Creation of NV Centers in Diamond under 155 MeV Electron Irradiation
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2023-11-20 |
| Journal | Advanced Physics Research |
| Authors | Elena Losero, V. Goblot, Yuchun Zhu, Hossein Babashah, Victor Boureau |
| Institutions | Ăcole Polytechnique FĂ©dĂ©rale de Lausanne, Deutsches Elektronen-Synchrotron DESY |
| Citations | 4 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research explores the use of extremely high-energy (155 MeV) electron irradiation for creating high-density, negatively charged Nitrogen-Vacancy (NV-) centers in bulk diamond, offering significant advantages for quantum sensing applications.
- Efficiency Breakthrough: The 155 MeV irradiation process demonstrated a 60-fold higher yield of NV- creation per electron compared to the conventional 200 keV low-energy method, using the same HPHT substrate and fluence.
- Macroscopic Volume Processing: The high energy allows for exceptional penetration depth, estimated to be greater than 10 cm in diamond. This enables the batch irradiation of hundreds of stacked substrates, drastically reducing processing costs and time.
- Sensitivity Enhancement: The treatment increased the NV- concentration in a nitrogen-rich HPHT diamond by over three orders of magnitude (up to 0.6 ppm) across the entire 3 mm sample depth, leading to a projected shot-noise limited sensitivity gain of ~45 times.
- Quality Preservation: The resulting NV ensembles maintained high quality, exhibiting unaffected ODMR contrast and linewidth, and achieving a near-ideal charge state conversion efficiency (Ο > 0.95) in the highly irradiated region.
- Modeling Advancement: A new simulation model was developed to accurately predict vacancy concentration across all energy regimes (keV to hundreds of MeV), crucially incorporating secondary cascade processes prevalent at ultra-high energies.
- Lattice Integrity: Raman spectroscopy confirmed that the high-energy irradiation and annealing process did not heavily affect the overall crystal structure, although increased background PL suggests the creation of infrared-emitting vacancy-related defects.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| High Energy (HE) Irradiation | 155 | MeV | Electron beam energy (ARES accelerator) |
| Low Energy (LE) Irradiation | 200 | keV | Electron beam energy (TEM) |
| HE Target Fluence | 1.5 · 1018 | e/cm2 | Sample 1 (HPHT) |
| LE Fluence Range | 1 · 1016 to 5 · 1020 | e/cm2 | TEM irradiation range |
| HPHT Substrate [N] | â€200 | ppm | Initial substitutional Nitrogen concentration |
| CVD Substrate [N] | â€1 | ppm | Initial substitutional Nitrogen concentration |
| Max NV- Concentration (155 MeV) | 0.6 | ppm | Achieved in HPHT sample (x=0) |
| Vacancy Creation Yield (155 MeV vs 200 keV) | ~60 | fold higher | Per electron, same substrate/fluence |
| Projected Sensitivity Gain (155 MeV) | ~45 | times | Compared to pristine HPHT sample |
| Charge State Conversion Efficiency (Ο) | >0.95 | N/A | [NV-]/[NVtot] in highly irradiated region |
| Annealing Temperature | 800 | °C | Post-irradiation thermal annealing |
| Annealing Duration | 4 | h | Post-irradiation thermal annealing |
| Estimated Penetration Depth (155 MeV) | >10 | cm | Simulation result in diamond |
| T1 Relaxation Time Range (155 MeV) | 2.5 to 3.2 | ms | Measured across the irradiated region |
| Carbon Displacement Threshold Energy | ~35 | eV | Minimum energy to create a vacancy |
| NV- ZPL Peak | 638 | nm | Zero Phonon Line |
| NV° ZPL Peak | 575 | nm | Zero Phonon Line |
Key Methodologies
Section titled âKey Methodologiesâ-
Simulation Model Development:
- An in-house Monte Carlo simulation tool (using GMS 3.5) was created to model electron energy loss (ionization and bremsstrahlung) and vacancy generation in diamond from keV to hundreds of MeV.
- The model explicitly accounts for secondary cascade processes (atom-atom collisions) and includes a 50% recombination rate for displaced carbon atoms.
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Extremely High Energy Irradiation (Sample 1):
- Facility: ARES linear accelerator (DESY, Hamburg).
- Substrate: Element6 HPHT diamond (3 x 3 x 0.3 mm3, [N] â€200 ppm).
- Beam Parameters: 155 MeV electrons, Gaussian profile (~500 ”m diameter), 10 Hz repetition rate, up to 120 pC per pulse.
- Process: Irradiated for 96 hours to achieve a target fluence of 1.5 · 1018 e/cm2.
-
Low Energy Irradiation (Comparison Samples):
- Facility: Talos F200S Transmission Electron Microscope (TEM) (EPFL).
- Substrates: HPHT ([N] â€200 ppm) and CVD ([N] â€1 ppm).
- Beam Parameters: 200 keV electrons, homogeneous 15 ”m diameter beam.
- Process: Ten different areas on each substrate were irradiated, covering fluences from 1 · 1016 to 5 · 1020 e/cm2.
-
Thermal Annealing:
- Method: Conventional low-pressure thermal annealing applied to all irradiated samples.
- Parameters: 800 °C for 4 hours under P = 10-6 mbar (to mobilize vacancies and form NV centers).
-
Characterization Techniques:
- Photoluminescence (PL) Spectroscopy: Used to quantify absolute [NV-] concentration (via ZPL peak area comparison to a calibrated sample) and determine the charge state conversion efficiency (Ο) using Debye-Waller decomposition.
- Raman Spectroscopy (785 nm): Used to assess lattice damage (monitoring FWHM, peak shift, and background signal).
- Optically Detected Magnetic Resonance (ODMR): Continuous wave measurements used to extract linewidth, contrast, and calculate the shot-noise limited sensitivity (η).
- T1 Relaxometry: All-optical protocol used to measure the longitudinal spin relaxation time T1, characterizing the spin environment quality.
Commercial Applications
Section titled âCommercial ApplicationsâThe ability to create high-density, uniform NV- ensembles over macroscopic volumes using ultra-high energy electron irradiation is highly relevant for several commercial and industrial sectors:
- Quantum Sensing and Metrology:
- High-Sensitivity Ensemble Sensors: Manufacturing high-performance magnetometers, electrometers, and gyroscopes that rely on large numbers of NV centers to achieve high signal-to-noise ratios (SNR) and absolute sensitivity.
- Medical and Biological Imaging: Developing large-area diamond sensors for high-resolution magnetic field mapping (e.g., in magnetoencephalography or NMR).
- Industrial Diamond Processing:
- Cost Reduction in Batch Production: The >10 cm penetration depth allows hundreds of diamond wafers or substrates to be stacked and irradiated simultaneously in a single accelerator run, significantly lowering the cost per substrate compared to low-energy methods.
- Advanced Materials Engineering:
- Heterostructure Integration: Enabling the creation of NV centers within complex diamond-based heterostructures (e.g., integrated quantum circuits) by minimizing electron absorption and scattering damage in surrounding non-diamond layers.
- Diamond Defect Engineering Research:
- The methodology provides a scalable tool for generating high concentrations of vacancies and studying their aggregation states (e.g., vacancy clusters) and their impact on spin properties (T1), which is crucial for optimizing future quantum materials.
View Original Abstract
Abstract Singleâcrystal diamond substrates presenting a high concentration of negatively charged nitrogenâvacancy centers (NV â ) are on high demand for the development of optically pumped solidâstate sensors such as magnetometers, thermometers, or electrometers. While nitrogen impurities can be easily incorporated during crystal growth, the creation of vacancies requires further treatment. Electron irradiation and annealing is often chosen in this context, offering advantages with respect to irradiation by heavier particles that negatively affect the crystal lattice structure and consequently the NV â optical and spin properties. A thorough investigation of electron irradiation possibilities is needed to optimize the process and improve the sensitivity of NVâbased sensors. In this work, the effect of electron irradiation is examined in a previously unexplored regime: extremely high energy electrons, at 155 MeV. A simulation model is developed to estimate the concentration of created vacancies and an increase of NV â concentration by more than three orders of magnitude following irradiation of a nitrogenârich HPHT diamond over a very large sample volume is experimentally demonstrated, which translates into an important gain in sensitivity. Moreover, the impact of electron irradiation in this peculiar regime on other figures of merits relevant for NV sensing is discussed, including charge state conversion efficiency and spin relaxation time. Finally, the effect of extremely high energy irradiation is compared with the more conventional low energy irradiation process, employing 200 keV electrons from a transmission electron microscope, for different substrates and irradiation fluences, evidencing 60âfold higher yield of vacancy creation per electron at 155 MeV.