Structures And Properties Of Co-implanted ZnO Films

As an important wide-band gap semiconductor, ZnO is a kind of promising materials for application of spintronics owing to its theoretically predicted ferromagnetism above room temperature. However, the origin of the observed ferromagnetism has been contributed to impurities, defects and the second phase. The carrier-mediated ferromagnetism has not been demonstrated by experiments. According to the difficulty about the mechanism and origin of ferromagnetism for ZnO based diluted magnetism semiconductor (DMS), we employed low doping dose and high implantation energy to dope Co ions into the high quality ZnO films with the second phase, impurities and defects suppressed. The influence of high temperature annealing on the structure and properties of ZnO:Co films has been studied in detail.The main results of the thesis are as follows:(1)We employed relative low doping dose and high implantation energy to implant Co into the ZnO films and studied the dependence of structure and properties of ZnO:Co films with the annealing temperatures. We found that high temperature annealing obviously improved the damage layer induced by ion implantation. Co ions moved and substituted Zn site, at the same time, Zn vacancies and Zn interstitials may move and recombine, and partly may also evorporate from the ZnO surface due to their low migration energy, leading to the reduced density of interstitials and vacancies and the recovery of crystalline structure. The M-H measurement at low temperature showed paramagnetic characteristics for all ZnO:Co films. We didn’t find the carrier-mediated ferromagnetism for the different annealed ZnO:Co samples. The zero field cooling magnetization curves of the annealed films have varied from concave shape to convex one with the annealing temperature increasing, which possibly resulted from the changes of the ratio of the itinerant carriers over the localized spin density. The ferromagnetism is attributed to the substituted Co2+ions and Zn vacancy defects, while the paramagnetism comes from interstitial Zn defects.(2) The influence of the implantation energy and doses on the structure and properties of the Co-implanted ZnO films have been studied. Lower implantation energy is found to cause obviously heavy damage near surface and high density of defects. We found that the high temperature annealing had improved the structure and properties of samples for all samples, but the extent of the improvement may depend on the implantation energy. For the sample with low implantation energy, Zn vacancies is easier to evorporate from the ZnO surface through the annealing process owning to heavy damage and the distribution of Co near surface, leading to higher density of Zn interstitials which act as shallow donors and contribute to high carrier concentration. The M-H measurement at low temperature show paramagnetic characteristics for all ZnO:Co films. We didn’t find the carrier-mediated ferromagnetism for all ZnO:Co samples too. From the M-T curve, we found a strong dependence of the measured magnetism on the carrier concentration, actually the concentration of Zn interstitials, but not the Co doping concentration, indicating that the contribution of paramagnetism comes from the interstitial Zn defects.