Extending Spin Dephasing Time of Perfectly Aligned Nitrogen‐Vacancy Centers by Mitigating Stress Distribution on Highly Misoriented Chemical‐Vapor‐Deposition Diamond
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2023-11-10 |
| Journal | Advanced Quantum Technologies |
| Authors | Takeyuki Tsuji, T. Sekiguchi, Takayuki Iwasaki, Mutsuko Hatano |
| Institutions | Tokyo Institute of Technology |
| Citations | 9 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research details a critical method for synthesizing high-quality, large-volume diamond films suitable for highly sensitive quantum sensors by extending the spin-dephasing time (T2*) of ensemble Nitrogen-Vacancy (NV) centers.
- Core Achievement: The spin-dephasing time (T2*) of large-ensemble, perfectly aligned NV centers in thick (~60 µm) Chemical Vapor Deposition (CVD) diamond was successfully doubled, increasing from approximately 0.15 µs to 0.30 µs.
- Methodology: T2* extension was achieved by growing the CVD diamond on highly misoriented (111) substrates (up to 10.0° misorientation angle, @mis), promoting step-flow growth.
- Stress Mitigation: Increasing the misorientation angle effectively mitigated the inhomogeneous stress distribution within the thick CVD film, which is the primary factor limiting T2* in high-density NV ensembles.
- Quantitative Stress Reduction: The standard deviation of the spin-stress interaction component (ΔMz) decreased significantly—by a factor of ~24 in the depth (XZ) plane and 11 on the surface (XY) plane—when @mis was increased from 2.0° to 10.0°.
- Physical Mechanism: The reduced stress distribution is attributed to the suppression of dislocation formation in the CVD film at high @mis, confirmed by the absence of etch pits on the 10.0° CVD film surface after plasma exposure.
- Sensitivity Impact: This technique provides a pathway to synthesize large-volume diamond materials with high T2, enhancing the DC magnetic sensitivity (proportional to T2 * N) required for advanced quantum sensing applications.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Substrate Material | Type-Ib HPHT (111) | N/A | Used for homo-epitaxial growth |
| Misorientation Angle (@mis) Range | 2.0, 3.7, 5.0, 10.0 | ° | Polished along [112] direction |
| CVD Film Thickness | 54 to 65 | µm | Dependent on @mis |
| Growth Temperature | 800 | °C | Measured by pyrometer |
| Growth Pressure | 30 | kPa | N/A |
| Microwave Power | 2.1 | kW | N/A |
| T2* (Large Ensemble, @mis=2.0°) | ~0.15 | µs | Lowest measured value |
| T2* (Large Ensemble, @mis≥5.0°) | ~0.30 | µs | Saturated maximum value |
| N0 Concentration (@mis=2.0°) | 31.6 ± 0.2 | ppm | Calculated from T2 (Hahn echo) |
| N0 Concentration (@mis=10.0°) | 25.1 ± 0.2 | ppm | Calculated from T2 (Hahn echo) |
| 13C Nuclear Spin Bath Limit (1/T2*) | ~1.0 | MHz | Due to 1.1% natural abundance |
| Stress Inhomogeneity Reduction (XZ plane) | ~24 | Factor | Reduction in standard deviation of ΔMz (2° to 10°) |
| Stress Inhomogeneity Reduction (XY plane) | 11 | Factor | Reduction in standard deviation of ΔMz (2° to 10°) |
| Dislocation Density (Substrate, 10.0°) | 2.5 x 106 | cm-2 | Measured via etch pits before CVD |
| Dislocation Density (CVD Film, 10.0°) | Not identified | N/A | Etch pits suppressed after CVD growth |
| Applied Magnetic Field (Bz) | ~7 | mT | Used for T2* measurement |
Key Methodologies
Section titled “Key Methodologies”The study employed high-power microwave plasma CVD for diamond synthesis and two distinct optical setups for characterizing spin coherence and stress distribution.
1. CVD Synthesis Recipe
Section titled “1. CVD Synthesis Recipe”- Substrate Preparation: Type-Ib HPHT (111) diamond substrates were polished to specific misorientation angles (@mis: 2.0° to 10.0°). Cleaning involved a Sulfuric Acid/Hydrogen Peroxide mixture (3:1) and a Mixed Acid (H2SO4/HNO3, 3:1).
- CVD System: High-power microwave plasma CVD with a spherical chamber to concentrate microwaves reflectively.
- Gas Flow Rates (sccm): Hydrogen (H2): 500; Methane (CH4): 0.5; Nitrogen (N2): 0.01.
- Growth Conditions: Pressure: 30 kPa; Microwave Power: 2.1 kW; Temperature: 800 °C.
2. Large-Excitation-Volume T2* Measurement
Section titled “2. Large-Excitation-Volume T2* Measurement”This system was designed to measure the ensemble T2* across the entire thickness of the CVD film, simulating a large-volume sensor.
- Excitation: 532 nm laser focused to a 20 µm diameter, illuminating the full ~60 µm film depth.
- Microwave Delivery: A coplanar waveguide resonator provided strong, uniform microwave irradiation perpendicular to the NV axis ([111]).
- Magnetic Field: A ring magnet applied a uniform static magnetic field of ~7 mT along the [111] direction.
- Measurement: T2* was extracted by fitting the Ramsey fringes.
3. Microscopic Stress and Material Characterization
Section titled “3. Microscopic Stress and Material Characterization”- Stress Imaging: A confocal microscope was used to map the stress distribution (ΔMz) in the XY (surface) and XZ (cross-section) planes.
- ΔMz (deviation of the z component of spin-stress interaction) was calculated from the shift in the average resonance frequency (a(P)) measured using the Ramsey sequence.
- NV Alignment Confirmation: Optically Detected Magnetic Resonance (ODMR) spectra confirmed perfect NV alignment by observing only two resonance dips.
- Spin Bath Characterization: The N0 concentration was calculated from the spin coherence time (T2) measured via the Hahn echo sequence using the confocal microscope.
- Dislocation Density: Atomic Force Microscopy (AFM) was used to count etch pits created on the diamond surfaces (substrate and CVD film) after exposure to H2 and O2 plasma, correlating dislocation density with @mis.
Commercial Applications
Section titled “Commercial Applications”The development of thick, low-stress diamond films with extended T2* is crucial for scaling up quantum sensing technologies, particularly those requiring large detection volumes and high sensitivity.
- Quantum Sensing and Metrology:
- High-Sensitivity DC Magnetometry: Enabling the next generation of highly sensitive magnetic field sensors by maximizing the figure of merit (C * N * T2*).
- Large-Volume Sensors: Essential for applications requiring detection volumes greater than 0.1 mm3, where thick films are necessary to increase the number of active NV centers (N).
- Biomedical and Industrial Monitoring:
- Biomagnetic Measurements: Achieving the high magnetic sensitivity required for non-invasive detection of weak biological signals (e.g., magnetocardiography).
- Battery Current Monitoring: High-resolution, non-contact monitoring of current flow in large-scale battery systems and power electronics.
- Advanced Material Engineering:
- High-Quality Substrates: Providing low-stress, high-purity (111) diamond material for the fabrication of complex quantum devices and integrated circuits.
- NV Center Engineering: Establishing a reliable, scalable method for synthesizing perfectly aligned NV centers in thick films, maximizing the signal contrast for quantum applications.
View Original Abstract
Abstract Extending the spin‐dephasing time ( T 2 * ) of perfectly aligned nitrogen‐vacancy (NV) centers in large‐volume chemical vapor deposition (CVD) diamonds leads to enhanced DC magnetic sensitivity. However, T 2 * of the NV centers is significantly reduced by the stress distribution in the diamond film as its thickness increases. To overcome this issue, they developed a method to mitigate the stress distribution in the CVD diamond films, leading to a T 2 * extension of the ensemble NV centers. CVD diamond films of ≈60 µm thickness with perfectly aligned NV centers are formed on (111) diamond substrates with misorientation angles of 2.0°, 3.7°, 5.0°, and 10.0°. The study found that T 2 * of the ensemble of NV centers increased to approach its value limited only by the electron and nuclear spin bath with increasing the misorientation angle. Microscopic stress imaging revealed that the stress distribution is highly inhomogeneous along the depth direction in the CVD diamond film at low misorientation angles, whereas the inhomogeneity is largely suppressed on highly misoriented substrates. The reduced stress distribution possibly originates from the reduction of the dislocation density in the CVD diamond. This study provides an important method for synthesizing high‐quality diamond materials for use in highly sensitive quantum sensors.