It was found that the N vacancy center, in type-Ib material which had been irradiated (and annealed at 300C), decreased in density with increasing irradiation when the dose exceeded a certain value. The 575 and 594nm centers suddenly appeared instead. This was interpreted in terms of a change, in the charge state of defects, which was related to a large change (in the Fermi level) that was produced by heavy irradiation. On the basis of these results, a model was proposed for the 575nm center. It was observed that the growth behavior of the 594nm center during irradiation was different for types Ia and Ib. This difference was attributed to a difference in the Fermi levels of the 2 types.

Y.Mita: Physical Review B, 1996, 53[17], 11360-4