. For the solubility limit of Al in ZnO is reported to be as low as 0.3 mol% [18], only minute of the doped Al atoms were expected to be dissolved in ZnO and the rest were expected to react with ZnO and thus form spinel phase. The intensity of XRD peaks from the spinel phase increases with the increasing content of Al dopant. This indicates that with the increase of Al-doping amount more ZnAl2O4 precipitates have formed due to the reaction of ZnO and Al. The ZnAl2O4 spinel impurity has been generated by the following reactions: