Undoped and Ga-doped ZnO (002) films were grown c-sapphire using the pulsed laser deposition (PLD) method. Znvacancy related defects in the films were studied by different positron annihilation spectroscopy (PAS). These included Doppler broadening spectroscopy (DBS) employing a continuous monenergetic positron beam, and positron lifetime spectroscopy using a pulsed monoenergetic positron beam attached to an electron linear accelerator. Two kinds of Znvacancy related defects namely a monovacancy and a divacancy were identified in the films. In as-grown undoped samples grown with relatively low oxygen pressure P(O 2)≤1.3 Pa, monovacancy is the dominant Zn-vacancy related defect. Annealing these samples at 900 o C induced Zn out-diffusion into the substrate and converted the monovacancy to divacancy. For the undoped samples grown with high P(O 2)=5 Pa irrespective of the annealing temperature and the as-grown degenerate Ga-doped sample (n 10 20 cm-3), divacancy is the dominant Zn-vacancy related defect. The clustering of vacancy will be discussed.

The origin of room temperature ferromagnetism in undoped ZnO is still a question of debate. Experimental and theoretical findings are inconclusive as to the predominant contributor for the magnetic behavior of undoped ZnO. First principle calculation pseudopotential method was used to systematically determine the relaxed atomic geometry, the formation energies and the magnetic properties of the native point defects (vacancies, interstitials and antisites), and vacancy clusters (VZnVO, VZn − 2VO and 2VZn − VO) in ZnO. The results show that ZnO cells consisting of the VZn and the Oi have non-zero magnetic moments, energetically favoring ferromagnetic states and close-to-room-temperature Curie temperatures (294 K). VZn and Oi are also characterized by their low formation energies, in particular in the case of n-type (i.e. Fermi level close to the conduction band minimum) and O-rich conditions. The energy differences between the ferromagnetic state and anti-ferromagnetic state for VZn and Oi are larger than kT at room temperature but still relatively small (∼34 meV). Although VZn and Oi would contribute for the room temperature ferromagnetism, the ferromagnetism states would not be robustly stable for thermal excitation to the anti-ferromagnetic states.

Undoped and Cu-doped ZnO grown on sapphire using pulsed laser deposition (PLD) were studied by positron annihilation spectroscopy (PAS), photoluminescence (PL), high-resolution transmission electron microscopy (HRTEM) and secondary ion mass spectroscopy (SIMS). In the undoped samples, two kinds of VZn-related defects, namely VZn1 and VZn2 are identified. VZn1 was identified in as-grown samples grown at relatively low substrate (~300 °C). After annealing at 900 °C, VZn-2, the green luminescence (GL) peaking at 2.47 eV and the near band edge (NBE) emission at 3.23 eV in the low temperature photoluminescence (LT-PL) were simultaneously introduced. Another kind of VZn-related defect is identified in the Cu-doped ZnO sample, and is tentatively assigned to the VZn decorated with the Cu.