Influence of polycrystalline diamond on silicon-based GaN material

At a Glance

Metadata	Details
Publication Date	2023-01-01
Journal	Acta Physica Sinica
Authors	Qingbin Liu, Yu Cui, Jianchao Guo, Mengyu Ma, Zezhao He
Institutions	Hebei Semiconductor Research Institute
Citations	3
Analysis	Full AI Review Included

Technical Analysis and Documentation: MPCVD Diamond for GaN HEMT Thermal Management

Reference: Liu Qing-Bin, Yu Cui, et al. Influence of polycrystalline diamond on silicon-based GaN material. Acta Phys. Sin. Vol. 72, No. 9 (2023) 098104.

Executive Summary

This research successfully demonstrates the direct growth of Polycrystalline Diamond (PCD) on 2-inch Si-based GaN HEMT structures via Microwave Plasma Chemical Vapor Deposition (MPCVD) to address critical self-heating issues.

Core Application: Enhanced thermal management for high-power GaN HEMT devices using high-conductivity PCD.
Material Achievement: Uniform PCD films grown directly on Si-based GaN, with thicknesses ranging from 9 µm to 81 µm.
Critical Challenge Identified: Thermal expansion mismatch between PCD and GaN/Si introduces tensile stress (up to 0.07 GPa on GaN), leading to GaN lattice distortion.
Electrical Degradation: Increased PCD thickness correlates directly with increased XRD FWHM₀₀₂ (up to 654 arcsec) and significant electron mobility loss (up to 25.8% for 81 µm film).
Key Finding (Recoverability): Through laser cutting and acid etching, the PCD layer was successfully stripped. Post-stripping analysis confirmed that the GaN material’s electrical properties and lattice structure fully recovered to their intrinsic states, proving the degradation is reversible and non-destructive.
Engineering Insight: The study confirms that minimizing PCD thickness while maintaining sufficient thermal dissipation is crucial for mitigating stress-induced performance degradation in directly grown GaN-on-Diamond structures.

Technical Specifications

The following hard data points were extracted from the experimental results, focusing on the 81 µm thick PCD sample (#3) which exhibited the maximum stress and degradation.

Parameter	Value	Unit	Context
Diamond Growth Method	MPCVD	N/A	Direct growth on 2-inch Si-based GaN HEMT
Microwave Power	3500	W	MPCVD recipe parameter
Growth Temperature	800	°C	MPCVD recipe parameter
Carbon Source Concentration	6	%	Likely CH₄/H₂ ratio
Cooling Rate (Slow)	~9	°C/min	Used to mitigate thermal stress
PCD Thickness Range	9 - 81	µm	Tested thicknesses
PCD Growth Rate (Average)	1.16 - 1.50	µm/h	Dependent on growth time
GaN Intrinsic Mobility (µ_intrinsic)	1588.8	cm²/(V·s)	Before diamond growth
GaN Mobility (81 µm PCD)	1178.2	cm²/(V·s)	After diamond growth
Mobility Loss (81 µm PCD)	25.8	%	Electrical performance degradation
GaN Tensile Stress (Calculated)	0.07	GPa	Stress induced by 81 µm PCD layer
GaN FWHM₀₀₂ (Intrinsic)	550	arcsec	Before diamond growth
GaN FWHM₀₀₂ (81 µm PCD)	654	arcsec	After diamond growth (Stress-induced increase)
GaN FWHM₀₀₂ (Recovered)	530	arcsec	After PCD stripping (Returned to intrinsic state)
PCD Raman Peak Position	1333.0 - 1333.9	cm^-1	Indicating high crystalline quality
PCD Pressure Stress (Calculated)	0.57 - 1.08	GPa	Calculated internal compressive stress in PCD

Key Methodologies

The experiment utilized precise MPCVD growth and specialized post-processing techniques to isolate the effects of the diamond layer on the GaN structure.

Substrate Preparation: 2-inch Si-based GaN HEMT structures were used. A 30 nm SiN layer was deposited via PECVD to protect the GaN from hydrogen etching during high-temperature diamond growth.
Nucleation: Substrates were ultrasonically treated for 20 minutes in a 1% diamond powder (20 nm particle size) suspension, followed by a 20-minute soak to ensure uniform nucleation density for continuous film growth.
MPCVD Growth: Polycrystalline diamond was grown using the following parameters:
- Equipment: MPCVD system.
- Carbon Source: Methane (CH₄).
- Carrier Gas: Hydrogen (H₂).
- Power/Temperature: 3500 W / 800 °C.
- Carbon Concentration: 6%.
- Growth Times: 6 h, 18 h, and 70 h (yielding 9 µm, 25 µm, and 81 µm thicknesses, respectively).
Stress Mitigation: A slow cooling process (~9 °C/min) was implemented post-growth to minimize thermal stress between the diamond and the GaN/Si stack.
PCD Stripping (Exfoliation):
- Laser Cutting: Infrared laser cutting was used on the edges of the composite material to initiate separation via thermal effects.
- Acid Etching: The samples were immersed in a mixed acid solution (50% Hydrofluoric Acid (HF) and 50% Nitric Acid (HNO₃) at a 3:1 ratio). This process etched the SiN nucleation layer and the Si substrate, allowing complete separation of the GaN/Si stack from the PCD layer.

6CCVD Solutions & Capabilities

The research highlights the critical need for highly controlled, high-quality PCD materials and precise integration techniques to successfully implement GaN-on-Diamond thermal solutions. 6CCVD is uniquely positioned to supply the necessary materials and engineering support to replicate and advance this research.

Applicable Materials for GaN-on-Diamond Research

To replicate or extend this work, researchers require high-purity, high-thermal-conductivity PCD wafers with exceptional thickness uniformity and surface quality.

6CCVD Material Solution	Specification & Relevance to Paper	Customization Potential
High Thermal Grade PCD	Required material for high-power GaN thermal spreading. We offer plates up to 125mm in diameter, significantly exceeding the 2-inch (50.8 mm) wafers used in this study.	Large Area: Enables scaling up of GaN HEMT production and larger device integration.
Precision Thickness PCD	The paper showed thickness (9 µm vs 81 µm) is the primary factor controlling stress. 6CCVD offers PCD thickness control from 0.1 µm to 500 µm.	Stress Engineering: Allows researchers to precisely tune the diamond thickness to optimize the trade-off between thermal performance and stress-induced mobility degradation.
Ultra-Smooth PCD Surfaces	Interface quality is critical for thermal boundary conductance (TBC). 6CCVD guarantees PCD polishing to Ra < 5 nm (for inch-size plates).	Enhanced TBC: Superior polishing minimizes interface defects, potentially improving heat transfer efficiency compared to as-grown surfaces.
Boron-Doped Diamond (BDD)	While the paper focused on intrinsic PCD, BDD is often used for integrated sensing or active device layers.	Integrated Functionality: We offer custom BDD films for projects requiring integrated electrical functionality alongside thermal management.

Customization Potential for Advanced Integration

The successful exfoliation method used in this paper (laser cutting followed by acid etching) demonstrates the need for precise material handling and interface engineering.

Custom Dimensions and Shaping: 6CCVD provides custom laser cutting and shaping services to match specific device geometries or to facilitate post-growth processing steps like the edge separation used in this study.
Advanced Metalization: For subsequent device fabrication (e.g., bonding the exfoliated GaN or creating contacts on the diamond), 6CCVD offers internal metalization capabilities, including Au, Pt, Pd, Ti, W, and Cu deposition, ensuring robust electrical and thermal contacts.
Substrate Flexibility: We can supply SCD or PCD substrates up to 10 mm thick for robust handling or specialized thermal sink applications, far exceeding the thin films grown in this research.

Engineering Support

The findings regarding the recoverable nature of GaN degradation due to thermal mismatch stress are vital. However, optimizing the growth recipe (temperature, cooling rate, SiN layer thickness) to minimize this stress during growth remains a complex challenge.

6CCVD’s in-house PhD-level engineering team specializes in MPCVD diamond growth physics and material characterization (Raman, XRD, Hall). We offer consultation services to assist researchers and engineers in:

Material Selection: Determining the optimal PCD thickness and quality grade for specific GaN HEMT thermal requirements.
Process Optimization: Advising on nucleation layer design (like the SiN layer used here) and cooling protocols to minimize stress and maximize electron mobility in similar GaN-on-Diamond HEMT projects.
Global Logistics: Ensuring reliable, DDU or DDP global shipping of sensitive diamond materials directly to your research facility.

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

View Original Abstract

Self-heating has become a limited factor for the performance improvement of GaN electronics. Growing polycrystalline diamond directly on GaN material to solve the heating problem of GaN devices has become one of the research highlights. Polycrystalline diamond on Si-based GaN material has the advantages of being close to the channel region and high heat dissipation efficiency. However, there is a problem that the thermal expansion mismatch between polycrystalline diamond and GaN material leads to the deterioration of electrical characteristics of GaN. In this work, we adopt microwave plasma chemical vapor deposition (MPCVD) method to grow polycrystalline diamond on 2-inch Si-based GaN material. The test results show that the polycrystalline diamond is uniform as a whole. The average thickness is in the range of 9-81 μm. With the thickness of polycrystalline diamond increasing, the XRD (002) diffraction peak FWHM increment and mobility loss gradually increase for the Si-based GaN material. Through laser cutting and acid etching, the Si-based GaN material is successfully stripped from the polycrystalline diamond. It is found that during the process of diamond growth at high temperature, hydrogen atoms etch the defect positions of the silicon nitride epitaxial layer, forming a hole area in the GaN, and the etching depth can reach the intrinsic GaN layer. During the process of cooling, a crack area is formed around the hole area. Raman characteristic peaks, full widths at half maximum of XRD (002) diffraction peaks, and electrical properties of the stripped Si-based GaN materials are all returned to their intrinsic states. The above results show that the thermal expansion mismatch between polycrystalline diamond and Si-based GaN introduces stress into GaN, which leads to lattice distortion of GaN lattice and the degradation of electrical property of GaN material. The degradation of GaN material is recoverable, but not destructive.

Tech Support

Original Source

DOI: https://doi.org/10.7498/aps.72.20221942