Irradiation‐Induced Spin Bath Evolution and as‐Grown Hydrogen Defects in CVD Diamond Revealed by NV‐Based DEER Spectroscopy

ABSTRACT The aim of this paper is to provide the reader with a review and current state of the art of the fabrication of high T2 coherence diamond, optimised by the use of double electron‐ electron resonance (DEER) spectroscopy. Using DEER, we study the formation, transformation, and annealing of paramagnetic defects in as‐grown CVD diamond and after post‐processing. Electron irradiation leads to the formation of an additional S = 1/2 resonance in the DEER spectrum, which we consider to be a composite X ensemble. By tracking the concentrations of X and P1 point defects during annealing from 650°C to 1200°C, we find that the X ensemble initially consists of a mixture of V − spins and interstitial spins, which disappear at about 650°C. Vacancies migrate during annealing, forming clusters that persist to ∼1000°C and disappear upon annealing at 1200°C, contributing to the X ensemble signal. We have developed a model of the influence of the mixed spin bath on the coherence of NV centers, which includes independent couplings with P1 centers, V − , divacancies, and interstitials. Detailed DEER studies allowed us to reveal and resolve a weak signal from two additional S = 1/2 species associated with hydrogen: NVH − and consistent with a substitutional hydrogen defect, which overlaps the vacancy spectral line. Taken together, these results show that the NV‐DEER method is a powerful tool for investigating paramagnetic defects in diamond with high precision and nanoscale resolution, essential for material optimisation. The achieved high T2 coherence time is consistent with the spin bath model, and the crystals reach the quality required for advanced quantum sensing applications.