorporate with ZnO nanorods. It is know that N and K are not magnetic element. However, by delicate control of fabrication parameters, room temperature ferromagnetism has been observed. The ferromagnetism is due to the formation of Zn vacancies by Na or K doping. It is known that Sm is one of the rare earth elements, which possesses 4f orbital having large spin-orbital coupling. Experimental results indicate that a small amount of Sm doping in ZnO can induce room temperature ferromagnetism. High doping concentration can greatly decrease the magnetic moment, which may be due to the formation of antiferromagnetic coupling for high doping concentration.