It was shown that metallic secondary phase formation inside ZnO(00•1) single crystals implant-doped with Ni at an atomic concentration of 5% could be suppressed. All the Ni ions were in the 2+ valence state after mild post-annealing. The suppression was achieved by annealing the crystals in high vacuum prior to implantation and was correlated with the introduction of structural disorder. The observed ferromagnetic properties of the pre-annealed crystals were evidently induced by defects and not primarily by Ni doping. They degraded at ambient temperatures within several days.

Ni Implanted ZnO Single Crystals - Correlation Between Nanoparticle Formation and Defect Structure. Zhou, S., Potzger, K., Kuepper, K., Grenzer, J., Helm, M., Fassbender, J., Arenholz, E., Denlinger, J.D.: Journal of Applied Physics, 2008, 103[4], 043901