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Chinese Physics B

Ultrahigh concentration of NV− centers embedded in the CVD epi-diamond layer near the interface with an HPHT diamond substrate

Yuanjie Yang, Shengran Lin, Jiaxin Zhao, Changfeng Weng, Liren Lou, Wei 巍 Zhu 祝, Guanzhong 冠中 Wang 王

Published Apr 2, 2025• 2 citations
Publisher

Abstract

Abstract The negatively charged nitrogen vacancy (NV−) center ensemble in as-grown chemical vapor deposition (CVD) diamond is a promising candidate for quantum sensing due to its long coherence time and excellent optical properties. However, achieving a high concentration of NV− centers in as-grown CVD diamond remains a critical challenge, which constrains the performance of NV− based sensors. In this study, we observe that NV− center formation efficiency is significantly enhanced during the initial growth phase, with a coherence time T 2 * of 1.1 μs. These findings demonstrate that high-concentration NV− centers can be achieved in as-grown diamonds, greatly enhancing their utility in high-performance magnetometers and quantum sensing.

DOI
10.1088/1674-1056/adc7f5

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Abstract The negatively charged nitrogen vacancy (NV−) center ensemble in as-grown chemical vapor deposition (CVD) diamond is a promising candidate for quantum sensing due to its long coherence time and excellent optical properties. However, achieving a high concentration of NV− centers in as-grown CVD diamond remains a critical challenge, which constrains the performance of NV− based sensors. In this study, we observe that NV− center formation efficiency is significantly enhanced during the initial growth phase, with a coherence time T 2 * of 1.1 μs. These findings demonstrate that high-concentration NV− centers can be achieved in as-grown diamonds, greatly enhancing their utility in high-performance magnetometers and quantum sensing.
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.
Negatively charged nitrogen-vacancy (NV − ) centres formed in diamond crystals are point defects that have potential applications in various quantum devices such as highly sensitive magnetic sensors. To improve the sensitivity of magnetic sensors using NV − centres, it is essential to precisely control the nitrogen concentration in the crystals. In this paper, we demonstrated that nitrogen concentration in diamond can be controlled with high precision for the following two representative growth methods. One is the high-pressure/high-temperature (HPHT) synthesis method and the other is the chemical vapour deposition (CVD) method. The nitrogen concentration of HPHT-grown diamond decreased semi-logarithmically with increasing contents of titanium or aluminium as nitrogen getter materials. The nitrogen concentration of CVD-grown diamond increased linearly with increasing the flow rate ratio of nitrogen to carbon. NV − centres were formed by controlling the total fluence of electron beams so that approximately 20% of the nitrogen became NV − centres. The coherence time of electron spin of NV − centres obtained by the Hahn-echo pulse sequence T 2 of these diamond crystals was inversely proportional to the nitrogen concentration. A comparison of T 2 of the NV − centres for HPHT-synthesized and CVD-grown diamonds showed no significant difference between them. This article is part of the Theo Murphy meeting issue ‘Diamond for quantum applications’.
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.
We systematically investigate the NV center creation and properties in bulk chemical vapor deposition (CVD) diamond, throughout growth, electron-beam irradiation, and annealing steps. We study diamond series grown with a varying nitrogen flow over 4 orders of magnitude, resulting in a broad range of single substitutional nitrogen (P1 center) densities from 0.2 to 20 ppm. We investigate in-situ P1 and NV creations, as well as NV charge states distributions. As the next step, we optimize electron-irradiation fluences for a fixed nitrogen concentration, with two different accelerated electron energies. We bring up a general approach to determine the optimal irradiation conditions, and discuss criteria for P1 to NV conversion ratios in the context of stable NV charge states. Through all these optimizations, we enhance the NV- concentration remarkably in our CVD diamond samples, without sacrificing their Hahn-echo coherence time T2. We achieve NV concentrations from 1 to 168 ppb with T2 from 549 to 45.5 µs respectively. This study shows a pathway to engineer targeted, optimized properties of NV-ensembles in bulk CVD diamonds for improved sensitivity.
The negatively charged nitrogen−vacancy (NV − ) center ensembles in diamonds offer enormous potential for developing integrated sensors with an improved signal-to-noise ratio (SNR) and high sensitivity. However, the preparation and treatment of diamond samples with suitable NV − concentrations and dephasing time have remained challenging. This work provided insight into the NV − center formation mechanism and reconstruction via a comprehensive analysis of the concentration and dephasing time of a set of diamond samples treated by various parameters. By varying the electron irradiation dose and subsequent annealing duration, the conversion rate of nitrogen to NV − is up to 18.45%, and the corresponding maximum NV − concentration is 3.69 ppm. The dephasing time for all samples varies around 300 ns. The nitrogen-related NV − center ensemble dephasing rate per unit density is 146.4 (ppm·ms) −1 , indicating that the treatment did not substantially alter the paramagnetic spin environment around the NV − center. This study not only offers support to exquisite sensitivities of NV-based sensors but also provides valuable experience for the preparation of unique properties of synthetic diamonds.
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.