Evidence for vacancy-interstitial pairs in Ib-type diamond

Diamonds containing nitrogen in different forms have been irradiated by 3-MeV electrons or Co60 gamma photons and characterized by optical absorption (OA) and electron spin resonance (ESR). An unusually low production rate of vacancies (V) and interstitials (I) was observed in gamma-irradiated Ib-type diamonds (those containing isolated nitrogen) and pure IIa-type diamonds as compared to Ia diamonds (containing nitrogen clusters). Postirradiation annealing at temperatures above 300°C strongly increased the V and I concentrations in Ib diamond, but not in IIa diamond. These results are explained as gamma irradiation of diamond predominantly produces V-I complexes instead of individual V and I defects. Strong effect of charge state on V-I recombination is revealed: In Ib diamond, V-I complexes are negatively charged and dissociate upon annealing. On the contrary, V-I pairs are neutral in IIa diamond, and they annihilate during irradiation. The OA, ESR, and positron annihilation signatures of the V^--I pairs are identified.