Patterned Formation of Highly Coherent Nitrogen-Vacancy Centers Using a Focused Electron Irradiation Technique

At a Glance

Metadata	Details
Publication Date	2016-03-24
Journal	Nano Letters
Authors	Claire A. McLellan, Bryan Myers, Stephan Kräemer, Kenichi Ohno, D. D. Awschalom
Institutions	University of Chicago
Citations	124
Analysis	Full AI Review Included

Technical Analysis and Documentation: Deterministic NV Center Formation

Executive Summary

This research demonstrates a breakthrough technique for the deterministic, three-dimensional (3D) placement of highly coherent Nitrogen-Vacancy (NV) centers in diamond, a critical step for scalable quantum technologies.

Record Coherence: Achieved exceptionally long quantum coherence times (T₂) exceeding 1 ms in shallow, 50-nm-deep NV centers, indicating minimal lattice damage from the formation process.
3D Nanoscale Precision: Full 3D control is achieved by combining two techniques: vertical confinement via ¹⁵N $\delta$-doping (4 nm precision) and lateral confinement via focused TEM electron irradiation (< 1 µm precision).
Material Purity Requirement: The success relies on ultra-high purity, isotopically enriched (99.99% ¹²C) CVD diamond substrates to minimize decoherence from ¹³C nuclear spins.
Gentle Formation Mechanism: The TEM irradiation technique introduces minimal lattice defects, resulting in T₂ times limited only by the paramagnetic P1 center spin bath, unlike typical ion implantation methods.
Process Optimization: The study precisely determined the minimum threshold energy (E_T) for NV formation at 145 keV, corresponding to a carbon displacement energy (E_d) of 30 ± 4 eV.
Scalability for Nanostructures: The maskless, point-and-shoot TEM method is highly desirable for integrating high-quality NV centers into complex nanofabricated devices, such as photonic crystals and mechanical resonators.

Technical Specifications

Parameter	Value	Unit	Context
NV Coherence Time (T₂)	1.0 ± 0.15	ms	Characteristic T₂ for 50-nm-deep NV
Minimum T₂ Observed	100	µs	All measured 50-nm-deep NV centers
Vertical N Confinement	~4	nm	Thickness of the ¹⁵N $\delta$-doped layer
Lateral V Confinement	< 1	µm	Precision achieved via focused TEM spot size
Substrate Purity	99.99%	¹²C	Isotopically pure CVD diamond film
NV Depth (Tested Samples)	20, 50	nm	Set by the CVD-grown capping layer thickness
Electron Energy Range (TEM)	120 to 200	keV	Used for vacancy introduction
Electron Dose Range (TEM)	10¹⁴ to 10¹⁸	e/cm²	Used for vacancy introduction
Threshold Energy (E_T) for NV Formation	145 ± 16	keV	Minimum energy required at the surface
Carbon Displacement Energy (E_d)	30 ± 4	eV	Calculated from E_T
NV Conversion Efficiency (N to NV)	~50	%	High efficiency observed using 200 keV dose
Surface Roughness (Pre-Growth)	< 1	nm	RMS roughness required for substrate
Annealing Temperature	850	°C	In H₂/Ar forming gas (2 hours)

Key Methodologies

The deterministic formation of highly coherent NV centers relies on precise control over the CVD growth process and subsequent post-processing steps:

Substrate Preparation: Electronic-grade diamond substrates were polished to an RMS surface roughness of < 1 nm. A 500 nm etch using ArCl ICP ion etching was performed to remove polishing-induced strain, ensuring optimal material quality.
CVD Film Growth: A thin film (50-80 nm) of isotopically pure (99.99%) ¹²C diamond was grown using a plasma-enhanced chemical vapor deposition (PECVD) process.
Nitrogen $\delta$-Doping: A 4-nm-thick layer of ¹⁵N was introduced during the CVD growth process (known as $\delta$-doping) at specific depths (20 nm or 50 nm) below the surface, providing vertical confinement.
Vacancy Introduction: Focused electron irradiation was performed using a Transmission Electron Microscope (TEM) at room temperature. Electron energies were tuned between 120 keV and 200 keV, and doses ranged from 10¹⁴ to 10¹⁸ e/cm².
Annealing: Samples were annealed at 850 °C in H₂/Ar forming gas for 2 hours. This step mobilizes the TEM-induced vacancies, allowing them to migrate and combine with the localized ¹⁵N atoms to form NV centers.
Surface Stabilization: The diamond surface was oxidized using a boiling acid mixture (~210 °C) for one hour to stabilize the NV center charge state (NV^-).

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the advanced MPCVD diamond materials required to replicate and extend this high-coherence quantum research. Our capabilities directly address the stringent purity, doping, and surface quality demands of deterministic NV formation.

Applicable Materials

To achieve T₂ coherence times limited only by P1 centers (as demonstrated in the paper), researchers require ultra-pure material with precise doping control.

Material Requirement (Paper)	6CCVD Solution	Technical Advantage
Electronic-Grade Substrate	Optical Grade Single Crystal Diamond (SCD)	Ultra-low native nitrogen (< 1 ppb) and high crystalline quality, essential for long T₂ coherence.
Isotopically Pure ¹²C	Isotopically Enriched SCD	Standard offering for quantum applications, minimizing ¹³C nuclear spin bath decoherence.
Precise ¹⁵N $\delta$-Doping	Custom $\delta$-Doped SCD	We offer precise control over the thickness (down to 0.1 µm) and depth of the ¹⁵N layer, replicating the critical 4 nm $\delta$-layer and 20/50 nm capping layers used in this study.

Customization Potential

The success of TEM irradiation relies on integrating the diamond material into complex nanofabrication workflows (e.g., photonic crystals, mechanical resonators). 6CCVD provides the necessary physical and chemical customization:

Dimensional Control: We supply custom plates and wafers up to 125 mm in diameter (PCD) and offer precise laser cutting and shaping of SCD plates to fit specific TEM or nanofabrication holders.
Ultra-Low Roughness Polishing: The paper required Ra < 1 nm surface roughness to minimize strain. 6CCVD guarantees Ra < 1 nm polishing on SCD and Ra < 5 nm on inch-size PCD, ensuring minimal surface-induced decoherence for shallow NVs.
Metalization Services: While not used in this specific paper, future integration of NV centers into microwave circuits or electrodes (e.g., for Stark shift control) requires metalization. 6CCVD offers in-house deposition of standard quantum metals including Au, Pt, Pd, Ti, W, and Cu.

Engineering Support

The determination of optimal electron energy (145 keV) and dose is highly dependent on the initial material quality and doping profile.

Material Recipe Optimization: 6CCVD’s in-house PhD team specializes in optimizing MPCVD growth recipes to achieve specific nitrogen concentrations (e.g., the 20 ppb P1 concentration limit cited in the paper) and depth profiles required for deterministic NV formation projects.
Process Integration: We provide consultation on material selection and preparation (polishing, etching, doping) to ensure compatibility with advanced post-processing techniques like focused electron irradiation and subsequent high-temperature annealing (850 °C).

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

View Original Abstract

We demonstrate fully three-dimensional and patterned localization of nitrogen-vacancy (NV) centers in diamond with coherence times in excess of 1 ms. Nitrogen δ-doping during chemical vapor deposition diamond growth vertically confines nitrogen to 4 nm while electron irradiation with a transmission electron microscope laterally confines vacancies to less than 450 nm. We characterize the effects of electron energy and dose on NV formation. Importantly, our technique enables the formation of reliably high-quality NV centers inside diamond nanostructures with applications in quantum information and sensing.

Tech Support

Original Source

DOI: https://doi.org/10.1021/acs.nanolett.5b05304