Shallow NV centers augmented by exploiting n-type diamond

At a Glance

Metadata	Details
Publication Date	2021-03-10
Journal	Carbon
Authors	Aya Watanabe, Tetsuri Nishikawa, Hiromitsu Kato, Masahiro Fujie, Masanori Fujiwara
Institutions	Kyoto University Institute for Chemical Research, Kyoto University
Citations	34
Analysis	Full AI Review Included

Technical Documentation & Analysis: Shallow NV Centers in N-Type Diamond

Executive Summary

This research demonstrates a significant advancement in quantum device fabrication by utilizing phosphorus-doped (n-type) single crystal diamond (SCD) grown by MPCVD to enhance the performance of shallow Nitrogen-Vacancy (NV) centers created via ion-implantation.

Enhanced Coherence Time (T₂): The longest measured T₂ time for shallow NV centers (depth ~15 nm) in n-type diamond reached 579.0 µs, approaching the theoretical limit set by natural ¹³C abundance in bulk diamond (~0.6 ms).
Improved Creation Yield: The NV center density in the P-doped sample was 2.2 times higher (27.5/100 µm²) compared to the non-doped reference (12.5/100 µm²).
Charge State Stabilization: Phosphorus doping effectively stabilizes the desired negatively charged state (NV^-), with the highest measured population reaching 0.92.
Mechanism: The enhancements are attributed to the suppression of paramagnetic defects (multi-vacancy complexes) generated during ion-implantation, achieved via Coulomb repulsion in the n-type semiconductor environment.
Application Relevance: These results are critical for the development of integrated quantum devices, high-sensitivity nanoscale sensing, and spintronics, requiring high-quality, near-surface NV centers.
6CCVD Value Proposition: 6CCVD specializes in the custom MPCVD growth of doped SCD films (including n-type/P-doped and BDD) required to replicate and scale this foundational research.

Technical Specifications

The following table summarizes the critical material properties and performance metrics achieved using the phosphorus-doped MPCVD diamond (Sample I) compared to the non-doped reference (Sample II).

Parameter	Sample I (P-doped) Value	Sample II (Non-doped) Value	Unit	Context
Longest Spin Coherence Time (T₂)	579.0 ± 28.7	359.6 ± 10.3	µs	1.6x improvement over non-doped.
Average Spin Coherence Time (T₂)	325.7 ± 148.2	184.6 ± 76.5	µs	1.76x improvement over non-doped.
NV Creation Density (¹⁵NV)	(27.5 ± 17.1) / 100	(12.5 ± 9.6) / 100	µm²	2.2x higher yield in P-doped material.
Average NV^- Population Ratio	0.746 ± 0.017	0.747 ± 0.019	N/A	Stabilization of the negatively charged state.
Highest Measured NV^- Population	0.92	< 0.80 (Non-doped reference)	N/A	Significant stabilization observed in 10% of P-doped centers.
P-Doped Film Thickness	~700	N/A	nm	CVD grown layer on HPHT substrate.
Phosphorus Concentration	5 x 10¹⁶	N/A	cm^-3	Target doping level for n-type conductivity.
NV Center Depth (Simulated)	~15	~15	nm	Peak concentration via SRIM simulation.
Ion Implantation Energy	10	10	keV	Kinetic energy of ¹⁵N ions.
Ion Implantation Temperature	600	600	°C	Implantation process temperature.
Annealing Temperature	800	800	°C	Post-implantation annealing for 30 minutes.

Key Methodologies

The successful fabrication of high-performance shallow NV centers relied on precise MPCVD growth of the semiconductor layer followed by controlled ion-implantation and annealing.

Substrate Preparation: Use of IIa (111) HPHT diamond substrates (0.3 mm thickness) as the base material.
MPCVD Growth: A thin film (~700 nm) of phosphorus-doped (n-type) diamond was grown via Microwave Plasma-Enhanced CVD (MPCVD).
- Doping Target: Phosphorus concentration of 5 x 10¹⁶ cm^-3.
- Carbon Isotope: Methane with natural abundance of ¹³C (1.1%) was used.
Ion Implantation: Nitrogen isotope (¹⁵N) ions were simultaneously implanted into both the P-doped sample (I) and the non-doped reference (II).
- Parameters: Kinetic energy of 10 keV, density of 5 x 10⁸ atoms cm^-2, performed at 600 °C.
- Result: Peak NV center depth simulated at ~15 nm.
Annealing and Cleaning: Samples were annealed at 800 °C for 30 minutes to mobilize vacancies and form NV centers, followed by hot acid washing.
Characterization: Coherence times (T₂) were measured using Hahn-echo sequences under a static magnetic field (3.2 mT). Charge state stability was assessed via nondestructive single-shot charge-state measurements.

6CCVD Solutions & Capabilities

This research highlights the critical role of high-quality, doped CVD diamond films in achieving next-generation quantum performance metrics. 6CCVD is uniquely positioned to supply the custom materials and fabrication services necessary to replicate and advance this work, particularly for integrated quantum devices.

Applicable Materials

The core requirement of this research is a high-quality, low-defect, n-type semiconductor diamond film. 6CCVD offers the following solutions:

6CCVD Material Solution	Relevance to Research	Customization Potential
Phosphorus-Doped SCD (N-Type)	Direct material required for replicating Sample I. 6CCVD provides custom doping profiles (e.g., 5 x 10¹⁶ cm^-3) and ultra-low defect density growth necessary for long T₂.	Custom P-doping concentration and depth control (e.g., delta-doping or super-lattice structures, as suggested in the paper).
Optical Grade SCD Wafers	Ideal substrates for homoepitaxial growth of the doped layer, ensuring high crystalline quality (e.g., (111) orientation used in the paper).	Available in custom dimensions up to 125mm, with thickness control from 0.1 µm to 500 µm.
High-Purity ¹²C SCD	While this paper used natural ¹³C, future research requires isotopically purified ¹²C diamond to extend T₂ beyond the 0.6 ms limit.	6CCVD supplies high-purity ¹²C SCD, enabling T₂ times in the millisecond range, essential for advanced quantum computing.

Customization Potential for Integrated Devices

The fabrication of integrated quantum devices requires precision material engineering, which is a core capability of 6CCVD:

Custom Dimensions and Thickness: 6CCVD can supply the required thin films (e.g., 700 nm) or thicker substrates (up to 10 mm) in custom plate or wafer sizes up to 125 mm (PCD), facilitating industrial scaling and integration.
Precision Polishing: Achieving shallow NV centers (15 nm depth) requires an atomically smooth surface. 6CCVD guarantees ultra-low roughness: Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD.
Metalization Services: For creating electrical contacts or integrated photonic structures (as referenced in the paper), 6CCVD offers in-house metalization using materials including Au, Pt, Pd, Ti, W, and Cu. This is crucial for electrically driven NV centers and charge state control.
Dopant Profile Engineering: The paper suggests improvements via “optimization of the surface termination, the phosphorus concentration, or the dopant profile by using delta-doping or a super-lattice.” 6CCVD’s advanced MPCVD capabilities allow for precise control over dopant placement and concentration gradients to maximize NV^- stability near the surface.

Engineering Support

The successful extension of T₂ and stabilization of the NV^- charge state in this research depends heavily on minimizing defects during CVD growth and subsequent processing. 6CCVD’s in-house PhD-level engineering team specializes in defect engineering and material optimization for quantum applications. We offer consultation services to assist researchers in:

Selecting the optimal SCD orientation ((111) vs. (100)) for specific NV alignment requirements.
Designing custom doping recipes (P-doping concentration and depth) to maximize charge stability and NV creation yield for specific ion-implantation parameters.
Developing post-processing protocols (annealing temperature and duration) tailored to the material specifications.

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

Tech Support

Original Source

DOI: https://doi.org/10.1016/j.carbon.2021.03.010

References

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