Photoexcitation electron paramagnetic resonance studies on nickel-related defectsin diamond

Abstract

Measurements of the electron paramagnetic resonance (EPR) uponphotoexcitation are reported on Ni defects in diamonds grown with Ni-containingsolvent/catalysts. The temperature dependence of the W8 EPR spectrumphotoquenching shows that the relaxation of substitutional Nis−upon electron ionization is very small, corroborating the interpretation that thepreviously reported photoinduced effects with thresholds at 2.5 and 3.0 eVcorrespond to two complementary photoionization transitions involving Nis.Photoinduced behaviour of the NIRIM1 EPR centre favours the interstitial Nii+model for this defect and suggests that the Nii0/+level is located at 1.98 ± 0.03 eVbelow the conduction band. In N-doped diamond, Nii is morelikely to appear in the neutral state, undetectable by EPR, whereas at substitutional sitesNis−is revealed. Observation of a strong AB2 EPR signal photoquenching and simultaneousdetection of different spectral dependencies of the EPR intensity for other defectsdetermine an electron photoionization energy of 1.67 ± 0.03 eVfor the AB2. The implications of the obtained data for the identification of theAB defects’ structure are discussed. Our study shows that Ni defectsexhibit a weak electron–lattice interaction. The importance of the strongerspin–orbit coupling in these centres as compared to other defects in diamond isdiscussed. Assuming direct intercentre charge transfer from Ns,a theoretical description of the photoionization kinetics is proposed to explain theobserved photoresponse of Ni defects.