Coherence times of precise depth controlled NV centers in diamond

At a Glance

Metadata	Details
Publication Date	2016-01-01
Journal	Nanoscale
Authors	Junfeng Wang, Wenlong Zhang, Jian Zhang, Jie You, Yan Li
Institutions	Hefei National Center for Physical Sciences at Nanoscale, University of Science and Technology of China
Citations	42
Analysis	Full AI Review Included

Technical Documentation & Analysis: Depth Control of NV Centers in Diamond

This document analyzes the research paper “Coherence times of precisely depth controlled NV centers in diamond” and outlines how 6CCVD’s advanced MPCVD diamond materials and customization capabilities can support and extend this critical quantum sensing research.

Executive Summary

The research successfully demonstrated nanoscale control over the depth of Nitrogen-Vacancy (NV) centers in electronic grade diamond, providing crucial insights into the influence of the surface spin bath on quantum coherence.

Precision Depth Control: Achieved an ultra-slow and stable oxidative etching rate of 1.1 nm/h at 580 °C, enabling precise, step-by-step tracing of NV center evolution.
Critical Depth Threshold: Identified a critical depth of approximately 22 nm from the diamond surface where the NV center coherence time ($T_2$) rapidly collapses due to interaction with the fast fluctuating surface spin bath.
High Initial Coherence: Initial spin echo coherence times ($T_2$) reached up to 234.6 µs, comparable to deep, native NV centers in high-purity diamond.
Material Requirement: The experiment relied on high-quality (100) electronic grade Single Crystal Diamond (SCD) with ultra-low nitrogen ([N]<5 ppb) and natural ¹³C abundance (1.1%).
Application Relevance: The methodology is vital for creating shallow NV centers with optimized coherence, essential for high-sensitivity nanoscale magnetic field and spin detection.
6CCVD Value Proposition: 6CCVD provides the necessary high-purity SCD substrates, custom dimensions, and ultra-smooth polishing required to replicate and scale this foundational quantum research.

Technical Specifications

The following hard data points were extracted from the research paper detailing the material properties and experimental results:

Parameter	Value	Unit	Context
Initial Diamond Grade	Electronic Grade (100)	N/A	Element Six source material
Nitrogen Concentration ([N])	< 5	ppb	Required for long coherence times
Carbon Isotope Concentration ([¹³C])	1.1	%	Natural abundance
Implantation Energy	60	keV	¹⁴N molecules
Implantation Fluence	$0.55 \times 10^{11}$	¹⁴N/cm²	NV center generation
Annealing Temperature	1050	°C	Vacuum annealing for NV formation
Oxidative Etching Temperature	580	°C	Used for slow, precise etching
Oxidative Etching Rate	1.1	nm/h	Achieved rate in air
Critical Coherence Depth	$\approx 22$	nm	Depth where $T_2$ rapidly declines
Maximum Initial $T_2$ (Spin Echo)	234.6	µs	NV-10 before etching
Minimum Final $T_2$ (Spin Echo)	0.7	µs	NV-10 after 49 h etching
Maximum $T_2$ (CPMG-100)	360.4	µs	Before etching

Key Methodologies

The experiment focused on precise material preparation, NV center creation, and controlled nanoscale removal of the diamond surface layer.

Substrate Selection: Use of $2 \times 2 \times 0.5$ mm³ (100) electronic grade SCD with ultra-low nitrogen content.
Masking and Patterning: A 300 nm thick polymethyl methacrylate (PMMA) layer was deposited and patterned using electron beam lithography to create 60 nm diameter apertures and 10 µm wide position strips.
Implantation: 60 keV ¹⁴N molecules were implanted through the PMMA mask to generate NV precursors at a controlled depth profile (SRIM simulations used for verification).
High-Temperature Annealing: The sample was annealed at 1050 °C in high vacuum ($2 \times 10^{-5}$ Pa) for 2 hours to convert nitrogen defects into long-coherence NV centers.
Surface Cleaning: Post-annealing cleaning was performed using a 1:1:1 boiling mixture of sulfuric, nitric, and perchloric acid at 200 °C.
Nanoscale Depth Control: Successive oxidative etching was performed in a box furnace at a reduced temperature of 580 °C in air, achieving the critical slow etching rate of 1.1 nm/h.
Coherence Measurement: Spin echo ($T_2$), Ramsey ($T_2^*$), and CPMG dynamical decoupling measurements were performed after each etching step to trace the coherence time dependence on depth.

6CCVD Solutions & Capabilities

This research highlights the absolute necessity of high-quality, ultra-pure diamond substrates for advanced quantum applications. 6CCVD is uniquely positioned to supply the materials and customization required to replicate, scale, and advance this work.

Applicable Materials

Research Requirement	6CCVD Recommended Material	Rationale & Advantage
High Purity Substrate	SCD (Single Crystal Diamond)	Electronic grade SCD with [N] < 5 ppb (or lower, upon request) is essential to minimize bulk spin noise and maximize initial $T_2$.
Depth Control Starting Surface	Optical Grade SCD (Ra < 1 nm)	Ultra-smooth surface polishing is critical. Starting with Ra < 1 nm minimizes surface defects and reduces the influence of the rapid fluctuating surface spin bath, potentially extending the critical 22 nm depth threshold.
Alternative Sensing Platform	Heavy Boron Doped PCD (BDD)	For electrochemical or high-power applications requiring robust, conductive diamond electrodes, BDD offers a scalable alternative platform.

Customization Potential for Replication and Scaling

The paper used small $2 \times 2 \times 0.5$ mm³ chips. 6CCVD specializes in providing custom dimensions and processing services necessary for industrial scaling and advanced research geometries.

Custom Dimensions and Thickness: 6CCVD can supply (100) SCD plates in custom sizes far exceeding the $2 \times 2$ mm² used, up to 125 mm in diameter (PCD) or large-area SCD wafers. We offer precise thickness control for SCD from 0.1 µm up to 500 µm, and substrates up to 10 mm thick.
Ultra-Low Roughness Polishing: To ensure optimal starting conditions for implantation and subsequent oxidative etching, 6CCVD offers state-of-the-art polishing services, guaranteeing Ra < 1 nm on SCD surfaces. This is crucial for minimizing surface damage that contributes to $T_2$ degradation.
Metalization Services: While the paper focused on etching, future NV center experiments often require microwave delivery structures. 6CCVD offers in-house metalization capabilities, including deposition of Au, Pt, Pd, Ti, W, and Cu layers, allowing researchers to integrate microwave antennas directly onto the diamond surface.
Laser Cutting and Shaping: We provide custom laser cutting services to achieve precise geometries (e.g., diamond tips for AFM integration, as referenced in the paper’s conclusion [28]) necessary for high-sensitivity nanoscale magnetic imaging.

Engineering Support

6CCVD’s in-house team of PhD material scientists and engineers can assist researchers in optimizing material selection for similar shallow NV center quantum sensing projects. We provide consultation on:

Selecting the optimal SCD purity and orientation for specific $T_2$ targets.
Determining appropriate surface preparation (polishing grade) to minimize surface spin bath effects.
Designing custom geometries for integration into existing quantum setups.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. We offer global shipping (DDU default, DDP available) to ensure rapid delivery of mission-critical materials.

View Original Abstract

We investigated the depth dependence of coherence times of nitrogen-vacancy (NV) centers through precise depth control using oxidative etching at 580 °C in air. By successive nanoscale etching, NV centers could be brought close to the diamond surface step by step, which enabled us to track the evolution of the number of NV centers remaining in the chip and to study the depth dependence of coherence times of NV centers with diamond etching. Our results showed that the coherence times of NV centers declined rapidly with the depth reduction in the last about 22 nm before they finally disappeared, which revealed a critical depth for the influence of a rapid fluctuating surface spin bath. Moreover, by using the slow etching method combined with low-energy nitrogen implantation, NV centers with depths shallower than the initially implanted depths can be generated, which are preferred for detecting external spins with higher sensitivity.

Tech Support

Original Source

DOI: https://doi.org/10.1039/c5nr08690f