Extrinsic and radiation induced point defects in diamond

• Main Author: Tucker, Owain David
• Published: University of Oxford 1995
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.730625

Methods for measuring Electron Nuclear Double Resonance (ENDOR) from optically created paramagnetic states in diamond have been developed. These were used in conjunction with conventional Electron Paramagnetic Resonance (EPR) and ENDOR measurements to investigate extrinsic and radiation induced point defects in natural and synthetic diamond. ENDOR has unambiguously identified the ionized di-nitrogen (or A) centre as two identical nitrogen atoms in neighbouring substitutional sites. The nitrogen hyperfine and quadrupole parameters for the defect have been accurately measured. Molecular orbital theory calculations showed the unpaired electron to be almost 100% localized in the antibonding orbital between the nitrogen atoms. The N-N bond has extended to 1.86 Å. The ¹³C ligand hyperfine interaction with six equivalent nearest neighbours was measured. The centre is created by photons of E >3.0 eV, either by ionization of the A-centre, or by electron capture in [N-N]²⁺ . It decays logarithmically with respect to time, with a temperature independent rate below 50K indicating electron tunnelling through barriers of random width. The first ENDOR measurements on an optically excited state in diamond have been made. These identified the ⁵A₂ excited state of the neutral vacancy in radiation damaged natural diamond. The measured hyperfine interactions with nearest and next-nearest neighbour ¹³C ligand atoms showed the defect symmetry to be T_d. The unusual EPR line shape has been explained, and reasons for the non-Boltzmann population distribution within the spin manifold are suggested. The decay lifetime on removal of illumination has been measured. A combination of low temperatures and high frequency EPR has been applied to the W11-W14, and Rl radiation damage defects in neutron irradiated type Ib diamond. Boltzmann depopulation between the spin sublevels allowed measurement of the sign of the fine structure interaction and the determination of the effective spins. The Wll centre was identified as an excited state, and explanations for the unusual depopulation behaviour of the W14 centre were presented. This is the first time this technique has been applied to diamond. Intrinsic radiation damage defects in a 50% ¹³C enriched type IIa synthetic diamond were examined and the R1 defect was shown to be optically active.