Hydrothermally grown ZnO samples, annealed in Zn-rich and O-rich ambients, have been investigated by deep level transient spectroscopy. A clear correlation has been found between the annealing treatment and the formation/suppression of two defect levels at ∼0.19 (E2) and ∼0.54 (E4) eV below the conduction band edge (Ec). Moreover, a close proportionality over more than three orders of magnitude has been established between the concentration of E2 and that of Fe, as determined by secondary ion mass spectrometry. Based on the above observations and previous reports in the literature, Fe on Zn-site is a likely candidate for E2.
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