Window into NV center kinetics via repeated annealing and spatial tracking of thousands of individual NV centers
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-02-25 |
| Journal | Physical Review Materials |
| Authors | Srivatsa Chakravarthi, Chris Moore, April Opsvig, Christian Pederson, Emma Hunt |
| Institutions | University of Washington |
| Citations | 33 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Novel Methodology: The study introduces longitudinal spatial tracking of thousands of individual Nitrogen-Vacancy (NV) centers during repeated high-temperature vacuum annealing (800-1100 °C) to elucidate complex defect kinetics in ultra-pure CVD diamond.
- NV Density Enhancement: A significant 6-fold to 24-fold enhancement in NV- density was achieved by annealing at 980 °C, demonstrating a method to increase NV concentration without requiring damaging irradiation.
- Vacancy Source Identification: Results indicate the activation of a significant vacancy source within the CVD diamond bulk between 950 °C and 980 °C, enabling NV formation via the N + V â NV reaction path.
- Quenching Mechanism: Observed NV disappearances near 960 °C and 1050 °C suggest that native hydrogen, likely trapped in NVHx complexes (e.g., NVH3, NVH2), is the dominant spatially homogeneous quenching source.
- Orientation Control: Large-scale NV- orientation changes were observed at 1050 °C, even in the absence of full NV dissociation, opening a path toward engineering preferential NV alignment using strain.
- Kinetic Parameter Estimation: The direct observation of orientation changes allowed for the estimation of the NV diffusion barrier (re-orientation barrier) at 4.7 ± 0.9 eV, consistent with theoretical calculations.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| NV Density Enhancement (Max) | 24.1 | Fold | Sample C (50 ppb Ns) after 980 °C anneal. |
| NV Diffusion Barrier (Eb) | 4.7 ± 0.9 | eV | Estimated from re-orientation rate (1050 °C anneals). |
| NV Re-orientation Temperature | 1050 | °C | Temperature where orientation changes are significant. |
| Vacancy Source Activation Range | 950 - 980 | °C | Temperature range for maximum NV appearance. |
| Annealing Temperature Range | 800 - 1100 | °C | Range used for repeated vacuum annealing. |
| Annealing Environment | < 1e-7 | mbar | High vacuum conditions. |
| Electronic Grade [Ns] (Manufacturer) | < 1 | ppb | Substitutional nitrogen concentration (Samples A, B, D, E). |
| Estimated [Ns] (Sample B) | â 1 | ppt | Estimated NV density in electronic grade samples. |
| Confocal Imaging Volume | 350 x 350 x 25 | ”m3 | Area x Depth of Focus for longitudinal tracking. |
| Excitation Wavelength | 532 | nm | Diode-pumped solid-state laser. |
| NV Emission Detection Range | 660 - 800 | nm | Filtered phonon sideband emission. |
| Theoretical NV Diffusion Barrier | 4.85 | eV | Density Functional Theory (DFT) calculation reference value. |
Key Methodologies
Section titled âKey Methodologiesâ-
Sample Selection and Preparation:
- Used commercial Element Six CVD diamond, primarily electronic grade ([Ns] < 1 ppb) with {100} crystal orientation.
- Samples were cleaned in a fuming acid bath (H2SO4:HNO3:HCLO3) at 250 °C for 90 minutes to minimize surface fluorescence prior to annealing.
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Repeated Vacuum Annealing:
- Anneals were conducted sequentially in order of increasing temperature (800 °C, 900 °C, 950 °C up to 1050 °C, 1100 °C) under high vacuum (< 1e-7 mbar).
- Anneal times varied from 2 hours (initial steps) up to 150 hours (to observe saturation effects at 980 °C and 1050 °C).
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Confocal Microscopy and Imaging:
- A home-built confocal microscope (0.75 NA objective) was used for photoluminescence (PL) imaging.
- NV centers were excited non-resonantly with a linearly-polarized 532 nm laser (4-5 mW power).
- NV- emission was collected via the phonon sideband (660-800 nm filter).
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Longitudinal Tracking and Registration:
- Large-Area Scans: Individual NV centers were tracked longitudinally in the same large volume (350 x 350 ”m2 area, 25 ”m depth of focus) across multiple anneals.
- Image Registration: Affine homography transformation, aided by persistent luminescent defects and local NV constellations, was used to precisely align successive scans.
- Orientation Mapping: Excitation polarization was used to distinguish the two sets of NV orientations (encoded as green and magenta).
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Data Analysis and Kinetics Extraction:
- Image subtraction algorithms were applied to normalized, registered images to isolate and quantify NV appearances, disappearances, and orientation changes for every tracked defect.
- The fractional rate of orientation change was used in an Arrhenius fit (R = Μ exp(-Eb/kBT)) to estimate the NV re-orientation barrier (Eb).
Commercial Applications
Section titled âCommercial Applicationsâ-
Quantum Sensing and Metrology:
- High-Density NV Ensembles: The demonstrated 6- to 24-fold NV density enhancement at 980 °C provides a scalable, non-irradiation-based method for manufacturing high-sensitivity diamond sensors (e.g., magnetometers, thermometers) requiring dense NV populations.
- Ultra-Pure Qubit Hosts: The focus on electronic-grade CVD diamond ensures minimal background defects, preserving the long spin coherence times (T2) necessary for high-fidelity quantum sensing applications.
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Quantum Information Processing (QIP):
- Defect Engineering: Detailed understanding of formation and quenching kinetics (N + V â NV and NVHx dissociation) allows manufacturers to optimize post-growth annealing recipes to maximize NV yield and minimize unwanted hydrogen-related defects, crucial for creating reliable solid-state qubits.
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Integrated Photonics and Device Fabrication:
- Controlled NV Alignment: The observation of NV re-orientation at 1050 °C suggests a route for achieving preferential NV alignment using strain during annealing. This control is vital for maximizing the coupling efficiency of NV emission into integrated photonic waveguides and resonators.
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Fundamental Material Science:
- CVD Diamond Optimization: Provides critical benchmarks for theoretical models (e.g., DFT calculations of defect migration barriers) and informs the industrial processing of high-quality CVD diamond substrates used in advanced electronics and quantum technologies.
View Original Abstract
Knowledge of the nitrogen-vacancy center formation kinetics in diamond is critical to engineering sensors and quantum information devices based on this defect. Here we utilize the longitudinal tracking of single NV centers to elucidate NV defect kinetics during high-temperature annealing from 800-1100 $^\circ$C in high-purity chemical-vapor-deposition diamond. We observe three phenomena which can coexist: NV formation, NV quenching, and NV orientation changes. Of relevance to NV-based applications, a 6 to 24-fold enhancement in the NV density, in the absence of sample irradiation, is observed by annealing at 980 $^\circ$C, and NV orientation changes are observed at 1050 $^\circ$C. With respect to the fundamental understanding of defect kinetics in ultra-pure diamond, our results indicate a significant vacancy source can be activated for NV creation between 950-980 $^\circ$C and suggests that native hydrogen from NVH$_y$ complexes plays a dominant role in NV quenching, in agreement with recent {\it ab initio} calculations. Finally, the direct observation of orientation changes allows us to estimate an NV diffusion barrier of 5.1~eV.