Two-dimensional photonic crystal slab nanocavities on bulk single-crystal diamond

At a Glance

Metadata	Details
Publication Date	2018-04-02
Journal	Applied Physics Letters
Authors	Noel H Wan, Sara Mouradian, Dirk Englund
Institutions	Massachusetts Institute of Technology
Citations	60
Analysis	Full AI Review Included

Technical Documentation and Analysis: Two-Dimensional Photonic Crystal Slab Nanocavities on Bulk Single-Crystal Diamond

Executive Summary

This research demonstrates a scalable fabrication technique for creating high-Quality (Q) factor two-dimensional (2D) Photonic Crystal (PhC) slab nanocavities directly within bulk Single-Crystal Diamond (SCD). This advancement significantly expands the platform for quantum nanophotonics utilizing inherent diamond color centers.

Core Innovation: Successful fabrication and suspension of 2D PhC slabs (~200 nm thick, ~4 µm wide) directly from bulk SCD substrates (Type IIa, 100 orientation), bypassing complex thinning or ion-slicing methods.
Material Foundation: Experiment relied on high-purity, low-nitrogen CVD SCD ([N] < 1ppm) sourced from Element Six, requiring stringent material consistency for reproducible results.
Performance Metrics (L3 Cavity): Achieved an experimental Q factor of 6,080 near the Nitrogen Vacancy (NV) center Zero-Phonon Line (ZPL), limited primarily by intrinsic cavity radiative losses.
Fabrication Process: A multi-step hybrid etch process combining deep directional Inductively Coupled Plasma (ICP)-Reactive Ion Etching (RIE), Atomic Layer Deposition (ALD) for sidewall masking (Al₂O₃), and controlled quasi-isotropic oxygen plasma undercutting.
Application Impact: This planar PhC slab architecture provides robust light confinement and improved Purcell factors, accelerating development of efficient spin-photon interfaces necessary for scalable quantum computing and quantum networking based on NV and Silicon Vacancy (SiV) centers.
Scalability: The demonstrated methodology allows for high-yield fabrication consistency across standard bulk diamond chips, making it a viable platform for compact, integrated nanophotonic devices.

Technical Specifications

The following table summarizes the critical material parameters, fabrication conditions, and achieved performance metrics extracted from the research.

Parameter	Value	Unit	Context
Initial Material	Single Crystal Diamond (SCD)	N/A	Type IIa CVD, 100 face, [N] < 1ppm
Initial Dimensions	3 x 3 x 0.3	mm	Bulk substrate size
Final Slab Thickness (H)	~200	nm (0.2 µm)	Suspended PhC membrane
L3 Lattice Constant (a)	214	nm	For optimal resonance at 637 nm
HS Lattice Constant (a)	210	nm	For optimal resonance at 637 nm
L3 Slab Undercut Width	~4	µm	Distance required for full suspension
Directional RIE ICP Power	500	W	Initial deep etch using O₂ plasma
Directional RIE RF Power	240	W	Initial deep etch using O₂ plasma
Directional RIE Temperature	32	°C	For initial vertical etching
Directional RIE Pressure	0.15	Pa	For initial vertical etching
Directional RIE Selectivity	30:1	N/A	Diamond to Silicon Nitride (SiN) mask
Isotropic Undercut ICP Power	900	W	Quasi-isotropic O₂ plasma undercut
Isotropic Undercut RF Power	0	W	Quasi-isotropic O₂ plasma undercut
Isotropic Undercut Temperature	200	°C	Elevated temperature for undercut
Isotropic Undercut Pressure	3	Pa	Elevated pressure for undercut
L3 Q Factor (Experimental)	6,080	N/A	Measured near NV ZPL (639.5 nm)
HS Q Factor (Experimental)	2,670	N/A	Measured resonance
L3 Mode Volume (Simulated)	0.76(λ/n)³	N/A	Highly confined TE-like mode
HS Mode Volume (Simulated)	1.28(λ/n)³	N/A	Highly confined TE-like mode
Excitation Wavelength	532	nm	Confocal PL microscopy laser

Key Methodologies

The fabrication of the suspended 2D PhC slabs relies on a carefully sequenced process combining advanced lithography with tailored dry and wet etching techniques.

Hard Mask Preparation:
- A 230 nm thick Silicon Nitride (SiN) hard mask was deposited via Plasma-Enhanced Chemical Vapor Deposition (PECVD).
- Patterns were defined using Electron-Beam Lithography (EBL) and ZEP-520A resist.
- The SiN was etched using Tetrafluoromethane (CF₄) RIE (RF = 200W).
Directional Diamond RIE:
- Bulk SCD was etched deep (~7× deeper than the desired final slab thickness) using an Inductively Coupled Oxygen Plasma (ICP = 500W, RF = 240W, T = 32°C, P = 0.15 Pa).
- This step established near-vertical sidewalls with a high diamond:SiN selectivity (30:1).
Sidewall Masking (ALD):
- Atomic Layer Deposition (ALD) was used to conformally coat the entire structure, including the sidewalls and holes, with Aluminum Oxide (Al₂O₃) for protection.
Al₂O₃ Top Layer Breakthrough:
- A brief CF₄ RIE step selectively removed the Al₂O₃ only from the top surface of the chip, leaving the SiN mask protected area and the hole sidewalls protected by Al₂O₃.
Quasi-Isotropic Undercut (Suspension Step):
- A specialized quasi-isotropic oxygen plasma RIE (ICP = 900 W, RF = 0 W, T = 200°C, P = 3 Pa) was utilized to undercut the diamond through the Al₂O₃ sidewall coatings at the bottom of the etched patterns.
- This process resulted in the suspension of the PhC slabs. Undercut time was approximately 8 hours for a 4 µm-wide slab.
Final Mask Removal:
- A Hydrofluoric (HF) acid wet etch removed the residual SiN and Al₂O₃, revealing the air-clad diamond PhC devices.

6CCVD Solutions & Capabilities

The successful replication and extension of this crucial quantum nanophotonics research hinges entirely on the quality and specifications of the starting single-crystal diamond material. 6CCVD is uniquely positioned to supply the precise, high-specification substrates required for robust 2D PhC slab fabrication.

Applicable Materials

To replicate or extend the results concerning NV (637 nm) and SiV (637 nm) defect centers, researchers require ultra-high-purity, low-strain material with optimal crystal orientation.

Material Requirement (Paper)	6CCVD Solution & Specifications	Value Proposition
Material Type	Optical Grade Single Crystal Diamond (SCD)	Our SCD is grown via MPCVD, ensuring superior purity, low strain, and highly stable crystal lattice structures essential for long coherence times.
Purity/Defects	High-Purity SCD (Electronic or Optical Grade)	We provide materials with native or intentionally controlled defect densities. For NV/SiV centers, our SCD offers [N] < 1 ppb (if required) or controlled concentration for enhanced emission.
Orientation & Dimensions	100 Orientation Wafers & Custom Plates	We supply 100-face SCD wafers up to 125 mm in diameter, perfectly matching the required crystal directionality for anisotropic RIE processing.
Substrate Preparation	Custom Thinning/Slab Service	While the paper used 300 µm bulk, we offer SCD substrates pre-thinned to specific required depths (e.g., 500 µm down to 0.1 µm) to reduce total etch time and improve yield in demanding deep-etch applications.
Surface Finish	Ultra-Smooth SCD Polishing	Our in-house polishing capability guarantees Ra < 1 nm, minimizing surface defects that can degrade Q factors and NV/SiV spectral stability near the etched surface.

Customization Potential

The complexity of PhC slab fabrication demands absolute control over material processing before and after etching. 6CCVD offers specialized services critical for engineering high-performance quantum devices:

Precision Thickness Control: 6CCVD supplies SCD wafers tailored to the required final thickness (e.g., 0.1 µm - 500 µm), optimizing the starting material for aggressive deep-etch or lift-off techniques used in PhC fabrication.
Metalization Services: Although this paper focuses on suspension, future integration of heaters or waveguides requires custom metal contacts. 6CCVD provides in-house deposition of metals including Ti, Pt, Au, and W, directly on SCD surfaces, ensuring strong adhesion compatible with high-temperature processing.
Laser Cutting and Shaping: We provide custom cutting and shaping services, including precise laser machining of initial 3 mm x 3 mm substrates or larger format wafers, ensuring compatibility with cleanroom lithography tools (EBL).

Engineering Support

The successful demonstration of high-Q PhC cavities relies heavily on mitigating fabrication mismatch and thickness gradients, challenges explicitly noted in the paper for the HS design. 6CCVD’s in-house PhD material science team understands these dependencies:

Material Selection for Quantum Applications: Our experts assist engineers in selecting the optimal SCD grade, nitrogen concentration, and crystal orientation necessary to maximize NV/SiV coherence and Purcell enhancement, ensuring the material meets the tight tolerances required by the 30:1 etch selectivity utilized here.
Process Optimization Consultation: We offer consultation regarding how SCD surface preparation (polishing, cleaning) can influence subsequent ALD and RIE processes, maximizing device yield for highly sensitive structures like 2D PhC slabs.
Global Logistics: We ensure rapid, reliable global shipping (DDU default, DDP available) to keep critical quantum research projects on schedule.

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

View Original Abstract

Color centers in diamond are promising spin qubits for quantum computing and quantum networking. In photon-mediated entanglement distribution schemes, the efficiency of the optical interface ultimately determines the scalability of such systems. Nano-scale optical cavities coupled to emitters constitute a robust spin-photon interface that can increase spontaneous emission rates and photon extraction efficiencies. In this work, we introduce the fabrication of 2D photonic crystal slab nanocavities with high quality factors and cubic wavelength mode volumes—directly in bulk diamond. This planar platform offers scalability and considerably expands the toolkit for classical and quantum nanophotonics in diamond.

Tech Support

Original Source

DOI: https://doi.org/10.1063/1.5021349

References

2013 - The nitrogen-vacancy colour centre in diamond [Crossref]
2013 - Diamond nv centers for quantum computing and quantum networks [Crossref]
2013 - Heralded entanglement between solid-state qubits separated by three metres [Crossref]
2017 - Entanglement distillation between solid-state quantum network nodes [Crossref]
2010 - Strongly enhanced photon collection from diamond defect centers under microfabricated integrated solid immersion lenses [Crossref]
2011 - Nanofabricated solid immersion lenses registered to single emitters in diamond [Crossref]
2014 - Microscopic diamond solid-immersion-lenses fabricated around single defect centers by focused ion beam milling [Crossref]
2014 - Low-loss broadband antenna for efficient photon collection from a coherent spin in diamond [Crossref]
2015 - Efficient photon collection from a nitrogen vacancy center in a circular bullseye grating [Crossref]
2010 - A diamond nanowire single-photon source [Crossref]