Mössbauer spectra were presented for Co-related point defects, with the emphasis being placed on Co dimers. The Mössbauer isomer shift was compatible with a substitutional lattice site and a highly negative charge state of the Mössbauer active atom. There was an unusually large electric field gradient. All of the experimental results were incompatible with a microscopic dimer model which involved just two Co atoms on next-nearest neighbour lattice positions. With the aid of  ab initio  total energy calculations, it was shown that dimers which were constructed from 2 interstitial Coi point defects would not be stable. Dimers which involved substitutional CoSi point defects were also not stable unless they were in aggregates with donors. It was concluded that an aggregate which involved one Sii self-interstitial was the most likely candidate for the experimentally observed Co dimer. This was because it was present in the damaged region after implantation; regardless of the doping level. It had a high aggregate formation energy, and it led to a complex having a strongly negative charge state of the 57Fe atom and an unusually large isomer shift. Calculations predicted a paramagnetic ground state for such a defect complex, and it was therefore expected that the dimer would be detectable by using electron paramagnetic resonance techniques.

A.M.Van Bavel, G.Langouche, H.Overhof: Semiconductor Science and Technology, 1998, 13[1], 108-15