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

Nitrogen-vacancy (NV) centers in diamond are powerful quantum sensors, but their sensitivity is fundamentally limited by magnetic noise from surrounding paramagnetic defects in the host lattice, collectively known as the spin bath. While post-processing via high-energy electron irradiation and thermal annealing is routinely used to create NV centers, the intermediate defect states immediately after irradiation and their systematic transformation during annealing have remained poorly understood. This paper investigates the formation, transformation, and annealing of paramagnetic defects across three distinct stages: before irradiation, immediately after irradiation, and following stepwise thermal annealing up to 1200 °C.

To overcome the spatial averaging limitations of conventional electron paramagnetic resonance (EPR), the authors employ NV-based double electron-electron resonance (DEER) spectroscopy. By utilizing the optical initialization and readout of individual NV centers as probe spins, DEER provides high spin sensitivity and nanoscale spatial selectivity. The researchers analyzed custom-grown multilayer chemical vapor deposition (CVD) diamond samples with varying nitrogen concentrations and growth temperatures, alongside a commercial reference diamond, allowing them to map out the microscopic composition of the spin bath at each stage of fabrication.

Electron irradiation creates vacancies and interstitial defects that drastically alter the diamond lattice. Photoluminescence and DEER measurements reveal that electron irradiation introduces an additional $S = 1/2$ resonance, designated as a composite X ensemble. By tracking the concentrations of substitutional nitrogen (P1 centers) and the X ensemble during stepwise thermal annealing from 650 °C to 1200 °C, the authors determine that the X ensemble initially consists of a mixture of negatively charged vacancies and interstitial spins that anneal out around 650 °C.

As annealing temperatures increase toward 1000 °C and 1200 °C, vacancies migrate and aggregate into vacancy clusters and divacancies that continue to contribute to the X ensemble signal until final high-temperature remediation. Furthermore, the DEER spectra resolve weak signals from two additional $S = 1/2$ species associated with hydrogen—specifically $\text{NVH}^-$ and a substitutional hydrogen defect—which overlap with the vacancy spectral line. The authors successfully model the influence of this complex mixed spin bath on NV center coherence, confirming that achieving high $T_2$ coherence times requires accounting for independent couplings with P1 centers, vacancies, divacancies, and interstitial defects.