The investigation of the defect structures for Co2+ in ZnO microwires, thin films and bulks

The defect structures for Co2+ in ZnO microwires, thin films and bulks are theoretically investigated by analyzing the electron paramagnetic resonance (EPR) parameters (zero-field splitting D, g factors g(//) and g(perpendicular to) and hyperfine structure constants A(//) and Al-perpendicular to) for trigonally distorted tetrahedral 3d(7) clusters in a consistent way. The impurities Co2+ are found to suffer the displacements (approximate to 0.046, 0.044 and 0.045 angstrom) away from the ligand triangles along the C-3 axis in ZnO microwires, thin films and bulks, respectively, which considerably reduce the trigonal distortions of the Zn2+ sites in the hosts. Apart from the contributions from the conventional crystal field (CF) mechanism, those from the charge-transfer (CT) mechanism are important and should be included in the EPR analysis. The defect structures and spectral properties of the three ZnO:Co2+ systems are discussed.