Optical and Spin Properties of NV Center Ensembles in Diamond Nano-Pillars

At a Glance

Metadata	Details
Publication Date	2022-04-29
Journal	Nanomaterials
Authors	Kseniia Volkova, Julia Heupel, Sergei Trofimov, Fridtjof Betz, Rémi Colom
Institutions	Zuse Institute Berlin, Freie Universität Berlin
Citations	21
Analysis	Full AI Review Included

Executive Summary

This technical documentation analyzes the fabrication and characterization of Nitrogen-Vacancy (NV) center ensembles within diamond nano-pillars, focusing on the material requirements and spin properties relevant to quantum sensing applications.

Core Achievement: Successful fabrication of high-density NV center ensembles in diamond nano-pillars (diameters up to 1000 nm) using Electron Beam Lithography (EBL) and Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE).
Material Basis: Nitrogen-rich Type Ib HPHT diamond ([100] and [111] orientations) was used, containing 65-68 ppm P1 substitutional nitrogen centers.
NV Creation Method: Vacancies were introduced via 6 keV He⁺ ion bombardment (8 x 10¹³ ions/cm² dose) followed by high-temperature annealing (1000 °C in UHV).
Density & Enhancement: Estimated NV density was high, averaging 4300 ± 300 NVs per pillar ([100] sample). The pillar structure significantly enhanced photon collection efficiency (up to 8x enhancement factor) into the objective (NA = 0.95).
Spin Properties: Measured spin coherence times (T₂) were 420-560 ns, typical for nitrogen-rich diamond. Spin relaxation times (T₁) were short (162-174 µs), indicating the presence of paramagnetic defects introduced during irradiation.
Application Potential: The resulting structures enable calculated magnetic field sensitivities up to 11.6 nT/√Hz, suitable for wide-field and scanning probe magnetic field imaging applications.

Technical Specifications

The following hard data points were extracted from the experimental results regarding material processing and measured NV properties:

Parameter	Value	Unit	Context
Base Material Type	Type Ib (HPHT)	N/A	Nitrogen-rich diamond
P1 Concentration	65 - 68	ppm	Substitutional Nitrogen
Ion Implantation Energy	6	keV	He⁺ ions for vacancy creation
Ion Implantation Dose	8 x 10¹³	ions/cm²	Used for both [100] and [111] samples
Annealing Temperature	1000	°C	Required for NV center formation
Annealing Pressure	<10^-7	mbar	Ultra-High Vacuum (UHV) environment
Nominal Pillar Diameter Range	100 - 1000	nm	Range of fabricated structures
Pillar Height ([100] sample)	2.2 ± 0.2	µm	Achieved with 11 min ICP-RIE etch
Average NVs per Pillar ([100])	4300 ± 300	NVs	Estimated ensemble size
Fluorescence Lifetime (τ₁)	1.9 - 2.5	ns	Bi-exponential short component
Fluorescence Lifetime (τ₂)	6.5 - 9.5	ns	Bi-exponential long component
Spin Coherence Time (T₂) [100]	420 ± 40	ns	Pulsed ODMR measurement
Spin Relaxation Time (T₁) [100]	162 ± 11	µs	Pulsed ODMR measurement
Maximum ODMR Contrast	5	%	Optically Detected Magnetic Resonance
Calculated Sensitivity ([100])	11.6	nT/√Hz	Maximum achievable magnetic field sensitivity

Key Methodologies

The fabrication of NV center ensembles in diamond nano-pillars involved a multi-step lithography and etching process combined with ion implantation and thermal treatment:

Substrate Selection: Use of HPHT Type Ib diamond samples with [100] and [111] crystal orientations, characterized for P1 center concentration (65-68 ppm).
Vacancy Introduction: 6 keV He⁺ ion bombardment was performed at a dose of 8 x 10¹³ ions/cm² to create vacancies, simulated to be distributed up to 50 nm deep (SRIM).
NV Center Formation: Samples were annealed at 1000 °C for two hours in ultra-high vacuum (<10^-7 mbar) to mobilize vacancies and form NV centers by coupling with substitutional nitrogen.
EBL Mask Preparation: A 7 nm Au conductive layer was deposited, followed by spin-coating of a positive EBL resist (AR-P 617.06).
Patterning and Hard Mask Deposition: EBL defined the pillar arrays. A 200 nm Au hard mask (with a 5 nm Ti adhesion layer) was deposited via evaporation.
Lift-off: The resist and excess metal were removed using dimethyl sulfoxide (DMSO) or sulfuric acid (H₂SO₄).
Diamond Etching (ICP-RIE): Nano-pillars were transferred into the diamond using an O₂ plasma etch. Key parameters included 1000 W ICP power, 200 W RF power, 10 sccm O₂ flow, and 5 mTorr pressure, resulting in etch rates of 150-220 nm/min.
Final Cleaning: The Au mask was removed with potassium iodide solution, and the Ti adhesion layer was removed with 10% hydrofluoric acid (HF).

6CCVD Solutions & Capabilities

The research highlights the potential of NV ensembles in structured diamond for quantum sensing but notes that the use of HPHT diamond limits the achievable spin coherence time (T₂) and relaxation time (T₁). 6CCVD’s expertise in MPCVD diamond growth and advanced processing directly addresses these limitations, enabling next-generation quantum sensor development.

Research Requirement / Limitation	6CCVD Solution & Capability	Technical Advantage
Material Limitation: Short T₁/T₂ due to high defect density in HPHT diamond.	Optical Grade SCD (MPCVD)	MPCVD allows precise control over nitrogen concentration (P1 centers) during growth, enabling the creation of NV centers with significantly longer T₂ and T₁ times (potentially milliseconds), crucial for maximizing magnetic field sensitivity.
Substrate Orientation: Need for both [100] and [111] orientations.	Custom SCD Substrates	6CCVD supplies high-quality Single Crystal Diamond (SCD) substrates in both [100] and [111] orientations, necessary for optimizing NV alignment and fabrication yield.
Pillar Dimensions: Need for thin films (up to 2.2 µm etched depth) on robust substrates.	Custom Thickness SCD/PCD Films	We offer SCD and PCD films ranging from 0.1 µm to 500 µm, ideal for shallow NV creation via implantation. Substrates up to 10 mm thick ensure mechanical stability during complex lithography and etching steps.
Hard Mask Fabrication: Requirement for Ti/Au metalization layers.	Internal Metalization Services	6CCVD offers in-house deposition of standard hard mask materials (Au, Ti, Pt, Pd, W, Cu). We can supply pre-metalized diamond wafers, reducing processing steps and contamination risk for the end user.
Scaling Potential: Need for larger arrays for wide-field imaging.	Large Area PCD Wafers	For scaling up nano-pillar arrays, 6CCVD provides Polycrystalline Diamond (PCD) plates up to 125 mm in diameter, polished to Ra < 5 nm, suitable for industrial-scale EBL/RIE processing.
Surface Quality: Need for ultra-smooth surfaces to minimize etching defects.	Precision Polishing	Our polishing capabilities achieve Ra < 1 nm for SCD, ensuring the highest quality starting surface necessary for high-resolution EBL patterning of features down to 100 nm.

Engineering Support

The paper suggests that coherence time can be significantly prolonged if NV centers are created via nitrogen doping during the CVD growth process. 6CCVD’s in-house PhD team specializes in tailoring MPCVD recipes to achieve optimal nitrogen incorporation, balancing high NV ensemble density with extended spin coherence times. We provide expert material consultation to transition similar Quantum Sensing and Scanning Probe Magnetic Field Imaging projects from research-grade HPHT materials to high-performance MPCVD diamond.

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

View Original Abstract

Nitrogen-vacancy (NV) color centers in diamond are excellent quantum sensors possessing high sensitivity and nano-scale spatial resolution. Their integration in photonic structures is often desired, since it leads to an increased photon emission and also allows the realization of solid-state quantum technology architectures. Here, we report the fabrication of diamond nano-pillars with diameters up to 1000 nm by electron beam lithography and inductively coupled plasma reactive ion etching in nitrogen-rich diamonds (type Ib) with [100] and [111] crystal orientations. The NV centers were created by keV-He ion bombardment and subsequent annealing, and we estimate an average number of NVs per pillar to be 4300 ± 300 and 520 ± 120 for the [100] and [111] samples, respectively. Lifetime measurements of the NVs’ excited state showed two time constants with average values of τ1 ≈ 2 ns and τ2 ≈ 8 ns, which are shorter as compared to a single color center in a bulk crystal (τ ≈ 10 ns). This is probably due to a coupling between the NVs as well as due to interaction with bombardment-induced defects and substitutional nitrogen (P1 centers). Optically detected magnetic resonance measurements revealed a contrast of about 5% and average coherence and relaxation times of T2 [100] = 420 ± 40 ns, T2 [111] = 560 ± 50 ns, and T1 [100] = 162 ± 11 μs, T1 [111] = 174 ± 24 μs. These pillars could find an application for scanning probe magnetic field imaging.

Tech Support

Original Source

DOI: https://doi.org/10.3390/nano12091516

References

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