Nitrogen-related point defects in homoepitaxial diamond (001) freestanding single crystals

At a Glance

Metadata	Details
Publication Date	2023-04-24
Journal	Journal of Applied Physics
Authors	Tokuyuki Teraji, Chikara Shinei
Institutions	National Institute for Materials Science
Citations	19
Analysis	Full AI Review Included

Technical Documentation & Analysis: Nitrogen Defect Control in MPCVD Diamond

This document analyzes the research on controlling nitrogen-related point defects in homoepitaxial diamond (001) using Microwave Plasma Chemical Vapor Deposition (MPCVD) under oxygen-adding conditions. The findings are critical for optimizing single-crystal diamond (SCD) materials for advanced quantum sensing applications requiring high concentrations of negatively charged Nitrogen-Vacancy (NV^-) centers and long coherence times (T₂).

Executive Summary

Quantum Material Optimization: Successful synthesis of ¹⁵N-doped, ¹²C-enriched, free-standing homoepitaxial diamond (001) optimized for quantum sensing applications.
Precise Doping Control: Nitrogen concentration in the diamond crystal was precisely controlled over three orders of magnitude by adjusting the N/C gas ratio from 10 ppb to 10 ppm.
High Purity & Isotopic Enrichment: Achieved high isotopic purity using >99.9% ¹²C-enriched methane and 98% ¹⁵N-enriched molecular nitrogen, crucial for maximizing electron spin coherence time (T₂).
Defect Engineering: Oxygen-adding growth conditions suppressed non-epitaxial crystallites and growth hillocks, maintaining high crystalline quality even at high nitrogen doping levels.
NV Precursor Dominance: Approximately 70% of the incorporated nitrogen was confirmed to be neutral substitutional nitrogen (N_s⁰, the P1 center), the necessary donor precursor for forming high-density NV^- centers.
Charge State Confirmation: Photoluminescence (PL) and Electron Paramagnetic Resonance (EPR) confirmed that both NV and NVH centers were predominantly in the desired negatively charged state (NV^- and NVH^-).

Technical Specifications

The following hard data points were extracted from the MPCVD growth parameters and material characterization results:

Parameter	Value	Unit	Context
Growth Method	Microwave Plasma CVD (MPCVD)	N/A	Homoepitaxial growth on (100) substrates
Carbon Isotope Purity	>99.9	%	¹²C enrichment in methane source
Nitrogen Isotope Purity	98	%	¹⁵N enrichment in molecular nitrogen source
N/C Gas Ratio (Control Range)	10 ppb to 10 ppm	N/C ratio	Achieved precise control over 3 orders of magnitude
Nitrogen Incorporation Efficiency	(1.9 ± 0.2) x 10^-4	Ratio	Constant efficiency using molecular N₂
Substrate Temperature	1020-1090	°C	Measured during CVD growth
Reaction Pressure	110	Torr	Standard CVD condition
Methane Concentration (CH₄/Total)	10	%	Flow rate ratio
Oxygen Concentration (O₂/Total)	2	%	Used to suppress defects/hillocks
Substitutional Nitrogen [N_s⁰]	4.9 to 12.9	ppm	Measured via SIMS and EPR (P1 center)
N_s⁰ Fraction of Total N	~70	%	Predominantly neutral charge state
NV^- Center Ratio ([NV^-]/[NV_Total])	0.84 to 0.90	Ratio	Confirmed negative charge state dominance
Maximum Thickness Grown	1.2	mm	Free-standing SCD plate (Sample 5)

Key Methodologies

The research utilized a highly controlled MPCVD process combined with comprehensive defect analysis:

Substrate Preparation: HPHT-grown Type-Ib (001) diamond substrates (3.5 x 3.5 mm², 1 mm thick) were used for homoepitaxial growth.
Isotopic Source Gases: High-purity ¹²C-enriched methane (>99.9%) was used as the carbon source, and ¹⁵N₂ (98% enriched) was used as the nitrogen dopant.
Oxygen-Adding Growth: A 2% oxygen concentration was maintained in the gas flow to suppress the formation of non-epitaxial crystallites and growth hillocks, preserving high crystalline quality.
Recipe Parameters: Growth was conducted at 110 Torr, 1.4 kW microwave power, 10% CH₄ concentration, and substrate temperatures between 1020-1090 °C.
Free-Standing Plate Creation: After growth, the CVD layer was laser-cut from the side and top surface to remove the substrate and polycrystalline rim, yielding free-standing SCD plates up to 1.2 mm thick.
Defect Characterization Suite:
- SIMS (Secondary Ion Mass Spectrometry): Used for depth profiling of ¹⁵N, ¹⁴N, H, B, and Si concentrations, confirming uniform nitrogen incorporation.
- EPR (Electron Paramagnetic Resonance): Used to quantify the neutral substitutional nitrogen (P1 center, N_s⁰) and the NVH^- center concentrations.
- FTIR (Fourier Transform Infrared Spectroscopy): Used to quantify N_s⁰, N_s⁺, and NVH⁰ concentrations based on characteristic absorption peaks (e.g., 1130 cm^-1 for N_s⁰).
- PL (Photoluminescence): Used to determine the charge state ratio of NV centers ([NV^-]/[NV⁰]) via zero-phonon line (ZPL) intensity measurements (637 nm for NV^-, 575 nm for NV⁰).

6CCVD Solutions & Capabilities

This research demonstrates the critical need for highly controlled, isotopically pure, and defect-engineered single-crystal diamond. 6CCVD is uniquely positioned to supply the materials required to replicate, scale, and advance this quantum sensing research.

Applicable Materials for Quantum Sensing

To achieve the high sensitivity and long coherence times demonstrated in this study, researchers require materials with exceptional purity and precise doping control.

6CCVD Material Solution	Specification & Relevance to Research
Isotopic Single Crystal Diamond (SCD)	Requirement: ¹²C enrichment (>99.9%) to minimize nuclear spin decoherence (T₂). 6CCVD supplies high-purity ¹²C SCD wafers, ensuring maximum coherence time for NV^- centers.
Custom N-Doped SCD	Requirement: Precise control of nitrogen concentration (10 ppb to 10 ppm range) and use of ¹⁵N. 6CCVD offers custom nitrogen doping (using either ¹⁴N or ¹⁵N) to target specific NV precursor concentrations (N_s⁰) necessary for high-density NV^- formation.
Optical Grade SCD	Requirement: Low defect density and high crystalline quality, maintained via oxygen-adding growth. Our Optical Grade SCD features Ra < 1 nm polishing and minimal non-nitrogen impurities, ideal for high-fidelity optical measurements (PL/FTIR).
Custom Substrates	Requirement: Low-strain HPHT substrates (Type-Ib (001)) were used. 6CCVD can supply high-quality, low-dislocation SCD substrates optimized for subsequent homoepitaxial growth.

Customization Potential for Scaling and Device Integration

The research utilized small, thick samples (up to 3.3 x 3.3 mm², 1.2 mm thick). 6CCVD offers the capability to scale these dimensions and integrate device features:

Large Area SCD: While the paper focused on small samples, 6CCVD can provide SCD plates up to 500 µm thick and PCD wafers up to 125 mm in diameter for large-scale device fabrication.
Custom Thickness: We offer SCD and PCD layers ranging from 0.1 µm (for surface-sensitive NV applications) up to 500 µm, and substrates up to 10 mm thick, allowing researchers to tailor material volume to specific magnetic sensing requirements.
Precision Processing: The paper required laser cutting to create free-standing plates. 6CCVD provides in-house laser cutting, shaping, and high-quality polishing (Ra < 1 nm for SCD) services to deliver ready-to-use components.
Metalization Services: For subsequent device integration (e.g., microwave strip lines or electrodes for charge state control), 6CCVD offers custom metalization layers including Au, Pt, Pd, Ti, W, and Cu.

Engineering Support

6CCVD’s in-house PhD team specializes in defect engineering and MPCVD growth optimization. We can assist researchers in similar Quantum Sensing and NV Center projects by:

Optimizing N_s⁰/N_s⁺ Ratios: Consulting on growth parameters (like oxygen concentration and temperature) to maximize the neutral substitutional nitrogen fraction (N_s⁰), which is crucial for maximizing the final NV^- yield after irradiation and annealing.
Hydrogen Management: Providing guidance on growth recipes to minimize unwanted hydrogen incorporation, which was found to be 1.5-4 times higher than nitrogen and may affect defect stability.
Isotopic Material Sourcing: Ensuring reliable, globally shipped supply of high-purity ¹²C and ¹⁵N source materials for reproducible results.

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

View Original Abstract

Controllability of nitrogen doping, types of nitrogen-related defects, and their charge states in homoepitaxial diamond (001) crystals were investigated. For these purposes, 15N-doped 12C-enriched free-standing chemical vapor deposited diamond (001) crystals were grown through long-time growth using 12C-enriched methane as the carbon source gas and 15N-enriched molecular nitrogen as the nitrogen source gas. The formation of non-epitaxial crystallites and growth hillocks was suppressed by the application of the oxygen-adding growth condition. Nitrogen was incorporated uniformly into the crystals, with a concentration variation of less than 10%. About 70% of the total nitrogen was substitutional nitrogen in a neutral charge state Ns0. Hydrogen was incorporated at approximately the same concentration as nitrogen. Both NV and NVH centers were predominantly negatively charged defect structures, i.e., NV− and NHV− centers. The concentrations of NHV− centers were less than 5% of the total nitrogen concentration. Nitrogen concentration in diamond crystals was controlled by changing the N/C gas ratio over a wide doping range from 10 ppb to 10 ppm. Nitrogen incorporation efficiency was found to be (1.5 ± 0.5) × 10−4 in this study.

Tech Support

Original Source

DOI: https://doi.org/10.1063/5.0143652

References

2015 - High-quality and high-purity homoepitaxial diamond (100) film growth under high oxygen concentration condition [Crossref]