Large Platform Growth Effect of Single-Crystal Diamond on the Regulation of Its Dielectric Properties and Stress for THz Applications

At a Glance

Metadata	Details
Publication Date	2025-10-16
Journal	Materials
Authors	Pengwei Zhang, Jun Zhou, Hui Song, Chenxi Liu, He Li
Institutions	University of Electronic Science and Technology of China, Ningbo Institute of Industrial Technology
Analysis	Full AI Review Included

Executive Summary

The research successfully implemented a “Two-Step Method” to regulate the growth morphology of single-crystal diamond (SCD), achieving a novel large platform growth pattern optimized for Terahertz (THz) window applications.

Core Achievement: Transitioning SCD growth from a defect-prone step-like mode to a uniform large platform mode via seed crystal plasma etching and precise CH₄/H₂ ratio control.
Stress Mitigation: The growth step height difference was reduced tenfold (from 30 nm to 3-4 nm), resulting in a 50% reduction in surface stress (from 0.3914 GPa to 0.1976 GPa).
Surface Quality: Root Mean Square (RMS) roughness plummeted from 5 nm (step growth) to 0.4-1.0 nm (large platform growth) across a 5 µm x 5 µm area.
Dielectric Enhancement: Optimized SCD exhibited significantly lower dielectric constant (reduced from 6.6 to 5.6) and decreased dielectric loss tangent across the 0.1-3 THz band.
Crystalline Quality: X-ray Diffraction (XRD) FWHM improved substantially, dropping to 0.0196° for the optimal growth condition (CH₄/H₂ = 3%).
Manufacturing Advantage: The low-stress material enabled low-damage, ultra-precision laser machining of small THz windows (1 mm diameter, 200 µm thickness), suppressing edge chipping and cracking.

Technical Specifications

Parameter	Value	Unit	Context
Operating Frequency Range	0.1 to 3	THz	Dielectric property testing
Dielectric Constant (Platform Growth)	5.6	N/A	Measured across 0.1-3 THz
Dielectric Constant (Step Growth)	6.6	N/A	Measured across 0.1-3 THz
RMS Roughness (Platform Growth)	0.4-1.0	nm	5 µm x 5 µm area
RMS Roughness (Step Growth)	5	nm	5 µm x 5 µm area
Growth Step Height Difference (Platform)	3-4	nm	After optimization
Growth Step Height Difference (Step)	30	nm	Traditional growth mode
Surface Stress (Platform Growth)	0.1976	GPa	Calculated via Raman shift
Surface Stress (Step Growth)	0.3914	GPa	Calculated via Raman shift
XRD FWHM (Optimal G2)	0.0196	°	Indicator of crystalline quality
Raman FWHM (Optimal G2)	2.04	cm^-1	Indicator of material quality
Optimal CH₄/H₂ Ratio	3	%	For large platform growth (G2)
Growth Rate (Optimal G2)	2.67	µm/h	Total growth 32.04 µm in 12 h
THz Window Diameter	1	mm	Target size for ultra-precision machining
THz Window Thickness	200	µm	Target size for ultra-precision machining
Diamond Thermal Conductivity	2000-2200	W/(mK)	Natural diamond reference

Key Methodologies

The “Two-Step Method” combines substrate preparation and precise epitaxial control to achieve the large platform growth effect:

First Step: Seed Crystal Optimization (Pre-etching Treatment)
- Purpose: Eliminate surface impurities (dust, organic residues), graphite phase (sp²), and hidden defects (point/line defects) that act as nucleation sites for pyramid growth.
- Cleaning: Immersion in Piranha solution (H₂SO₄:H₂O₂ = 7:3) for 12 h at 80 °C, followed by ultrasonic cleaning.
- Plasma Etching: Hydrogen (H₂) plasma etching was performed using the following parameters:
  - Gas Flow: H₂ at 400 sccm.
  - Pressure: 8 kPa.
  - Temperature: 700-800 °C.
  - Duration: 30 min.
  - Power: 2000 W (Microwave).
Second Step: Epitaxial Layer Growth Regulation
- Purpose: Control the growth pattern to favor horizontal expansion (Vx) over vertical deposition (Vy), achieving layer-by-layer growth and inhibiting step bunching.
- Control Parameter: Methane concentration (CH₄/H₂ ratio) was meticulously regulated between 2% and 4%.
- Optimal Growth Recipe (G2):
  - CH₄/H₂ Ratio: 3% (Optimal for high quality and flatness).
  - H₂ Flow: 400 sccm.
  - Temperature: 1000 °C (±10 °C).
  - Pressure: 16 kPa.
  - Power: 3.8 kW (Microwave).
  - Duration: 12 h.

Commercial Applications

The development of high-quality, low-stress SCD with superior dielectric properties in the THz band directly addresses critical material needs in high-frequency and high-power electronic systems.

High Power THz Sources: SCD is the ideal material for THz Traveling Wave Tube (TWT) windows and other high-power vacuum electronic devices, ensuring minimal transmission loss and high thermal stability.
High-Frequency RF Components: Use in dielectric material windows for extremely high operating frequencies (e.g., 220 GHz and 693 GHz systems), where low dielectric loss (Tan(δ) < 10^-4) is essential for efficiency.
Defense and Radar Detection: Application in advanced radar systems and national defense technologies requiring high-efficiency transmission windows for THz imaging and detection.
Precision Optics and Photonics: The ultra-smooth surface (0.4-1.0 nm RMS) and low internal stress make this material suitable for high-precision optical components and substrates in THz spectroscopy and sensing.
Advanced Manufacturing: The low-stress nature of the material significantly improves the yield and quality of ultra-precision machining processes (e.g., laser cutting) for small, thin diamond components.

View Original Abstract

The single-crystal diamond (SCD) possessing both favorable dielectric properties and low stress is esteemed as the ideal material for terahertz windows. The intrinsic step-like growth pattern of SCD can easily lead to stress concentration and a decrease in dielectric performance. In this study, a “two-step method” was designed to optimize the growth mode of SCD. A novel large platform growth pattern has been achieved by controlling diamond seed crystal etching and the epitaxial layer growth process. The experimental results indicate that, compared with the traditional step-like growth model, the root mean square (RMS) roughness of as-prepared SCD reduced from 5 nanometers (step growth) to 0.4~~1.0 nanometers (platform growth) within a 5 μm × 5 μm area. Furthermore, the growth step height difference diminished from 30 nm to 3~~4 nm, thereby mitigating stress induced by steps to a mere 0.1976 GPa. Additionally, at frequencies ranging from 0.1 to 3 THz, the diamond windows exhibit lower refractive index, dielectric constant, and dielectric loss. Finally, large platform growth effectively reduces phenomena such as dislocation pile-up brought about by step growth, achieving low-damage ultra-precision machining of diamond windows measuring 1 mm in diameter.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma18204745

References

1974 - High-Resolution Submillimeter-Wave Fourier-Transform Spectrometry of Gases [Crossref]
2022 - Seven Defining Features of Terahertz (THz) Wireless Systems: A Fellowship of Communication and Sensing [Crossref]
2017 - Cavity-Enhanced Optical Hall Effect in Epitaxial Graphene Detected at Terahertz Frequencies [Crossref]
2008 - Plasma Physics and Related Challenges of Millimeter-Wave-to-Terahertz and High Power Microwave Generation [Crossref]
2017 - The 2017 Terahertz Science and Technology Roadmap [Crossref]
2004 - Enhanced Nucleation and Post-Growth Investigations on HFCVD Diamond Films Grown on Silicon Single Crystals Pretreated with Zr:Diamond Mixed Slurry [Crossref]
2018 - Homo-Epitaxial Growth of Single Crystal Diamond in the Purified Environment by Active O Atoms [Crossref]