Tunable and Transferable Diamond Membranes for Integrated Quantum Technologies

At a Glance

Metadata	Details
Publication Date	2021-12-13
Journal	Nano Letters
Authors	Xinghan Guo, Nazar Delegan, Jonathan C. Karsch, Zixi Li, Tianle Liu
Institutions	Argonne National Laboratory, University of Chicago
Citations	50
Analysis	Full AI Review Included

Technical Documentation and Analysis: Tunable and Transferable Diamond Membranes for Integrated Quantum Technologies

This document analyzes the attached research concerning the synthesis and characterization of ultra-high quality, transferable single-crystal diamond (SCD) membranes hosting coherent color centers. The findings directly support the demand for precision SCD materials in integrated Quantum Information Science (QIS) platforms, a core market for 6CCVD.

Executive Summary

Pioneering Integration Platform: Developed a high-yield, scalable platform for fabricating tunable, nanoscale-thick SCD membranes (demonstrated 50 nm to 250 nm) designed for integration into hybrid quantum heterostructures (nanophotonics, superconducting devices).
Achieved Bulk-Exceeding Quality: Membranes utilized He⁺-based “smart-cut” processing combined with high-purity, isotopically engineered ¹²C PE-CVD overgrowth, resulting in record-narrow Raman linewidths (1.375 cm^-1).
Atomically Flat Surfaces: Achieved exceptional bilateral surface quality (R_q ≤ 0.3 nm) across the membrane, critical for protecting near-surface color centers and ensuring efficient device integration.
Robust Qubit Coherence: Room-temperature NV^- centers exhibited spin coherence times (T₂) up to 400 µs, competitive with the best bulk materials, demonstrating the viability of the membrane platform as a quantum host.
Group IV Coherence Demonstrated: Successfully incorporated Group IV color centers (GeV^-, SiV^-, SnV^-) via both ion implantation and in-situ ¹⁵N δ-doping, with GeV^- showing optical transition linewidths as low as 70 MHz at 5.4 K.
Deterministic Transferability: Implemented a novel dry-transfer technique allowing deterministic placement (>80% yield) of custom-patterned 200 µm x 200 µm membranes onto arbitrary carrier wafers (e.g., fused silica, thermal oxide/Si).

Technical Specifications

Parameter	Value	Unit	Context
Membrane Thickness (Tunable Range)	50 to 250	nm	Synthesized thickness demonstrated via smart-cut + overgrowth.
Final Surface Roughness (R_q)	≤ 0.3	nm	Bilaterally flat surfaces after multi-step ICP etching.
Isotopically Purified Overgrowth	99.99	at.% ¹²C	Achieved during PE-CVD using ¹²CH₄ precursor.
Narrowest Raman Linewidth (FWHM)	1.375(2)	cm^-1	Measured on isotopically purified, strain-released SCD.
NV^- Spin Coherence Time (T₂)	Up to 400	µs	Measured at Room Temperature (RT).
NV^- Spin Dephasing Time (T₂*)	Up to 150	µs	Measured at RT, enhanced by ¹²C purification.
GeV^- Optical Linewidth (Min.)	70(1)	MHz	Single-scan measurement at 5.4 K.
He⁺ Implantation Energy	150	keV	Used to define graphitized layer depth.
He⁺ Implantation Depth	≈410	nm	Target depth for graphitized sacrificial layer.
Color Center Implantation Dose (Group IV)	2 x 10⁸	cm^-2	Target dose for individual-level color center density.
CVD Plate Temperature (High Purity Growth)	500 to 700	°C	Range maintained during homoepitaxial PE-CVD.

Key Methodologies

The highly controlled fabrication of these diamond membrane quantum platforms relies on a synergistic process combining advanced ion implantation, high-purity homoepitaxial PE-CVD, and precision etching/transfer techniques.

Substrate Preparation & Graphitization:
- Starting Material: Electronic/optical grade SCD wafers (Element Six, R_q ≤ 0.3 nm).
- “Smart-Cut” Layer: Low energy He⁺ ions (150 keV, 5 x 10¹⁶ cm^-2 dose) implanted at 7° incidence to create a sub-surface graphitized layer ≈410 nm deep.
- Annealing: Multi-step high temperature anneal (up to 1200 °C) in forming gas (Ar:H₂) to mobilize vacancies and crystallize the top layer.
Isotopically Controlled PE-CVD Overgrowth:
- Growth System: Custom configured SEKI DIAMOND SDS6350 system.
- Conditions: Microwave power 900 W, Process pressure 25 Torr.
- Precursor Ratio: H₂:¹²CH₄ kept constant at 0.05% for morphology-preserving step-flow growth.
- Growth Rates: 6.2(4) nmh^-1 (700 °C) to 9.3(8) nmh^-1 (500 °C).
Color Center Incorporation:
- Implantation: Simultaneous ion implantation (N⁺, ²⁸Si⁺, ⁷⁴Ge⁺, ¹²⁰Sn⁺) at a dose of 2 x 10⁸ cm^-2 to create individual Group IV centers. Followed by 1200 °C anneal.
- In-Situ δ-Doping (NV^-): Introduction of ¹⁵N₂ gas (0.06 sccm) for 2 minutes during growth, creating a localized ≈2 nm deep δ-layer. Followed by electron irradiation (2 MeV) and 850 °C anneal.
Membrane Definition and Release:
- Patterning: Al₂O₃ hard mask (25 nm ALD), lithography (200 µm x 200 µm squares), followed by multi-step Cl-based ICP etching (e.g., Ar/Cl₂, O₂/Cl₂).
- Undercut: Electrochemical (EC) etching of the graphitic layer in deionized (DI) water (15 V - 36 V), leaving a small diamond tether for stabilization.
Deterministic Dry Transfer and Post-Processing:
- Transfer: PDMS/PC dry-adhesion stamp technique used to break the membrane from the tether and place it deterministically onto carrier wafers (e.g., fused silica or thermal oxide/Si coated with HSQ).
- Thickness Tuning: Final backside etching using ICP (Ar/Cl₂, O₂/Cl₂) to precisely control thickness down to 50 nm and remove residual graphitized residue.

6CCVD Solutions & Capabilities

The production of these highly coherent, ultra-thin diamond membranes demands exceptional precision in material quality, isotopic control, and dimensional engineering—all areas where 6CCVD excels. We provide the foundational materials and advanced engineering services necessary to replicate and scale this QIS platform.

Research Requirement/Application	6CCVD Solution & Capability	Core Value Proposition for Engineers/Scientists
Ultra-High Purity SCD Substrates & Overgrowth	We supply Optical Grade SCD wafers and custom ¹²C Isotopically Purified epitaxial films (99.99 at.% ¹²C guaranteed).	Qubit Coherence Guarantee: Material tailored to minimize magnetic noise (P1 centers) and maximize spin coherence (T₂ & T₂*) for high-performance quantum devices.
Tunable Thickness Membranes (50 nm - 250 nm)	Custom Thin Film Manufacturing: SCD film thicknesses achievable from 0.1 µm up to 500 µm with industry-leading uniformity, perfect precursors for smart-cut techniques.	Precision & Yield: Reliable MPCVD growth ensures the thickness uniformity (σ < 10 nm) required for deterministic, high-yield membrane release and nanophotonic integration.
Atomically Flat Surfaces (R_q ≤ 0.3 nm)	Advanced Polishing Services: We guarantee surface roughness R_a < 1 nm (SCD) and < 5 nm (Inch-size PCD), minimizing surface-related decoherence (a critical challenge noted in the research).	Near-Surface Qubit Protection: Deliver material with the pristine surface finish necessary to protect shallow NV^- centers and other near-surface Group IV emitters.
Integration of Group IV Centers (GeV^-, SiV^-, SnV^-)	In-Situ Doping and Defect Engineering: 6CCVD offers controlled in-situ doping of precursor elements (N, Si, Ge, B) during CVD growth, including precise δ-doping capabilities for localized qubit layers.	Targeted Qubit Placement: Achieve highly localized dopant concentration (ppb level) at specific depths, bypassing the crystal damage and straggle associated with high-dose implantation.
Custom Patterning and Heterostructure Integration	Custom Dimensions & Fabrication Services: We offer laser cutting and Cl-based ICP etching to pre-pattern membranes (up to 125 mm PCD/SCD) into complex geometries required for photonic circuits (e.g., the 200 µm squares demonstrated).	Reduced R&D Time: Receive device-ready membranes and wafers with pre-defined trenches and patterns, accelerating the transition from material synthesis to functional device testing.
Integration onto Non-Diamond Substrates	Custom Metalization & Bonding: Our internal capabilities include the deposition of key metal layers (Au, Pt, Pd, Ti, W, Cu) required for the demonstrated EC etching or for subsequent device integration (e.g., superconducting circuits).	Hybrid System Compatibility: Provide turnkey SCD solutions compatible with integration onto silicon, quartz, or III-V platforms for magnetometry, thermal management, or hybrid quantum systems.

Engineering Support

6CCVD’s in-house PhD team provides expert consultation on material selection, customized growth recipes, and post-growth processing (including implantation guidance and metalization stacks) required to replicate or extend high-coherence diamond projects like the tunable membrane platform demonstrated here.

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

View Original Abstract

Color centers in diamond are widely explored as qubits in quantum technologies. However, challenges remain in the effective and efficient integration of these diamond-hosted qubits in device heterostructures. Here, nanoscale-thick uniform diamond membranes are synthesized via “smart-cut” and isotopically (12C) purified overgrowth. These membranes have tunable thicknesses (demonstrated 50 to 250 nm), are deterministically transferable, have bilaterally atomically flat surfaces (Rq ≤ 0.3 nm), and bulk-diamond-like crystallinity. Color centers are synthesized via both implantation and in situ overgrowth incorporation. Within 110-nm-thick membranes, individual germanium-vacancy (GeV-) centers exhibit stable photoluminescence at 5.4 K and average optical transition line widths as low as 125 MHz. The room temperature spin coherence of individual nitrogen-vacancy (NV-) centers shows Ramsey spin dephasing times (T2*) and Hahn echo times (T2) as long as 150 and 400 μs, respectively. This platform enables the straightforward integration of diamond membranes that host coherent color centers into quantum technologies.

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Original Source

DOI: https://doi.org/10.1021/acs.nanolett.1c03703