Ensemble Negatively-Charged Nitrogen-Vacancy Centers in Type-Ib Diamond Created by High Fluence Electron Beam Irradiation

At a Glance

Metadata	Details
Publication Date	2021-12-30
Journal	Quantum Beam Science
Authors	Shuya Ishii, Seiichi Saiki, Shinobu Onoda, Y. MASUYAMA, Hiroshi Abe
Institutions	Takasaki Advanced Radiation Research Institute, National Institutes for Quantum Science and Technology
Citations	17
Analysis	Full AI Review Included

Technical Documentation & Analysis: High Concentration NV^- Centers via Electron Beam Irradiation

Executive Summary

This research successfully demonstrates the creation of high-concentration, negatively-charged Nitrogen-Vacancy (NV^-) centers in Type-Ib diamond using high-fluence Electron Beam Irradiation (EBI) followed by thermal annealing. This methodology is critical for fabricating highly sensitive ensemble quantum sensors.

Core Achievement: Creation of up to 10 ppm of NV^- centers in Type-Ib diamond, confirming the effectiveness of EBI for high-density quantum sensing applications.
High Conversion Efficiency: The process achieved a P1 (substitutional nitrogen) to NV^- conversion efficiency ranging from 12% to 19%, significantly higher than typical unmodified CVD processes.
Quantum Metric (T₂): Measured spin coherence times (T₂) ranged from 1.3 µs to 2.7 µs, confirming that T₂ remains dominated by the total nitrogen concentration ([N_T]).
Material Requirement: The study utilized Type-Ib diamond with high initial P1 concentrations (46-80 ppm), highlighting the need for precise control over nitrogen incorporation during synthesis.
Mechanism Insight: Analysis suggests that while EBI is highly effective, not all P1 centers convert to NV^-, indicating the formation of residual defects that limit maximum T₂ and require optimization of irradiation/annealing protocols.
6CCVD Value Proposition: 6CCVD provides the necessary high-quality, high-nitrogen SCD and PCD substrates, along with custom processing and metalization capabilities, required to replicate and advance this quantum sensor fabrication technique.

Technical Specifications

The following hard data points were extracted from the research regarding material properties and process outcomes:

Parameter	Value	Unit	Context
Initial P1 Concentration ([P1]_initial)	46 - 80	ppm	Type-Ib HPHT Diamond Substrates
Electron Beam Energy	2	MeV	Irradiation source
Maximum Irradiation Fluence	8.0 x 10¹⁸	e/cm²	Total electron dose applied
Fluence Rate	1.0 x 10¹⁷	e/cm²/h	Irradiation speed
Annealing Temperature	1000	°C	Post-irradiation thermal treatment
Annealing Vacuum	~1.0 x 10^-4	Pa	High vacuum environment
Maximum NV^- Concentration ([NV^-])	10	ppm	Achieved in Ib-80 sample (80 ppm initial P1)
P1 to NV^- Conversion Efficiency	12 - 19	%	Ratio of [NV^-] to [P1]_initial
Spin Coherence Time (T₂) Range	1.3 ± 0.48 to 2.7 ± 0.96	µs	Measured via Hahn-echo sequence
NV^- Zero-Phonon Line (ZPL)	638	nm	Measured via Photoluminescence (PL)
NV⁰ Zero-Phonon Line (ZPL)	575	nm	Measured via Photoluminescence (PL)

Key Methodologies

The creation of high-density NV^- ensembles relied on precise control over the initial material state and subsequent defect engineering steps:

Material Selection: Commercially available Type-Ib HPHT diamonds were used, characterized by high initial substitutional nitrogen concentrations ([P1]_initial) ranging from 46 ppm to 80 ppm.
Surface Preparation: Samples were treated with a mixture of sulfuric acid and nitric acid at approximately 200 °C to remove surface contamination prior to irradiation.
Vacancy Creation (EBI): Electron beam irradiation was performed at 2 MeV energy in atmosphere, with fluences up to 8.0 x 10¹⁸ e/cm². Samples were water-cooled during the 80-hour irradiation period to prevent heating.
Vacancy Diffusion & Trapping (Annealing): Irradiated samples were subsequently annealed in a furnace at 1000 °C for 2 hours in high vacuum (~1.0 x 10^-4 Pa). This step allows vacancies (V⁰) to diffuse and be trapped by P1 centers, forming neutral NV⁰ centers, which then capture an electron (from another P1 center) to form the desired NV^- state.
Characterization:
- [P1] and [NV^-] concentrations were quantified using X-band Electron Spin Resonance (ESR).
- Charge state (NV⁰ vs. NV^-) and total NV concentration were analyzed using Photoluminescence (PL) excited by a 532 nm laser.
- Spin Coherence Time (T₂) was measured using Optically Detected Magnetic Resonance (ODMR) and Hahn-echo pulse sequences.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the advanced diamond materials and customization services necessary to replicate and optimize this high-fluence EBI technique for quantum sensing applications.

Applicable Materials

To achieve the high NV^- concentrations demonstrated in this study, researchers require diamond with tightly controlled, high substitutional nitrogen content (P1 centers).

Research Requirement	6CCVD Solution	Technical Advantage
High [P1]_initial (46-80 ppm)	High-Nitrogen SCD (Single Crystal Diamond)	MPCVD synthesis allows precise, tunable nitrogen incorporation (Type Ib equivalent) for optimal P1 concentration control.
Large Area Ensemble Sensing	Optical Grade PCD (Polycrystalline Diamond)	Available in plates/wafers up to 125mm diameter, ideal for large-scale ensemble magnetometers and sensors.
Decoherence Reduction	Ultra-Low Strain SCD	SCD substrates with Ra < 1nm polishing minimize surface defects and strain, potentially improving T₂ beyond the limits imposed by nitrogen spin bath.
Charge State Control	Boron-Doped Diamond (BDD)	For applications requiring specific charge state manipulation (e.g., creating NV⁰ or controlling Fermi level), 6CCVD offers BDD layers.

Customization Potential

The EBI process requires robust, precisely dimensioned substrates. 6CCVD offers comprehensive customization capabilities that streamline the fabrication workflow:

Custom Dimensions: We provide plates and wafers up to 125mm (PCD) and custom-cut SCD substrates, ensuring compatibility with specific irradiation chambers and device geometries.
Thickness Control: SCD and PCD layers are available from 0.1 µm up to 500 µm, allowing researchers to select the optimal thickness for MeV-range electron beam penetration depth.
Advanced Polishing: Our SCD substrates achieve surface roughness (Ra) < 1nm, critical for minimizing surface-related decoherence effects in quantum sensing devices. Inch-size PCD can be polished to Ra < 5nm.
Integrated Metalization: For implementing the ODMR and Hahn-echo sequences used in this study (requiring microwave strip lines), 6CCVD offers in-house metalization services, including Au, Pt, Pd, Ti, W, and Cu deposition, directly onto the diamond surface.

Engineering Support

The research highlights that T₂ is limited by residual nitrogen and other defects created during irradiation. 6CCVD’s in-house PhD team specializes in defect engineering and can provide critical support:

Material Optimization: Consultation on balancing the initial [P1] concentration to maximize NV yield (12-19% conversion) while minimizing the residual nitrogen spin bath that limits T₂ coherence time.
Process Integration: Assistance in selecting the optimal diamond grade (SCD vs. PCD) and geometry for high-fluence EBI and subsequent high-temperature annealing (1000 °C).
Global Logistics: We offer reliable global shipping (DDU default, DDP available) to ensure prompt delivery of specialized substrates to research facilities worldwide.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Electron beam irradiation into type-Ib diamond is known as a good method for the creation of high concentration negatively-charged nitrogen-vacancy (NV−) centers by which highly sensitive quantum sensors can be fabricated. In order to understand the creation mechanism of NV− centers, we study the behavior of substitutional isolated nitrogen (P1 centers) and NV− centers in type-Ib diamond, with an initial P1 concentration of 40-80 ppm by electron beam irradiation up to 8.0 × 1018 electrons/cm2. P1 concentration and NV− concentration were measured using electron spin resonance and photoluminescence measurements. P1 center count decreases with increasing irradiation fluence up to 8.0 × 1018 electrons/cm2. The rate of decrease in P1 is slightly lower at irradiation fluence above 4.0 × 1018 electrons/cm2 especially for samples of low initial P1 concentration. Comparing concentration of P1 centers with that of NV− centers, it suggests that a part of P1 centers plays a role in the formation of other defects. The usefulness of electron beam irradiation to type-Ib diamonds was confirmed by the resultant conversion efficiency from P1 to NV− center around 12-19%.

Tech Support

Original Source

DOI: https://doi.org/10.3390/qubs6010002

References

2011 - Dynamic Jahn-Teller effect in the NV− center in diamond [Crossref]
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2020 - Sensitivity optimization for NV− diamond magnetometry [Crossref]
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2000 - Radiation damage of diamond by electron and gamma irradiation [Crossref]
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