Spin coherence and depths of single nitrogen-vacancy centers created by ion implantation into diamond via screening masks

At a Glance

Metadata	Details
Publication Date	2020-06-24
Journal	Journal of Applied Physics
Authors	Shuntaro Ishizu, Kento Sasaki, Daiki Misonou, Tokuyuki Teraji, Kohei M. Itoh
Institutions	National Institute for Materials Science, RIKEN Center for Emergent Matter Science
Citations	6
Analysis	Full AI Review Included

Technical Documentation & Analysis: Near-Surface NV Creation via Screening Masks

Executive Summary

This documentation analyzes the successful creation and characterization of single Nitrogen-Vacancy (NV) centers near the diamond surface using 10 keV N⁺ ion implantation through thin SiO₂ screening masks. This method is critical for advancing nanoscale quantum sensing applications, particularly nanoNMR.

Core Achievement: Demonstrated a method using comparatively high-energy (10 keV) N⁺ implantation combined with a SiO₂ mask to concentrate the highest N⁺ density at the diamond surface, enabling shallow NV creation.
Material Requirement: The study utilized high-purity, isotopically enriched ¹²C CVD diamond, essential for suppressing nuclear spin bath noise and achieving measurable coherence times (T₂).
Coherence Results: Observed T_2,echo up to 27.1 µs for NV centers located at a depth (d_NV) of 17.4 nm, establishing a T₂-d_NV relationship comparable to state-of-the-art low-energy implantation methods.
Depth Profiling: Confirmed that a large fraction of NV centers are located within 10 nm of the surface, consistent with Monte Carlo simulations (SRIM).
Noise Limitation: Noise spectroscopy confirmed that T₂ is primarily limited by magnetic noise, presumably due to surface defects, highlighting the need for ultra-smooth, stable diamond surfaces.
6CCVD Value Proposition: 6CCVD provides the necessary high-purity Single Crystal Diamond (SCD) substrates, custom dimensions, and superior surface polishing (Ra < 1 nm) required to stabilize and extend the T₂ coherence times of these critical near-surface NV centers.

Technical Specifications

The following hard data points were extracted from the research paper detailing the NV creation and characterization process:

Parameter	Value	Unit	Context
Implantation Energy	10	keV	N⁺ ions
Implantation Dose	10¹¹	cm^-2	Incident angle normal to surface
Screening Mask Material	SiO₂	N/A	Thickness varied: 52.3, 57.6, 64.1, 69.1 nm
Max Observed T_2,echo	27.1	µs	Measured via Hahn echo decay
Corresponding NV Depth (d_NV)	17.4	nm	Determined via proton NMR spectroscopy
NV Yield (Efficiency)	0.1 - 0.4	%	Conversion from N⁺ ions to NV centers
Initial Annealing	800	°C	2 hours in vacuum (9.7 x 10^-7 torr)
Oxidation Annealing	450	°C	9 hours in oxygen atmosphere
Average Surface Roughness (R_rms)	0.17	nm	Measured via AFM after processing
Static Magnetic Field (B₀)	23.2	mT	Applied parallel to NV symmetry axis

Key Methodologies

The creation of near-surface NV centers relied on precise material preparation and controlled implantation/annealing steps:

Substrate Preparation: Began with a Type-IIa (001) natural abundant diamond substrate (2 x 2 x 0.5 mm³).
Isotopic Enrichment: An undoped, high-purity ¹²C layer (99.95% isotopic purity, few µm thickness) was grown via Chemical Vapor Deposition (CVD) to minimize nuclear spin bath noise.
Screening Mask Deposition: SiO₂ layers were deposited onto the diamond surface using electron beam evaporation, with thicknesses precisely controlled (52.3 nm to 69.1 nm).
Ion Implantation: ¹⁴N⁺ ions were implanted at 10 keV energy and a dose of 10¹¹ cm^-2 through the SiO₂ mask.
Mask Removal: The SiO₂ layers were removed post-implantation using hydrofluoric (HF) acid.
Annealing & Charge State Conversion:
- Vacancy Diffusion: Annealed at 800 °C in vacuum to mobilize vacancies to pair with implanted N atoms, forming NV centers.
- Charge State Conversion: Subsequent annealing at 450 °C in an oxygen atmosphere to convert neutral NV⁰ centers into the desired negatively charged NV^- state.
Characterization: Single NV centers were characterized using continuous wave Optically Detected Magnetic Resonance (CW ODMR), Hahn echo decay (T_2,echo), and depth determination via proton NMR spectroscopy (XYk sequences).

6CCVD Solutions & Capabilities

The research successfully demonstrated a robust method for creating shallow NV centers. However, the conclusion explicitly notes that T₂ coherence is limited by surface noise and instability, suggesting that improved material quality and surface engineering are the next critical steps. 6CCVD is uniquely positioned to supply the advanced diamond materials and processing services required to stabilize and extend the T₂ coherence times for next-generation quantum devices.

Applicable Materials

Research Requirement	6CCVD Solution	Material Specification	Value Proposition
High Purity Substrate	Optical Grade SCD	SCD (001) orientation, few µm thick layer, high isotopic purity (e.g., >99.99% ¹²C available upon request).	Minimizes intrinsic spin bath noise (¹³C), crucial for achieving long T₂ and clear NMR spectra.
Surface Quality	Precision Polished SCD	Ra < 1 nm (Standard SCD polishing).	Directly addresses the paper’s finding that surface morphology (R_rms = 0.17 nm) limits T₂ stability. Superior polishing is essential for stabilizing near-surface NV centers.
Future Integrated Devices	Polycrystalline Diamond (PCD)	Plates/wafers up to 125 mm diameter.	Enables scaling up from small research substrates (2x2 mm²) to industrial-scale quantum sensor arrays.

Customization Potential

The researchers used small, custom-processed substrates. 6CCVD’s capabilities directly support the replication and extension of this work:

Custom Dimensions: 6CCVD supplies SCD and PCD plates/wafers in custom sizes, significantly larger than the 2x2 mm² samples used in the study, up to 125 mm in diameter (PCD).
Thickness Control: We offer SCD layers from 0.1 µm up to 500 µm, allowing researchers to precisely match the CVD layer thickness used in the experiment (a few µm).
Metalization Services: The experiment utilized external copper wires for microwave delivery. For integrated quantum devices, 6CCVD offers in-house custom metalization (e.g., Ti/Pt/Au, W, Cu) directly patterned onto the diamond surface, enabling on-chip microwave delivery and enhanced coupling efficiency for NV centers.

Engineering Support

The paper concludes that optimizing annealing (potentially up to 1200 °C) and maintaining ultra-pure surface morphology (R_rms < 0.06 nm) are key to stabilizing T₂ for NV centers < 5 nm deep.

R&D Partnership: 6CCVD’s in-house PhD team specializes in MPCVD growth and post-processing optimization. We can assist researchers in selecting the optimal SCD grade and surface preparation protocols (including custom polishing and cleaning chemistries) for similar Nanoscale Quantum Sensing projects.
Material Selection for Yield Improvement: The observed low NV yield (0.1-0.4%) suggests optimization is needed. We provide consultation on material selection and surface treatments (e.g., plasma etching) to maximize N⁺-to-NV conversion efficiency.

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

View Original Abstract

We characterize single nitrogen-vacancy (NV) centers created by 10-keVN+ ion implantation into diamond via thin SiO2 layers working as screening masks. Despite the relatively high acceleration energy compared with standard ones (&lt;5keV) used to create near-surface NV centers, the screening masks modify the distribution of N+ ions to be peaked at the diamond surface [Ito et al., Appl. Phys. Lett. 110, 213105 (2017)]. We examine the relation between coherence times of the NV electronic spins and their depths, demonstrating that a large portion of NV centers are located within 10 nm from the surface, consistent with Monte Carlo simulations. The effect of the surface on the NV spin coherence time is evaluated through noise spectroscopy, surface topography, and x-ray photoelectron spectroscopy.

Tech Support

Original Source

DOI: https://doi.org/10.1063/5.0012187

References

2013 - Nanoscale nuclear magnetic resonance with a nitrogen-vacancy spin sensor [Crossref]
2013 - Nuclear magnetic resonance spectroscopy on a (5-Nanometer)3 sample volume [Crossref]
2014 - Nuclear magnetic resonance spectroscopy with single spin sensitivity [Crossref]
2015 - Nanoscale nuclear magnetic imaging with chemical contrast [Crossref]
2015 - Nanoscale NMR spectroscopy and imaging of multiple nuclear species [Crossref]
2016 - Nuclear magnetic resonance detection and spectroscopy of single proteins using quantum logic [Crossref]
2017 - Nanoscale nuclear magnetic resonance with chemical resolution [Crossref]
2013 - Quantum plasmonics [Crossref]
2016 - Quantum nanophotonics in diamond [Crossref]
2018 - Material platforms for spin-based photonic quantum technologies [Crossref]