Study Of Zno-based Diluted Magnetic Semiconductors By X-ray Absorption Fine Structure

In this dissertation, the structures and magnetic nature of the transition metal (TM) doped ZnO-based Diluted Magnetic Semiconductors (DMSs) are investigated in experimentally and theoretically using synchrotron radiation x-ray absorption fine structure (XAFS:EXAFS and XANES), combined with x-ray diffraction(XRD), x-ray photoelectron spectrometer (XPS), and SQUID (Superconducting Quantum Interference Device) methods. We fabricated a series of the doped and co-doped ZnO-based DMSs by Pulsed Laser Deposition(PLD). The XAFS technique and the SQUID method have been employed to probe the change of local structures and magnetic property of the as-deposited film and annealing treatment at 1073 K of Zn0.98Co0.02O thin film, we also use XAFS and XPS technique to analyse the mechanism that Cr mediate the distribution of Co in Co:ZnO DMS thin film. This study provides further evidence that O vacancy indeed plays an important role in activating the ferromagnetic interactions in Co-doped ZnO. It also providing experimental and theoretical evidence on the design and fabrication of the ZnO DMSs with good structure and performance.1. Determination of the role of O vacancy in Co:ZnO dilute magnetic semiconductor thin filmsAnnealing-induced changes in structural and magnetic property of Zn0.98Co0.02O thin film prepared at a low oxygen pressure by pulsed laser deposition(PLD) have been studied with x-ray absorption fine structure (XAFS), x-ray diffraction (XRD) and magnetization measurement. The results of XRD and EXAFS indicate that there is no obvious change in either the long-range crystallographic order, the short-range order surrounding Co, or the formal Co charge state, the room-temperature ferromagnetism (RTFM) observed in the as-grown film disappears upon annealing in air. X-ray absorption near-edge spectroscopy (XANES) analysis and multiple-scattering calculations reveal that the change of magnetic property caused by annealing in air is due to the annihilation of the preformed oxygen vacancy. This study provides further evidence that O vacancy indeed plays an important role in activating the ferromagnetic interactions in Co-doped ZnO.2. the mechanism that Cr atoms mediate the distribution of Co atoms in Co:ZnO DMS thin film.we investigated the distribution of Co atoms in Zn0.92Co0.08O thin film under the influence of Cr ions. For the sample of Zn0.92Co0.08O thin film, the results of XAFS indicate that a secondary phase of the Co clusters is formed,while in the sample of Zn0.84Co0.08Cr0.08O DMS thin film, only a single phase of the substitutional Co atoms occupied Zn sites in the ZnO matrix. So it suggested that the preimplantation with Cr plays a important role in the sample of Zn0.84Co0.08Cr0.08O thin film, it can effective to mediate the distribution of Co atoms in Co:ZnO DMS thin film.Based on the analysis and calculations of X-ray absorption near-edge spectroscopy (XANES) and XPS reveal that Cr is exist in the form of trivalent state and occupied Zn sites.The first-principles calculation further evidence that the Cr3+ ions could cause a shallow donor impurity band. The electron transfer from the donor impurity band to Co2+ ions and make Co2+ ion deviates from neutral. As a result, Coulomb exclusion interactions of Co2+ ions terminates the spin-split of the acceptor band and reduction in the implanted Co to a metallic state. The results provide an experimental guidance on the design and synthesis of the co-doped ZnO based DMSs.