Change of absorption spectra in type-Ib diamond with heavy neutron irradiation

For type-Ib diamond neutron irradiated and annealed at 900 degrees C, it has been found that the nitrogen vacancy (NV) center is reduced in density with increasing irradiation in the dose range above a certain value, and instead the 575 and 594 nm centers appear abruptly. This fact is interpreted in terms of change in the charge state of defects correlated with large change in the Fermi level produced by heavy irradiation. Based on the present results, a model is proposed for the 575 nm center: (NV)(0), a defect released one electron from the NV center. It has been observed that the growth nature of the 594 nm center with irradiation is different between types Ia and Ib. This difference is explained by difference in the Fermi level of both types.