The chemical origin of the defects responsible for the visible luminescence in ZnO remains controversial. Debate centres on the assignment of these luminescence bands to specific radiative recombination centres, such as oxygen vacancies and zinc interstitials or complexes involving these defects. Of current interest is the role these defects play in the incorporation of high concentrations of transition metals (Fe, Cr, Mn, Ni) in ZnO to produce dilute magnetic semiconductors. In the present work, ZnO films were deposited on Si (100) and glass substrates by RF magnetron sputtering using a high purity ZnO target with different O₂/Ar partial pressure ratios. The cathodoluminescence (CL) and optical properties of these films were investigated as a function of substrate temperature, O₂ concentration, working pressure (1 - 10 mtorr) and post-deposition thermal annealing in N₂ at 500^oC for 1 hour. The position of the optical absorption edge was observed to red shift with increasing O₂ concentration. Post-growth thermal annealing significantly increased the CL intensity of the near band edge CL emission but had virtually no effect on the defect CL emission. The relative intensity of the green, orange and red defect CL bands exhibited a complex dependence on growth conditions. CL measurements from the ZnO films have been compared with monochromatic CL images of defect distributions around indents in high quality single crystal ZnO before and after thermal annealing. Each defect CL emission displayed a distinct spatial distribution around the indent, indicating that each centre has a different chemical origin. Furthermore, the centre responsible for the green CL was observed to have a higher mobility those involved in the orange and red emission. This result suggests that zinc interstitials are involved in the green luminescence as they are the most mobile centres in ZnO. Relationships between the CL emission in ZnO and structural point defects will be discussed.

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