Doping ZnO with Fe far from thermal equilibrium

The fabrication of diluted magnetic semiconductors (DMS) by transition-metal (TM) doping of ZnO has attracted tremendous interest within the last 3 years. However, there are still vivid discussions if the ferromagnetic state stems from Zener-interaction between diluted TM ions or from magnetic secondary phases. In order to prove or exclude the possible formation of TM-secondary phases in ZnO we have investigated iron doping, since Fe-ZnO (n-type) DMS are theoretically predicted to exhibit ferromagnetism [1].
For these investigations Fe-ions have been implanted with an ion energy of 180 keV (projected range Rp=80 nm) at 420 K into ZnO single crystals. Two fluences of either 0.4 or 4x1016 ions per cm2 were chosen which correspond to 0.5 and 5 at%, respectively. The samples were characterized by CEMS (conversion electron Mössbauer spectroscopy), XRD (X-ray diffraction) using synchrotron radiation, RBS (Rutherford back scattering), TEM (transmission electron microscopy) and SQUID (superconducting quantum interference device) magnetometry.
For the as implanted sample a high solubility of Fe was found, e.g. 100% for the low fluence and ~90% for the high fluence sample. The ionic states are 2+ and 3+ but none of the ionic fractions could be clearly determined to occupy substitutional lattice sites. The high fluence implanted sample exhibits ferromagnetic behavior at room temperature as was observed by CEMS and SQUID. However, CEMS and XRD measurements confirm that the origin of the ferromagnetic behavior is due to Fe-nanocluster formation. These nanoclusters show the magnetic moment and hyperfine field of metallic bcc-Fe. After annealing the samples at 800°C secondary phases (mainly zinc ferrite clusters) form and no ferromagnetic behavior could be detected. For the low fluence implanted sample neither as-implanted nor after annealing a ferromagnetic behavior could be detected although after annealing the Fe-ions develop a 2+ state suggesting an occupation of substitutional lattice sites.

[1] K. Sato, H. Katayama-Yoshida, Jpn. J. Appl. Phys. 40, L334 (2001)