Impurity control in Co-doped ZnO films through modifying cooling atmosphere

In order to understand the role of impurities in Co-doped ZnO, while at the same time minimizing the possibility of Co nanocluster formation, Zn1−xCoxO films were grown under relatively oxidizing conditions and then cooled under either N2 or air. Up to the highest doping level x=0.04, after cooling in N2 no measurable second phases were found whereas after cooling in air, ZnCo2O4 (nonferromagnetic at room temperature) appeared even for the lowest doping level x=0.0015. In addition, stronger ferromagnetism was observed in the N2 cooled samples, indicating an intrinsic origin to the ferromagnetism in Zn1−xCoxO.