Impact of Helium Ion Implantation Dose and Annealing on Dense Near-Surface Layers of NV Centers

At a Glance

Metadata	Details
Publication Date	2022-06-29
Journal	Nanomaterials
Authors	Andris Bērziņš, Hugo Grube, Einārs Sprūģis, G. Vaivars, Ilja Fescenko
Institutions	University of Latvia
Citations	8
Analysis	Full AI Review Included

Executive Summary

This study investigates the optimization of helium (He⁺) ion implantation dose and subsequent high-temperature annealing steps to maximize the magnetic sensitivity of dense, near-surface Nitrogen-Vacancy (NV) layers fabricated in inexpensive High-Pressure/High-Temperature (HPHT) diamond.

Core Achievement: Tripling the He⁺ implantation dose (from 8 to 24 x 10¹² He⁺/cm²) increased the magnetic sensitivity of the NV sensor ensemble by 28 ± 5%.
Material and Structure: Sensors were fabricated in HPHT Type Ib diamond (100 ppm initial N) to create a dense, near-surface NV layer approximately 200 nm thick.
Dose Optimization: Data suggests a linear growth in NV^- concentration and sensitivity up to the maximum tested dose. Extrapolation projects a potential sensitivity improvement of up to 70% at the estimated saturation dose (0.5 x 10¹⁴ He⁺/cm²).
Annealing Impact: While initial annealing at 800 °C is necessary, subsequent annealing steps at 1100 °C provided only a modest average sensitivity gain of 6.6 ± 2.7%, primarily by enhancing the transverse relaxation time (T₂).
Spin Coherence Trade-offs: Higher implantation doses successfully convert paramagnetic P1 centers (substitutional nitrogen) into NV centers, improving T₂ (coherence). However, the increased density of the NV^- bath simultaneously reduces the longitudinal relaxation time (T₁).
Conclusion for Fabrication: High implantation doses (up to 24 x 10¹² He⁺/cm²) are highly lucrative for maximizing sensitivity, and annealing at 1100 °C should not be neglected for optimal T₂ performance.

Technical Specifications

Parameter	Value	Unit	Context
Diamond Type	HPHT Type Ib	N/A	Initial material for fabrication.
Initial N Concentration	~100	ppm	Substitutional nitrogen (P1 centers).
NV Layer Thickness	~200	nm	Fabricated using multi-energy He⁺ implantation.
Implantation Dose (F1, Standard)	8 x 10¹²	He⁺/cm²	Lowest dose tested.
Implantation Dose (F3, Triple)	24 x 10¹²	He⁺/cm²	Highest dose tested.
Optimal Projected Dose (Saturation)	0.5 x 10¹⁴	He⁺/cm²	Estimated dose where P1 = NV^- concentration.
He Ion Energies	33, 15, 5	keV	Used to achieve uniform 200 nm depth profile.
Max Sensitivity Improvement	28 ± 5	%	Achieved by tripling the implantation dose (F3 vs F1).
Projected Max Sensitivity Improvement	Up to 70	%	Extrapolated to the optimal saturation dose.
Transverse Relaxation Time (T₂) Range	70 - 85	ns	Homogeneous decoherence time (increases with dose).
Longitudinal Relaxation Time (T₁) Range	1.0 - 1.6	ms	Spin-lattice relaxation time (decreases with dose).
Estimated NV^- Concentration (F3)	~10	ppm	Estimated post-annealing concentration.
NV⁰-to-NV^- Conversion Efficiency	~25	%	Conservative value used for concentration estimates.

Key Methodologies

The fabrication process involved multi-energy He⁺ implantation followed by three sequential annealing steps under vacuum, separated by acid cleaning.

Material Preparation: HPHT Type Ib diamond crystals (2 mm x 2 mm x 0.06 mm) with a (110) surface polish were used.
SRIM Simulation: Stopping Range of Ions in Matter (SRIM) simulations were performed to determine the necessary He⁺ energies (33 keV, 15 keV, 5 keV) and doses to create a uniform 200 nm vacancy-depth profile near the surface.
Ion Implantation: Three samples (F1, F2, F3) were irradiated with cumulative doses of 8, 16, and 24 x 10¹² He⁺/cm², respectively.
Acid Cleaning: All crystals underwent 6 hours of boiling at 200 °C in triacid (1:1:1 mixture of nitric, perchloric, and sulfuric acids) before and after each annealing step to remove surface impurities.
First Annealing Step: Performed at 800 °C for 2 hours under a vacuum of 1 x 10^-2 ± 0.1 x 10^-2 mbar.
Subsequent Annealing Steps: Two additional steps were performed sequentially at 1100 °C for 2 hours each, under a higher vacuum of 1 x 10^-5 ± 0.3 x 10^-5 mbar.
Ramp/Cool-down: All annealing steps included a 4-hour ramp-up and cool-down time.
Characterization: Optically Detected Magnetic Resonance (ODMR) measurements were performed to quantify fluorescence intensity, contrast, and Full Width at Half Maximum (FWHM). Longitudinal (T₁) and transverse (T₂) relaxation times were measured using pulsed relaxometry sequences (Hahn echo for T₂).

Commercial Applications

The development of dense, near-surface NV ensembles in diamond is critical for applications requiring high spatial resolution and high sensitivity, particularly in magnetic sensing.

Widefield Magnetic Imaging: Creating thin, dense NV layers (200 nm) is optimal for widefield magnetic imaging where the sensor plane is adjacent to the sample, offering spatial resolution close to the layer thickness.
Quantum Sensing: Fabrication of high-sensitivity quantum sensors, benefiting from the NV center’s long coherence times and room-temperature operation.
Biomagnetism and Medical Diagnostics: High-sensitivity detection of weak magnetic fields generated by biological samples (e.g., imaging Malarial Hemozoin nanocrystals, as referenced in related work).
Solid-State Physics Research: Providing high-quality, dense NV ensembles for studying spin dynamics, cross-relaxation, and decoherence mechanisms in solid-state systems.
Mass Production of Sensors: Utilizing inexpensive HPHT diamond with high initial nitrogen concentration makes this fabrication method viable for the large-scale production of high-performance magnetic sensors.

View Original Abstract

The implantation of diamonds with helium ions has become a common method to create hundreds-nanometers-thick near-surface layers of NV centers for high-sensitivity sensing and imaging applications; however, optimal implantation dose and annealing temperature are still a matter of discussion. In this study, we irradiated HPHT diamonds with an initial nitrogen concentration of 100 ppm using different implantation doses of helium ions to create 200-nm thick NV layers. We compare a previously considered optimal implantation dose of ∼1012 He+/cm2 to double and triple doses by measuring fluorescence intensity, contrast, and linewidth of magnetic resonances, as well as longitudinal and transversal relaxation times T1 and T2. From these direct measurements, we also estimate concentrations of P1 and NV centers. In addition, we compare the three diamond samples that underwent three consequent annealing steps to quantify the impact of processing at 1100 °C, which follows initial annealing at 800 °C. By tripling the implantation dose, we have increased the magnetic sensitivity of our sensors by 28±5%. By projecting our results to higher implantation doses, we demonstrate that it is possible to achieve a further improvement of up to 70%. At the same time, additional annealing steps at 1100 °C improve the sensitivity only by 6.6 ± 2.7%.

Tech Support

Original Source

DOI: https://doi.org/10.3390/nano12132234

References

2020 - Nitrogen in Diamond [Crossref]
2020 - Sensitivity optimization for NV-diamond magnetometry [Crossref]
2016 - Diamond Quantum Devices in Biology [Crossref]
2018 - Nanodiamonds and Their Applications in Cells [Crossref]
2020 - Novel color center platforms enabling fundamental scientific discovery [Crossref]
2020 - Sensitive magnetometry in challenging environments [Crossref]
2018 - Tutorial: Magnetic resonance with nitrogen-vacancy centers in diamond—Microwave engineering, materials science, and magnetometry [Crossref]
2019 - Principles and techniques of the quantum diamond microscope [Crossref]
2016 - Optically detected magnetic resonance of high-density ensemble of NV- centers in diamond [Crossref]
2014 - Magnetometry with nitrogen-vacancy defects in diamond [Crossref]