Photoexcitation electron paramagnetic resonance studies on nickel-related defects in diamond

Measurements of the electron paramagnetic resonance (EPR) upon photoexcitation are reported on Ni defects in diamonds grown with Ni-containing solvent/catalysts. The temperature dependence of the W8 EPR spectrum photoquenching shows that the relaxation of substitutional Ni-s(-) upon electron ionization is very small, corroborating the interpretation that the previously reported photoinduced effects with thresholds at 2.5 and 3.0 eV correspond to two complementary photoionization transitions involving Ni-s. Photoinduced behaviour of the NIRIM1 EPR centre favours the interstitial Ni-i(+) model for this defect and suggests that the Ni-i(0/+) level is located at 1.98+/-0.03 eV below the conduction band. In N-doped diamond, Ni-i is more likely to appear in the neutral state, undetectable by EPR, whereas at substitutional sites Ni-s(-) is revealed. Observation of a strong AB2 EPR signal photoquenching and simultaneous detection of different spectral dependencies of the EPR intensity for other defects determine an electron photoionization energy of 1.67+/-0.03 eV for the AB2. The implications of the obtained data for the identification of the AB defects' structure are discussed. Our study shows that Ni defects exhibit a weak electron-lattice interaction. The importance of the stronger spin-orbit coupling in these centres as compared to other defects in diamond is discussed. Assuming direct intercentre charge transfer from N-s, a theoretical description of the photoionization kinetics is proposed to explain the observed photoresponse of Ni defects.