Electron paramagnetic resonance of the ${\mathrm{N}}_2{\mathrm{V}}^{-}$ defect in ${}^{15}\mathrm{N}$-doped synthetic diamond

Nitrogen is the dominant impurity in the majority of natural and synthetic diamonds, and the family of nitrogen vacancy-type (${\mathrm{N}}_n\mathrm{V}$) defects are crucial in our understanding of defect dynamics in these diamonds. A significant gap is the lack of positive identification of ${\mathrm{N}}_2{\mathrm{V}}^{-}$, the dominant charge state of ${\mathrm{N}}_2\mathrm{V}$ in diamond that contains a significant concentration of electron donors. In this paper, we employ isotopically-enriched diamond to identify the EPR spectrum associated with ${}^{15}{\mathrm{N}}_2{\mathrm{V}}^{-}$ and use the derived spin Hamiltonian parameters to identify ${}^{14}{\mathrm{N}}_2{\mathrm{V}}^{-}$ in a natural isotopic abundance sample. The electronic wave function of the ${\mathrm{N}}_2{\mathrm{V}}^{-}$ ground state and previous lack of identification is discussed. The ${\mathrm{N}}_2{\mathrm{V}}^{-}$ EPR spectrum intensity is shown to correlate with the H2 optical absorption over an order of magnitude in concentration.

Figure 1

(a) Structure of the ${\mathrm{N}}_2\mathrm{V}$ defect in diamond. Blue (light) atoms are nitrogen, and nominal $sp$-type orbitals are drawn for illustration. The defect has ${\mathcal{C}}_{2\mathrm{v}}$ symmetry; the $\left(1\overline{1}0\right)$ mirror plane is shown. (b) Effective one-electron picture produced by starting with the structure of the vacancy, lowering the symmetry and adding two (three) electrons for H3 (H2) using the procedure described by Lowther [17]. Electronic occupancy of orbitals indicated by vertical bars, with arrows indicating electronic absorption transitions between ground and excited states of symmetry ${\mathrm{A}}_1$ or ${\mathrm{B}}_1$. Adapted from Lawson [15].

Figure 2

(a) NIR-visible absorption spectrum of the sample following high-temperature, high-pressure annealing, electron irradiation, and further annealing. Strong absorption bands include H2 (1.257 $\mathrm{eV})$ and ${\mathrm{NV}}^{-}$ (1.945 $\mathrm{eV})$. (b) Correlation between the integrated absorption of the H2 optical band and the ${}^{15}{\mathrm{N}}_2{\mathrm{V}}^{-}$ EPR intensity in neutron-irradiated and annealed samples.

Figure 3

A comparison of experimental (a) and calculated spectra (b) at a microwave frequency of approximately $9.755\mathrm{GHz}$. Calculated spectra generated with easyspin [26] using the spin Hamiltonian parameters determined by fitting the line positions (see Table 1); EPR linewidth was adjusted to fit the experimental data. The orientation of the magnetic field for each spectrum is given in the center of the figure. The calculated spectra include ${}^{15}{\mathrm{N}}_2{\mathrm{V}}^{-}$ and ${}^{14}{\mathrm{N}}_{\mathrm{s}}^0$ (visible at 348.82 $\mathrm{mT})$, and have been referenced to ${\mathrm{N}}_{\mathrm{s}}^0$ assuming that this defect possesses an isotropic $g$ value: 2.0024.

Figure 4

(a) Experimental $B\parallel \langle 001\rangle$ EPR spectrum of a ${}^{14}\mathrm{N}$-doped diamond post-irradiation and annealing; calculated spectrum of ${}^{14}{\mathrm{N}}_{\mathrm{s}}^0$ and an additional line at $g=2.00269$; difference spectrum. (b) Difference spectrum (as bottom left); ${}^{14}{\mathrm{N}}_2{\mathrm{V}}^{-}$ spectrum generated using the Hamiltonian parameters given in Table 2.