NITROGEN AND NITROGEN-VACANCY COMPLEXES AND THEIR FORMATION IN DIAMOND

Using a semiempirical molecular-orbital technique, incorporating Car-Parrinello molecular dynamics, the structure and properties of the common aggregates of nitrogen atoms in diamond have been investigated. Nitrogen-vacancy complexes have been examined in the same way, showing that, as is observed, it is energetically favorable for the nitrogen to form aggregates and for the vacancies to be trapped at the nitrogen, whatever its state of aggregation. The activation energy for migration of the vacancy was found to be 2.4 eV, in close agreement with experiment, and the processes by which the nitrogen complexes trap vacancies were modeled. The mechanisms involved in the migration of the single nitrogen atom and its aggregation to form pairs are explored. It is found that a vacancy-assisted mechanism has an activation energy of 4.5 eV, and a direct atomic exchange of the nitrogen with its carbon neighbor has an energy of 6.3 eV. These values straddle the experimental estimate of 5 eV and would indicate why radiation damage enhances the nitrogen aggregation. The mechanisms involved in the further aggregation of nitrogen pairs to form well-defined clusters of four nitrogen atoms were also modeled, and an activation energy of 7-8 eV found.