Depth-dependent decoherence caused by surface and external spins for NV centers in diamond
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2017-12-29 |
| Journal | Physical review. B./Physical review. B |
| Authors | Wenlong Zhang, Jian Zhang, Junfeng Wang, Fupan Feng, Shengran Lin |
| Institutions | Chinese Academy of Sciences, University of Science and Technology of China |
| Citations | 23 |
| Analysis | Full AI Review Included |
Technical Analysis and Commercial Solutions Brief: Depth-Dependent NV Decoherence
Section titled âTechnical Analysis and Commercial Solutions Brief: Depth-Dependent NV Decoherenceâ6CCVD Reference Document: QST-2017-NV-Decoherence-Depth
Executive Summary
Section titled âExecutive SummaryâThis research establishes a critical quantitative relationship between the depth of negatively charged Nitrogen-Vacancy (NV) centers in diamond and their spin coherence time ($T_2$), particularly when interacting with external spin baths. The findings are essential for optimizing ultra-shallow NV sensors required for nanoscale magnetic and electric field detection.
- Core Achievement: Demonstrated the capability to track the $T_2$ coherence time of the exact same single NV center as its depth was precisely reduced step-by-step using nanoscale ICP RIE etching. This eliminates variables associated with internal adjacent environments.
- Material Requirement: Experiments required high-purity, electronic-grade Single Crystal Diamond (SCD) with extremely low nitrogen ([N]<5ppb) and carbon-13 ([13C]=1.1%) impurity concentrations to maximize initial $T_2$.
- Characteristic Depth Found: Identified a characteristic depth ($d_0 \approx 6$ nm) where external spins (nuclear or electronic) cause the relatively strongest decoherence, providing a design threshold for optimizing shallow NV sensor performance.
- Methodology: NV centers were formed via 50 keV N2+ ion implantation, annealed at 1050 °C, and progressively brought closer to the surface via highly controlled, efficient Inductively Coupled Plasma Reactive Ion Etching (ICP RIE) at a rate of 11.8 $\pm$ 1 nm/min.
- External Spin Effects: Measured decoherence influence from external nuclear spins (immersion oil) and external electronic spins (Cu2+ solution), confirming that electronic spins cause stronger decoherence, especially near the characteristic depth.
- Commercial Relevance: Validates the critical necessity of ultra-high quality SCD substrates and precision nanoscale etching for the next generation of solid-state quantum sensing devices.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Diamond Grade | Electronic Grade (100) | N/A | Substrate orientation |
| Substrate Dimensions | 2 x 2 x 0.5 | mm3 | Sample size |
| Nitrogen Impurity ([N]) | < 5 | ppb | Specified purity from Element Six |
| Carbon-13 Isotope ([13C]) | 1.1 | % | Natural abundance spin bath |
| Implantation Energy | 50 | keV | N2+ molecule energy for NV creation |
| Implantation Fluence | 0.65 x 1011 | 14N2+ per cm2 | Nitrogen dose |
| Annealing Temperature | 1050 | °C | Required temperature for NV formation |
| Annealing Environment | 2 x 10-5 | Pa | High vacuum condition |
| Etching Method | ICP RIE | N/A | Oxford PlasmaPro NGP80 machine |
| ICP Power | 200 | W | RIE plasma parameter |
| Chamber Pressure | 30 | mTorr | RIE process parameter |
| Gas Flow (Oxygen) | 10 | sccm | O2 flow rate |
| Gas Flow (Argon) | 5 | sccm | Ar flow rate |
| Average Etching Rate | 11.8 $\pm$ 1 | nm/min | Highly controlled removal rate |
| Characteristic Depth ($d_0$) | ~6 | nm | Depth for minimum coherence time ratio (strongest external decoherence) |
| Magnetic Field (B) | 55 $\pm$ 5 | G | Applied field for spin measurement (paralleling NV center axes) |
| Coherence Time (Deep NV) | 214.1 | ”s | Initial $T_{2,air}$ at $d \sim 38$ nm |
| Coherence Time (Shallow NV) | 6.84 | ”s | Final $T_{2,air}$ at $d \sim 2$ nm |
Key Methodologies
Section titled âKey MethodologiesâThe highly precise depth profiling and NV tracking relied on specialized material preparation and controlled plasma etching:
-
Substrate Preparation:
- Used electronic-grade (100) SCD (2 x 2 x 0.5 mm3) with low impurities ([N]<5ppb, [13C]=1.1%).
- Coated surface with 300 nm Polymethyl Methacrylate (PMMA).
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NV Center Patterning and Formation:
- Electron beam lithography was used to pattern arrays of 60 nm diameter apertures and 2 ”m wide vacant strips (serving as position markers) onto the PMMA mask.
- Ion implantation performed using 50 keV N2+ through the mask.
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Post-Implantation Processing:
- Annealing conducted at 1050 °C in high vacuum (2 x 10-5 Pa) for 2 hours to activate NV centers.
- Thermal oxidation (430 °C in air for 2 h) and rigorous acid cleaning (sulfuric, nitric, perchloric acid 1:1:1) at 200 °C were performed to remove surface contaminants.
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Controlled Depth Etching (ICP RIE Recipe):
- Plasma etching performed using an Oxford PlasmaPro NGP80 reactor.
- Recipe: 200 W ICP power, 30 mTorr chamber pressure, 10 sccm O2, and 5 sccm Ar.
- The etching rate was precisely calibrated (11.8 $\pm$ 1 nm/min) using AFM analysis to ensure accurate depth control.
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Spin Decoherence Measurement:
- Fluorescence images (using position marks) tracked the exact same NV center through multiple etching steps (0 nm, 20 nm, 44 nm, etc.).
- Spin echo measurements were conducted before ($T_{2,air}$) and after applying external spin baths (T2,oil or T2,Cu2+) at each depth interval.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThe findings in this paper underscore the critical role of ultra-pure, precisely manufactured diamond substrates in advancing quantum technology, particularly in achieving shallow, high-coherence NV centers for robust sensing applications. 6CCVD is uniquely positioned to supply materials that meet or exceed the demanding specifications required for this research.
Applicable Materials
Section titled âApplicable MaterialsâTo replicate or extend this research, the highest quality diamond material is paramount. 6CCVD recommends:
- Optical Grade Single Crystal Diamond (SCD): Required for achieving maximum initial $T_2$ coherence times. Our SCD wafers feature superior crystal quality necessary for post-processing steps like ion implantation and annealing.
- Ultra-Low Nitrogen/Low [13C] Substrates: 6CCVD focuses on MPCVD growth capable of delivering SCD with impurity levels commensurate with or better than the commercial electronic grade material used in the study, ensuring high spin coherence.
- Precision Polishing (Surface Engineering): The paper highlights the dependence of decoherence on surface spins. 6CCVD provides SCD polishing down to Ra < 1 nm, which is essential for maximizing the performance and predictability of ultra-shallow NV centers (d < 10 nm).
Customization Potential for Advanced Sensing
Section titled âCustomization Potential for Advanced Sensingâ6CCVDâs in-house capabilities directly address the manufacturing challenges implied by this cutting-edge research:
| Requirement from Paper/Field | 6CCVD Specific Capability | Technical Advantage |
|---|---|---|
| Custom Wafer Size | Plates/wafers up to 125 mm (PCD), and large format SCD. | Supports scale-up from lab samples (2x2 mm3) to industrial, inch-sized quantum devices. |
| Precise Depth Control | SCD Thickness control from 0.1 ”m up to 500 ”m. | Ideal for manufacturing substrates with precisely tailored active layers or specialized membranes. |
| Electrode Integration (Future Devices) | Full internal metalization capability: Au, Pt, Pd, Ti, W, Cu. | Enables direct integration of quantum devices with on-chip microwave and control electrodes (e.g., for applying RF fields or mitigating electric field noise). |
| Patterning & Isolation | Custom laser cutting and patterning services. | Allows creation of the micro-scale mesas, trenches, or protected reference areas required for targeted ICP RIE etching and high-density NV arrays. |
Engineering Support
Section titled âEngineering SupportâThe discovery of the characteristic depth ($d_0$) depending on surface spin density ($\sigma$) requires careful material selection and surface preparation. 6CCVDâs in-house PhD team can provide specialized technical consultation covering:
- Optimizing substrate selection (e.g., nitrogen concentration, crystal orientation) for specific Quantum Sensing projects.
- Consulting on surface treatments and cleaning protocols to minimize $\sigma$ and thus lower $d_0$, enabling more sensitive External Spin Detection projects (e.g., detecting electronic spins in biological samples or materials science).
- Guidance on material requirements for high-temperature Annealing processes (up to 1200 °C) post-implantation.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
By efficient nanoscale plasma etching, the nitrogen-vacancy (NV) centers in diamond were brought to the sample surface step by step successfully. At each depth, we used the relative ratios of spin coherence times before and after applying external spins on the surface to present the decoherence, and investigated the relationships between depth and ratios. The values of relative ratios declined and then rised with the decreasing depth, which was attributed to the decoherence influenced by external spins, surface spins, discrete surface spin effects and electric field noise. Moreover, our work revealed a characteristic depth at which the NV center would experience relatively the strongest decoherence caused by external spins in consideration of inevitable surface spins. And the characteristic depth was found depending on the adjacent environments of NV centers and the density of surface spins.