The transport properties of Co-doped ZnO epilayers were investigated by using thermally stimulated current and deep-level transient spectroscopy. A defect center located at Ec-0.5eV was identified in all of the studied materials. Annealing of the present samples in nitrogen ambient or in air revealed that the density of the center increased when the annealing was performed in a nitrogen ambient. Illumination of the samples with ultra-violet light at low temperatures before the deep-level transient spectroscopy measurements reduced the density of the observed defect significantly, thus revealing the metastable nature of the center. It was therefore attributed to the isolated oxygen vacancy. Furthermore, comparison with reports on the magnetic properties of ZnO:Co suggested a correlation between the observed ferromagnetism in these materials and the oxygen vacancy.

On the Oxygen Vacancy in Co-Doped ZnO Thin Films. Seghier, D., Gislason, H.P.: Physica B, 2009, 404[23-24], 4800-2