Local structures of copper-doped ZnO films

We report the local structures of a series of copper-doped zinc oxide films using polarization-dependent x-ray-absorption spectroscopy. The films were grown by pulsed-laser ablation under various conditions. The results show that films where copper exists solely as clusters are not ferromagnetic. The results also show that some of the copper-doped zinc oxide films are not ferromagnetic despite the fact that the copper substitution for zinc in the ZnO lattice is in the Cu2+ state, which provides the necessary unpaired spins for ferromagnetism. Therefore, Cu2+/Zn2+ substitution is not the only imperative condition for ferromagnetism to occur. We present characteristics unique to the electronic and atomic structure of ferromagnetic films and argue that the increased covalence of the CuZn -O bond found in these films is a prerequisite for the spin alignments in a substitutionally copper-doped zinc oxide film.