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	Keio University, RIKEN Center for Emergent Matter Science
Citations	6
Analysis	Full AI Review Included

Executive Summary

This research characterizes single Nitrogen-Vacancy (NV) centers created near the diamond surface using a novel ion implantation technique, focusing on the relationship between NV depth (d_NV) and spin coherence time (T₂).

Novel Implantation Method: Near-surface NV centers were successfully created using a comparatively high energy (10 keV) N⁺ ion implantation combined with a thin Silicon Dioxide (SiO₂) screening mask.
Depth Profile Validation: The resulting NV depth profile was concentrated toward the surface (unlike standard Gaussian profiles), with a large portion of centers located within 10 nm, consistent with Monte Carlo (SRIM) simulations.
Coherence-Depth Correlation: A clear trend was established: deeper NV centers exhibit longer T_2,echo. The measured T_2,echo vs. d_NV relation is comparable to results from other low-energy implantation methods.
Yield and Density: The conversion efficiency (yield) from implanted N⁺ ions to active NV centers was low (0.1-0.4%), similar to standard low-energy implantation results.
Noise Limitation: Noise spectroscopy confirmed that magnetic noise, likely originating from paramagnetic surface defects, is the dominant factor limiting the spin coherence time (T₂) in these near-surface NV centers.
Surface Instability: The T₂ of shallow NV centers was observed to be unstable or degrade over time, a phenomenon potentially linked to slight morphological changes (wrinkles) observed on the diamond surface after the SiO₂ deposition and processing steps.

Technical Specifications

Parameter	Value	Unit	Context
Ion Implantation Species	¹⁴N⁺	N/A	Used instead of ¹⁵N⁺ due to use of ¹²C CVD layer
Ion Implantation Energy	10	keV	Relatively high energy for near-surface creation
Ion Implantation Dose	10¹¹	cm^-2	Target dose for single NV resolution
SiO₂ Mask Thickness (t) Range	52.3 to 69.1	nm	Varied to control implantation profile
NV Center Yield (Efficiency)	0.1 - 0.4	%	Conversion efficiency from N⁺ to NV^-
Maximum T_2,echo Measured	27.1	µs	Achieved using Hahn echo sequence
Shallowest d_NV Measured	14.4	nm	Determined via proton NMR (XY16-128 sequence)
Magnetic Field (B₀)	23.2	mT	Applied parallel to NV axis for ODMR/NMR
Proton NMR Frequency (ω_n/2π)	0.988	MHz	Calculated value for 23.2 mT field
Magnetic Noise Rate (Ω)	316	Hz	Deduced from Single Quantum (SQ) relaxation
Electric Noise Rate (γ)	272	Hz	Deduced from Double Quantum (DQ) relaxation
Surface Roughness (R_rms, t=0)	0.11	nm	AFM measurement on blocked area
Surface Roughness (R_rms, t≠0)	0.14	nm	AFM measurement on implanted area
Longest T_2,drive Achieved	43.0	µs	Under N-pulse dynamical decoupling (N=512)

Key Methodologies

The creation and characterization of single NV centers involved precise material growth, implantation, and multi-stage annealing and cleaning processes:

Substrate Selection: Began with a natural abundant (1.1% ¹³C) Type-IIa (001) diamond substrate.
CVD Growth: An undoped, isotopically pure ¹²C layer (99.95%) was grown by Chemical Vapor Deposition (CVD) to a thickness of a few microns to suppress nuclear spin bath noise (¹³C).
Screening Mask Deposition: SiO₂ layers were deposited using electron beam evaporation. A metal plate with four apertures was used to create areas with controlled thicknesses (t = 52.3, 57.6, 64.1, and 69.1 nm).
Ion Implantation: ¹⁴N⁺ ions were implanted at 10 keV energy with a dose of 10¹¹ cm^-2, passing through the deposited SiO₂ screening masks.
Mask Removal: The SiO₂ layers were subsequently removed using hydrofluoric acid (HF).
Annealing (Vacancy Diffusion): The sample was annealed at 800 °C for 2 hours in vacuum (9.7 x 10^-7 torr) to promote vacancy diffusion and NV center formation.
Annealing (Charge State Conversion): A second annealing step was performed at 450 °C for 9 hours in an oxygen atmosphere to convert neutral NV⁰ centers into the desired negatively charged NV^- state.
Chemical Cleaning: The sample underwent rigorous chemical cleaning using a triacid mixture (sulfuric, nitric, and perchloric acids) and a piranha solution (H₂O₂/sulfuric acid).
Quantum Characterization: Single NV centers were measured using Optically Detected Magnetic Resonance (ODMR), Hahn Echo (T_2,echo), Dynamical Decoupling (XYk sequences) for noise spectroscopy, and Proton Ensemble NMR for d_NV determination.

Commercial Applications

The precise engineering of shallow NV centers is critical for applications requiring strong coupling between the quantum sensor and external analytes or fields.

Nano-scale NMR and MRI: The primary application is high-sensitivity Nuclear Magnetic Resonance (NMR) spectroscopy and imaging of analyte ensembles (e.g., proteins, chemicals) placed directly on the diamond surface, requiring NV centers within 5-10 nm of the surface.
Quantum Sensing and Magnetometry: Development of high-resolution quantum sensors for detecting magnetic and electric fields, particularly in environments where surface proximity is necessary (e.g., measuring current flow in 2D materials).
Quantum Communication and Photonics: Integration of shallow NV centers into nanophotonic and plasmonic structures to enhance light coupling efficiency, crucial for quantum network nodes and quantum light sources.
Diamond Material Processing: Provides a validated, high-energy implantation recipe combined with screening masks, offering an alternative to standard low-energy implantation for creating high-quality, shallow defect layers in CVD diamond.
Surface Science Research: The methodology provides a tool for investigating and characterizing surface defects and noise sources that limit quantum coherence, informing future surface passivation and cleaning protocols.

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

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