Annealing of electron radiation damage in a wide range of Ib and IIa diamond samples

• The long range diffusion of vacancies does not occur until 850 °C.
• A ZPL line at 733.1 nm is identified with the divacancy in diamond.
• Another at 517.6 nm is identified with the positive nitrogen vacancy centre.
• The well-known 3H photoluminescence centre is found to be negatively charged.

The diffusion of vacancies in diamond is of considerable practical and fundamental interest. This work undertakes a new investigation of this property, based on photoluminescence of electron irradiated and annealed samples, exploiting the recent availability of ultra-pure diamond where the very low nitrogen level dictates the necessity for long-distance migration of vacancies on annealing if nitrogen-vacancy complexes are to be formed. The results reveal that an annealing temperature of 850 °C is required for long-range vacancy diffusion rather higher than the generally accepted 700 °C, but that in Ib samples with high nitrogen concentrations marked reductions of vacancy concentrations can occur at temperatures as low as 500 °C by short range diffusion to nearby nitrogen atoms. As a result of this study, two new optical centres have been discovered and evidence is provided for the hypotheses that they are the divacancy and the positively charged nitrogen-vacancy complex.Interstitials created by the electron irradiation are well known to produce several optically active defects, but in particular, the centre known in the literature as 3H exhibits properties that have so far eluded convincing explanations. As a by-product of this investigation, a number of the properties of this centre were encountered that may assist a final determination of the atomic structure of this complex but very common defect. It is deduced that, contrary to conclusions in the literature, the centre is negatively charged.