Creation of nitrogen-vacancy centers in chemical vapor depositionn diamond for sensing applications
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-11-15 |
| Journal | arXiv (Cornell University) |
| Authors | Tingpeng Luo, Lukas Lindner, Julia Langer, V. Cimalla, Xavier Vidal |
| Institutions | UniversitÀt Ulm, Fraunhofer Institute for Applied Solid State Physics |
| Citations | 60 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research systematically investigates the creation and optimization of nitrogen-vacancy (NV) centers in Chemical Vapor Deposition (CVD) diamond to maximize sensitivity for quantum sensing applications.
- Material Optimization Goal: Achieve the optimal balance between high NV concentration (for signal intensity, I) and long spin coherence time (T2) to maximize the sensitivity factor (proportional to âI·T2).
- CVD Control: P1 (substitutional nitrogen) concentrations were precisely controlled during CVD growth, spanning 0.2 to 20 ppm by varying the N/C ratio over four orders of magnitude.
- Irradiation Optimization: Systematic electron irradiation (1 MeV and 2 MeV) and annealing treatments were optimized to maximize P1-to-NV- conversion (up to 8.4%) while ensuring high charge stability (NV-/NV ratio maintained between 66% and 86%).
- Charge State Control: The optimal irradiation fluence was found to scale positively with the initial P1 concentration. A conversion rate (Rcon = NV/P1grown) less than 10% was established as a critical criterion for achieving robust NV- charge state stability.
- Coherence Achievement: T2 coherence times were achieved ranging from 549 ”s (for 1 ppb NV- centers) down to 45.5 ”s (for 168 ppb NV- centers), confirming the expected inverse relationship between T2 and P1 concentration.
- Pathway Demonstrated: The study provides a clear pathway to engineer NV-doped CVD diamond properties for specific sensitivity requirements by modulating the combination of NV concentration and T2 time.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| P1 Concentration Range (As-Grown) | 0.2 to 20 | ppm | Nitrogen Series (CVD growth variation) |
| NV- Concentration Range (Treated) | 1 to 168 | ppb | Achieved after 2 MeV irradiation and annealing |
| Maximum Spin Coherence Time (T2) | 549 | ”s | Achieved at 1 ppb NV- concentration |
| Minimum Spin Coherence Time (T2) | 45.5 | ”s | Achieved at 168 ppb NV- concentration |
| NV-/P1 Ratio (As-Grown) | ~0.25 | % | Constant ratio before treatment |
| Optimal P1-to-NV- Conversion Rate (Rcon) | 7.3 to 8.4 | % | Achieved under optimized irradiation |
| Target NV-/NV Ratio (Charge Stability) | 66 to 86 | % | Achieved under optimized irradiation |
| Electron Irradiation Energy (Series 1) | 2 | MeV | Used for vacancy creation |
| Electron Irradiation Energy (Series 2) | 1 | MeV | Used for vacancy creation |
| Optimal Fluence (2 MeV) | 1E17 to 2E17 | e/cm2 | For 2.2 ppm initial P1 concentration |
| Optimal Fluence (1 MeV) | 1E18 to 3E18 | e/cm2 | For 2.2 ppm initial P1 concentration |
| Annealing Temperature | 1000 | °C | Post-irradiation treatment |
| Annealing Duration | 2 | h | Post-irradiation treatment |
| CVD Growth Pressure | 210 | mbar | Growth condition |
| CVD Growth Temperature | 800 to 900 | °C | Substrate temperature |
| Methane Concentration (CH4) | 2.2 to 2.7 | % | In precursor gas |
| NV Absorption Cross-Section (Ï532) | 0.95 ± 0.25 · 10-16 | cm2 | Used for NV concentration calibration |
Key Methodologies
Section titled âKey MethodologiesâThe experiment involved three main stages: CVD growth, electron irradiation, and thermal annealing, followed by comprehensive optical and spin characterization.
-
CVD Growth (Synthesis):
- Reactor Type: Ellipsoidal-shaped CVD reactor operating at 2.45 GHz microwave frequency (6 kW generator).
- Substrates: (100) oriented diamond, using both HPHT Type IIa and CVD substrates.
- Nitrogen Doping: Nitrogen flow was varied to achieve P1 concentrations from 0.2 to 20 ppm (N/C ratio varied from 150 to 106 ppm).
- Gas Mixture: 210 mbar pressure, 800-900 °C, 2.2-2.7% CH4, and 0.1 sccm O2.
-
Defect Creation (Irradiation):
- Purpose: Introduce isolated vacancies (V) homogeneously throughout the bulk diamond.
- Energy Levels: Two series were irradiated using 1 MeV and 2 MeV accelerated electrons at room temperature.
- Fluence Variation: Fluences were varied from 1E16 to 3E18 e/cm2 to study the conversion efficiency and charge state distribution.
-
NV Formation (Annealing):
- Process: Post-irradiation annealing was conducted at 1000 °C for 2 hours in vacuum.
- Mechanism: Thermal energy mobilizes vacancies, allowing them to combine with P1 centers (substitutional nitrogen) to form NV centers (P1 + V â NV).
-
Characterization:
- P1 Concentration: Measured using X-band continuous wave EPR spectroscopy.
- NV Concentration and Charge State: Determined via Photoluminescence (PL) mapping and spectroscopy (532 nm laser excitation, 10 ”W power), calibrated using UV-Visible absorption cross-section.
- Defect Analysis: UV-Visible absorption spectroscopy was used to monitor defect transformations (ND1 band, GR1 band) during irradiation and annealing, helping to identify optimal fluence before annealing.
- Coherence Time (T2): Measured using the Hahn-echo protocol (MW Ï time of 120 ns) on a home-built widefield microscope.
Commercial Applications
Section titled âCommercial ApplicationsâThe engineered NV-doped CVD diamond materials are critical components for advanced quantum technologies and high-sensitivity classical sensing applications.
- Quantum Magnetometry: Enabling high-precision AC and DC magnetic field sensing, particularly for applications requiring high spatial resolution (nanoscale) or high sensitivity (leveraging the optimized âI·T2 product).
- Biomedical Sensing: Used for nanoscale magnetic imaging of biological processes, such as mapping magnetic fields generated by single neurons or tracking magnetic nanoparticles within living cells.
- Temperature Sensing: Providing highly sensitive nanoscale thermometry, applicable in microelectronics, materials science, and biological systems.
- Strain and Pressure Metrology: Utilizing the NV centerâs sensitivity to local lattice strain for monitoring mechanical stress in devices or materials.
- Quantum Computing Substrates: Serving as a platform for solid-state quantum information processing where long spin coherence times and controlled defect densities are essential.
- Material Quality Control: The established methodology provides a reproducible recipe for manufacturing high-quality, NV-doped diamond for commercial quantum sensor production.
View Original Abstract
The nitrogen-vacancy (NV) center in diamond is a promising quantum system for\nmagnetometry applications exhibiting optical readout of minute energy shifts in\nits spin sub-levels. Key material requirements for NV ensembles are a high\nNV$^-$ concentration, a long spin coherence time and a stable charge state.\nHowever, these are interdependent and can be difficult to optimize during\ndiamond growth and subsequent NV creation. In this work, we systematically\ninvestigate the NV center formation and properties in chemical vapor deposition\n(CVD) diamond. The nitrogen flow during growth is varied by over 4 orders of\nmagnitude, resulting in a broad range of single substitutional nitrogen\nconcentrations of 0.2-20 parts per million. For a fixed nitrogen concentration,\nwe optimize electron-irradiation fluences with two different accelerated\nelectron energies, and we study defect formation via optical characterizations.\nWe discuss a general approach to determine the optimal irradiation conditions,\nfor which an enhanced NV concentration and an optimum of NV charge states can\nboth be satisfied. We achieve spin-spin coherence times T$_2$ ranging from 45.5\nto 549 $\mu$s for CVD diamonds containing 168 to 1 parts per billion NV$^-$\ncenters, respectively. This study shows a pathway to engineer properties of\nNV-doped CVD diamonds for improved sensitivity.\n