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Orientation Enhancement of NV Centers in Diamond by Microwave Annealing

M. Haruyama, S. Fujii, Abe Hiroshi, Onoda Shinobu, Ohshima Takeshi, H. Kato, Y. Miyamoto, M. Ogura, T. Makino

Published Mar 4, 2026• 0 citations
OpenAlex

Abstract

Nitrogen-Vacancy (NV) centers in diamond are expected as highly sensitive magnetic sensors operable at room temperature. One major challenge in improving sensitivity is controlling the NV axis orientation. In diamond crystals, NV centers are typically randomly oriented along four crystallographic axes. Since only NV centers on a specific axis are utilized, NV centers on other axes act as fluorescence noise. Therefore, selectively forming NV centers along a single crystallographic axis is expected to enhance sensitivity. Perfectly oriented NV centers can be fabricated by nitrogen doping during chemical vapor deposition (CVD) growth, however, achieving a high NV center density remains challenging. In contrast, a high NV center density can be achieved by electron-beam irradiation followed by post-annealing, although orientation control remains difficult. In this study, we attempted to control NV center orientation by applying microwave annealing after electron-beam irradiation. Generally, microwave irradiation of material induces heating via Joule, dielectric and magnetic losses. Diamond has negligible dielectric and magnetic losses, so a boron doped diamond layer was formed as conductive layer and microwave annealing was performed via Joule loss. Because the conductive layer is thin, an electric field is applied throughout the whole diamond, resulting in a drift current flowing across the diamond. We thought that by controlling this current in the [111] direction, it would be possible to preferentially form [111]-oriented NV centers.

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Nitrogen-Vacancy (NV) centers in diamond are expected as highly sensitive magnetic sensors operable at room temperature. One major challenge in improving sensitivity is controlling the NV axis orientation. In diamond crystals, NV centers are typically randomly oriented along four crystallographic axes. Since only NV centers on a specific axis are utilized, NV centers on other axes act as fluorescence noise. Therefore, selectively forming NV centers along a single crystallographic axis is expected to enhance sensitivity. Perfectly oriented NV centers can be fabricated by nitrogen doping during chemical vapor deposition (CVD) growth, however, achieving a high NV center density remains challenging. In contrast, a high NV center density can be achieved by electron-beam irradiation followed by post-annealing, although orientation control remains difficult. In this study, we attempted to control NV center orientation by applying microwave annealing after electron-beam irradiation. Generally, microwave irradiation of material induces heating via Joule, dielectric and magnetic losses. Diamond has negligible dielectric and magnetic losses, so a boron doped diamond layer was formed as conductive layer and microwave annealing was performed via Joule loss. Because the conductive layer is thin, an electric field is applied throughout the whole diamond, resulting in a drift current flowing across the diamond. We thought that by controlling this current in the [111] direction, it would be possible to preferentially form [111]-oriented NV centers.
Introduction Nitrogen-vacancy (NV) centers in diamond are widely recognized as highly promising solid-state quantum sensors due to their long room temperature coherence times and atomic-scale size, which enable exceptional sensitivity and nanoscale spatial resolution under ambient conditions. Ultrafast laser writing has demonstrated the deterministic spatial control of individual NV− centers, however, the resulting random orientation of the defect axis limits the magnetic field sensitivity and signal contrast. Methods We developed an all-optical approach for reorienting laser-written NV− centers to lie along a specific crystallographic axis using femtosecond laser annealing. The orientation is determined by polarization analysis, and the annealing and subsequent polarization analysis are repeated until the desired orientation is observed. Results Our method achieves deterministic alignment of NV− centers along the optical axis in (111)-oriented diamond substrates and allows selection between two observable orientation classes in (100)-oriented substrates. The reorientation preserves spatial ordering while producing uniform orientation across arrays of NV− centers. Discussion This approach enables scalable fabrication of orientation-controlled NV− arrays, and paves the way for scalable, high performance quantum devices based on orientation-controlled NV− centers.
The negatively charged nitrogen-vacancy (${\mathrm{NV}}^{\ensuremath{-}}$) center in diamond is an attractive candidate for applications that range from magnetometry to quantum information processing. Here we show that only a fraction of the nitrogen (typically <0.5<0.5<0.5%) incorporated during homoepitaxial diamond growth by chemical vapor deposition (CVD) is in the form of undecorated ${\mathrm{NV}}^{\ensuremath{-}}$ centers. Furthermore, studies on CVD diamond grown on (110)(110)(110)-oriented substrates show a near 100% preferential orientation of NV centers along only the [111][111][111] and [1‾1‾1][\overline{1}\overline{1}1][111] directions, rather than the four possible orientations. The results indicate that NV centers grow in as units, as the diamond is deposited, rather than by migration and association of their components. The NV unit of the ${\mathrm{NVH}}^{\ensuremath{-}}$ is similarly preferentially oriented, but it is not possible to determine whether this defect was formed by H capture at a preferentially aligned NV center or as a complete unit. Reducing the number of NV orientations from four orientations to two orientations should lead to increased optically detected magnetic resonance contrast and thus improved magnetic sensitivity in ensemble-based magnetometry.
To improve the sensitivity of magnetic sensors using negatively charged nitrogen-vacancy (NV−) centers, it is essential to improve the quality of diamond crystals and precisely control the nitrogen concentration in the crystals. We have utilized high-pressure, high-temperature synthesis [1] and chemical vapor deposition [2] for the growth of nitrogen-doped diamond. The concentration of substitutional nitrogen in the neutral charge state [NS0] were evaluated by electron paramagnetic resonance (EPR) measurements. As shown in Fig. 1, [NS0] were precisely controlled by the weight ratio of nitrogen getter material in the metal solvent for the HPHT method (Fig. 1(a)) and by the carbon to nitrogen gas ratio for the CVD method (Fig. 1(b)). Then, the diamond crystals were irradiated with electron beams followed by vacuum annealing so that 10% of the nitrogen at the substitutional site became NV− centers, and the negatively charged NV− centers were successfully formed preferentially [3]. T2 of these diamond crystals was inversely proportional to the nitrogen concentration. The product of T2 and nitrogen concentration of the obtained HPHT diamond crystal is slightly higher than the maximum value reported so far [4], suggesting that the crystal is expected to have high magnetic sensitivity.
Nitrogen-vacancy (NV) centers in diamond have attracted significant interest because of their excellent spin and optical characteristics for quantum information and metrology. To exploit these characteristics, precise control of the orientation of the NV axis in the lattice is essential. Here we show that the orientation of more than 99% of the NV centers can be aligned along the [111] axis by chemical vapor deposition homoepitaxial growth on (111) substrates. We also discuss the alignment mechanisms. Our result enables a fourfold improvement in the magnetic field sensitivity and opens new avenues to the optimum design of NV center devices.
Negatively charged nitrogen-vacancy (NV-) center in diamond has been paid attention as a highly sensitive detector for magnetic field. The magnetic sensitivity is proportional to the reciprocal of √(NT_2 ), where N is the number of NV centers and T2 is the spin coherence time. To improve the sensitivity, high concentration of NV- centers and extended coherence time are necessary. It is generally known that electron beam irradiation into type-Ib diamond which include high initial concentration of isolated substitutional nitrogen (P1 center) is a useful method to create high concentration of NV- centers. The electron beam irradiation generates vacancies in diamond and the subsequently annealing enables vacancies to diffuse in the lattice. Then diffused vacancies are captured by nitrogen atoms, as a result, NV centers are created. The post annealing has been performed at 1000°C or higher temperature for the purpose of improving the spin properties of NV- centers. In current study, we focused on post-annealing at high temperature to improve magnetic sensitivity of quantum sensing. We investigated annealing temperature and time dependence of the creation behavior of NV centers in type-Ib diamonds to understand appropriate annealing process for enhancing sensitivity of NV- center.
Abstract The nitrogen-vacancy (NV) center in diamond is a promising quantum system for magnetometry applications exhibiting optical readout of minute energy shifts in its spin sub-levels. Key material requirements for NV ensembles are a high NV − concentration, a long spin coherence time and a stable charge state. However, these are interdependent and can be difficult to optimize during diamond growth and subsequent NV creation. In this work, we systematically investigate the NV center formation and properties in bulk chemical vapor deposition (CVD) diamond. The nitrogen flow during growth is varied by over four orders of magnitude, resulting in a broad range of single substitutional nitrogen concentrations of 0.2–20 parts per million. For a fixed nitrogen concentration, we optimize electron-irradiation fluences with two different accelerated electron energies, and we study defect formation via optical characterizations. We discuss a general approach to determine the optimal irradiation conditions, for which an enhanced NV concentration and an optimum of NV charge states can both be satisfied. We achieve spin–spin coherence times T 2 ranging from 45.5 to 549 μ s for CVD diamonds containing 168 to 1 parts per billion NV − centers, respectively. This study shows a pathway to engineer properties of NV-doped CVD diamonds for improved sensitivity.