Identification of the neutral carbon 〈100〉-split interstitial in diamond

A systematic study has been made of some of the properties of R2, the most dominant paramagnetic defect produced in type-IIa diamond by electron irradiation. R2 has been produced in high-purity synthetic diamonds, which have been irradiated with 2 MeV electrons in a specially developed dewar, allowing irradiation down to a measured sample temperature of 100 K, at doses of $2\times {10}^{17}$ to $4\times {10}^{18}$ electrons ${\mathrm{cm}}^{-2}.$ The production rate of vacancies $\left[1.53\right(10\left)\mathrm{}{\mathrm{cm}}^{-1}\right]$ was the same for irradiation at 100 K as at 350 K, but the production rate of the R2 electron-paramagnetic-resonance (EPR) center is $1.1\left(1\right)\mathrm{}{\mathrm{cm}}^{-1}$ at 100 K and only $0.10\left(5\right)\mathrm{}{\mathrm{cm}}^{-1}$ at 350 K. Measurements have been made of the angular variation of the EPR linewidth, ${}^{13}\mathrm{C}$ hyperfine structure of samples grown with enriched isotopic abundance of ${}^{13}\mathrm{C},$ and of the EPR of samples annealed under uniaxial stress (for which a special equipment was developed). A combination of these data with the previously measured data has shown that R2 is the neutral $〈100〉$-split self-interstitial. This is an identification of an isolated stable self-interstitial in a group IV material. This shows that the self-interstitial is not mobile in type-IIa diamond under normal conditions (i.e., without the irradiation) until the annealing temperature of 700 K.