Impedance analysis of secondary phases in a Co-implanted ZnO single crystal.

Co ions with 100 keV energy with a fluence of 1 × 10(15) cm(-2) are implanted into ZnO(0001) single crystals at 300 °C under vacuum. The resulting Co-implanted ZnO single crystals and the subsequent 750 °C and 900 °C annealed samples are analysed with respect to their structural, optical, electronic, magnetic and ac electrical properties. Photoluminescence and X-ray photoelectron spectroscopy results indicate the signatures of the Co(2+) state and its substitution at the tetrahedrally coordinated Zn-sites. X-ray diffraction and X-ray photoelectron spectroscopy identify the presence of the ZnCo2O4 and Co3O4 phases in the 900 °C annealed sample. By comparing the resistance response of the identified phases towards different magnetic environments, the impedance spectroscopy results successfully identify two magnetic phases (ZnCo2O4 and Co3O4) and a paramagnetic (CoZn) phase for the 750 °C and 900 °C annealed samples, implying the extrinsic nature of room temperature ferromagnetism. The observed ferromagnetism in each sample is not of single origin, instead the mutual effects of the secondary phases embedded in the paramagnetic host matrix are in competition with each other.

Vacancy cluster in ZnO films grown by pulsed laser deposition.

Undoped and Ga-doped ZnO films were grown on c-sapphire using pulsed laser deposition (PLD) at the substrate temperature of 600 °C. Positron annihilation spectroscopy study (PAS) shows that the dominant VZn-related defect in the as-grown undoped ZnO grown with relative low oxygen pressure P(O2) is a vacancy cluster (most likely a VZn-nVO complex with n = 2, 3) rather than the isolated VZn which has a lower formation energy. Annealing these samples at 900 °C induces out-diffusion of Zn from the ZnO film into the sapphire creating the VZn at the film/sapphire interface, which favors the formation of vacancy cluster containing relatively more VZn. Increasing the P(O2) during growth also lead to the formation of the vacancy cluster with relatively more VZn. For Ga-doped ZnO films, the oxygen pressure during growth has significant influence on the electron concentration and the microstructure of the VZn-related defect. Green luminescence (GL) and yellow luminescence (YL) were identified in the cathodoluminescence study (CL) study, and both emission bands were quenched after hydrogen plasma treatment. The origin of the GL is discussed.

ZnO Luminescence and scintillation studied via photoexcitation, X-ray excitation, and gamma-induced positron spectroscopy.

The luminescence and scintillation properties of ZnO single crystals were studied by photoluminescence and X-ray-induced luminescence (XRIL) techniques. XRIL allowed a direct comparison to be made between the near-band emission (NBE) and trap emissions providing insight into the carrier recombination efficiency in the ZnO crystals. It also provided bulk luminescence measurements that were not affected by surface states. The origin of a green emission, the dominant trap emission in ZnO, was then investigated by gamma-induced positron spectroscopy (GIPS) - a unique defect spectroscopy method that enables positron lifetime measurements to be made for a sample without contributions from positron annihilation in the source materials. The measurements showed a single positron decay curve with a 175 ps lifetime component that was attributed to Zn vacancies passivated by hydrogen. Both oxygen vacancies and hydrogen-decorated Zn vacancies were suggested to contribute to the green emission. By combining scintillation measurements with XRIL, the fast scintillation in ZnO crystals was found to be strongly correlated with the ratio between the defect luminescence and NBE. This study reports the first application of GIPS to semiconductors, and it reveals the great benefits of the XRIL technique for the study of emission and scintillation properties of materials.