Nitrogen-vacancy centers created by N+ ion implantation through screening SiO2 layers on diamond

At a Glance

Metadata	Details
Publication Date	2017-05-22
Journal	Applied Physics Letters
Authors	Kazuki Ito, Hiroshi Saito, Kento Sasaki, Hideyuki Watanabe, Tokuyuki Teraji
Institutions	Spintronics Research Network of Japan, National Institute for Materials Science
Citations	13
Analysis	Full AI Review Included

Technical Analysis and Material Sourcing Solutions for Shallow NV Center Fabrication

Leveraging MPCVD Diamond for Advanced Quantum Sensing and Computing

The analyzed research details an effective method for creating shallow, high-coherence nitrogen-vacancy (NV^-) centers in diamond using a standard 10 keV ion implanter combined with an optimized screening mask of amorphous SiO₂. This technique successfully suppresses ion channeling and controls the depth profile, placing single NV centers close to the surface with measured coherence times (T₂) up to 23 µs.

This method is highly relevant to quantum applications requiring large-scale, cost-effective fabrication of near-surface NV centers for magnetometry and quantum networking. 6CCVD provides the next-generation MPCVD diamond substrates required to maximize the quantum performance (specifically T₂ coherence) of devices fabricated using this robust implantation technique.

Executive Summary

Novel Fabrication Method: Shallow NV centers were created using 10 keV ¹⁵N⁺ ion implantation screened by custom-thickness SiO₂ masks, successfully suppressing ion channeling.
Performance Achieved: Single NV centers were isolated, achieving coherence times (T₂) up to 23 µs close to the diamond surface ($t$ = 72 nm mask thickness).
Manufacturing Advantage: The screening mask allows high-fluence, standard implantation systems (10¹¹ cm^-2) to effectively reduce the dose by more than three orders of magnitude, simplifying the process by eliminating the need for expensive low-energy implanters or complex post-implantation plasma etching.
Depth Control: The method successfully places the highest NV density profile at the surface by tuning the SiO₂ thickness ($t \ge 40$ nm).
Material Limitation Addressed: The authors suggest replacing the natural HPHT diamond substrate with isotopically pure ¹²C diamond to improve T₂ coherence properties—a core strength of 6CCVD’s Single Crystal Diamond (SCD) catalog.

Technical Specifications

The following hard data points define the parameters and results of the shallow NV center fabrication process:

Parameter	Value	Unit	Context
Substrate Type	(100) HPHT IIa Diamond	N/A	Natural abundant, ¹³C content approx. 1.1%
Implantation Energy	10	keV	Standard system energy
Implantation Dose (D_N)	10¹¹	cm^-2	Required high starting dose
Implanted Ion	¹⁵N⁺	N/A	Used to discriminate from bulk ¹⁴N
Optimal Screening Mask Thickness ($t$)	53 - 72	nm	Range for isolating discrete single NV centers
Effective Dose Reduction	> three	orders of magnitude	Achieved by SiO₂ screening layer
Highest Coherence Time (T₂)	23	µs	Measured for single NV centers at $t$ = 72 nm
Single NV Center Yield	~0.2	%	Conversion efficiency (NNV / NN) for isolated spots
Vacancy Anneal	800 °C for 2h	N/A	Performed in vacuum
NV^- Conversion Anneal	450 °C for 9h	N/A	Performed in oxygen (O₂) atmosphere

Key Methodologies

The experiment successfully employed a multi-step fabrication sequence for controlled, shallow NV creation:

Substrate Preparation: Use of (100)-oriented HPHT IIa diamond as the starting material.
Screening Mask Deposition: Multiple layers of amorphous SiO₂ were deposited onto the diamond via electron beam evaporation. Thickness ($t$) was precisely monitored via ellipsometry.
Ion Implantation: 10 keV ¹⁵N⁺ ions were implanted at a dose of 10¹¹ cm^-2 through the SiO₂ mask.
Mask Removal: The SiO₂ screening layers were removed post-implantation using hydrofluoric acid (HF).
Annealing for NV Formation: A two-stage thermal annealing process was utilized:
- Stage 1 (Vacancy Diffusion): 800 °C for 2 hours in vacuum.
- Stage 2 (Negative Charge Conversion): 450 °C for 9 hours in an oxygen atmosphere to maximize the NV^- yield.
Characterization: NV properties were examined using fluorescence imaging, Optically-Detected Magnetic Resonance (ODMR), and Hahn-echo sequences to measure T₂ coherence time.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the advanced diamond materials and engineering services necessary to replicate this successful fabrication method and significantly improve T₂ coherence, accelerating the development of robust quantum devices.

Applicable Materials for Coherence Enhancement

The researchers noted that the relatively short T₂ is partly indicative of the near-surface nature but also highlighted that using isotopically pure ¹²C diamond as a starting material will “improve the coherence properties of the NV spins.”

Requirement from Research	6CCVD Solution (Material Grade)	Technical Benefit
Need for isotopically pure ¹²C material	Quantum Grade Single Crystal Diamond (SCD)	Drastically reduces the concentration of ¹³C (I = 1/2) nuclear spins in the bulk, minimizing magnetic noise and maximizing T₂ coherence time, potentially reaching millisecond scale.
Substrate for scaling fabrication	Large Area SCD Wafers (up to 125mm)	Supports standard lithography and high-throughput ion implantation methods necessary for industrial application (e.g., using standard 10 keV implanters).
Optimal starting surface	Optical Grade SCD Polishing	Guaranteed polishing standard of Ra < 1 nm (SCD), critical for uniform thin film deposition (SiO₂ mask) and precise shallow implantation depth control.

Customization Potential

Replicating the research requires precise control over the diamond surface and the deposited screening mask thickness ($t$ = 53-72 nm). 6CCVD offers comprehensive engineering support:

Custom Dimensions and Thickness: We can supply (100) or (111) SCD wafers up to 10mm thickness, ensuring compatibility with virtually any implantation tooling platform. Our SCD layer thickness ranges from 0.1 µm up to 500 µm.
Advanced Surface Preparation: Beyond standard polishing, 6CCVD’s internal capabilities ensure atomically smooth surfaces, crucial for high-quality, defect-free deposition of amorphous screening layers (SiO₂, AlO₂, etc.).
Integrated Metalization Services: While this study used SiO₂, future research may explore metal masks (as suggested for focused ion beam work). 6CCVD offers in-house deposition of standard masking materials including Ti, Au, Pt, and W, customized to specifications.

Engineering Support

6CCVD’s in-house PhD team specializes in Diamond Quantum Material Science and can assist research groups with process optimization:

Material Selection and Orientation: Consultation on the optimal crystal orientation ((100) or (111)) and nitrogen concentration for specific shallow NV projects.
Implantation Recipe refinement: Assistance in calculating required substrate thickness and thermal management protocols for high-temperature (800 °C) annealing cycles.

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

View Original Abstract

We report on an ion implantation technique utilizing a screening mask made of SiO2 to control both the depth profile and the dose. By appropriately selecting the thickness of the screening layer, this method fully suppresses the ion channeling, brings the location of the highest nitrogen-vacancy (NV) density to the surface, and effectively reduces the dose by more than three orders of magnitude. With a standard ion implantation system operating at the energy of 10 keV and the dose of 1011 cm2 and without an additional etching process, we create single NV centers close to the surface with coherence times of a few tens of μs.

Tech Support

Original Source

DOI: https://doi.org/10.1063/1.4984060