On intrinsic defects in Co-doped ZnO thin films

We report on electrically active point defects in Co-doped ZnO epilayers grown by molecular beam epitaxy. Using thermally stimulated current and deep level transient spectroscopy (DLTS), we identify a deep electron trap with a thermal activation energy of 0.5 eV. The density of this centre seems to decrease upon annealing of the samples in air ambient, while it remains unchanged when the annealing is performed in nitrogen ambient. It also decreases if the samples have been illuminated with UV light at a low temperature prior to the DLTS measurements, thus showing the metastable nature of the centre. We attribute this electron trap to the isolated oxygen vacancy. Comparison of our findings with what has been reported in the literature on the magnetic properties of ZnO:Co suggests a correlation between the observed ferromagnetism in these materials and the oxygen vacancy.