It was recalled that interstitial atoms from Ti to Fe, and Cu, were electrically active and introduced multi-levels into the band gap. A survey was presented here of the diffusion and electrical properties of various elements. Among the 3d transition metal elements, Co exhibited a peculiar behaviour in that, while the majority species were interstitial atoms, only the minority substitutional atoms were electrically active.

Diffusion and Electrical Properties of 3d Transition-Metal Impurities in Silicon H.Kitagawa: Solid State Phenomena, 2000, 71, 51-72