Diamond Crystal Growth for Weak Magnetic Field Detection

The formation of negatively charged nitrogen-vacancy (NV⁻) centers in diamond and the control of electron spin have attracted considerable attention for the realization of next-generation quantum devices. In sensing applications, relatively large amounts of NV⁻ centers are needed to increase sensitivity. This corresponds to an increase in the total number of sensors. Typically, 0.1-3 ppm [NV⁻] is desired to detect weak magnetic fields. The electron spin dephasing time T₂^* is also an important factor for increasing sensitivity of DC magnetometry, and it has been reported that this value is inversely proportional to the nitrogen concentration [1].<br/><br/>Considering these facts, we optimized diamond growth conditions both by chemical vapor deposition (CVD) [2, 3] and by high-pressure high-temperature (HPHT) method [4]. ¹²C isotope enrichment was applied to lengthen T₂^*. In the CVD method, nitrogen-doped homoepitaxial diamond thick films were grown and made freestanding, then cut diagonally to obtain diamond {111} single crystals; the dimensions of the CVD single crystal plates were typically 5×1×0.5 mm³. For HPHT crystals, bulk diamond crystals were grown, then {111} single crystals were obtained by cutting parallel to the {111} crystal facet planes; typical dimensions of HPHT {111} single crystal plates were 1.5×1.5×0.4 mm³. Concentration of NV⁻ centers and other defects were characterized by electron paramagnetic resonance, photoluminescence, secondary ion mass spectrometry, and Fourier transform infrared spectroscopy measurements [3, 5]. T₂^* have been characterized by Ramsey measurements using columnar excitation fluorescence microscope (CEFM) [6,7]. Diamonds with [NV⁻] of approximately 1 ppm are most suitable for weak-field measurements. Our characterization results indicate that in this nitrogen concentration range, the strain distribution formed in the crystal is the predominant limiting factor for T₂^*.<br/><br/>The author would like to thank Dr. C. Shinei, Dr. T. Taniguchi, Dr. M. Miyakawa, Ms. S. Manako of NIMS and Dr. Y. Masuyama, Dr. H. Abe, Dr. T. Ohshima of QST for crystal growth, characterization, and electron beam irradiation processes.<br/>This work was partially supported by MEXT Q-LEAP (JPMXS0118068379, JPMXS0118067395), JST Moonshot R&D (JPMJMS2062), MIC R&D for construction of a global quantum cryptography network (JPMI00316), CSTI SIP “Promoting the application of advanced quantum technology platforms to social issues”, JSPS KAKENHI (No. 20H05661, 24H00406).<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>., phys. stat. sol. (a) <b>212</b>, 2365 (2015).<br/>[3] T. Teraji <i>et al</i>., J. Appl. Phys. <b>133, </b>165101 (2023).<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).<br/>[6] Y. Masuyama <i>et al</i>., arXiv:2301.12441 (2023).<br/>[7] T. Teraji<i> et al</i>., Philos Trans A Math Phys Eng Sci <b>382</b>, 20220322 (2024).