Study Of Electronic Structure And Magnetism In ZnO-based Diluted Magnetic Semiconductors

In this dissertation, the structures and magnetic nature of the transition metal (TM) doped ZnO-based Diluted Magnetic Semiconductors (DMSs) are investigated using mainly synchrotron radiation x-ray absorption fine structure (XAFS) in experimentally and theoretically. We fabricated a series of the doped and co-doped ZnO-based DMSs by different method, such as equilibrium state (sol-gel) and non-equilibrium state (PLD). The XAFS technique has been employed to probe the local structures of the TM atoms in ZnO host matrix, and analyse the change of the local structure around Zn atoms which influenced by these doping atoms. We obtained the structure information of doping atoms, such as the occupation sites and their coordination environment. Besides, we investigate the distributions over different positions of TM atoms and their solubility in ZnO host matrix. Combining with magnetic properties obtained from the superconducting quantum interference device (SQUID) and vibrating sample magnetometer (VSM), we study the the origin of the ferromagnetic and magnetic mechanism. Which providing experimental and theoretical evidence on the design and fabrication of the ZnO DMSs with ferromagnetic at room temperature.1. Role of Co clusters in wurtzite Co:ZnO dilute magnetic semiconductor thin films Zn_1-xCo_xO (x=0.02, 0.05, 0.10) dilute magnetic semiconductor (DMS) thin films had been deposited on Si (001) substrate using PLD. The local structures and magnetic nature of the films are investigated by x-ray absorption fine structure spectroscopy and x-ray diffraction. The results indicate that a single phase of the substitutional Co atoms occupied Zn sites in the ZnO matrix exists in the Zn_0.98Co_0.02O DMS thin film, while a secondary phase of the Co clusters is formed in Zn_0.95Co_0.05O and Zn0.90Co0.10O thin films. Despite the formation of Co clusters, the average magnetic moment MS per Co atom is sharply decreased with increasing Co concentration, which suggests that the small Co clusters are superparamagnetic. For the Zn_0.98Co_0.02O DMS thin film, the local structural distortion around the substitutional Co atoms induced by the existence of oxygen vacancies is interpreted as the origin of intrinsic weak room-temperature ferromagnetism.2. Solubility and structures of codoped Cu ions in Zn_0.95Co_0.05O diluted magnetic semiconductorsCu and Co ions codoped Zn_0.95-xCu_xCo_0.05O (0≤x≤0.08) dilute magnetic semiconductors (DMS) prepared by sol-gel method were investigated by x-ray absorption fine structure (XAFS). Although the Co K-edge XAFS data indicate that the doped Co ions are located substitutionally at the Zn sites for all the Zn_0.95-xCu_xCo_0.05O DMSs and which would result in the expansion of the local structures, the Cu K-edge XAFS analysis combined with XRD results reveals that the doped Cu ions are separated to form CuO phase for samples calcined at 1073 K. After sintered at a higher temperature of 1473 K, the Cu ions with low Cu concentration (x≤0.02) are substantially incorporated into the ZnO host lattice, while a small proportion of CuO clusters still coexist in the samples with a higher Cu concentration (x≥0.05). The solubility of the doped Cu ions in Zn_0.95Co_0.05O DMS is estimated as about 0.04. On the other hand, the variety of the peak in the O K edge XANES shows that there is a great deal of Co dimer in the samples calcined at 1073 K, while for the samples sintered at a higher temperature, the substitutional Co and Cu ions in Zn_0.95-xCu_xCo_0.05O are homogeneously and randomly distributed in the host. The results provide an experimental guidance on the design and synthesis of the co-doped ZnO based DMSs.3. Structures and magnetic properties of Zn_0.95-xCu_xCo_0.05O dilute magnetic semiconductor nanocompositesThe local structures and magnetic properties of Zn_0.95-xCu_xCo_0.05O (0≤x≤0.08) dilute magnetic semiconductors (DMSs) prepared by sol-gel method have been investigated by x-ray absorption fine structure (XAFS), XRD, XPS and VSM. The magnetic measurement show that the Zn_0.95-xCu_xCo_0.05O have weak room temperature ferromagnetic. And the XAFS and XRD results indicated that the Co atoms are incorporated substitutionally into the ZnO lattice at Zn sites and cause the disorder and expansion of the lattice. The Cu atoms with low Cu concentration (x≤0.02) replace Zn atoms in the wurtzite lattice., while a small proportion of CuO clusters still coexist in the samples with a higher Cu concentration (x≥0.05). The local structural distortion around the substitutional Co atoms and the defects at/near grain boundaries possibly induced during the pressing process are interpreted as the origin of intrinsic weak room-temperature ferromagnetism for Zn_0.95Co_0.05O. As for the case of Zn_0.95-xCu_xCo_0.05O, XAFS results clearly show that the Cu ion substitutes into the host and occupies at the Zn site. However, its 3d orbitals are mainly mixed with the 3d states of Co ion and shifted upwards in energy, resulting in localized states within the band gap. As a result, the 3d states of Cu ion could not be substantially hybridized with the 2p states of the O ion, making the ferromagnetic interaction not effectively function in the Zn_0.95-xCu_xCo_0.05O system. Therefore, Cu²⁺ /Zn²⁺ substitution is not the only imperative condition for ferromagnetism to occur. We find evidence that the ferromagnetic moment of Zn_0.95-xCu_xCo_0.05O is due to Cu-O planar nanophase inclusions in Zn_0.95Co_0.05O basal planes or small CuO paricles.