Search of the origin of ferromagnetism in DMS

Transition metal doped wide band gap semiconductors like GaN, ZnO and TiO2 are promising candidates to be diluted magnetic semiconductors (DMS) with Curie temperatures (Tc) above room temperature (RT). In most of the studies reported in literature the presence of metallic precipitates could not be excluded and may play an important role in the discussion of the origin of the ferromagnetism.
Hence, one of the main obstacles while creating a DMS is the formation of secondary phases. The detection of those is very problematic too.
Since solubility limits for the doping atoms, especially in GaN, are rather low, non-equilibrium doping techniques like low temperature film growth or ion implantation are commonly used.
Ion implantation gives the possibility of doping at different temperatures and with high dopand concentrations but is always connected with lattice damage of the target material. Subsequent annealing in a time scale of some minutes mostly results in phase separation. Combining ion implantation with faster techniques, i.e. rapid thermal annealing or flash lamp annealing, leads to a diluted state while the crystallinity of the target material can be partially restored.
Sample sets were implanted with 57Fe ions and concentrations up to 18 at. % and subsequently annealed at different temperatures and time scales.
Conversion electron Mössbauer spectroscopy (CEMS), superconducting quantum interference device (SQUID), x-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and Rutherford backscattering spectroscopy (RBS) were used to characterize the (micro)-structure, magnetic response and radiation damage of specimens as well as the charge state and site location of Fe. The origin of ferromagnetism is discussed on base of these results.