Indirect overgrowth as a synthesis route for superior diamond nano sensors

At a Glance

Metadata	Details
Publication Date	2020-12-29
Journal	Scientific Reports
Authors	Christoph Findler, Johannes Lang, Christian Osterkamp, Miloš Nesládek, Fedor Jelezko
Institutions	Center for Integrated Quantum Science and Technology
Citations	20
Analysis	Full AI Review Included

Indirect Overgrowth for Superior Diamond Nano Sensors: A 6CCVD Technical Analysis

This document analyzes the research detailing the “Indirect overgrowth” method for fabricating high-coherence, depth-confined Nitrogen-Vacancy (NV^-) centers in diamond, and outlines how 6CCVD’s advanced MPCVD capabilities directly support the replication and scaling of this critical quantum sensing technology.

Executive Summary

Core Achievement: The research successfully implemented an “Indirect Overgrowth” method (Implantation → CVD Overgrowth → Annealing) to create shallow, highly stable NV^- centers, overcoming the passivation and etching losses associated with traditional direct overgrowth.
Enhanced Coherence: The technique achieved a significant enhancement in spin coherence time (T_2,avg) up to 100 µs at room temperature—a five-fold increase over reference samples—by burying the NV centers under a protective diamond capping layer.
Nanoscale Precision: Precise depth control was demonstrated, achieving capping layer thicknesses of 6.5 nm and 13 nm, confirming the ability to tailor NV^- depth for optimal sensitivity in nanoscale quantum metrology.
Material Requirement: Success hinges on the use of ultra-high purity, electronic-grade, isotopically enriched ¹²C Single Crystal Diamond (SCD) substrates, which 6CCVD provides as a core product.
Mechanism Insight: The study identifies hydrogen passivation (conversion of NV to non-fluorescing NVH centers) as the primary loss mechanism during growth, showing that higher ¹⁵N implantation doses slow down this conversion kinetics.
Scalability: This methodology is critical for the reliable, repeatable engineering of depth-confined NV-qubits, paving the way for advanced diamond quantum sensors and quantum registers.

Technical Specifications

The following table summarizes the critical material and process parameters extracted from the research:

Parameter	Value	Unit	Context
Substrate Material	Electronic-grade ¹²C SCD (100)	N/A	High purity (99.999 % ¹²C enriched)
Implantation Species	¹⁵N⁺	N/A	Isotope used to distinguish from native ¹⁴NV centers
Implantation Energy Range	2.5 - 5.0	keV	Used for shallow defect creation
Implantation Dose (Medium)	10¹¹	¹⁵N⁺/cm²	Primary dose for T₂ enhancement study
CVD Growth Temperature	900	°C	Capping layer deposition
CVD Methane Concentration	0.05	%	Low concentration ¹²CH₄ in H₂
CVD Working Pressure	22.5	mbar	MPCVD operational parameter
Annealing Temperature	1000	°C	UHV annealing for NV formation
Achieved Capping Thickness	6.5 and 13	nm	Resulting from 1 h and 2 h overgrowth, respectively
Diamond Growth Rate (2.5 keV)	5	nm/h	NV^- calibrated growth rate
Maximum T_2,avg Achieved	100	µs	5x enhancement over reference
Maximum T_2,avg* Achieved	20	µs	High value for DC magnetic sensing
Nuclear Hyperfine Splitting	3	MHz	Confirms ¹⁵N nucleus (I_15N = 1/2)

Key Methodologies

The successful fabrication of superior diamond nano sensors relies on the precise control of the MPCVD growth environment and post-processing steps:

Substrate Preparation: Commercial electronic-grade (100) SCD diamond is rigorously cleaned using a hot acid mixture (Nitric, Sulphuric, and Perchloric acid) in a microwave reactor (200 °C, 30 min) to ensure an atomically clean surface.
Buffer Layer Growth: An initial ultrapure ¹²C-diamond layer (approx. 150 nm) is grown via MPCVD (0.2% ¹²CH₄ in H₂ at 900 °C) to provide a high-quality, low-defect starting point.
Low-Energy Ion Implantation: ¹⁵N⁺ ions are implanted at low energies (2.5 keV or 5 keV) using 98 %-enriched ¹⁵N₂ gas to create shallow nitrogen defects and associated vacancies.
Indirect Overgrowth (Capping Layer Deposition): The sample is heated (700 °C), exposed to H₂ plasma (5 min), and then a thin ¹²C capping layer is grown via MPCVD (0.05% ¹²CH₄ in H₂) at 900 °C. This step buries the implanted nitrogen before NV formation.
UHV Annealing: The final step involves annealing in an Ultra High Vacuum (UHV) oven at 1000 °C for 3 hours to mobilize vacancies, allowing them to combine with the implanted ¹⁵N atoms to form stable ¹⁵NV^- centers.
Spin Characterization: Confocal microscopy and pulsed ODMR techniques (Hahn-echo, Ramsey, XY8) are used to measure the resulting NV density, depth distribution (via NMR sensing of ¹H spins), and critical spin lifetimes (T₂ and T₂*).

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the foundational materials and engineering services required to replicate and advance the “Indirect Overgrowth” technique for high-performance quantum sensors.

Applicable Materials

The research demands ultra-high purity, low-strain diamond material with precise isotopic control. 6CCVD offers the following solutions:

6CCVD Material	Specification	Relevance to Research
Optical Grade SCD	SCD (100) orientation, low nitrogen content (< 1 ppb).	Provides the necessary low-defect, high-quality crystal structure required for long coherence times.
Isotopically Pure ¹²C SCD	99.999 % enriched ¹²C SCD wafers.	Essential for minimizing decoherence caused by native ¹³C nuclear spins, maximizing T₂ and T₂* performance, as demonstrated in the paper.
Custom Thickness SCD	SCD plates from 0.1 µm up to 500 µm.	Ideal for preparing the initial buffer layer (150 nm) and ensuring the final device thickness meets application requirements.
Polycrystalline Diamond (PCD)	Wafers up to 125 mm diameter.	While the paper used SCD, 6CCVD can provide large-area PCD substrates for scaling up sensor arrays or for applications where large-area coverage is prioritized over ultimate T₂ performance.

Customization Potential

The success of indirect overgrowth relies on nanometer-scale control of the capping layer and the integration of microwave delivery structures. 6CCVD’s in-house capabilities ensure seamless integration:

Nanoscale Thickness Control: 6CCVD’s MPCVD expertise allows for the precise deposition of diamond capping layers with thickness control down to the nanometer scale (0.1 µm), perfectly matching the 6.5 nm and 13 nm layers achieved in this study.
Ultra-Smooth Polishing: Shallow NV centers are highly sensitive to surface defects. 6CCVD guarantees surface roughness (Ra) of < 1 nm for SCD and < 5 nm for inch-size PCD, ensuring minimal surface-induced decoherence.
Integrated Metalization Services: The research utilized a copper wire for microwave (MW) control. For integrated quantum devices, on-chip metalization is necessary. 6CCVD offers custom deposition of standard contact and antenna metals, including Ti/Pt/Au, Pd, W, and Cu, allowing researchers to fabricate optimized MW delivery structures directly onto the diamond surface.
Custom Dimensions and Shaping: 6CCVD provides custom laser cutting and shaping services for plates/wafers up to 125 mm, enabling the creation of application-specific geometries for microfluidic devices or integrated quantum circuits.

Engineering Support

6CCVD’s in-house PhD team specializes in MPCVD growth recipes and material optimization for quantum applications. We offer consultation services to assist researchers in:

Material Selection: Guiding the choice between SCD and PCD, and determining the optimal ¹²C enrichment level based on target T₂ performance.
Recipe Optimization: Assisting with the selection of appropriate methane concentrations and growth temperatures to control NV formation kinetics and minimize hydrogen passivation for similar shallow NV center quantum sensing projects.
Interface Engineering: Consulting on surface termination and polishing techniques necessary to maintain the high spin coherence achieved through indirect overgrowth.

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

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41598-020-79943-2