| Recently, the emerging scientific field of spintronics becomes an active area because spin-based multifunctional electronic devices have several advantages over the conventional charge-based devices regarding data-processing speed, nonvolatility, higher integration densities, and decreased electric power consumption. As the most potential candidates for these spin-based electronic devices, ZnO-based diluted magnetic semiconductors (DMS) have aroused significant interest. Despite the successful fabrication of ferromagnetic ZnO-based DMS, the origin of ferromagnetism is still under controversy. To further understand the origin of ferromagnetism, detailed microstructural characterization is needed.In this dissertation, microstructural defects in ZnO-based DMS including transition metal (TM) ions implanted ZnO single crystal, pure ZnO nanocrystals, and Fe-doped ZnO nanocrystals were carefully investigated using positron annihilation spectroscopy and other experimental methods. The main contents and results are listed as follows:TM ions (Fe/Co) were implanted into ZnO single crystals with multiple energies between 50 and 380 keV up to a total fluence of 1.25×1017 cm-2. The crystal quality of TM implanted ZnO was investigated by X-ray diffraction 2θscans andω-rocking curve measurements. The appearance of Fe related diffraction peaks in Fe+-implanted samples after 700℃annealing indicates possibly formation of Fe nanonparticles. Superconducting quantum interference device measurements revealed ferromagnetic behavior below 250K for the Fe+-implanted ZnO, while the Co+-implanted ZnO samples manifested stronger ferromagnetism at room temperature. Photoluminescence and Raman scattering as well as X-ray rocking curve measurements all indicate introduction of structural defects after Fe/Co implantation. Some of the defects act as nonradiative recombination centers, and suppress the visible and ultraviolet emission in ZnO. These defects also break the Raman selection rule, and lead to the activation of some silent phonon modes. The slow positron beam based Doppler broadening results suggest that annealing of the Fe+-implanted samples at 700℃causes partial recovery of the defects, while partial recovery of the defects in the Co+-implanted samples start at 900℃. The defects in Fe+-implanted samples can be annealed out above 1000℃, while defects in Co+-implanted samples can stay up to 1100℃. The discrepancy between the recovery process of Fe+-implantation induced defects and the variation in ferromagnetism in Fe+-implanted samples suggests that the ferromagnetism in Fe+-implanted ZnO single crystals might originate from the substitutional Fe on Zn sites.High purity ZnO nanopowders were pressed into pellets and annealed in air between 100 and 1200℃. The crystal quality and grain size of the ZnO nanocrystals were investigated by x-ray diffracton 2θscans. Annealing induces an increase in the grain size from 25 to 165 nm with increasing temperature from 400 to 1200℃. Scanning electron microscopy and high-resolution transmission electron microscopy observations also confirm the grain growth during annealing. Positron annihilation measurements reveal vacancy defects including Zn vacancies, vacancy clusters, and voids in the grain boundary region. The voids show an easy recovery after annealing at 100-700℃. However, Zn vacancies and vacancy clusters observed by positrons remain unchanged after annealing at temperatures below 500℃and begin to recover at higher temperatures. After annealing at temperatures higher than 1000℃, no positron trapping by the interfacial defects can be observed. Raman spectroscopy studies confirm the recovery of lattice disorder after annealing. Hysteresis loops are observed for the 100 and 400℃annealed samples, which indicate ferromagnetism in ZnO nanocrystals. However, the ferromagnetism disappears after annealing above 700℃, suggesting that it might originate from the surface defects such as Zn vacancies.High purity ZnO/Fe2O3 nanocomposites were annealed in air at different temperatures between 100 and 1200℃. The structure and grain size of the nanocomposites were investigated by X-ray diffraction 2θscans. Annealing induces an increase of the grain size from 25 nm to 195 nm and appearance of franklinite phase of ZnFe2O4. Positron annihilation measurements reveal large number of vacancy defects in the interface region of the ZnO/Fe2O3 nanocomposites, and they are gradually recovered with increasing annealing temperature. After annealing at temperatures higher than 1000℃, the number of vacancies decreases to the lower detection limit of positrons. Room temperature ferromagnetism can be observed in Fe-doped ZnO nanocrystals using physical properties measurement system. The ferromagnetism remains after annealing up to 1000℃, suggesting that it is not related with the interfacial defects.
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