[Paper Review] Creation of nitrogen-vacancy centers in chemical vapor deposition diamond for sensing applications

Tingpeng Luo, Lukas Lindner+13 more|arXiv (Cornell University)|Nov 15, 2021

Diamond and Carbon-based Materials Research72 references69 citations

TL;DR

This study systematically optimizes nitrogen-vacancy (NV) center creation in chemical vapor deposition (CVD) diamond for quantum sensing by tuning nitrogen doping during growth and electron-beam irradiation fluence. By varying nitrogen flow over four orders of magnitude (0.2–20 ppm) and optimizing 2 MeV and 1 MeV electron irradiation, the authors achieve a balance between high NV⁻ concentration (up to 168 ppb) and long spin coherence times (T₂ up to 549 µs), with an optimal NV⁻/NV ratio of 67–86% and a P1-to-NV⁻ conversion rate of ~7–8%.

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 chemical vapor deposition (CVD) diamond. The nitrogen flow during growth is varied by over 4 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 $\mu$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.

Motivation & Objective

• To systematically investigate NV center formation in CVD diamond across a wide range of nitrogen doping levels (0.2–20 ppm) during growth.
• To optimize electron-beam irradiation fluence and energy (2 MeV and 1 MeV) to maximize NV⁻ concentration while preserving long spin coherence times (T₂).
• To identify the optimal irradiation conditions that balance high NV⁻ density with a high NV⁻/NV ratio, critical for enhanced sensing sensitivity.
• To correlate P1 center concentration with T₂ and demonstrate that irradiation and annealing preserve long T₂ despite increased NV density.
• To develop a UV-Vis absorption-based method to predict optimal irradiation fluence before annealing, using the GR1 band as an indicator of NV⁰ formation.

Proposed method

• Grew (100)-oriented CVD diamond samples using a microwave plasma CVD reactor with nitrogen flow varied over 4 orders of magnitude (0.1–2.7 sccm), achieving P1 center concentrations from 0.2 to 20 ppm.
• Performed electron irradiation at two energies (1 MeV and 2 MeV) with multiple fluences (1×10¹⁷ to 3×10¹⁸ e/cm²) on fixed-P1 samples to map NV creation and charge state evolution.
• Conducted post-irradiation annealing at 1000 °C for 2 hours to stabilize NV centers and enable charge-state conversion.
• Used confocal photoluminescence (PL) spectroscopy to measure NV⁻ concentration and T₂ coherence times via Rabi oscillations and Hahn echo decay.
• Applied UV-Vis absorption spectroscopy to monitor defect evolution, particularly the appearance of V⁻ (ND1) and V⁰ (GR1) centers during irradiation.
• Correlated GR1 band intensity after irradiation with subsequent NV⁰ formation after annealing to establish a predictive method for optimal fluence.

Experimental results

Research questions

• RQ1 How does nitrogen doping during CVD growth affect the initial P1 center concentration and its correlation with T₂ in as-grown diamond?
• RQ2 What is the optimal electron-beam irradiation fluence and energy for maximizing NV⁻ concentration while maintaining a high NV⁻/NV ratio?
• RQ3 How does the P1 center density influence the maximum achievable T₂ and the trade-off between NV⁻ concentration and coherence time?
• RQ4 Can UV-Vis absorption spectra before annealing predict the optimal irradiation fluence to avoid over-irradiation and excessive NV⁰ formation?
• RQ5 To what extent can the combination of high NV⁻ concentration and long T₂ be simultaneously achieved in CVD diamond for improved sensing performance?

Key findings

• The P1 center concentration in as-grown CVD diamond scales approximately as ∼0.09 × √(N/C), with measured values ranging from 0.2 to 20 ppm across the nitrogen series.
• After irradiation and annealing, T₂ values ranged from 45.5 µs (168 ppb NV⁻) to 549 µs (1 ppb NV⁻), demonstrating that long coherence times can be preserved even at high NV⁻ densities.
• For an initial P1 concentration of ~2.2 ppm, the optimal irradiation fluence was 1×10¹⁷–2×10¹⁷ e/cm² at 2 MeV and 1×10¹⁸–3×10¹⁸ e/cm² at 1 MeV, achieving a P1-to-NV⁻ conversion rate of ~7–8%.
• The NV⁻/NV ratio reached 67–86% at optimal fluence, with a critical threshold: to maintain >80% NV⁻/NV ratio, the P1-to-NV⁻ conversion rate must remain below 10%.
• The appearance of the GR1 band (500–550 nm) in UV-Vis spectra after irradiation correlates strongly with subsequent NV⁰ formation after annealing, enabling pre-annealing prediction of optimal fluence.
• The combination of high NV⁻ concentration (168 ppb) and long T₂ (45.5 µs) was achieved, with the longest T₂ (549 µs) being comparable to state-of-the-art single-NV and ensemble-NV systems, but with significantly higher NV⁻ density.