Creation of nitrogen-vacancy centers in chemical vapor depositionn diamond for sensing applications

At a Glance

Metadata	Details
Publication Date	2021-11-15
Journal	arXiv (Cornell University)
Authors	Tingpeng Luo, Lukas Lindner, Julia Langer, V. Cimalla, Xavier Vidal
Institutions	The University of Melbourne, Fraunhofer Institute for Applied Solid State Physics
Citations	60
Analysis	Full AI Review Included

Technical Documentation & Analysis: NV Center Optimization in CVD Diamond

This document analyzes the research paper “Creation of nitrogen-vacancy centers in chemical vapor deposition diamond for sensing applications” to provide technical specifications and demonstrate how 6CCVD’s advanced MPCVD diamond capabilities can support and extend this critical quantum sensing research.

Executive Summary

This study successfully demonstrates the systematic engineering of Nitrogen-Vacancy (NV) center ensembles in Chemical Vapor Deposition (CVD) diamond, a crucial step for high-sensitivity quantum magnetometry.

Material Control: CVD growth parameters (specifically the N/C ratio) were precisely modulated over four orders of magnitude (150 to 10⁶ ppm) to control the initial single substitutional nitrogen (P1) concentration from 0.2 to 20 ppm.
NV Creation Optimization: Optimal electron-beam irradiation fluences (using 1 MeV and 2 MeV electrons) were determined for a fixed P1 density (~2.2 ppm) to maximize P1-to-NV conversion while maintaining high NV⁻ charge state stability (up to 86% NV⁻/NV ratio).
Coherence Time Achievement: The resulting treated NV ensembles exhibited long spin coherence times ($T_2$) ranging from 45.5 µs (at 168 ppb NV⁻) to 549 µs (at 1 ppb NV⁻), confirming the inverse relationship between P1 density and $T_2$.
Sensitivity Pathway: The combined optimization of high NV concentration and long $T_2$ time provides a direct pathway to significantly improve the shot-noise limited sensitivity ($\eta$) for NV-ensemble-based magnetometry.
Method Validation: The study validates a novel approach using UV-Vis absorption spectroscopy (specifically the appearance of the GR1 band) as an indicator for determining the optimal irradiation fluence before annealing.

Technical Specifications

The following table summarizes the key quantitative results and material parameters achieved in the study.

Parameter	Value	Unit	Context
P1 Concentration Range (As-Grown)	0.2 - 20	ppm	Controlled by N/C ratio (150 to 10⁶ ppm)
NV Concentration Range (As-Grown)	0.03 - 33.9	ppb	Proportional to P1 concentration
NV Concentration Range (Treated)	1 - 168	ppb	After optimal irradiation and 1000 °C annealing
Maximum Spin Coherence Time ($T_2$)	549 ± 332	µs	Achieved at lowest NV⁻ concentration (1 ppb)
Minimum Spin Coherence Time ($T_2$)	45.5 ± 1.2	µs	Achieved at highest NV⁻ concentration (168 ppb)
Optimal NV⁻/NV Ratio	66 - 86	%	Achieved by balancing P1 conversion and remaining P1 donors
Optimal Irradiation Fluence (2 MeV)	1E17 - 2E17	e/cm²	For initial P1 concentration of ~2.2 ppm
Optimal Irradiation Fluence (1 MeV)	1E18 - 3E18	e/cm²	For initial P1 concentration of ~2.2 ppm
Annealing Temperature/Duration	1000 °C / 2h	°C / h	Post-irradiation treatment
CVD Growth Temperature	800 - 900	°C	Measured by radiation thermometer

Key Methodologies

The experimental procedure involved precise CVD growth control followed by systematic post-growth processing and characterization.

CVD Synthesis: Diamond was grown in an ellipsoidal-shaped MPCVD reactor (2.45 GHz, 6 kW) at 210 mbar pressure and 800-900 °C substrate temperature.
Nitrogen Doping: The N/C ratio was varied using adjustable nitrogen doping gas flow, resulting in P1 concentrations ranging from 0.2 ppm to 20 ppm in the (100) oriented SCD material.
Vacancy Creation: Samples were subjected to high-energy electron irradiation at room temperature using two different energies (1 MeV and 2 MeV) across a range of fluences (1E16 to 3E18 e/cm²).
NV Formation: Irradiated samples underwent subsequent vacuum annealing at 1000 °C for 2 hours to mobilize vacancies, allowing them to combine with P1 centers to form NV centers.
P1 Characterization: P1 concentration was measured using X-band Continuous Wave Electron Paramagnetic Resonance (EPR) spectroscopy.
NV Characterization: NV concentration and NV charge state (NV⁻/NV⁰ ratio) were determined via Photoluminescence (PL) spectroscopy using a 532 nm laser (10 µW power) and calibrated against the UV-Visible absorption cross-section.
Coherence Measurement: Spin coherence time ($T_2$) was measured using the Hahn-echo protocol under a home-built widefield microscope.

6CCVD Solutions & Capabilities

6CCVD specializes in providing the high-quality, customized MPCVD diamond materials and processing support necessary to replicate and advance the findings of this research, particularly in optimizing NV ensemble performance for quantum sensing.

Applicable Materials

To achieve the precise doping control and high material quality demonstrated in this study, 6CCVD recommends the following materials:

Optical Grade Single Crystal Diamond (SCD): Required for achieving the long $T_2$ coherence times (up to 549 µs). Our SCD material features ultra-low strain and high purity, ensuring minimal decoherence sources beyond the intentionally introduced nitrogen.
Custom Nitrogen Doped SCD: We offer precise control over the N/C ratio during MPCVD growth to target specific P1 concentrations (e.g., 0.2 ppm for long $T_2$ or 20 ppm for high NV density), enabling researchers to fine-tune the balance between NV concentration and coherence time for specific applications.

Customization Potential

The research highlights the need for specific material dimensions, orientation, and post-processing preparation. 6CCVD directly addresses these needs:

Research Requirement	6CCVD Capability	Technical Specification
Custom Dimensions	Plates/Wafers up to 125mm	SCD thickness: 0.1 µm to 500 µm. Substrates up to 10 mm.
Surface Quality	Ultra-low roughness polishing	SCD: Ra < 1 nm. Essential for high-resolution optical readout.
Metalization for MW Delivery	In-house deposition services	Custom metal stacks (Au, Pt, Pd, Ti, W, Cu) for fabricating microwave delivery structures (e.g., the omega-shaped resonator used for $T_2$ measurement).
Post-Processing Preparation	Custom SCD thickness and polishing	We deliver pre-polished, high-purity, doped SCD plates ready for immediate electron irradiation and subsequent high-temperature annealing (1000 °C).

Engineering Support

The optimization of NV ensembles is a complex process involving precise control over growth, irradiation fluence, and annealing protocols.

Doping Recipe Consultation: 6CCVD’s in-house PhD team can assist researchers in defining the optimal nitrogen doping recipe to achieve the desired initial P1 concentration, crucial for subsequent fluence calculations.
Fluence Optimization Modeling: We provide material consultation to help estimate the required electron irradiation fluence based on the target P1 concentration and desired NV⁻/NV ratio, accelerating the optimization process for similar quantum sensing and magnetometry projects.
Global Logistics: We ensure reliable global shipping (DDU default, DDP available) of sensitive, high-value diamond materials, minimizing logistical delays for international research collaborations.

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

View Original Abstract

The nitrogen-vacancy (NV) center in diamond is a promising quantum system for\nmagnetometry applications exhibiting optical readout of minute energy shifts in\nits spin sub-levels. Key material requirements for NV ensembles are a high\nNV$^-$ concentration, a long spin coherence time and a stable charge state.\nHowever, these are interdependent and can be difficult to optimize during\ndiamond growth and subsequent NV creation. In this work, we systematically\ninvestigate the NV center formation and properties in chemical vapor deposition\n(CVD) diamond. The nitrogen flow during growth is varied by over 4 orders of\nmagnitude, resulting in a broad range of single substitutional nitrogen\nconcentrations of 0.2-20 parts per million. For a fixed nitrogen concentration,\nwe optimize electron-irradiation fluences with two different accelerated\nelectron energies, and we study defect formation via optical characterizations.\nWe discuss a general approach to determine the optimal irradiation conditions,\nfor which an enhanced NV concentration and an optimum of NV charge states can\nboth be satisfied. We achieve spin-spin coherence times T$_2$ ranging from 45.5\nto 549 $\mu$s for CVD diamonds containing 168 to 1 parts per billion NV$^-$\ncenters, respectively. This study shows a pathway to engineer properties of\nNV-doped CVD diamonds for improved sensitivity.\n

Tech Support

Original Source

DOI: https://doi.org/10.1088/1367-2630/ac58b6