Structure of Diamond Films Grown Using High-Speed Flow of a Thermally Activated CH4-H2 Gas Mixture

At a Glance

Metadata	Details
Publication Date	2020-01-04
Journal	Materials
Authors	Yu. V. Fedoseeva, Dmitriy V. Gorodetskiy, Kseniya I. Baskakova, Igor Asanov, Lyubov G. Bulusheva
Institutions	Freie Universität Berlin, Siberian Branch of the Russian Academy of Sciences
Citations	11
Analysis	Full AI Review Included

Technical Analysis and Documentation: High-Speed Gas-Jet CVD Diamond Films

Executive Summary

This paper details a successful advancement in diamond synthesis using a High-Speed Gas-Jet Hot Filament (HF) Chemical Vapor Deposition (CVD) method, resulting in high-quality microcrystalline diamond (MCD) films with controlled morphology.

Enhanced Growth Rate: The method achieved highly accelerated diamond growth rates up to 20 µm/h, representing a seven-fold increase compared to classic HF CVD methods, demonstrating potential for rapid industrial production.
Methodology Innovation: Spatially separated, high-speed flows of methane ($\text{CH}{4}$) and hydrogen ($\text{H}{2}$), thermally activated by a hot tungsten filament (up to 2700 K), minimized filament carbidization and maximized radical concentration.
Morphology Control: Crystal shape and film density were precisely controlled by synthesis parameters. Films transitioned from defective octahedral crystals (30 µm) to dense, high-quality rhombic-dodecahedron crystals (5-30 µm).
Structural Quality: The resulting films exhibited excellent crystalline quality, confirmed by sharp Raman diamond peaks (FWHM of 11 $\text{cm}^{-1}$ for the optimal sample).
Surface Purity: XPS and NEXAFS confirmed minimal non-diamond components, resulting in the thinnest hydrogenated $\text{sp}^{2}$-carbon surface coating observed ($\sim$2nm), critical for minimizing surface defects in electronic and optical applications.
Application Relevance: The controlled synthesis of diamond films with specified structural and surface properties is essential for advancing applications in thermophysical, optical, and high-power electronic devices.

Technical Specifications

The synthesis parameters and performance characteristics of the three experimental samples (S1, S2, S3) using the Gas-Jet CVD system are summarized below.

Parameter	Value	Unit	Context
Synthesis Method	Gas-Jet HF CVD	N/A	High-speed, thermally activated flow deposition
Max Growth Rate	20	µm/h	Achieved with high power (S2: 1700 W, 1.5 h duration)
Max Crystal Size	30	µm	Octahedral crystals (S2)
Substrate Material	Molybdenum (Mo)	Foil	20 mm diameter, 250 µm thickness
Reactor Pressure	20	Torr	Constant operating pressure
Activator Temperature (Max)	$\sim$2700	K	Estimated temperature at 1800 W power (S3)
Substrate Temperature ($\text{T}_{s}$)	1073 - 1273	K	$800^{\circ}$C to $1000^{\circ}$C
Hydrogen Flow ($\text{R}_{H}$)	1500, 3500	sccm	External and internal channels
Methane Flow ($\text{R}_{m}$)	10, 30	sccm	Inner channel flow
Raman FWHM (S3)	11	$\text{cm}^{-1}$	Full Width at Half Maximum (indicates high quality)
Minimum $\text{sp}^{2}$ Layer Thickness (S3)	$\sim$2	nm	Thin hydrogenated coating on diamond surface

Key Methodologies

The core of the research focused on modifying classical Hot Filament CVD (HF CVD) using a specialized Gas-Jet reactor design to achieve high radical concentration and fast deposition rates.

Spatially Separated Activation: Methane ($\text{CH}{4}$) and Hydrogen ($\text{H}{2}$) precursor gases were fed through separate high-speed channels (internal/external), activated by a hot tungsten spiral filament.
High-Temperature Activation: The tungsten filament was resistively heated to temperatures ranging from 2400 K to 2700 K (900 W to 1800 W power input). This high temperature promoted the efficient dissociation of $\text{H}{2}$ and $\text{CH}{4}$.
Filament Protection: The separate gas injection successfully prevented rapid carbidization of the tungsten filament, allowing the use of higher heating power and maintaining high concentrations of atomic hydrogen and methyl radicals ($\text{CH}_{3}$°).
High-Speed Jet Deposition: The activated gas mixture was rapidly deposited onto a transverse Molybdenum (Mo) substrate positioned only 1 cm from the reactor exit. This minimized the spontaneous recombination of active radicals before reaching the substrate surface, enabling the high growth rates.
Growth Control via Parameters:
- Power/Flow Ratio: Increasing power (from 900 W to 1800 W) and flow rates ($\text{R}_{H}$ from 1500 to 3500 sccm) enhanced atomic hydrogen concentration, etching the $\text{sp}^{2}$ phase faster than the diamond phase.
- Duration/Temperature: Doubling the synthesis duration (from 1.5 h to 3 h) and decreasing the substrate temperature (from 1273 K to 1073 K) led to increased nucleation density and a shift from octahedral to rhombic-dodecahedron morphology.
Advanced Characterization: Morphology, bulk, and surface chemistry were studied rigorously using multi-modal spectroscopy:
- Scanning Electron Microscopy (SEM) for crystal size and shape.
- Raman spectroscopy (514 nm laser) for bulk crystallinity ($\text{sp}^{3}/\text{sp}^{2}$ ratio, FWHM).
- Near-Edge X-ray Absorption Fine Structure (NEXAFS TEY/AEY) for surface ($\lt$1 nm) and near-surface ($\sim$10 nm) bonding environment.
- X-ray Photoelectron Spectroscopy (XPS, 1486.74 eV) for surface chemical states ($\lt$10 nm depth).

6CCVD Solutions & Capabilities

The research demonstrates the effectiveness of controlled thermal activation and high flow rate techniques to produce high-quality microcrystalline diamond (MCD) materials tailored for severe environments. 6CCVD’s MPCVD expertise and customizable fabrication capabilities are ideally suited to replicate, refine, and scale the material requirements outlined in this study.

Applicable Materials & Purity

The study achieved high-quality PCD with FWHM of 11 $\text{cm}^{-1}$ and minimal $\text{sp}^{2}$ contamination ($\sim$2nm surface layer).

Polycrystalline Diamond (PCD): 6CCVD offers high-purity PCD wafers, superior in homogeneity and structural integrity to the microcrystalline films produced by the Gas-Jet method. Our MPCVD films provide dense, columnar structures required for reliable thermophysical applications identified in the paper (e.g., heat sinks and wear-resistance coatings).
Optical Grade Single Crystal Diamond (SCD): For applications requiring the highest thermal conductivity and optical transparency (where the 2nm $\text{sp}^{2}$ surface layer observed here would be prohibitive), 6CCVD provides Optical Grade SCD up to 500 µm thick, featuring minimal defects and Ra < 1nm surface finish.
Boron-Doped Diamond (BDD): Although the paper focuses on insulating diamond, the derived applications (electrochemical) require conductive diamond. 6CCVD offers highly controlled Boron-Doped Diamond (BDD) materials, customizable for specific conductivity requirements.

Customization Potential

The synthesis used specific Mo substrates (20mm diameter, 250 µm thickness). 6CCVD specializes in matching substrate requirements and custom fabrication for research scaling.

Requirement from Research	6CCVD Capability	Benefits to Engineer
Substrate/Material Scaling	Plates/wafers up to 125mm (PCD) and custom SCD dimensions. Substrate thickness up to 10 mm.	Enables scale-up from lab-scale 20mm samples to commercial-grade wafers for device integration.
Substrate Handling	Capability to deposit diamond films on diverse engineering materials (e.g., Silicon, Mo, W, ceramics).	Offers flexibility to replicate the use of Mo substrates or transition to more complex, application-specific host materials.
Surface Finish	Polishing down to Ra < 1nm (SCD) and Ra < 5nm (Inch-size PCD).	Essential for reducing friction (wear applications) and minimizing light scattering (optical applications) beyond the as-grown roughness seen in the Gas-Jet method.
Metalization	Internal capabilities for custom metal contacts (Au, Pt, Pd, Ti, W, Cu) patterning via lithography.	Allows researchers to immediately fabricate devices (e.g., electrodes, thermistors, electronic contacts) directly onto the high-quality diamond surface.

Engineering Support

The ability to control diamond morphology (octahedral vs. rhombic-dodecahedron) and minimize $\text{sp}^{2}$ content requires sophisticated process control. 6CCVD’s commitment to scientific rigor ensures project success.

6CCVD’s in-house PhD engineering team possesses deep expertise in MPCVD growth kinetics, surface chemistry (analogous to the XPS/NEXAFS analysis in this paper), and structural defect management.
We offer crucial assistance with material selection and process optimization for complex projects, including diamond films intended for advanced thermophysical, high-power electronic, and wear-resistant coating applications.
We provide global logistical support with DDU (Delivered Duty Unpaid) default shipping, and DDP (Delivered Duty Paid) available upon request, ensuring reliable delivery of sensitive materials worldwide.

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

View Original Abstract

Diamond films are advanced engineering materials for various industrial applications requiring a coating material with extremely high thermal conductivity and low electrical conductivity. An approach for the synthesis of diamond films via high-speed jet deposition of thermally activated gas has been applied. In this method, spatially separated high-speed flows of methane and hydrogen were thermally activated, and methyl and hydrogen radicals were deposited on heated molybdenum substrates. The morphology and structure of three diamond films were studied, which were synthesized at a heating power of 900, 1700, or 1800 W, methane flow rate of 10 or 30 sccm, hydrogen flow rate of 1500 or 3500 sccm, and duration of the synthesis from 1.5 to 3 h.The morphology and electronic state of the carbon on the surface and in the bulk of the obtained films were analyzed by scanning electron microscopy, Raman scattering, X-ray photoelectron, and near-edge X-ray absorption fine structure spectroscopies. The diamond micro-crystals with a thick oxidized amorphous sp2-carbon coating were grown at a heating power of 900 W and a hydrogen flow rate of 1500 sccm. The quality of the crystals was improved, and the growth rate of the diamond film was increased seven times when the heating power was 1700-1800 W and the methane and hydrogen flow rates were 30 and 3500 sccm, respectively. Defective octahedral diamond crystals of 30 μm in size with a thin sp2-carbon surface layer were synthesized on a Mo substrate heated at 1273 K for 1.5 h. When the synthesis duration was doubled, and the substrate temperature was decreased to 1073 K, the denser film with rhombic-dodecahedron diamond crystals was grown. In this case, the thinnest hydrogenated sp2-carbon coating was detected on the surface of the diamond crystals.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma13010219

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