Effects of cobalt doping on the electrical properties of MBE-grown ZnO

We investigated n-conducting Co-doped ZnO epilayers grown by MBE with a cobalt content of nominal 4, 12, and 18 at.% by means of deep level transient spectroscopy (DLTS) and resistivity measurements. We found that in materials grown at the same substrate temperature, 500 A degrees C, the resistivity decreases with the cobalt mole fraction. The free electron concentration accounts for most of the change in the resistivity. The DLTS measurements reveal the presence of a defect center located at E(c)-0.5 eV, which is (1) metastable; (2) sensitive to annealing in oxygen-free ambient, and (3) decreasing in density in the samples with higher cobalt content. We therefore assign the center to the isolated oxygen vacancy. Furthermore, our results support the hypothesis that ferromagnetism in ZnO:Co is mediated by Co-oxygen vacancy pairs.