Electron irradiation and the formation of vacancy interstitial pairs in diamond

A large number (>60) of nitrogen-containing type Ia natural diamonds, and nitrogen-free type IIa natural and synthetic diamonds, have been electron irradiated and their optical absorption spectra have been analysed in detail. The analysis reveals that the concentrations of vacancies [V] and interstitials [I⁰ ] in type IaA diamonds increase linearly with the concentration [N_A ] of the A-form of nitrogen. This increase of [V] and [I⁰] with [N_A ] is attributed to the trapping of vacancies and interstitials in the strain field of nitrogen. The trapping produces 'strained' vacancies and partly immobilizes interstitials, thus reducing the vacancy-interstitial recombination during electron irradiation, thereby increasing [V] and [I⁰ ]. Those strained vacancies and interstitials recombine upon annealing at temperatures ~390-420 °C. In type IaB diamonds no correlation has been found between [V] and the concentration [N_B ] of the B-form of nitrogen; furthermore [I⁰] was found to decrease linearly with [N_B ]. In both type IaA and type IaB diamonds the width of the GR1 zero-phonon line, associated with the neutral vacancies [V⁰ ], increases with nitrogen concentration. A new, more appropriate and accurate formula to calculate [V⁰] is proposed based on measuring the height H of the GR1 absorption band at 2.0 eV.