The Highest Concentration NV Ensembles Formed by Heavily Nitrogen-Doping CVD System with Plasma Confinement and High Density Vacancy Formation by Focused Electron Beam Irradiation

Highly concentrated NV ensembles are expected to be applied to magnetic sensing.Only method for high concentration of nitrogen above 10²⁰ cm^-3 in diamond is limited to CVD method, because the damage of heavily implanted diamond cannot recover easily.<br/>We have successfully fabricated diamond thin films with the world's highest nitrogen concentration of 8.0 × 10²⁰ cm^-3[1] using a microwave plasma chemical vapor deposition (MPCVD) system that is specially designed to confine plasma to the thickness of a microwave waveguide. This method can also introduce 1 × 10²²cm^-3 of boron, and diamond with the world's highest superconducting transition temperature has been synthesized. HPHT synthetic (111)-oriented diamond substrates were used. That’s because (111) diamond substrate have the high impurity incorporation efficiency [2].These CVD films exhibit extremely high crystallinity, as indicated by the Raman spectrum measurement (Laser 532nm), which shows a diamond peak FWHM of 8.6 cm^-1, comparable to HPHT diamonds. After fabricating the films, we performed electron beam irradiation using a 300 keV TEM with an electron fluence of 10^18~22 cm^-2, resulting in the world's highest NV concentration of 8.5 × 10¹⁸ cm^-3 [3]. This method is necessary for electron irradiation for high density nitrogen above 1 × 10²⁰ cm^-3. The quantification of NV concentration was done using the DEER(Double Electron-Electron Resonance) based on NV interactions. DEER is a method of measuring NV concentration from the magnetic dipole interaction strength between NV’s four orientation groups by applying microwaves simultaneously to groups of each different resonance frequencies, so this method utilizes the fact that the decay rate of coherence time is proportional to the number of resonant NV groups in the concentration range of 8.5 × 10¹⁷~ 1.7 × 10¹⁹ cm^-3,as confirmed by simulations and experiments [4]. By manipulating the number of resonant NV groups, we determined the NV concentration [3]. Previously, the concentration was estimated from the emission intensity, but when the NV concentration exceeded 10¹⁸ cm^-3, the saturation of NV concentration occurred without an increase in irradiation fluence. As a result, the estimated values from the emission intensity using the DEER method were higher than those estimated from the emission intensity. However, the obtained NV concentration of 1.0 × 10¹⁸ cm^-3 is only about 1% yield relative to the nitrogen concentration. Understanding the behavior of nitrogen in CVD diamond could potentially lead to higher NV concentrations. Despite such a high nitrogen concentration, the T₂^*was 50 ns, and T₂ remained at 0.5 μs measured by Ramsey and Hahn echo, respectively. This value is long compared to experimentally reported values [5]. This is because many reports showed that coherence time is inversely proportional to nitrogen concentration between 2 × 10¹⁶ and 1 × 10²⁰cm^-3. At the nitrogen concentration of our nitrogen-doped CVD diamond ([N] = 8.0 × 10²⁰ cm^-3), the coherence time is calculated to be approximately 0.03 μs, which is much shorter than our measured value, suggesting that CVD diamond has advantages for high-density NV ensemble.<br/><br/>This work was supported by MEXT Quantum Leap Flagship Program (MEXT Q-LEAP) Grant Number JPMXS0118067395.<br/>[1] M. Ueda, H. Kawarada, et al., 2022 MRS Fall Meeting, EQ07.08.01<br/>[2] J Hiromitsu Kato, Toshiharu Makino, Satoshi Yamasaki and Hideyo Okushi. Phys. D: Appl. Phys. 40 (2007) 6189- 6200<br/>[3] K. Hayasaka, H. Kawarada, et al., 2022 MRS Fall Meeting, EQ07.05.04<br/>[4] G. Kucsko, M. D. Lukin, et. al., Phys. Rev. Lett. 121 (2018).<br/>[5] E. Bauch, R. L. Walsworth, et al., arXiv:1904.08763 (2019).