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Materials for Quantum Technology

Characterisation of CVD diamond with high concentrations of nitrogen for magnetic-field sensing applications

Andrew Mark Edmonds, Connor Hart, Matthew J. Turner, Pierre-Olivier Colard, Jennifer M. Schloss, Kevin Olsson, Raisa Trubko, Matthew Markham, Adam Rathmill, Ben Horne-Smith, Wilbur Lew, Arul Manickam, Scott Bruce, Peter G. Kaup, Jon Carmelo Russo, Michael DiMario, Joseph T. South, Jay T. Hansen, Daniel J. Twitchen, Ronald Lee Walsworth

Published Jan 5, 2021• 77 citations
Publisher

Abstract

Abstract Ensembles of nitrogen-vacancy (NV) centres in diamond are a leading platform for practical quantum sensors. Reproducible and scalable fabrication of NV-ensembles with desired properties is crucial, as is an understanding of how those properties influence performance. This work addresses these issues by characterising nitrogen-doped diamond produced by the chemical vapour deposition (CVD) method across a range of synthesis conditions. This is shown to produce material with widely differing absorption characteristics, which is linked to the level of defects other than substitutional nitrogen (NS) and NV. In such material, the achievable concentration of NV− ([NV−]) is found to be influenced by the as-grown properties. At the 10–20 ppm level for [NS], the production of CVD-grown material with strain levels sufficient not to limit achievable device sensitivity is demonstrated and a favourable product of [NV−] and T 2 * is obtained. Additionally, reproducible properties over a batch of 23 samples from a single synthesis run are achieved, which appears promising for the scalability efforts underway in this area of research.

DOI
10.1088/2633-4356/abd88a

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Philosophical Transactions of the Royal Society A Mathematical Physical and Engineering Sciences•Dec 3, 2023•20 citations
Abstract Ensembles of nitrogen-vacancy (NV) centres in diamond are a leading platform for practical quantum sensors. Reproducible and scalable fabrication of NV-ensembles with desired properties is crucial, as is an understanding of how those properties influence performance. This work addresses these issues by characterising nitrogen-doped diamond produced by the chemical vapour deposition (CVD) method across a range of synthesis conditions. This is shown to produce material with widely differing absorption characteristics, which is linked to the level of defects other than substitutional nitrogen (NS) and NV. In such material, the achievable concentration of NV− ([NV−]) is found to be influenced by the as-grown properties. At the 10–20 ppm level for [NS], the production of CVD-grown material with strain levels sufficient not to limit achievable device sensitivity is demonstrated and a favourable product of [NV−] and T 2 * is obtained. Additionally, reproducible properties over a batch of 23 samples from a single synthesis run are achieved, which appears promising for the scalability efforts underway in this area of research.
Ensembles of nitrogen-vacancy (NV) centers in diamond are a leading platform for practical quantum sensors. Reproducible and scalable fabrication of NV-ensembles with desired properties is crucial. This work addresses these challenges by developing a chemical vapor deposition (CVD) synthesis process to produce diamond material at scale with improved NV-ensemble properties for a target NV density. The material reported in this work enables immediate sensitivity improvements for current devices. In addition, techniques established in this work for material and sensor characterization at different stages of the CVD synthesis process provide metrics for future efforts targeting other NV densities or sample geometries.
Abstract Diamond hosts a wide variety of colour centres that have demonstrated outstanding optical and spin properties. Among them, the nitrogen-vacancy (NV) centre is by far the most investigated owing to its superior characteristics, which promise the development of highly sophisticated quantum devices, in particular for sensing applications. Nevertheless, harnessing the potential of these centres mainly relies on the availability of high quality and purity diamond single crystals that need to be specially designed and engineered for this purpose. Plasma assisted chemical vapour deposition (CVD) has become a key enabling technology in this field of research. Nitrogen can indeed be directly doped in situ into a high crystalline quality diamond matrix in a controlled way, allowing the production of single isolated centres or ensembles that can potentially be integrated into a device. In this paper we will provide an overview of the requirements for synthesizing ‘quantum-grade’ diamond films using CVD. These include the reduction of impurities and surrounding spins that limit coherence times, the control of NV density in a wide range of concentrations as well as their spatial localization within the diamond. Enhancing the charge state and preferential orientation of the colour centres is also discussed. These improvements in material fabrication have contributed to positioning diamond as one of the most promising solid-state quantum systems and the first industrial applications in sensing are just starting to emerge.
Ensembles of nitrogen-vacancy (NV) centers in diamond are versatile quantum sensors with broad applications in the physical and life sciences. The concentration of neutral substitutional nitrogen ( [ N s 0 ]) strongly influences NV electronic spin coherence times, sensitivity, and optimal sensing strategies. Diamonds with [ N s 0 ] ∼ 1–10 ppm are a focus of recent material engineering efforts, with higher concentrations being favorable for continuous-wave optically detected magnetic resonance (CW-ODMR) and lower concentrations expected to benefit pulsed magnetometry techniques through extended NV spin coherence times and improved sensing duty cycles. In this work, we synthesize and characterize low- [ N s 0 ] ( ∼ 0.8 ppm), NV-enriched diamond material, engineered through low-strain chemical vapor deposition (CVD) growth on high-quality substrates, 12 C isotopic purification, and controlled electron irradiation and annealing. Our results demonstrate good strain homogeneity in diamonds grown on CVD substrates and spin-bath-limited NV dephasing times. By measuring NV spin and charge properties across a wide range of optical NV excitation intensity, we provide direct comparisons of photon-shot-noise-limited magnetic field sensitivity between the current low- [ N s 0 ] and previously studied higher- [ N s 0 ] ( ∼ 14 ppm) NV-diamond sensors. We show that low- [ N s 0 ] diamond can outperform higher- [ N s 0 ] diamond at moderate and low optical NV excitation intensity. Our results provide practical benchmarks and guidance for selecting NV-diamond sensors tailored to specific experimental constraints and sensing requirements.
Ensembles of negatively-charged nitrogen-vacancy (NV-) centres in diamond constitute a promising platform for magnetometry and other sensing applications. However, the sensitivity of present NV-ensemble devices and the need for diamond material with reproducible properties have hindered progress toward many envisioned applications. The work presented here addresses these challenges by optimizing a chemical vapor deposition (CVD) growth process to produce material at scale with improved NV-ensemble properties for both bulk and widefield magnetic sensing modalities.
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’.