Nitrogen concentration dependent optical defects transition in single crystal diamond through low pressure high temperature annealing

The effects of nitrogen concentration on the transition of optical defects in single crystal diamond under low pressure high temperature (LPHT) annealing are extensively investigated for the first time. The origin of the brown color is attributed to the superposition absorption of Ns0, vacancy clusters, NVH0, as well as unsaturated bonds. Diamonds with higher nitrogen concentration require elevated annealing temperature and prolonged durations to enhance optical properties. Our findings reveal that Ns0 can be captured by vacancies to form NV centers at lower annealing temperature, whereas undergoing aggregation into Hydrogen-modified A-type nitrogen aggregation center (Am center) and N2VH0 at elevated temperature. In highly nitrogen-doped diamond, sufficient densities of Ns0 and vacancies favor NV center formation. Conversely, aggregation processes dominate in lightly doped diamonds. This nitrogen-dependent transition mechanism provides some insights for the controversial photoluminescence (PL) variations observed during LPHT annealing.