Determination of optical properties of single crystal diamond substrates grown via welding-assisted microwave plasma enhanced chemical vapour deposition

At a Glance

Metadata	Details
Publication Date	2024-05-11
Journal	Functional Diamond
Authors	Khyati Upadhyay, Abhay Dasadia, Sandip V. Bhatt, M.P. Deshpande
Institutions	Sardar Patel University
Citations	2
Analysis	Full AI Review Included

Technical Documentation & Analysis: Welding-Assisted MPCVD for Optical Grade SCD

This document analyzes the research paper, “Determination of optical properties of single crystal diamond substrates grown via welding-assisted microwave plasma enhanced chemical vapour deposition,” to provide technical specifications and highlight how 6CCVD’s advanced MPCVD capabilities can support and extend this research.

Executive Summary

The research successfully demonstrates a novel “welding-assisted” MPCVD technique using copper foil to significantly enhance the quality and growth rate of Single Crystal Diamond (SCD) substrates for optical applications.

Thermal Management Innovation: Insertion of a thin copper foil between the SCD seed and the Molybdenum holder reduced thermal contact resistance, stabilizing substrate temperature during long-duration growth.
Superior Optical Quality: The welding method (Sample S2) produced SCDs with exceptionally high optical transmission (up to 72% maximum) across the critical 200 nm to 2700 nm range.
Ultra-Low Impurity: Optical analysis confirmed a minimum nitrogen impurity concentration of < 10 ppm in the grown SCDs, resulting in a low average absorption coefficient of 1.11 cm^-1.
Enhanced Growth Rate: The stabilized thermal environment increased the average growth rate to 12-14 µm/hr, compared to 10-12 µm/hr achieved via the conventional method.
High Crystalline Purity: Raman spectroscopy confirmed high sp³ purity, evidenced by a narrow Full Width at Half Maximum (FWHM) of 5.40 cm^-1 at the 1332.72 cm^-1 diamond peak.
Application Relevance: These findings confirm the viability of welding-assisted MPCVD for producing high-quality, technological-grade diamond suitable for advanced optical and high-power electronic devices.

Technical Specifications

The following hard data points were extracted from the analysis of Sample S2 (Welding Method):

Parameter	Value	Unit	Context
Growth Method	Welding-Assisted MPCVD	N/A	Utilized Copper (Cu) foil
Average Growth Rate	12-14	µm/hr	Enhanced rate due to thermal stability
Obtained Thickness	1.85	mm	Grown over 154 hours
Substrate Temperature Range	950-1050	°C	Controlled range during growth
Nitrogen Impurity Concentration (N)	&lt; 10	ppm	Estimated via 270 nm absorption
Average Absorption Coefficient (α)	1.11	cm^-1	Measured in the 240 nm to 400 nm range
Maximum Optical Transmission	72	%	Observed at 412 nm (200 nm to 2700 nm range)
Raman FWHM (sp³ peak)	5.40	cm^-1	Indicates high crystalline quality
Raman Peak Position (sp³)	1332.72	cm^-1	Characteristic diamond peak
SCD Seed Dimensions	10 x 10 x 0.5	mm	(100) oriented CVD grown
Welding Material Dimensions	8 x 8 x 0.014	mm	Copper foil thickness
MPCVD Pressure Range	160-180	Torr	During growth phase

Key Methodologies

The experiment utilized a specialized MPCVD setup incorporating a thermal stabilization technique.

Seed Pre-treatment: (100) oriented SCD seeds were cleaned using a hot wet chemical mixture (HNO₃:H₂SO₄:HCl, 2:1:1) at 300 °C, followed by ultrasonic cleaning in acetone and methanol to remove metallic and organic residues.
Welding Setup: Thin Copper (Cu) foils (8 mm x 8 mm x 0.014 mm) were precisely cut to be slightly smaller than the 10 mm x 10 mm SCD seeds. These foils were inserted between the SCD seed and the Molybdenum (Mo) substrate holder.
MPCVD Etching: The chamber was charged with clean Hydrogen (H₂, 99.9998%) gas and ionized to perform a 30-minute hydrogen plasma etch.
Growth Initiation: Methane (CH₄, 99.9998%) and Nitrogen (N₂, 99.9998%) were introduced. Gas flow rates were set with H₂ at 500 sccm, CH₄ concentration up to 5%, and N₂ concentration up to 1% of the total flow.
Temperature Control: Substrate temperature was maintained between 950 °C and 1050 °C. The successful melting of the Cu foil (Melting Point: 1082 °C) was confirmed by a sudden 50 °C to 100 °C temperature drop during the 30-40 hour growth cycle.
Post-Growth Cleansing: Grown SCDs were subjected to a thermal etching process using oxidizing agents (H₂SO₄ and K₂Cr₂O₇ in a 5:1 ratio) at 350 °C to remove residual polycrystalline inclusions and graphitic (sp²) carbon.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the high-purity diamond materials and custom engineering services required to replicate, scale, and advance the welding-assisted MPCVD technique for optical applications.

Applicable Materials

To replicate or extend this research, 6CCVD recommends the following materials:

Optical Grade Single Crystal Diamond (SCD): We supply high-purity, low-nitrogen SCD substrates, achieving nitrogen concentrations significantly lower than the &lt; 10 ppm reported in this study, enabling superior UV-Vis-NIR transparency.
Custom SCD Seeds: We can provide (100) oriented SCD seeds in the exact dimensions used (10 mm x 10 mm x 0.5 mm) or in larger formats up to 125 mm diameter (PCD) or 500 µm thick SCD plates.
High-Purity Substrates: Our SCD substrates are available with polishing down to Ra &lt; 1 nm, minimizing the initial thermal contact resistance and potentially simplifying or optimizing the welding material requirements.

Customization Potential

The success of this research relies on precise material handling and interface engineering. 6CCVD offers comprehensive customization services:

Requirement from Paper	6CCVD Capability	Technical Advantage
SCD Dimensions	Custom plates/wafers up to 125 mm (PCD) or 500 µm thickness (SCD).	Enables scaling of the welding method for industrial applications.
Substrate Thickness	SCD available from 0.1 µm up to 500 µm.	Allows optimization of thermal mass and growth time.
Interface Engineering	High-precision polishing (Ra &lt; 1 nm for SCD).	Provides a near-perfect interface, reducing reliance on welding materials to fill gaps.
Post-Growth Metalization	In-house capability for Au, Pt, Pd, Ti, W, Cu.	Essential for subsequent device integration (e.g., creating thermal sinks or electrical contacts on the grown diamond).
Doping Options	Boron-Doped Diamond (BDD) available.	Allows researchers to explore the welding method for semiconducting or electrochemical diamond applications.

Engineering Support

6CCVD’s in-house team of PhD material scientists specializes in optimizing MPCVD growth recipes and material selection for demanding applications.

Thermal Management Consultation: We assist engineers in selecting the optimal substrate material and interface preparation techniques to manage thermal contact resistance for high-power or long-duration growth projects.
Impurity Control: We provide expert guidance on achieving ultra-low nitrogen levels (N &lt; 1 ppm) necessary for cutting-edge quantum and deep-UV optical applications, extending the quality achieved in this research.
Global Logistics: We offer reliable global shipping (DDU default, DDP available) to ensure prompt delivery of custom-engineered diamond materials worldwide.

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

View Original Abstract

Lab grown single crystal diamonds (SCDs) offer unrivalled hardness, a wide range of optical transparency, and supremely high thermal conductivity reliable materials to be a part of devices run at high frequency, temperature, and power. Effective synthesis techniques are essential in enhancing the potential applications of high-quality SCDs. This study aims to decrease the thermal contact resistance between diamond seeds and the molybdenum holder by utilizing welding material. Quality of grown diamond substrates (plates) were assessed by analysing optical properties through Raman, UV-Vis, and FT-IR spectroscopic methods. The findings revealed that the grown single-crystal diamonds have excellent transmittance (>70%) and absorption at 270 nm. Calculations also showed an average absorption coefficient of 1.1 cm−1, indicating the high quality of the grown SCDs with nitrogen impurities below 10 ppm. The absorption observed in the FTIR spectra, ranging from 1600 cm−1 to 2700 cm−1 with a peak at 2354.13 cm−1, is referred to as the “two phonon region,” which is a distinctive feature of the diamond phase.