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

Technical Documentation & Analysis: Dense Near-Surface NV Layers via He+ Implantation

6CCVD Reference Document: ANL-2022-NV-001 Source Paper: Berzins et al., Impact of Helium Ion Implantation Dose and Annealing on Dense Near-Surface Layers of NV Centers, Nanomaterials 2022, 12, 2234.

Executive Summary

This research successfully optimized helium ion implantation and high-temperature annealing protocols to maximize the magnetic sensitivity of near-surface Nitrogen-Vacancy (NV) ensembles in diamond.

Core Achievement: Creation of dense, 200 nm thick NV layers in HPHT diamond (100 ppm N) using optimized He+ ion implantation doses.
Performance Gain: Tripling the standard implantation dose (from $8 \times 10^{12}$ to $24 \times 10^{12}$ He+/cm²) resulted in a significant 28 ± 5% improvement in magnetic sensitivity (B_min).
Optimal Projection: Extrapolation suggests that an optimal dose of $0.5 \times 10^{14}$ He+/cm², where P1 donor concentration matches NV^- concentration, could yield up to 70% sensitivity improvement.
Critical Processing: A two-step high-temperature annealing process (800 °C followed by 1100 °C under high vacuum) was confirmed as essential for defect healing, maximizing fluorescence intensity, and achieving optimal transverse relaxation time (T₂).
Material Requirement: The study underscores the need for high-purity, precisely polished diamond substrates capable of withstanding extreme high-vacuum, high-temperature processing required for advanced quantum sensing applications.

Technical Specifications

The following hard data points were extracted from the study regarding material properties, processing parameters, and performance metrics.

Parameter	Value	Unit	Context
Substrate Material	HPHT Type Ib Diamond	N/A	Initial N concentration: 100 ppm
Substrate Orientation	(110)	N/A	Surface polish
NV Layer Depth	200	nm	Near-surface layer thickness
Implantation Ion	Helium (He+)	N/A	Used to create vacancies
Implantation Energies	33, 15, 5	keV	Used for uniform depth profile
Minimum Dose (F1)	$8 \times 10^{12}$	He+/cm²	Standard reference dose
Maximum Dose (F3)	$24 \times 10^{12}$	He+/cm²	Triple dose tested
Optimal Projected Dose	$0.5 \times 10^{14}$	He+/cm²	Projected dose for P1/NV^- saturation
Initial Annealing Temp.	800	°C	2 hours, $1 \times 10^{-2}$ mbar vacuum
Final Annealing Temp.	1100	°C	Two subsequent 2-hour steps
Final Annealing Vacuum	$1 \times 10^{-5}$	mbar	High vacuum environment
Sensitivity Improvement	28 ± 5	%	Measured B_min improvement (F3 vs F1)
Projected Sensitivity Gain	Up to 70	%	Extrapolated at optimal dose
T₂ Relaxation Time Range	70 - 85	ns	Dependent on dose and annealing

Key Methodologies

The fabrication of high-density NV ensembles requires precise control over implantation energy, dose, and subsequent thermal processing.

Substrate Selection and Preparation:
- HPHT Type Ib diamond crystals (100 ppm N) were used, polished to a (110) surface orientation.
- Dimensions were small (2 mm x 2 mm x 0.06 mm), requiring high precision cutting.
He+ Ion Implantation:
- SRIM simulations were used to determine three specific energies (33 keV, 15 keV, 5 keV) required to achieve a uniform 200 nm vacancy-depth profile.
- Doses were scaled linearly (F1, F2, F3) to test the effect of vacancy concentration on NV yield.
Chemical Cleaning:
- A 6-hour boiling triacid mixture (nitric: perchloric: sulfuric) was used before and after each annealing step to remove surface impurities and graphitized carbon.
Thermal Annealing Protocol (Three Steps):
- Step 1 (800 °C): Performed for 2 hours under vacuum ($1 \times 10^{-2}$ mbar) to initiate vacancy migration and NV formation.
- Steps 2 & 3 (1100 °C): Two subsequent 2-hour steps performed under high vacuum ($1 \times 10^{-5}$ mbar). This high-temperature step was crucial for reducing vacancy-related paramagnetic defects and maximizing T₂ coherence time.
Characterization:
- Optically Detected Magnetic Resonance (ODMR) spectroscopy was used to measure fluorescence intensity, contrast (NV^-/NV⁰ ratio), and FWHM.
- Relaxometry measurements (T₁ and T₂) were performed using microwave impulse sequences (Hahn echo) to assess spin coherence properties.

6CCVD Solutions & Capabilities

6CCVD provides the high-purity materials and advanced processing services necessary to replicate and significantly extend the performance achieved in this research, enabling the fabrication of next-generation quantum sensors.

Applicable Materials

The paper utilized HPHT diamond. For optimal magnetic sensitivity and coherence, 6CCVD recommends materials with superior purity and crystalline quality:

Optical Grade Single Crystal Diamond (SCD):
- Benefit: Offers inherently lower defect density and superior lattice quality compared to HPHT, which is critical for achieving longer T₂ coherence times—the limiting factor for pulse magnetometry sensitivity.
- Customization: 6CCVD can supply Low-Nitrogen SCD (N < 1 ppm) for researchers who prefer to control the nitrogen concentration solely via implantation, or Controlled-Nitrogen SCD for vacancy creation methods requiring specific P1 donor levels.
Polycrystalline Diamond (PCD):
- Benefit: For large-area imaging applications requiring scalability beyond the small 2 mm x 2 mm substrates used in the study, 6CCVD offers high-quality PCD wafers up to 125 mm in diameter.

Customization Potential

6CCVD’s in-house fabrication capabilities directly address the precise requirements of NV ensemble creation:

Requirement from Paper	6CCVD Capability	Technical Specification
Substrate Dimensions	Custom Plates/Wafers	SCD up to 10 mm x 10 mm; PCD up to 125 mm diameter.
Surface Orientation	Custom Orientation	Standard (100) and (111), plus the required (110) orientation for specific NV axis alignment.
Surface Quality	Ultra-Polishing	SCD surface roughness (Ra) < 1 nm; Inch-size PCD Ra < 5 nm. Essential for near-surface NV creation.
Thickness Control	Precise Layer Engineering	SCD/PCD thickness from 0.1 µm (for ultra-thin sensing) up to 500 µm (for bulk applications).
Integrated Components	Custom Metalization	In-house deposition of Au, Pt, Pd, Ti, W, Cu for creating on-chip microwave delivery lines (ODMR) or electrical contacts.

Engineering Support

The research highlights the critical nature of the 1100 °C high-vacuum annealing step for maximizing NV^- yield and T₂ time.

Thermal Processing Expertise: 6CCVD’s in-house PhD team specializes in optimizing post-growth processing, including high-vacuum, high-temperature annealing protocols necessary for maximizing NV^- conversion efficiency and minimizing lattice damage in quantum sensing projects.
Application Consultation: We provide material selection and engineering consultation for projects targeting high-sensitivity applications, such as magnetic imaging, quantum computing, and solid-state spin ensembles, ensuring the optimal diamond grade is selected for specific ion implantation and annealing recipes.

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

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

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