Defect evolution in nitrogen-implanted CVD diamond during thermal annealing: The formation of NV centers and vacancy clusters

We investigate the thermal evolution of implantation-induced defects in single-crystal CVD diamond using depth-resolved positron annihilation lifetime spectroscopy (PALS). Samples were implanted with 0.5MeV N+ ions at a fluence of 1×1014cm−2 and annealed between 600∘C and 1200∘C. We probe defect populations as a function of depth and quantify their types and concentrations. In the pristine material, small vacancies, predominantly divacancies, are detected at ∼ppm levels together with a low concentration of larger vacancy clusters. Nitrogen implantation increases the abundance of mono-/divacancies. In nitrogen-rich regions, fewer isolated vacancies are observed despite higher displacement damage. Upon annealing, small vacancies become mobile. In nitrogen-poor regions, they agglomerate and grow pre-existing clusters. In contrast, in nitrogen-rich zones, they are efficiently captured by substitutional nitrogen to form NV centers, which limits the formation of new vacancy clusters. At annealing above 1000∘C, positron annihilation occurs predominantly in perfect bulk or small open-volume defects consistent with NV center-related positron lifetimes. These results reveal a nitrogen content- and temperature-dependent competition between vacancy clustering and NV center formation.