Ni implanted ZnO single crystals: correlation between nanoparticle formation and defect structure

We show that metallic secondary phase formation inside ZnO(0001) single crystals implant-doped with Ni at an atomic concentration of 5 % can be suppressed. All the Ni ions are in 2+ valence state after mild post-annealing. The suppression is achieved by means of annealing of the crystals in high vacuum prior to implantation and is correlated with structural defects of the ZnO single crystals. The observed ferromagnetic properties of the pre-annealed crystals degrade at ambient temperature within several days.