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 2×10¹⁷ to 4×10¹⁸ electrons cm^-2. The production rate of vacancies [1.53(10)cm^-1] 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(1)cm^-1 at 100 K and only 0.10(5)cm^-1 at 350 K. Measurements have been made of the angular variation of the EPR linewidth, ¹³C hyperfine structure of samples grown with enriched isotopic abundance of ¹³C, 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.