Stabilizing shallow color centers in diamond created by nitrogen delta-doping using SF6 plasma treatment

At a Glance

Metadata	Details
Publication Date	2015-03-16
Journal	Applied Physics Letters
Authors	Christian Osterkamp, Johannes Lang, Jochen Scharpf, Christoph Müller, Liam P. McGuinness
Institutions	Universität Ulm
Citations	48
Analysis	Full AI Review Included

Technical Documentation and Analysis: Diamond NV Stabilization via Delta-Doping

Executive Summary

This research demonstrates a critical advance in creating stable, shallow Nitrogen-Vacancy (NV) centers in CVD diamond, essential components for quantum sensing and solid-state qubits.

Precise Defect Placement: Stable NV centers were fabricated at an ultra-shallow depth of approximately 5 nm using the Nitrogen delta-doping technique during the Chemical Vapor Deposition (CVD) process.
Critical Surface Engineering: Stabilization of these near-surface NVs was achieved exclusively through surface termination using SF₆ plasma, resulting in robust fluorine coverage. Standard hydrogen or acid terminations failed to produce stable, charged NV centers.
Quantum Performance: The shallow NVs exhibited excellent Optically Detected Magnetic Resonance (ODMR) contrast (> 20%), confirming their potential for use in high-sensitivity sensing applications.
Depth Verification: Nuclear Magnetic Resonance (NMR) measurements of protons in the immersion oil precisely confirmed the NV average depth at 5 nm.
Material Foundation: The process required the reproducible growth of thick (5 µm), high-purity Type IIa diamond films via Plasma Enhanced CVD (PECVD), minimizing intrinsic nitrogen concentration.
Application Focus: The findings pave the way for designing next-generation surface-based sensors utilizing the high magnetic and electric field sensitivity of ultra-shallow NV centers.

Technical Specifications

The following hard data points define the material preparation and key results of the shallow NV fabrication process.

Parameter	Value	Unit	Context
NV Center Depth	~5	nm	Measured via Proton NMR
Coherence Time (T₂)	4 ± 1	µs	Short-term average limited by paramagnetic surface defects
Buried Layer T₂	Few Hundreds	µs	Limited solely by natural ¹³C nuclear spins
ODMR Contrast	> 20	%	Achieved on stable, shallow NVs
CVD Temperature	750	°C	Plasma Enhanced CVD (PECVD)
CVD Pressure	20	mbar	Used for both growth and hydrogenation
Microwave Power / Frequency	1.2 kW / 2.45 GHz	-	PECVD operating parameters
H₂ Flow Rate	200	sccm	Main carrier gas
CH₄ Flow Rate	1	sccm	Methane precursor (99.9995% purity)
N₂ Delta-Doping Flow	5	sccm	Added for 5 minutes during layer growth
SF₆ Stabilization Flow	100	sccm	Used for plasma termination (4 minutes)
ICP Power (SF₆)	500	W	Inductively Coupled Plasma power for fluorine termination
Diamond Layer Thickness	~5	µm	Type IIa layer grown on Type Ib substrate

Key Methodologies

The experimental procedure relies heavily on precise control over MPCVD parameters and subsequent surface plasma treatment to stabilize the defect charge state.

Substrate Pre-Treatment: Commercial diamond substrates (Type Ib) underwent extensive chemical cleaning using piranha acid, chromium sulfuric acid, aqua regia, ammonia-hydrogen peroxide, potassium hydroxide, and final piranha acid wash.
PECVD Preparation & Initial Growth:
- Substrates were mounted in the reactor and exposed to a Hydrogen plasma (200 sccm H₂) for 5 minutes at 750°C and 20 mbar to achieve steady-state conditions and initial H-termination.
- A high-purity Type IIa layer (~5 µm thick) was grown using 1 sccm CH₄/200 sccm H₂, targeting ppb-range nitrogen concentration.
Nitrogen Delta-Doping: N₂ gas (5 sccm) was pulsed into the reactor chamber for 5 minutes to create the highly localized nitrogen layer near the surface.
Surface Stabilization Procedure:
- The CVD process was stopped, leaving the surface H-terminated (which causes NV fluorescence disappearance/blinking).
- The sample was treated with SF₆ plasma (100 sccm flow) in an Inductively Coupled Plasma (ICP) system at 500 W for 4 minutes to achieve robust Fluorine termination.
Characterization: X-Ray Photoelectron Spectroscopy (XPS) confirmed the presence of fluorine (F1s peak at 685 eV) and the complete absence of oxygen, indicating superior surface coverage compared to CF₄ treatments. Proton NMR measured the NV depth.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the ultra-high purity materials and complex processing services required to replicate and scale this critical quantum sensing research. The precise control over thickness, doping, and surface termination achieved in this paper aligns perfectly with 6CCVD’s core MPCVD capabilities.

Applicable Materials

To replicate the ultra-pure IIa growth and minimize spin-bath decoherence, the following 6CCVD materials are required:

Electronic Grade Single Crystal Diamond (SCD) Substrates:
- Advantage: While the paper used Type Ib substrates, 6CCVD recommends using our Electronic Grade SCD (N < 5 ppb, B < 1 ppb) to ensure the highest initial purity. This minimizes background nitrogen incorporation, making the delta-doping layer highly discrete.
¹²C Enriched SCD Wafers:
- Value Proposition: The short T₂ coherence time of buried NVs in the paper was limited by the dynamics of the natural ¹³C nuclear spin bath (1.1% abundance). 6CCVD offers Isotopically Enriched ¹²C SCD to reduce ¹³C content to < 0.1%, potentially increasing the NV coherence time (T₂) from hundreds of µs to the millisecond range, enabling superior qubit and sensor performance.
Custom Delta-Doping Diamond Layers:
- 6CCVD Capability: We specialize in controlled gas injection during MPCVD, allowing for precise SCD or PCD layer thickness control from 0.1 µm to 500 µm and reproducible delta-doping profiles to place NVs exactly at the 5 nm required depth.

Advanced Customization Potential

The success of this research hinges on precise material dimensions and surface chemistry, areas where 6CCVD provides comprehensive engineering services:

Custom Service	Paper Requirement Match	6CCVD Fulfillment
Thickness Control	5 µm Type IIa layer grown on substrate.	SCD/PCD layers available from 0.1 µm up to 500 µm, matching experimental needs exactly.
Surface Termination	Critical SF₆ plasma (Fluorine) stabilization.	6CCVD provides custom post-growth plasma treatments (H-termination, O-termination, or F-termination simulation/processing) to stabilize near-surface defect charge states.
Polishing Requirements	Need for extremely flat surfaces for subsequent quantum fabrication.	SCD polishing to Ra < 1 nm standard, ensuring minimal surface roughness interference with ultra-shallow NV sensing.
Metalization	None specified, but usually required for microwave delivery (ODMR).	Full in-house metalization capability: Au, Pt, Pd, Ti, W, Cu deposition for on-chip microwave guides or electrode fabrication.
Dimensions & Scaling	Experiment performed on small samples.	6CCVD offers plates/wafers up to 125 mm (PCD) for scalable manufacturing and sensor array development.

Engineering Support

6CCVD’s in-house PhD materials science team provides dedicated consultation to translate complex academic findings into scalable production. We can assist researchers and engineers with:

Optimizing gas flow rates and pressure parameters for reproducible delta-doping in high-volume MPCVD reactors.
Selecting the ideal starting material (Electronic Grade SCD vs. ¹²C enriched SCD) to maximize the T₂ coherence time for quantum computing and nanoscale sensing projects.
Designing and fabricating wafers with custom metalization patterns suitable for efficient microwave delivery for ODMR protocols.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. We ship globally, with DDU default and DDP options available.

View Original Abstract

Here we report the fabrication of stable, shallow (&lt;5 nm) nitrogen-vacancy (NV) centers in diamond by nitrogen delta doping at the last stage of the chemical vapor deposition growth process. The NVs are stabilized after treating the diamond in SF6 plasma, otherwise the color centers are not observed, suggesting a strong influence from the surface. X-ray photoelectron spectroscopy measurements show the presence of only fluorine atoms on the surface, in contrast to previous studies, indicating very good surface coverage. We managed to detect hydrogen nuclear magnetic resonance signal from protons in the immersion oil, revealing a depth of the NVs of about 5 nm.

Tech Support

Original Source

DOI: https://doi.org/10.1063/1.4915305