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6CCVD Research

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

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2020-01-04
JournalMaterials
AuthorsYu. V. Fedoseeva, Dmitriy V. Gorodetskiy, Kseniya I. Baskakova, Igor Asanov, Lyubov G. Bulusheva
InstitutionsFreie UniversitÀt Berlin, Novosibirsk State University
Citations11
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Structure of Diamond Films Grown Using High-Speed Flow of a Thermally Activated CH4-H2 Gas Mixture

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

Executive Summary

Section titled “Executive Summary”

This study details the synthesis of high-quality diamond films using an advanced gas-jet deposition technique, a modification of Hot Filament Chemical Vapor Deposition (HF CVD).

  • Core Innovation: The method utilizes spatially separated, high-speed jets of methane (CH4) and hydrogen (H2) activated by a tungsten spiral heated up to 2700 K, preventing activator carbidization and maximizing atomic hydrogen concentration.
  • Growth Rate Enhancement: Optimization of heating power and gas flow rates resulted in a significant increase in diamond growth rate, achieving up to 20 ”m/h, which is seven times higher than initial low-power conditions (3 ”m/h).
  • Morphological Control: By adjusting synthesis duration and substrate temperature, the morphology was controlled, transitioning from small, amorphous carbon-coated spheres (<10 ”m) to large, high-quality microcrystals (up to 30 ”m).
  • Optimal Crystal Structure: The best quality films (Sample 3) consisted of dense, rhombic-dodecahedron crystals (5-30 ”m) with a narrow Raman FWHM of 11 cm-1 for the diamond peak.
  • Surface Chemistry: High-quality films exhibited minimal non-diamond carbon content. Surface analysis (NEXAFS AEY) confirmed the thinnest possible coating, consisting primarily of hydrogenated sp2-carbon, crucial for maintaining functional properties.
  • Substrate Selection: Molybdenum (Mo) was used as the substrate due to its coefficient of linear expansion (5 x 10-6 K-1) being similar to that of diamond (~10-6 K-1), promoting better film adhesion.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Activator Heating Power (P)900 to 1800WRange tested for thermal activation
Activator Temperature (TA)2400 to 2700KEstimated temperature of the tungsten spiral
Substrate Temperature (Ts)1073 or 1273KMo substrate temperature during deposition
H2 Flow Rate (RH)1500 or 3500sccmLow vs. High flow conditions
CH4 Flow Rate (Rm)10 or 30sccmLow vs. High flow conditions
Reactor Pressure20TorrConstant operating pressure
Maximum Growth Rate20”m/hAchieved in Sample 2 (1700 W, 1.5 h)
Crystal Size Range5 to 30”mSize of microcrystals in high-quality films
Diamond Peak FWHM (Sample 3)11cm-1Measure of crystal quality (lower is better)
Mo Substrate CTE5 x 10-6K-1Used for adhesion matching with diamond
NEXAFS TEY Probing Depth~10nmBulk/subsurface analysis depth
NEXAFS AEY Probing Depth~1nmSurface analysis depth
Surface sp2 Carbon (XPS)<10at%Maximum concentration detected in surface layer

Key Methodologies

Section titled “Key Methodologies”

The diamond films were synthesized using a gas-jet deposition system, separating the activation of precursor gases to enhance radical concentration and growth rate.

  1. Gas Activation System: A high-speed jet reactor was employed, featuring a hot spiral tungsten filament (activator). H2 and CH4 were injected separately through internal and external channels to prevent tungsten carbidization.
  2. Thermal Activation: The activator was resistively heated to high temperatures (up to 2700 K) to maximize the dissociation of H2 into atomic hydrogen (H°) and CH4 into methyl radicals (CH3°).
  3. Deposition: The activated gas mixture was deposited onto a transverse Molybdenum (Mo) substrate positioned 1 cm (L=1 cm) from the reactor exit.
  4. Parameter Variation: Three distinct synthesis conditions (Samples 1, 2, 3) were tested, varying heating power (900-1800 W), gas flow rates (RH 1500-3500 sccm, Rm 10-30 sccm), synthesis duration (1.5-3 h), and substrate temperature (1073-1273 K).
  5. Morphological Analysis (SEM): Scanning Electron Microscopy was used to observe crystal shape (spherical, octahedral, rhombic-dodecahedron) and density.
  6. Structural Analysis (Raman): Raman spectroscopy (514 nm) identified the diamond phase (1333 cm-1 peak) and non-diamond components (G-mode 1582 cm-1, D-mode 1350 cm-1).
  7. Chemical Bonding Analysis (XPS): X-ray Photoelectron Spectroscopy (Al Kα) determined the concentration and chemical state of carbon atoms (sp3, sp2, C-O, C=O) in the near-surface region (<10 nm).
  8. Depth-Resolved Bonding (NEXAFS): Near-Edge X-ray Absorption Fine Structure spectroscopy was performed in Total Electron Yield (TEY, ~10 nm depth) and Auger Electron Yield (AEY, ~1 nm depth) modes to differentiate between bulk diamond structure and surface coatings.

Commercial Applications

Section titled “Commercial Applications”

The synthesized diamond films, characterized by high quality, rapid growth, and controlled surface chemistry, are suitable for demanding engineering applications:

  • High-Power Thermal Management: Used as heat spreaders or substrates for high-frequency and high-power electronic devices (e.g., GaN/SiC systems) due to diamond’s superior thermal conductivity.
  • Mechanical and Wear Resistance: Coatings for cutting tools, drills, and components requiring extreme hardness and abrasion resistance.
  • Advanced Sensors: Application in fiber-optic low coherence sensors and biosensors, leveraging diamond’s stability and optical transparency.
  • Optoelectronics: Fabrication of ultraviolet photodetectors and optical windows due to transparency across a wide wavelength range.
  • Electrochemical Systems: Use as conductive diamond electrodes for harsh environment electrochemistry, electrosynthesis, and water treatment (if doped).
  • High-Frequency Electronics: Substrates requiring low dielectric constant for microwave and RF applications.
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 ÎŒm 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

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2000 - Diamond Thin Films: A 21st-Century Material [Crossref]
  2. 2000 - Optical, Thermal and Mechanical Properties of CVD Diamond [Crossref]
  3. 2009 - Chemical Vapour Deposition Synthetic Diamond: Materials, Technology and Applications [Crossref]
  4. 2018 - Optoelectronic Diamond: Growth, Properties, and Photodetection Applications [Crossref]
  5. 1996 - Diamond Coated Cutting Tools [Crossref]
  6. 2016 - Application of Thin Diamond Films in Low-Coherence Fiber-Optic FabryPĂ©rot Displacement Sensor [Crossref]
  7. 2004 - Protein-Modified Nanocrystalline Diamond Thin Films for Biosensor Applications [Crossref]
  8. 2019 - Conductive Diamond: Synthesis, Properties, and Electrochemical Applications [Crossref]
  9. 1995 - Studies on Nucleation Process in Diamond CVD: An Overview of Recent Developments [Crossref]
  10. 2006 - Growth Rate Improvements in the Hot-Filament CVD Deposition of Nanocrystalline Diamond [Crossref]

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