Stability of the Fe3 + state in ZnO

We have applied implantation of radioactive 57Mn + (T1/2 = 1.5 min) at the ISOLDE facility at CERN with 50-60 keV energy to fluences <1012 /cm2 to study the emission Mössbauer spectra from the 14.4 keV transition of the 57Fe daughter atoms. For fluences >=5 × 1011 57Mn/cm2, the spectra are dominated by the Fe3 + state exhibiting a slow paramagnetic relaxation. We show that the Fe3 + state in ZnO saturated by <1012 57Mn/cm2 implantations is unstable. The crystals can be reset by annealing at T > 760 K or storage at room temperature for 12 months, after which they show dominantly the Fe2 + state for <2 × 1010 57Mn/cm2 implantations. These findings are discussed in terms of diffusion and/or annihilation of implantation-induced defects.