Boron-Doped Diamond/GaN Heterojunction—The Influence of the Low-Temperature Deposition
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2021-10-23 |
| Journal | Materials |
| Authors | Michał Sobaszek, Marcin Gnyba, Sławomir Kulesza, Mirosław Bramowicz, Tomasz Klimczuk |
| Institutions | Gdańsk University of Technology, University of Warmia and Mazury in Olsztyn |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: Boron-Doped Diamond/GaN Heterojunction
Section titled “Technical Documentation & Analysis: Boron-Doped Diamond/GaN Heterojunction”Executive Summary
Section titled “Executive Summary”This research demonstrates a critical advancement in diamond-based thermal management for high-power electronics by achieving a direct, intermediate-layer-free Boron-Doped Diamond (BDD) on epitaxial Gallium Nitride (GaN) heterojunction using low-temperature Microwave Plasma-Assisted Chemical Vapor Deposition (MPACVD).
- Direct Heterojunction: Successful deposition of polycrystalline BDD directly onto GaN, eliminating the need for complex, thermally resistive intermediate layers (e.g., SiNx or AlGaN).
- Low-Temperature Synthesis: The process utilized a substrate temperature of only 500 °C, effectively preventing the thermal decomposition and etching of the sensitive GaN substrate, a major hurdle in GaN-on-Diamond technology.
- Enhanced Adhesion: Hydrogen plasma pre-treatment and nanodiamond seeding resulted in high nucleation density and increased surface roughness, crucial for strong adhesion and mitigating delamination/cooling cracks.
- High-Quality BDD: The resulting films were closed, isotropic polycrystalline diamond with well-developed grains (average size ~100 nm) and minimal internal stress, confirmed by Raman spectroscopy.
- Superior Electronic Properties: Electrical analysis of the BDD-7k@GaN structure yielded a low activation energy (Ea) of 93.8 meV, demonstrating semiconducting behavior highly suitable for integration into high-electron-mobility transistors (HEMTs).
- Relevance to Power Electronics: This methodology provides a viable path for manufacturing high-power density GaN HEMTs by leveraging diamond’s extreme thermal conductivity for efficient heat dissipation.
Technical Specifications
Section titled “Technical Specifications”Hard data extracted from the MPACVD growth and characterization of the BDD/GaN heterojunctions.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Deposition Temperature | 500 | °C | Substrate/Stage Temperature |
| Microwave Power (PMW) | 1100 | W | MPACVD System Power |
| Chamber Pressure | 50 | Torr | CVD Growth Condition |
| Methane Concentration | 1 | % vol. | CH4/H2 Gas Mixture |
| Boron Doping Levels ([B]/[C]) | 2000, 7000 | ppm | Diborane (B2H6) Precursor |
| Growth Time | 2 | hours | Resulted in sub-microcrystalline films |
| BDD Film Thickness (7k ppm) | 483 | nm | Average thickness |
| Nanodiamond Seed Size | 4-5 | nm | Water-based suspension |
| Mean Grain Size (DAC, 7k ppm) | 130 ± 18 | nm | Autocorrelation Function |
| Surface Roughness (Sq, 7k ppm) | 13.6 ± 1.4 | nm | RMS (Root Mean Square) |
| Mean Young’s Pseudo-Modulus (Ymod, 7k ppm) | 1280 ± 640 | MPa | Mechanical Stiffness |
| Activation Energy (Ea) | 93.8 | meV | BDD-7k@GaN Heterojunction |
| Relaxed GaN E2 Phonon Frequency | 567.2 | cm-1 | Reference value (No stress observed) |
Key Methodologies
Section titled “Key Methodologies”The following steps outline the low-temperature MPACVD process used to achieve the BDD/GaN heterojunction:
- Substrate Cleaning: Epitaxial GaN substrates (grown on Si via MBE) were cleaned using acetone and isopropanol in an ultrasonic bath.
- Hydrogen Plasma Pre-treatment: Substrates were exposed to H2 plasma for 5 minutes to modify the GaN surface, increasing nucleation density and enhancing adhesion by changing surface oxygen to hydroxyl groups.
- Nanodiamond Seeding: Substrates were sonicated in a water-based suspension containing 4-5 nm diamond particles (H-terminated seeds, positive $\zeta$ potential) to achieve high seeding density.
- MPACVD Deposition: Films were synthesized in a Seki Technotron AX5200S system under the following conditions:
- Substrate Temperature: 500 °C
- Pressure: 50 Torr
- Microwave Power: 1100 W
- Gas Mixture: 1% CH4 in H2 (300 sccm total flow)
- Boron Doping: Diborane (B2H6) was introduced as the precursor to achieve gas-phase [B]/[C] ratios of 2000 ppm and 7000 ppm.
- Characterization: Structural, morphological, and electrical properties were analyzed using SEM, AFM (PeakForce Tapping mode for QNM), Raman spectroscopy, XRD, and two-point probe resistance measurements.
6CCVD Solutions & Capabilities
Section titled “6CCVD Solutions & Capabilities”6CCVD is uniquely positioned to support and extend this research into scalable, high-performance GaN-on-Diamond devices. Our expertise in MPCVD diamond synthesis ensures the precise material control necessary for low-temperature integration with sensitive semiconductor substrates.
Applicable Materials
Section titled “Applicable Materials”To replicate and advance the BDD/GaN heterojunction demonstrated in this paper, 6CCVD recommends:
- Heavy Boron-Doped Polycrystalline Diamond (BDD-PCD): The research confirms that high doping (7000 ppm [B]/[C]) yields optimal electronic properties (Ea = 93.8 meV) and enhanced mechanical stiffness (Ymod 1280 MPa). 6CCVD offers BDD-PCD wafers tailored for heavy doping, ensuring the required semiconducting behavior and high thermal conductivity necessary for high-power applications.
- Polycrystalline Diamond (PCD) Wafers: For thermal management layers, 6CCVD provides high-quality PCD plates up to 125 mm in diameter, allowing for the scale-up of GaN-on-Diamond technology from research samples to commercial wafer sizes.
Customization Potential
Section titled “Customization Potential”The success of this low-temperature deposition hinges on precise control over film thickness, surface morphology, and subsequent device processing. 6CCVD provides the following custom capabilities:
| Research Requirement | 6CCVD Capability | Technical Advantage |
|---|---|---|
| Sub-Micron Thickness | SCD and PCD thickness control from 0.1 µm up to 500 µm. | Precisely match the 483 nm film thickness used, optimizing thermal boundary resistance (TBR) and mechanical stress. |
| Interface Quality (Adhesion) | Expertise in custom surface pre-treatments (e.g., plasma etching, seeding optimization). | Replicate the high-adhesion, low-stress interface achieved via H2 plasma pre-treatment, critical for preventing delamination in high-power operation. |
| Surface Finish | Ultra-smooth polishing services: Ra < 5 nm for inch-size PCD wafers. | Essential for subsequent photolithography and metal contact deposition required for HEMT fabrication. |
| Device Integration | In-house metalization capabilities (Au, Pt, Pd, Ti, W, Cu). | Apply custom ohmic and Schottky contacts directly onto the BDD layer, streamlining the device fabrication process (e.g., Ti/Pt/Au stacks). |
| Custom Dimensions | Plates/wafers up to 125 mm (PCD). | Facilitate the transition from small-scale research samples to industry-standard wafer sizes for mass production feasibility studies. |
Engineering Support
Section titled “Engineering Support”The integration of diamond with sensitive materials like GaN requires deep expertise in MPCVD process chemistry and thermal dynamics.
6CCVD’s in-house PhD team specializes in optimizing diamond growth recipes (gas flow, pressure, and temperature profiles) specifically for GaN High-Electron-Mobility Transistor (HEMT) projects. We provide consultation on material selection to ensure the BDD properties (doping concentration, grain size, and film stress) are perfectly matched to the thermal and electrical requirements of the target device architecture.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
We report a method of growing a boron-doped diamond film by plasma-assisted chemical vapour deposition utilizing a pre-treatment of GaN substrate to give a high density of nucleation. CVD diamond was deposited on GaN substrate grown epitaxially via the molecular-beam epitaxy process. To obtain a continuous diamond film with the presence of well-developed grains, the GaN substrates are exposed to hydrogen plasma prior to deposition. The diamond/GaN heterojunction was deposited in methane ratio, chamber pressure, temperature, and microwave power at 1%, 50 Torr, 500 °C, and 1100 W, respectively. Two samples with different doping were prepared 2000 ppm and 7000 [B/C] in the gas phase. SEM and AFM analyses revealed the presence of well-developed grains with an average size of 100 nm. The epitaxial GaN substrate-induced preferential formation of (111)-facetted diamond was revealed by AFM and XRD. After the deposition process, the signal of the GaN substrate is still visible in Raman spectroscopy (showing three main GaN bands located at 565, 640 and 735 cm−1) as well as in typical XRD patterns. Analysis of the current-voltage characteristics as a function of temperature yielded activation energy equal to 93.8 meV.
Tech Support
Section titled “Tech Support”Original Source
Section titled “Original Source”References
Section titled “References”- 2005 - Monte Carlo Study of Self-Heating Effect in GaN/AlGaN HEMTs on Sapphire, SiC and Si Substrates [Crossref]
- 2020 - Integration of GaN and Diamond Using Epitaxial Lateral Overgrowth [Crossref]
- 2002 - Surface Polarity Dependence of Decomposition and Growth of GaN Studied Using in Situ Gravimetric Monitoring [Crossref]
- 2006 - Deposition of CVD Diamond onto GaN [Crossref]
- 2018 - Study on Electronic Properties of Diamond/SiNx-Coated AlGaN/GaN High Electron Mobility Transistors Operating up to 500 °C [Crossref]
- 2021 - Development of Polycrystalline Diamond Compatible with the Latest N-Polar GaN Mm-Wave Technology [Crossref]
- 2020 - Mixed-Size Diamond Seeding for Low-Thermal-Barrier Growth of CVD Diamond onto GaN and AlN [Crossref]