Preparation And Physical Properties Of ZnO-based Diluted Magnetic Semiconductors

The field of spintronics has attracted significant attention since it promises new semiconductor devices through functionalization of both electron charge and spin degrees of freedom. Theoretical predictions of room temperature ferromagnetism in Mn-doped ZnO systems have generated considerable interest in studying transition-metal-doped ZnO. So it is very important to investigate material preparation, ferromagnetic mechanism and spin-dependent transport properties from the viewpoints of fundamental physics and technology applications.In this dissertation, polycrystalline CrxZn1-xO thin films, Zn/ZnO bilayers, [Zn/ZnO]3 mulitlayers and Znx(ZnO)1-x granular thin films were prepared by magnetron sputtering. The structure, composition and spin-related transport properties have been studied systemically and the ferromagnetism mechanism has also been discussed.Polycrystalline CrxZn1-xO films were fabricated by co-sputtering. Within the limits of detection, all the films have wurtzite hexagonal structure and no ferromagnetic impurity was observed. Magnetic measurements show that the magnetization decreases monotonously with increasing Cr concentration and reaches its maximum value of about 0.65μB/Cr at x=0.011. Microstructure is improved after annealing both in air and vacuum, while the magnetization decreases rapidly. Among the impurity phases related to Cr-ZnO systems, CrO2 is the only ferromagnetic phase with the Curie temperature of 386 K. However, because the TC of the CrxZn1-xO films is well above 395 K, the ferromagnetism does not originate from the CrO2 clusters but is the intrinsic material properties.The magnetizations show a strong dependence on the Cr concentration and, especially, on oxygen partial pressure. Fluorescence measurements show that Cr0.01Zn0.99O thin films deposited under different oxygen partial pressures include the main point defects of VZn, Oi, Zni,和Vo All the concentrations of point defects increase with increased oxygen partial pressures. The relative areas of VZn, Oi, and Vo decrease slightly with oxygen partial pressures, while Zni increases, which is consistent with the increasing trend of the magnetic moment. The results prove that the existence of intrinsic point defects Zni plays an important role in the ferromagnetism origin. Changing the transition metal Cr concentration and oxygen partial pressure can change the concentration of Zni and lead to the changes of macro-ferromagnetism.Preparations of Zn/ZnO bilayers, [Zn/ZnO]3 multilayers and Znx(ZnO)1-x (x=0～0.31) granular films construct Zn and ZnO heterostructures. Magnetic measurements show that the as-deposited Zn/ZnO bilayer is antiferromagnetic. The samples become ferromagnetic after annealing in vacuum at 300℃. The as-deposited [Zn/ZnO]3 multilayer is ferromagnetic. In Zn/ZnO film, ferromagnetism disappears after annealing in air at 600℃, corresponding to the leakage of Zni out of ZnO lattice. However, in the [Zn/ZnO]3 multilayer, the ferromagnetism does not disappear after the same annealing process, due to the plenty of Zni. These results indicate that Zni together with the heterostructure of Zn/ZnO play an important role in the ferromagnetic origin. As-deposited Znx(ZnO)1-x(x=0.07～0.31) granular films show room temperature ferromagnetism. The room-temperature saturated magnetization increases with increasing x and reaches its maximum value of about 3.34 emu/cc at x=0.31. Room temperature photoluminescence analysis and high-temperature X-ray diffraction measurement show conclusive evidence that the native point defect of Zni plays a crucial role in the observed magnetic behaviors. The temperature-dependent magnetization curve measured in a large temperature range from 50 to 800 K indicates that magnetization comes from the combination effect of single particle excitations and local spin-density fluctuations in 50 to 600 K. At temperatures higher than 600 K, the magnetization is contributed from the local spin-density fluctuations. Ferromagnetism could be qualitatively explained based on weak itinerant ferromagnetism theory.