Boron-Doped Diamond/GaN Heterojunction—The Influence of the Low-Temperature Deposition

At a Glance

Metadata	Details
Publication Date	2021-10-23
Journal	Materials
Authors	Michał Sobaszek, Marcin Gnyba, Sławomir Kulesza, Mirosław Bramowicz, Tomasz Klimczuk
Institutions	University of Warmia and Mazury in Olsztyn, Gdańsk University of Technology
Analysis	Full AI Review Included

Executive Summary

The research reports the successful direct deposition of Boron-Doped Diamond (BDD) films onto epitaxial Gallium Nitride (e:GaN) substrates using low-temperature Microwave Plasma-Assisted Chemical Vapor Deposition (MPACVD).

Direct Heterojunction: A key achievement is the formation of a BDD/GaN interface without the use of intermediate transition layers (like SiN_x or AlGaN), simplifying fabrication and potentially improving thermal transfer.
Low-Temperature Process: The deposition was performed at a low substrate temperature (500 °C), successfully mitigating the risk of GaN decomposition and etching typically associated with high-temperature CVD diamond growth (800-900 °C).
Stress-Free Interface: Raman spectroscopy confirmed that the MPACVD process did not induce internal stress in the underlying GaN substrate, evidenced by the relaxed E₂ phonon frequency (564-565 cm^-1).
Enhanced Electrical Performance: The BDD-7k@GaN heterojunction exhibited semiconducting behavior with a low activation energy (E_a) of 93.8 meV, which is significantly lower than typical values reported for SiC MOSFETs (0.9-1.1 eV).
Morphology and Quality: The films are polycrystalline, closed, and isotropic, with well-developed grains averaging 100 nm in diameter, confirming high quality despite the low deposition temperature.
Mechanical Properties: Higher boron doping (7k ppm) resulted in increased mechanical stiffness (Young’s pseudo-modulus of 1280 MPa) and greater tip-surface adhesion compared to the 2k ppm sample.

Technical Specifications

Parameter	Value	Unit	Context
Deposition Temperature	500	°C	MPACVD heated stage setting
Microwave Power (P_MW)	1100	W	High-density direct plasma
Chamber Pressure	50	Torr	CVD process condition
Methane Concentration (CH₄)	1	% vol.	H₂/CH₄ gas mixture
Total Gas Flow	300	sccm	Overall gas flow rate
Boron Doping Ratio ([B]/[C])	7000	ppm	Highest doping level tested (BDD-7k@GaN)
Activation Energy (E_a)	93.8	meV	Derived from Arrhenius plot (BDD-7k@GaN)
Film Thickness (7k ppm)	483	nm	Average thickness after 2 hours growth
Mean Grain Size (DAC, 7k ppm)	130 ± 18	nm	Derived from autocorrelation function
Crystallite Size (XRD)	29.9	nm	Calculated via Scherrer Equation
Surface Roughness (S_q, 7k ppm)	13.6 ± 1.4	nm	Root Mean Square (RMS) roughness
Young’s Pseudo-Modulus (Y_mod, 7k ppm)	1280 ± 640	MPa	Mechanical stiffness (AFM PF-QNM)
Adhesion Force (F_adh, 7k ppm)	2.84 ± 0.87	nN	Tip-surface adhesion force (AFM PF-QNM)
GaN E₂ Phonon Frequency	564-565	cm^-1	Confirms relaxed (stress-free) GaN substrate
Diamond Lattice Band Position (7k ppm)	1330	cm^-1	Shifted due to boron doping

Key Methodologies

The BDD/GaN heterojunctions were synthesized using a multi-step process optimized for low-temperature deposition and high nucleation density:

Substrate Preparation: Epitaxial GaN (e:GaN) grown on silicon via Molecular-Beam Epitaxy (MBE) was cleaned using acetone and isopropanol in an ultrasonic bath.
Hydrogen Plasma Pre-treatment: Substrates were exposed to hydrogen plasma for 5 minutes. This step increases the surface roughness and adhesion by expanding the GaN surface topography and changing absorbed oxygen to hydroxyl groups.
Nanodiamond Seeding: Substrates were sonicated in a water-based suspension containing 4-5 nm nanodiamond particles (H-terminated seeds, which have a positive ζ potential).
MPACVD Growth: Films were grown in a Seki Technotron AX5200S system under the following conditions:
- Temperature: 500 °C.
- Pressure: 50 Torr.
- Microwave Power: 1100 W.
- Gas Mixture: 1% CH₄ in H₂ (300 sccm total flow).
- Doping: Diborane (B₂H₆) precursor used to achieve 2000 ppm or 7000 ppm [B]/[C] ratios.
Characterization: Structural, morphological, and electrical properties were analyzed using:
- Scanning Electron Microscopy (SEM) for cross-section and morphology.
- Atomic Force Microscopy (AFM) in PeakForce Quantitative Nanomechanical Mapping (PF-QNM) mode for surface texture, modulus, and adhesion.
- Raman Spectroscopy (532 nm laser) for molecular composition and stress analysis.
- X-ray Diffraction (XRD) for crystallite size and lattice structure.
- Two-point probe measurements for resistance and activation energy (293-573 K).

Commercial Applications

This technology, focusing on creating highly thermally conductive interfaces on GaN devices, is critical for next-generation high-power electronics.

High-Electron-Mobility Transistors (HEMTs): Direct diamond integration is essential for thermal management in AlGaN/GaN HEMTs, allowing operation at higher current densities and junction temperatures, thereby increasing device reliability and power output (e.g., for 5G/6G infrastructure).
High-Power RF Devices: Diamond substrates are crucial for dissipating heat generated by high-frequency, high-power radio frequency amplifiers used in radar, satellite communications, and electronic warfare.
Thermal Management Substrates: The low thermal boundary resistance achieved by the direct, stress-free interface makes this BDD/GaN structure ideal for advanced heat spreading in microelectronic packages.
Extreme Environment Electronics: Boron-doped diamond is a robust semiconductor suitable for sensors and electronics operating in harsh environments (high temperature, high radiation) where traditional silicon or SiC devices fail.
Semiconductor Manufacturing: The demonstrated low-temperature PACVD process offers a viable, scalable method for integrating diamond layers directly onto temperature-sensitive compound semiconductors like GaN.

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

Original Source

DOI: https://doi.org/10.3390/ma14216328

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]