High-Q Nanophotonic Resonators on Diamond Membranes using Templated Atomic Layer Deposition of TiO2

At a Glance

Metadata	Details
Publication Date	2020-05-22
Journal	Nano Letters
Authors	Amy Butcher, Xinghan Guo, Robert Shreiner, Nazar Delegan, Kai Hao
Institutions	Argonne National Laboratory, University of Chicago
Citations	16
Analysis	Full AI Review Included

Technical Documentation & Analysis: High-Q Nanophotonic Resonators on Diamond Membranes

Executive Summary

This research demonstrates a highly scalable and deterministic platform for integrating high-performance nanophotonic resonators with single-crystal diamond (SCD) membranes, crucial for advancing solid-state quantum technologies.

Core Achievement: Fabrication of high-Quality factor (Q) nanophotonic resonators (up to Q = 33,260 on silica, Q = 4,410 on diamond) using templated Atomic Layer Deposition (ALD) of TiO₂.
Material Innovation: The platform utilizes ultra-thin (50 nm) SCD membranes, avoiding the conventional, damaging process of etching wavelength-scale features directly into diamond.
Quantum Application: The resulting devices enable high-cooperativity spin-photon interfacing, specifically targeting color centers like the Silicon-Vacancy (SiV) center (ZPL 737 nm).
Surface Quality: The ALD approach minimizes surface roughness, which is critical for reducing scattering loss (proportional to $\sigma^2/\lambda^3$) in the visible and near-infrared regimes.
Process Flexibility: The fabrication method is highly reproducible, non-invasive to the underlying diamond, and can be iterated multiple times on a single membrane sample without damaging the crystal lattice.
6CCVD Value Proposition: This work relies fundamentally on ultra-smooth, high-purity SCD membranes, a core specialty of 6CCVD, ensuring optimal starting material quality for quantum applications.

Technical Specifications

Parameter	Value	Unit	Context
Diamond Material	Single Crystal Diamond (SCD)	N/A	Membrane substrate
SCD Membrane Thickness	50	nm	Used for 1D photonic crystal cavities
Maximum Q Factor (Silica)	33,260	N/A	Microring resonator (729 nm)
Maximum Q Factor (Diamond)	4,410	N/A	1D cavity on 50 nm SCD membrane
Target ZPL Wavelength	737	nm	Silicon-Vacancy (SiV) color center
TiO₂ Refractive Index (n)	> 2.3	N/A	High index guiding material
TiO₂ Bandgap	~3.3	eV	Suitable for visible operation
Waveguide Width (Ring)	300	nm	TiO₂ structure
Waveguide Height (SCD Cavity)	300	nm	Taller structure required for n=2.4 diamond
ALD Deposition Temperature	90	°C	Ensures amorphous TiO₂ phase
Post-Fabrication Annealing	250	°C	For 2 hours, reducing optical loss
Simulated Purcell Factor (F_Purcell)	Up to 175	N/A	At cavity mode maximum (Q=4,400)
Cavity Mode Volume (V)	2.0($\lambda$/n)³	N/A	Simulated at SiV ZPL (737 nm)

Key Methodologies

The fabrication relies on a non-invasive, templated ALD process to create high-index TiO₂ structures on the diamond surface.

E-Beam Lithography (EBL):
- Resist: 950K PMMA A4 (270 nm - 330 nm thickness).
- Conduction Layer: 20 nm Au (thermally evaporated) to mitigate charge build-up.
- Exposure: 100 kV, 1200 µC/cm² dose, low beam current (0.3 nA).
- Development: 1:3 MIBK:IPA solution at 7 °C for 90 s.
ALD Template Preparation:
- Residual Polymer Removal: Brief 3-second O₂ plasma exposure (10 sccm O₂, 50 W ICP power).
- Etch Rate (PMMA): Roughly 2.5 nm/s (ensures minimal substrate damage).
Atomic Layer Deposition (ALD) of TiO₂:
- Chamber Temperature: 90 °C (below PMMA glass transition, ensures amorphous TiO₂).
- Precursors: Tetrakis(dimethylamido) titanium (TDMAT) and water.
- Pulse Times: 0.08 s (TDMAT) and 0.10 s (Water).
- Deposition Rate: Typically 0.6 Å/cycle.
- Process: Templates are significantly over-filled to planarize the top surface.
ICP RIE Etching and Stripping:
- Overfill Removal: Chlorine-based ICP RIE etch (150 W substrate bias, 400 W ICP power, Cl₂/BCl₃ gases).
- Etch Rate (TiO₂): Typically 1.5-1.7 nm/s (stops on the PMMA resist).
- Resist Removal: Chemical strip (Nanostrip/MicroChem) to reveal the templated TiO₂ devices.
Post-Fabrication Annealing:
- Process: Hot plate anneal at 250 °C for two hours to reduce material optical loss.
Diamond Membrane Preparation (Prior to ALD):
- Initial Thickness: 500 nm SCD generated via ion bombardment and CVD overgrowth.
- Mounting: Adhered to a Si carrier chip using ~500 nm of HSQ resist.
- Final Thinning: Back-etched to the desired 50 nm thickness using ICP etching (Ar/Cl₂ gases, 400 W ICP, 250 W bias).

6CCVD Solutions & Capabilities

This research highlights the critical need for ultra-high quality, precisely engineered diamond substrates to enable next-generation quantum devices. 6CCVD is uniquely positioned to supply the foundational materials required to replicate and scale this work.

Applicable Materials

The success of this platform hinges on the quality and smoothness of the Single Crystal Diamond (SCD) membrane.

Research Requirement	6CCVD Material Solution	Key Specification Match
Ultra-Thin Substrate	Optical Grade SCD	SCD thickness available from 0.1 µm up to 500 µm. We provide the high-purity SCD necessary for subsequent thinning to 50 nm membranes.
Minimal Scattering Loss	Ultra-Smooth Polishing	Guaranteed surface roughness (Ra) < 1 nm for SCD, essential for minimizing optical scattering loss ($\sigma^2/\lambda^3$) in the visible spectrum.
Color Center Integration	High-Purity SCD	Low-strain, high-coherence SCD material grown via MPCVD, ideal for hosting high-performance quantum emitters (NV, SiV, GeV, SnV).
Alternative Substrates	Boron-Doped Diamond (BDD)	For applications requiring integrated electrical control or sensing, 6CCVD offers BDD films with tunable conductivity.

Customization Potential

6CCVD’s in-house engineering capabilities directly address the dimensional and integration challenges faced in nanophotonics fabrication:

Custom Dimensions: The paper utilized 200 µm x 300 µm membranes. 6CCVD provides custom laser cutting and shaping services for SCD and PCD plates up to 125 mm, ensuring precise starting dimensions for membrane fabrication.
Metalization Services: While this study used TiO₂, future integrated photonics platforms often require electrical contacts or alignment marks. 6CCVD offers in-house metalization (Au, Pt, Pd, Ti, W, Cu) tailored to specific device geometries and processing requirements (e.g., alignment marks for EBL).
Thickness Control: We offer precise control over the initial SCD thickness (0.1 µm to 500 µm), allowing researchers to optimize the starting material for subsequent ion implantation and membrane thinning processes.

Engineering Support

6CCVD’s in-house PhD team specializes in MPCVD diamond growth and material characterization for quantum applications. We offer consultation services to assist researchers in material selection for similar integrated quantum photonics projects, ensuring the diamond substrate meets the stringent requirements for low-loss waveguiding and high spin-coherence.

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

View Original Abstract

Integrating solid-state quantum emitters with nanophotonic resonators is essential for efficient spin-photon interfacing and optical networking applications. While diamond color centers have proven to be excellent candidates for emerging quantum technologies, their integration with optical resonators remains challenging. Conventional approaches based on etching resonators into diamond often negatively impact color center performance and offer low device yield. Here, we developed an integrated photonics platform based on templated atomic layer deposition of TiO<sub>2</sub> on diamond membranes. Our fabrication method yields high-performance nanophotonic devices while avoiding etching wavelength-scale features into diamond. Moreover, this technique generates highly reproducible optical resonances and can be iterated on individual diamond samples, a unique processing advantage. Our approach is suitable for a broad range of both wavelengths and substrates and can enable high-cooperativity interfacing between cavity photons and coherent defects in diamond or silicon carbide, rare earth ions, or other material systems.

Tech Support

Original Source

DOI: https://doi.org/10.1021/acs.nanolett.0c01467