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	Chinese Academy of Sciences, Yunnan University
Analysis	Full AI Review Included

Technical Documentation & Analysis: Large Platform SCD for THz Windows

Executive Summary

This research successfully addresses critical material limitations (stress and dielectric loss) in Single-Crystal Diamond (SCD) required for high-efficiency Terahertz (THz) windows. 6CCVD recognizes this advancement as pivotal for next-generation high-power THz devices.

Core Achievement: Implementation of a “Two-Step Method” (H₂ plasma etching followed by controlled MPCVD) to transition SCD growth from traditional step-like flow to a novel “Large Platform Growth” pattern.
Surface Quality Enhancement: Root Mean Square (RMS) roughness was dramatically reduced from 5 nanometers (step growth) to an ultra-smooth 0.4-1.0 nanometers, minimizing diffuse scattering losses in the THz band.
Stress Mitigation: The intrinsic stress induced by growth steps was reduced by nearly 50%, dropping from 0.3914 GPa to a mere 0.1976 GPa, significantly improving material stability for precision machining.
Dielectric Performance Boost: The dielectric constant ($\epsilon’$) decreased substantially from 6.6 to 5.6 across the critical 0.1-3 THz frequency range, confirming enhanced transmission efficiency.
Crystalline Purity: Optimized growth achieved an exceptionally low X-ray Diffraction (XRD) Full Width at Half Maximum (FWHM) of 0.0196°, indicating superior crystal quality and low dislocation density.
Machinability Validation: The low-stress material enabled ultra-precision laser cutting of small THz windows (1 mm diameter, 200 µm thickness) with minimal edge damage or chipping cracks.

Technical Specifications

The following hard data points were extracted, demonstrating the superior material properties achieved through the optimized growth process:

Parameter	Value	Unit	Context
RMS Roughness (Step Growth)	5	nm	Measured over 5 µm x 5 µm area
RMS Roughness (Platform Growth)	0.4-1.0	nm	Achieved via Two-Step Method
Growth Step Height Reduction	30 $\rightarrow$ 3-4	nm	Ten-fold reduction in height disparity
Intrinsic Stress (Step Growth)	0.3914	GPa	High stress state, prone to cracking
Intrinsic Stress (Platform Growth)	0.1976	GPa	Low stress state, ideal for processing
Dielectric Constant ($\epsilon’$) Reduction	6.6 $\rightarrow$ 5.6	N/A	Measured across 0.1-3 THz band
Optimal CH₄/H₂ Ratio	3	%	For highest crystal quality (G2 substrate)
XRD FWHM (Optimal G2)	0.0196	°	Indicator of low defect density
THz Window Diameter	1	mm	Final laser-cut device dimension
THz Window Thickness	200	µm	Final laser-cut device thickness
Deposition Temperature	1000 ± 10	°C	SCD epitaxial layer growth
Microwave Power (Growth)	3.8	kW	Used during epitaxial layer growth

Key Methodologies

The successful regulation of SCD growth morphology relied on a precise, two-stage MPCVD process:

Substrate Preparation & Cleaning:
- SCD seeds (3.5 mm x 3.5 mm x 1 mm, 100 orientation) were used.
- Piranha solution cleaning (H₂SO₄:H₂O₂ = 7:3) for 12 hours at 80 °C to remove surface impurities.
Step 1: H₂ Plasma Etching (Seed Optimization):
- Purpose: Selective erosion of graphite (sp²) phase, organic impurities, and surface defects to eliminate nucleation sites for “Pyramid” island growth.
- Gas Flow: H₂ at 400 sccm.
- Pressure: 8 kPa.
- Microwave Power: 2000 W.
- Temperature: 700-800 °C.
- Duration: 30 minutes.
Step 2: Epitaxial Layer Growth (Platform Regulation):
- Purpose: Precise control of the growth pattern to achieve uniform, layer-by-layer deposition (Large Platform Growth).
- Precursor Gases: H₂ and CH₄.
- Deposition Temperature: 1000 ± 10 °C.
- Pressure: 16 kPa.
- Microwave Power: 3.8 kW.
- Critical Parameter: Meticulous regulation of the CH₄/H₂ ratio (maintained at 2-4%), with 3% yielding the optimal low-stress, high-quality result.
- Duration: 12 hours.

6CCVD Solutions & Capabilities

The research demonstrates a critical need for ultra-high purity, low-stress Single-Crystal Diamond (SCD) with exceptional surface finish for advanced THz applications. 6CCVD is uniquely positioned to supply and customize materials that meet or exceed these stringent requirements.

Applicable Materials

To replicate or extend this research, customers require diamond material optimized for minimal dielectric loss and high structural integrity.

Research Requirement	6CCVD Material Solution	Key Benefit
Ultra-low Dielectric Loss (0.1-3 THz)	Optical Grade Single Crystal Diamond (SCD)	Guaranteed high purity, minimal nitrogen incorporation, and low sp² content, essential for minimizing THz absorption.
Low Intrinsic Stress (0.1976 GPa target)	Custom SCD Wafers	Our MPCVD process allows for precise control over growth parameters (similar to the Two-Step Method) to minimize dislocation pile-up and internal stress.
High Crystalline Quality (FWHM < 0.02°)	High-Purity SCD Substrates	We provide SCD with verified FWHM values, ensuring the low defect density necessary for high-power THz transmission windows.

Customization Potential & Precision Services

The successful fabrication of the 1 mm diameter THz window relied on ultra-precision machining of low-stress material. 6CCVD offers comprehensive services to support the entire device fabrication workflow.

Custom Dimensions and Thickness:
- The paper utilized 3.5 mm seeds and produced 200 µm thick windows. 6CCVD offers SCD plates up to 500 µm thick and substrates up to 10 mm thick.
- We provide custom laser cutting services to produce small, high-precision components (like the 1 mm diameter THz windows) from large-area SCD wafers, ensuring minimal edge damage, as validated by this research.
Surface Finish Guarantee:
- The research achieved an RMS roughness of 0.4-1.0 nm. 6CCVD guarantees ultra-precision polishing of SCD wafers to an average roughness (Ra) of < 1 nm, directly addressing the need to minimize diffuse reflection scattering of THz waves.
Metalization for Integration:
- While not the primary focus of this paper, THz TWT windows often require metalization for sealing or integration. 6CCVD offers in-house metalization capabilities including Au, Pt, Pd, Ti, W, and Cu, tailored to specific vacuum or RF sealing requirements.

Engineering Support

The complexity of regulating the SCD growth pattern via precise CH₄/H₂ ratio control and pre-etching requires deep material science expertise.

Process Optimization: 6CCVD’s in-house PhD engineering team specializes in optimizing MPCVD recipes to achieve specific material properties, such as the low-stress, high-purity characteristics required for THz window applications.
Material Consultation: We provide expert consultation on selecting the optimal SCD orientation and growth recipe to maximize dielectric performance and structural integrity for similar high-frequency projects (e.g., radar detection, military industry, high-power TWTs).

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

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]