Formation of Nitrogen Vacancy Center in Diamond for Quantum Sensing Applications

Formation and control of electron spin of negatively charged nitrogen vacancy center (NV⁻) in diamond is attracting much attention for next-generation quantum devices. For sensing applications, a relatively large amount of NV⁻ center is required to increase sensitivity. This corresponds to an increase of the total number of sensors. Typically, [NV⁻] of 0.1−3ppm is desired to detect weak magnetic fields. The coherence time of the electron spin T₂ is another important factor for increasing sensitivity, and this value has been reported to be inversely proportional to the density of nitrogen concentration [1].<br/> Considering these facts, we have optimized diamond growth condition for both chemical-vapor deposition (CVD) [2, 3] and high-pressure/high-temperature (HPHT) methods [4]. To prolong T₂, we applied ¹²C isotopic enrichment and improved a crystalline quality of diamond. For obtaining higher [NV⁻], first we improved controllability of nitrogen concentration in the doping range of 0.1−50ppm. Then, NV⁻ center is created in diamond crystals through electron beam irradiation and subsequent vacuum annealing. It is also important to elucidate the creation of point defects other than the NV⁻ center during NV⁻ center formation processes and to understand their effect on T₂, that is, on magnetic sensitivity. We performed electron paramagnetic resonance and photoluminescence measurements from this point of view [5].<br/> In the CVD method, a free-standing diamond (001) single crystals were obtained by cutting the substrate after a growth of nitrogen-doped homoepitaxial diamond thick layer. The dimension of CVD single-crystal plates was typically 3×3×0.5mm³. In the case of HPHT crystals, after the bulk diamond crystals were grown, these crystals were cut parallel to the {111} crystal plane to obtain {111} single crystals. Typical size of the HPHT {111} single-crystal plates was 1.5×1.5×0.4mm³.<br/> <br/> <br/> The author would like to thank Dr. C. Shinei, Dr. T. Taniguchi, Dr. M. Miyakawa, Dr. K. Watanabe of NIMS and Dr. Y. Masuyama, Dr. H. Abe, Dr. S. Onoda, Dr. T. Ohshima for crystal growth, characterization, and electron beam irradiation processes. This work was partially supported by MEXT Q-LEAP (JPMXS0118068379, JPMXS0118067395), JST CREST (JPMJCR1773), JST Moonshot R&D (JPMJMS2062), MIC R&D for construction of a global quantum cryptography network (JPMI00316), JSPS KAKENHI (No. 20H02187 and 20H05661).<br/> <br/>[1] J. F. Barry<i> et al</i>., Rev. Mod. Phys. <b>92</b>, 015004 (2020).<br/>[2] T. Teraji <i>et al</i>., J. Appl. Phys. <b>118</b>, 115304 (2015).<br/>[3] T. Teraji<i> et al</i>., phys. stat. sol. (a) <b>212</b>, 2365 (2015).<br/>[4] M. Miyakawa<i> et al</i>., Jpn. J. Appl. Phys.<b> 61</b>, 045507 (2022).<br/>[5] C. Shinei <i>et al</i>., Appl. Phys. Lett., <b>119</b>, 254001 (2021).