Contact resistance of various metallisation schemes to superconducting boron doped diamond between 1.9 and 300 K

At a Glance

Metadata	Details
Publication Date	2021-02-25
Journal	Carbon
Authors	Scott Manifold, Georgina M. Klemencic, Evan L. H. Thomas, Soumen Mandal, Henry A. Bland
Institutions	Cardiff University, Engineering and Physical Sciences Research Council
Citations	8
Analysis	Full AI Review Included

Technical Documentation & Analysis: Cryogenic Ohmic Contacts on Superconducting B-NCD

This document analyzes the research on metallization schemes for superconducting Boron-Doped Nanocrystalline Diamond (B-NCD) and outlines how 6CCVD’s advanced MPCVD diamond capabilities can support and extend this critical work in quantum and NEMS applications.

Executive Summary

Core Achievement: Verification of stable, low-resistance ohmic contacts on highly Boron-Doped Nanocrystalline Diamond (B-NCD) across a wide temperature range (1.9 K to 300 K).
Material Requirement: High boron doping concentration (> 2 x 10²¹ cm^-3) is confirmed as essential to maintain ohmic behavior at cryogenic temperatures by promoting carrier tunneling.
Optimal Cryogenic Schemes: Ta/Pt/Au and Mo/Pt/Au trilayers demonstrated the lowest contact resistivity in the superconducting regime (1.9 K), achieving values as low as (8.07 ± 0.62) x 10^-6 Ω·cm².
Role of Diffusion Barriers: The inclusion of a Platinum (Pt) interlayer successfully prevented diffusion of the carbide-forming metals (Ti, Cr, Mo, Ta) into the Gold (Au) cap, ensuring mechanical and electrical stability across multiple temperature cycles.
Application Relevance: These findings are crucial for the reliable fabrication and operation of superconducting Nanoelectromechanical Systems (NEMS) and other diamond-based quantum devices requiring stable interfaces below the critical temperature (T_c ~2.4 K).
Mechanical Stability: All tested carbide-forming and carbon-soluble (Pd) contacts maintained mechanical stability throughout numerous temperature cycles between 300 K and 1.9 K.

Technical Specifications

The following table summarizes the critical material and performance data extracted from the study, focusing on parameters relevant to device fabrication and cryogenic performance.

Parameter	Value	Unit	Context
Temperature Range Tested	1.9 - 300	K	Cryogenic stability verification
B-NCD Film Thickness	200	nm	Grown via MPCVD
Gas Phase B/C Ratio	12800	ppm	Used for B-NCD growth
Estimated Boron Doping	> 2 x 10²¹	cm^-3	Required for reliable superconductivity
Superconducting Critical Temperature (T_c)	~2.4	K	Consistent superconducting performance
Lowest Contact Resistivity (300 K, incl. L_T)	0.82 ± 0.1 x 10^-7	Ω·cm²	Achieved by Cr/Pt/Au scheme
Lowest Contact Resistivity (1.9 K, excl. L_T)	8.07 ± 0.62 x 10^-6	Ω·cm²	Achieved by Ta/Pt/Au scheme (Superconducting Regime)
Interface Metal Thickness (Ti, Cr, Mo, Ta, Pt)	50	nm	Sputtered layer thickness
Total Contact Thickness (Carbide Schemes)	300	nm	Including 150 nm thermally evaporated Au cap
Optimal Annealing (Carbide Formers)	600 °C for 10 min	N₂	Rapid Thermal Annealing (RTA)
Optimal Annealing (Pd)	400 °C for 3 min	N₂	Rapid Thermal Annealing (RTA)

Key Methodologies

The experimental approach utilized high-quality MPCVD growth and precise microfabrication techniques to test contact performance.

Substrate Preparation: High resistivity silicon wafer buffered with 500 nm SiO₂, seeded using ~5 nm diamond nanoparticles via electrostatic self-assembly.
B-NCD Growth: Microwave Plasma CVD (MPCVD) was used with low CH₄/H₂ chemistry (<3% CH₄) and high trimethylboron flow (12800 ppm B/C ratio) to achieve superconducting films.
Mesa Etching: Photolithography followed by Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE) using a Nickel mask to define 200 x 1200 µm mesas.
Surface Termination: Oxygen plasma ashing (30 W, 0.1 mT) was performed to achieve oxygen termination prior to metal deposition, enhancing adhesion.
Metal Deposition: Magnetron PVD sputtering was used to deposit five schemes:
- Carbide Schemes: Trilayer deposition of Interface Metal (Ti, Cr, Mo, or Ta) / Pt / Au (50 nm each layer).
- Pd Scheme: 200 nm Pd.
- Note: All carbide schemes were capped with an additional 150 nm of thermally evaporated Au for a total thickness of 300 nm.
Annealing: Samples were annealed in a Rapid Thermal Annealer (RTA) under a nitrogen atmosphere to promote carbide formation (600 °C) or diffusion (400 °C for Pd).
Measurement: Contact resistance was measured using a modified Transmission Line Model (TLM) pattern across 1.9 K to 300 K in a Quantum Design PPMS system.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the advanced Boron-Doped Diamond (BDD) materials and custom fabrication services required to replicate and advance this research into commercial superconducting devices.

Research Requirement	6CCVD Solution & Capability	Technical Advantage
Material: Highly Boron-Doped Nanocrystalline Diamond (B-NCD)	BDD Polycrystalline Diamond (PCD): We supply heavily doped PCD films (NCD/MCD structure) with precise, tunable boron concentrations up to 10²² cm^-3.	Achieve the critical doping level (> 2 x 10²¹ cm^-3) necessary for reliable superconductivity and dominant tunneling transport at cryogenic temperatures.
Dimensions & Scaling: Custom TLM patterns and large-area B-NCD films.	Custom Dimensions & Laser Cutting: We offer BDD wafers/plates up to 125mm in diameter. Our in-house laser cutting and lithography services ensure precise replication of custom mesa and contact geometries (e.g., 200 x 1200 µm).	Enables direct scaling of proven TLM patterns and integration into large-scale superconducting Nanoelectromechanical Systems (NEMS) or quantum device arrays.
Metallization Schemes: Complex trilayers (Ta/Pt/Au, Mo/Pt/Au) and Pd contacts.	Advanced Metalization Suite: 6CCVD offers internal deposition capabilities for all tested metals (Ti, Cr, Mo, Ta, Pd, Au, Pt, Cu, W) using PVD (sputtering/evaporation).	Provides turnkey fabrication of optimized low-resistance contacts, ensuring vacuum integrity between layers and precise control over diffusion barriers (e.g., Pt).
Surface Quality: Low surface roughness required for high-Q NEMS devices.	Precision Polishing: We provide polishing services for PCD films down to Ra < 5nm (inch-size wafers), meeting the stringent requirements for high-quality factor resonators.	Minimizes surface scattering and defect density, crucial for maximizing the performance and stability of superconducting devices.
Engineering Support: Optimization of B doping and interface chemistry for cryogenic stability.	In-House PhD Engineering Team: Our experts assist with material selection, B/C ratio tuning, and process optimization (e.g., annealing protocols, surface termination) for similar superconducting or high-power diamond projects.	Accelerate R&D cycles by leveraging proven MPCVD recipes and interface engineering expertise for stable, high-performance superconducting interfaces.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. We offer global shipping (DDU default, DDP available) for all custom SCD, PCD, and BDD solutions.

Tech Support

Original Source

DOI: https://doi.org/10.1016/j.carbon.2021.02.079

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