Structure,Optical Properties And Magnetic Origin Of Co-doped ZnO Diluted Magnetic Semiconductors

ZnO is II-VI group compound semiconductor with wurtzite structure and a wide direct band gap of 3.37 eV at room temperature. ZnO has large exciton binding energy of 60 meV and is much higher than the thermal ionization energy (26meV) at room temperature. In theory, it can achieve stimulated emission at room temperature. ZnO also has a good transparent conductivity, piezoelectricity, gas and pressure sensitive properties. So, it has broad application prospects in many fields, such as UV detectors, piezoelectric devices, surface acoustic wave devices, transparent electrodes, pressure-sensitive devices and gas sensors, GaN buffer layer Blue-ray films, etc. Additionally, ZnO also has good thermal stability, low temperature epitaxial growth, strong anti-radiation, rich source of low cost, non-toxic pollution, etc. Aslo, ZnO have the crystal growth characteristics of polarity, is doped with transition metallic ions easily, could be prepared as dilute magnetic semiconductors. Thus, it has become a major research material in recent decades.Co-doped ZnO films were deposited on sapphire substrates with pulsed laser deposition method. The microstructure, compositions, and optical, electrical, and magnetic properties of the films were characterized by x-ray diffraction (XRD), electron probe micro-analyzer (EPMA), transmittance spectra, photoluminescence spectra (PL) and physical properties measurement system (PPMS). The XRD analysis shows:the films have high c-axis preferred orientation and high crystallinity. It is found that Co doping can change the absorption of ZnO in the wavelength range of near ultraviolet and visible light largely. The band gap energy of ZnO films decreases with the increase of Co content, which is dependent on oxygen partial pressure. The contents of Co, Co2+ ions and phases in the films are quantitatively determined with XRD, EPMA and transmittance spectra and the correlations between room-temperature magnetism and the Co contents with different charge states are discussed. It is found that the Co content in the film decreases with the increase of oxygen pressure during deposition. The most of Co atoms are Co2+ ions entering the ZnO lattice to substitute for Zn2+ ions, but not being responsible for the magnetism of the films at room temperature. The metallic Co nano-clusters are detected by fine XRD analysis in the films deposited at 1.0×10-4 and 5.0 Pa and their contents are consistent with the estimation with the room temperature magnetism of the films. On the basis of quantitative analysis, a superparamagnetic magnetization mechanism of metallic Co nano-clusters is suggested and compared with experimental results by quantitative calculation.