Extraordinary Field Emission of Diamond Film Developed on a Graphite Substrate by Microwave Plasma Jet Chemical Vapor Deposition

At a Glance

Metadata	Details
Publication Date	2023-02-16
Journal	Applied Sciences
Authors	Hua–Yi Hsu, Jing-Shyang Yen, Chun‐Yu Lin, Chi-Wen Liu, Kaviya Aranganadin
Institutions	National Taipei University of Technology, Minghsin University of Science and Technology
Citations	4
Analysis	Full AI Review Included

Technical Documentation & Analysis: MPCVD Diamond for Advanced Field Emission Sources

Executive Summary

This documentation analyzes the successful reconditioning and optimization of a Microwave Plasma Jet Chemical Vapor Deposition (MPJCVD) system for fabricating high-performance diamond cold cathodes. The research leverages advanced 3D Finite Element Method (FEM) modeling to optimize plasma parameters, resulting in extraordinary field emission characteristics.

Core Achievement: Successful fabrication of diamond thin films exhibiting an extremely low turn-on electric field of ~4 V/µm on graphite substrates.
Methodology: Optimization of the MPJCVD reactor geometry (holder height) and operating conditions (600 W power, 70 Torr pressure) using self-consistent 3D plasma fluid modeling (COMSOL).
Material Performance: The optimized films demonstrated extraordinarily high current density, making them highly suitable for bright field electron emission sources and cold cathode applications.
Key Mechanism: The superior field emission performance is attributed to the combined effects of excellent electrical contact provided by the graphite substrate and the field enhancement generated by Carbon Nanopillars (CNPs) formed on the diamond surface at high methane concentrations (up to 32%).
Film Characteristics: Films were fabricated with thicknesses ranging from 435 nm (low CH₄) up to 115.5 µm (high CH₄), exhibiting high uniformity and continuity with particle sizes around 1 µm.
6CCVD Value: 6CCVD provides the high-purity Polycrystalline Diamond (PCD) and Single Crystal Diamond (SCD) materials necessary to replicate and scale this research, offering custom dimensions, thickness control, and integrated metalization services.

Technical Specifications

The following hard data points were extracted from the optimized simulation and experimental results:

Parameter	Value	Unit	Context
Deposition Method	MPJCVD	N/A	Optimized via 3D FEM modeling
Microwave Frequency	2.45	GHz	Standard operation
Optimized Microwave Power	600	W	Used for film fabrication
Optimized Gas Pressure	70	Torr	Used for film fabrication
Optimized Holder Height	118	mm	Based on electric field optimization
Substrate Size (Experimental)	15 x 15	mm	Graphite and Silicon (100)
Methane Concentration (Low)	0.25	%	Resulted in ~435 nm film thickness
Methane Concentration (High)	32	%	Resulted in lowest turn-on field
Film Thickness (Low CH₄)	~435	nm	Uniform, 1 µm particle size
Film Thickness (High CH₄)	~115.5	µm	Used for best field emission result
Turn-On Electric Field (Lowest)	~4	V/µm	Achieved on Graphite substrate (32% CH₄)
Electron Density (Simulated H₂)	1.2 x 10²⁰	m^-3	Concentrated at antenna tip
Electron Temperature (Simulated H₂)	10	eV	Simulated result

Key Methodologies

The research employed a combined numerical and experimental approach to optimize diamond film growth for field emission applications:

System Modeling and Optimization: A complete three-dimensional plasma fluid model was constructed using COMSOL Multiphysics to simulate the MPJCVD system. This model solved electromagnetic wave and plasma dynamics self-consistently.
Parameter Optimization: The model was used to optimize reactor geometry (sample holder height, optimized at 118 mm), microwave power (400 W-700 W), and gas pressure (5 Torr-90 Torr) to maximize electron density and electric field enhancement.
Substrate Pretreatment: Silicon (100) and graphite substrates were cleaned ultrasonically in acetone and methanol, followed by immersion in a TiD (nano titanium and nanodiamond powder) solution for 30 minutes to induce crystal nucleation.
MPJCVD Growth: Films were deposited for 2 hours using the optimized conditions (600 W, 70 Torr). The gas mixture was H₂ combined with CH₄, with concentrations varied from 0.25% (for thin film) up to 40% (for field emission testing).
Characterization: Scanning Electron Microscopy (SEM) was used to analyze surface morphology (1 µm particles) and measure film thickness (435 nm to 115.5 µm). Raman spectroscopy confirmed the presence of diamond (D peak) and carbon nanopillars (G peak).
Field Emission Testing: Electrical properties were measured to determine the field emission current density as a function of the applied electric field, confirming the low turn-on voltage of ~4 V/µm for the 32% CH₄ film on graphite.

6CCVD Solutions & Capabilities

This research demonstrates the critical role of high-quality, thick diamond films and precise surface engineering in achieving next-generation cold cathode performance. 6CCVD is uniquely positioned to supply the necessary materials and customization required to replicate and scale these results.

Research Requirement/Goal	6CCVD Solution & Capability	Value Proposition for Engineers
Material: High-purity diamond films with controlled grain structure (1 µm particles).	Applicable Materials: Optical Grade Polycrystalline Diamond (PCD) or Single Crystal Diamond (SCD).	Our MPCVD process delivers high-purity diamond with customizable grain sizes, ensuring the consistent electrical and thermal properties essential for reproducible field emission devices.
Thickness: Films up to 115.5 µm thick required for robust cathodes.	Thickness Capability: We supply PCD and SCD wafers from 0.1 µm up to 500 µm thick, and substrates up to 10 mm.	Easily accommodates the required 115.5 µm thickness and allows for the development of even thicker, self-supporting diamond structures for high-power applications.
Substrate Size & Scale-Up: Experiments used 15mm x 15mm samples; scale-up is required for commercialization.	Custom Dimensions: We offer custom plates and wafers up to 125mm in diameter (PCD).	Enables direct scale-up from R&D prototypes to large-area, commercial-grade electron emitters and cold cathodes.
Electrical Contact & Interface: Need robust electrical contact (e.g., to graphite or metal backings).	Integrated Metalization Services: We offer in-house deposition of critical metals including Au, Pt, Pd, Ti, W, and Cu.	Facilitates the creation of low-resistance ohmic contacts and complex electrode patterns directly on the diamond surface, streamlining the fabrication of functional cold cathodes.
Surface Finish: Need to control surface morphology (CNPs/Graphite) for field enhancement.	Polishing Services: We provide ultra-smooth finishes (Ra < 1 nm for SCD, Ra < 5 nm for inch-size PCD) or can supply as-grown surfaces tailored for specific surface treatments or CNP formation.	Provides the flexibility to either replicate the rough, field-enhancing surface or start with an atomically smooth surface for alternative device designs.
Process Optimization: Need expertise in MPCVD plasma chemistry (e.g., high CH₄ concentration recipes).	Engineering Support: 6CCVD’s in-house PhD team specializes in MPCVD optimization and can assist with material selection and recipe development for similar Field Electron Emission projects.	Accelerate your research timeline by leveraging our deep technical knowledge in diamond growth kinetics and plasma physics.

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View Original Abstract

This work reports both numerical and experimental studies of the reconditioning of a microwave plasma jet chemical vapor deposition (MPJCVD) system for the growth of diamond film. A three-dimensional plasma fluid model is constructed for investigating and conditioning the MPJCVD system and optimizing its operating conditions. The methodology solves electromagnetic wave and plasma dynamics self-consistently using an adaptive finite element method as implemented in COMSOL Multiphysics. The whole system has been modeled under varying parameters, including the reactor geometry, microwave power, and working gas pressure. Using an operating condition identical to the optimized simulation results, the MPJCVD system successfully fabricates a diamond-thin film on a graphite substrate. The SEM image reveals the presence of a diamond film uniformly distributed with particles of a size of ~1 μm. The field emission from the diamond film grown from our homemade MPJCVD system on the graphite substrate presents extraordinary properties, i.e., extremely high current density and relatively low turn-on voltage. The turn-on electric field observed could be as low as ~4 V/μm. This developed model provides valuable physical insights into the MPJCVD system, which guided performance improvements. The work may find applications in surface hardening and provide a better cold cathode for field electron emission.

Tech Support

Original Source

DOI: https://doi.org/10.3390/app13042531

References

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