By using a semi-empirical molecular orbital technique which incorporated Car-Parrinello molecular dynamics, the structure and properties of the common aggregates of N atoms were investigated. The N-vacancy complexes were examined in the same way, and this showed that it was energetically favorable for the N to form aggregates and for the vacancies to be trapped at N atoms; whatever its state of aggregation. The activation energy for migration of the vacancy was found to be 2.4eV. This value was in close agreement with experimental data. The mechanisms which were involved in the migration of the single N atom, and its aggregation to form pairs, were examined. It was found that a vacancy-assisted mechanism required an activation energy of 4.5eV, and a direct atomic exchange of the N with its C neighbor required an energy of 6.3eV. These values straddled the experimental estimate of 5eV, and indicated why radiation damage enhanced N aggregation. The mechanisms which were involved in the further aggregation of N pairs to form well-defined clusters of four N atoms were also modeled, and an activation energy of 7 to 8eV was found.

Nitrogen and Nitrogen-Vacancy Complexes and their Formation in Diamond. A.Mainwood: Physical Review B, 1994, 49[12], 7934-40