| 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 |
- Core Innovation: A welding-assisted Microwave Plasma Enhanced Chemical Vapour Deposition (MPCVD) method was developed using thin copper foil as a welding material to stabilize the substrate temperature during single crystal diamond (SCD) growth.
- Thermal Stability Improvement: The copper foil successfully reduced thermal contact resistance between the diamond seed and the molybdenum holder, mitigating the accumulation of graphitic carbon (sp2) which typically causes temperature instability.
- Enhanced Growth Rate: The welding method achieved a higher average growth rate of 12-14 ”m/hr, compared to 10-12 ”m/hr obtained using the conventional MPCVD technique.
- Optical Grade Quality: Grown SCDs (Sample S2) demonstrated excellent optical quality, achieving maximum transmittance greater than 70% (72% at 412 nm) across the UV-Vis-NIR spectrum.
- Low Impurity Levels: Calculations based on UV-Vis absorption confirmed a minimum nitrogen impurity concentration of less than 10 ppm (7.60 ppm for S2), classifying the material as high-quality optical grade.
- Structural Purity: Raman spectroscopy confirmed the growth of the pure diamond phase (sp3 bonded carbon) with a sharp peak at 1332.72 cm-1 and minimal evidence of non-diamond (sp2) components.
| Parameter | Value | Unit | Context |
|---|
| Growth Method | Welding-Assisted MPCVD | N/A | Optimized technique |
| Microwave Power Range | 3.5 - 5.5 | kW | For both S1 (Conventional) and S2 (Welding) |
| Growth Pressure (Welding) | 160 - 180 | Torr | Sample S2 |
| Substrate Temperature Range (Welding) | 950 - 1050 | °C | Controlled growth temperature |
| Average Growth Rate (Welding) | 12 - 14 | ”m/hr | Sample S2 (154h cycle) |
| SCD Thickness (Welding) | 1.85 | mm | Obtained thickness (Sample S2) |
| Maximum Transmittance (S2) | 72 | % | At 412 nm (UV-Vis-NIR) |
| Absorption Edge | 225 | nm | Corresponds to diamond wide band gap |
| Average Absorption Coefficient (S2) | 1.11 | cm-1 | Range 240 nm to 400 nm |
| Estimated Nitrogen Impurity (S2) | 7.60 | ppm | Calculated via 270 nm absorption peak |
| Diamond Raman Peak (S2) | 1332.72 | cm-1 | sp3 bonded carbon vibration |
| Two Phonon Region Peak (S2) | 2354.13 | cm-1 | Distinctive feature of diamond phase (FT-IR) |
| Copper Melting Point | 1082 | °C | Welding material |
| Copper Thermal Conductivity | 397 | W m-1 K-1 | High conductivity material |
| Seed Dimensions | 10 x 10 x 0.5 | mm | (100) oriented CVD grown SCD seeds |
- Seed Pre-Treatment: SCD seeds were cleaned using a wet chemical mixture (Nitric, Sulfuric, and Hydrochloric acids in 2:1:1 ratio) at 300°C for 1 hour, followed by ultrasonic cleaning in acetone and methanol.
- Welding Setup: Copper foils (8 mm x 8 mm x 0.014 mm) were inserted between the diamond seed and the molybdenum substrate holder to reduce thermal contact resistance. The smaller size prevented melting material from flowing out.
- MPCVD Initiation: Growth was performed using the Seki Diamond MPCVD system (Model SDS 6K). The chamber was first filled with clean Hydrogen gas (99.9998%) and ionized to create plasma.
- Hydrogen Etching: The substrate was etched for 30 minutes in hydrogen plasma to prepare the surface for homoepitaxial growth.
- Growth Gas Mixture: Methane (CH4) and Nitrogen (N2) were introduced into the chamber. Gas flow rates were set with H2 at 500 sccm, and CH4 and N2 concentrations up to 5% and 1% respectively, of the total flow.
- Process Control: Gas pressure was maintained between 160-180 Torr. Input microwave power was adjusted (3000 W-5500 W) to control the substrate temperature between 950°C and 1050°C.
- Welding Confirmation: Successful melting of the copper foil was indicated by a sudden drop in substrate temperature (50°C to 100°C) during the 30-40 hour process cycle.
- Post-Growth Purification: Grown SCDs were subjected to chemical cleansing using oxidizing agents (H2SO4 and K2Cr2O7) at 350°C to remove residual polycrystalline inclusions and graphitic carbon (sp2).
- Characterization: Material quality was verified using Raman spectroscopy (to confirm sp3 bonding), UV-Vis-NIR spectroscopy (to measure transmittance and absorption coefficient), and FT-IR spectroscopy (to identify impurities like nitrogen and OH-stretching vibrations).
- High-Power RF and Microwave Devices: The high thermal conductivity and low defect density of the SCD substrates make them superior heat sinks for high-frequency, high-power electronic devices, ensuring reliable operation at elevated temperatures.
- Advanced Optical Windows and Lenses: The exceptional optical transparency (>70%) across the UV (225 nm edge) to NIR range positions these SCDs as ideal materials for high-performance optical components in demanding environments, such as high-power laser systems.
- Quantum Sensing and Computing: The achievement of very low nitrogen impurity levels (< 10 ppm) is crucial for creating isolated Nitrogen-Vacancy (N-V) centers, which are fundamental components for solid-state quantum memory and high-sensitivity magnetic sensors.
- Semiconductor Substrates: High-quality, large-area SCDs serve as essential substrates for the fabrication of diamond-based power electronics (diodes, transistors) that exploit diamondâs wide bandgap for high-voltage and high-temperature operation.
- Abrasives and Grinding Tools: While focused on optical properties, the high growth rate and confirmed structural purity contribute to the efficient production of high-quality synthetic diamond material used in advanced industrial abrasive applications.
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 cmâ1, indicating the high quality of the grown SCDs with nitrogen impurities below 10 ppm. The absorption observed in the FTIR spectra, ranging from 1600 cmâ1 to 2700 cmâ1 with a peak at 2354.13 cmâ1, is referred to as the âtwo phonon region,â which is a distinctive feature of the diamond phase.
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