High-Density NV Ensemble Produced by Electron Irradiation of Ultra High-Concentration Nitrogen-Doped CVD Diamond

Negatively charged nitrogen vacancy (NV) centers have potential applications in magnetic sensors and quantum communications [1]. In particular, the sensitivity of magnetic sensors scales with the square root of the quantum coherence time and the total number of NV centers [1]. Therefore, to improve magnetic sensitivity, it is desirable to increase the concentration of NVs while maintaining a long coherence time. Moreover, highly concentrated NV centers have recently emerged as a promising platform for investigating non-equilibrium quantum many-body systems, contributing to the observation of discrete-time crystals [2]. <br/>It is desirable to use diamond with a high concentration of nitrogen for the fabrication of high-concentration NV centers. Currently, nitrogen concentrations of at most 2×10²⁰ [cm^-3] have been achieved in CVD diamond [3]. Furthermore, the higher the nitrogen concentration, the more atomic vacancies must be introduced. Electron irradiation with a single-ended accelerator [4] or TEM [5] is often used as a means of forming vacancies.<br/>With TEM, the amount of electron irradiation per unit time can be adjusted by reducing the beam diameter. Therefore, it is possible to irradiate with a high dose of electron beams, which cannot be achieved with an accelerator. TEMs also have the advantage of being more easily accessible than accelerators. Therefore, we irradiated CVD diamonds containing the world's highest concentration of nitrogen ([N]=8×10²⁰ [cm^-3]) prepared by the MPCVD method with electron beams using a TEM and investigated changes in NV density and spin properties in the irradiated area.<br/>CVD synthesis was performed using a waveguide confined MPCVD system with quartz tube where plasma was confined within the waveguide, with a pressure of 110 [Torr], a microwave power 300 [W], and at a temperature of 850 [°C]. CO2, N2, and CH4 gases were used for homoepitaxial growth. The gas mixture condition was 2%, 8%, and 2%, respectively. The total gas flow rate was fixed at 200 [sccm]. HPHT synthetic (111)-oriented diamond substrate was used. <br/>A 300-keV TEM (JEOL JEM-ARM300F) was used for electron beam irradiation because the minimum electron energy required for defect formation in (111) diamond is 220-keV [6]. After electron irradiation, vacuum annealing was performed at 1000°C for 2 hours. Next, a custom-made laser scanning confocal fluorescence microscope (CFM) was used to estimate the concentration of NV centers from the issue intensity. T^*₂ and T&lt;span style="font-size:10.8333px"&gt;2&lt;/span&gt; were also measured by Ramsey and Hahn-Echo measurements on the NV ensemble in the electron-irradiated region.<br/> As a result, the volume density of the NV ensemble in the electron irradiated region increased with increasing electron dose from 10¹⁷ to 10²⁰ [cm^-2]; at a dose of 1.0×10²⁰ [cm^-2], the NV volume density reached 2×10¹⁷ [cm^-3]. In HPHT diamond([N]=3×10¹⁹ [cm^-3]), which was used as a comparison, the NV volume density was 4×10¹⁷ [cm^-3] after 1.0×10²⁰ [cm^-2] electron doses. Since the NV volume density was on the same order of magnitude as that of HPHT diamond, the concentration of NV centers could be further improved by further irradiation with higher doses in the future. In addition, T^*₂ remained 50 ns and T₂ remained 0.5 μs despite such a high nitrogen concentration in the diamond.<br/><br/>This work was supported by MEXT Quantum Leap Flagship Program (MEXT Q-LEAP) Grant Number JPMXS0118067395.<br/><br/>[1] V. M. Acosta, D. Budker et al., Phys. Rev. B 80, 115202 (2009).<br/>[2] S. Choi, S., M. D. Lukin et al., Nature, 543(7644), 221-225 (2017).<br/>[3] Y. Nakano, N. Tokuda et al., Diamond and Related Materials, 125, 108997 (2022).<br/>[4] C. Zhang, J. C. Fang et al., J. Phys. D: Appl. Phys. 50 505104 (2017).<br/>[5] D. Farfurnik, N. Bar-Gill et al., Appl. Phys. Lett. 111, 123101 (2017).<br/>[6] J. Koike, D. M. Parkin, and T. E. Mitchell, Appl. Phys. Lett. 60, 1450 (1992).