Preparation and Characterization of GaN-on-Si HEMTs with Nanocrystalline Diamond Passivation

At a Glance

Metadata	Details
Publication Date	2025-02-28
Journal	Crystals
Authors	Yu Fu, Songyuan Song, Zeyang Ren, Liaoliang Zhu, Jinfeng Zhang
Institutions	Xidian University, Wuhu Institute of Technology
Citations	1
Analysis	Full AI Review Included

Technical Documentation: Nanocrystalline Diamond Passivation for High-Power GaN HEMTs

6CCVD Material Science Analysis & Sales Documentation Reference Paper: Preparation and Characterization of GaN-on-Si HEMTs with Nanocrystalline Diamond Passivation

Executive Summary

This study successfully demonstrates the use of Microwave Plasma Chemical Vapor Deposition (MPCVD) grown Nanocrystalline Diamond (NCD) as a passivation layer to significantly enhance the thermal management and performance of AlGaN/GaN-on-Si High-Electron Mobility Transistors (HEMTs).

Thermal Performance: NCD passivation resulted in a 36% improvement in heat dissipation efficiency, effectively suppressing self-heating effects crucial for high-power GaN devices.
Material Compatibility: The NCD film was grown using a low-temperature MPCVD process (650 °C), ensuring minimal degradation of the underlying AlGaN/GaN heterostructure.
Electrical Gains: Device performance saw a 24% increase in maximum current density (I_Dmax) to 555 mA/mm and a 34% reduction in on-resistance (R_on) to 13.2 Ω·mm.
Reliability Enhancement: The NCD layer contributed to improved device reliability, increasing the off-state breakdown voltage (V_br) from 400 V to 500 V.
NCD Specifications: The optimized NCD film thickness ranged from 250-383 nm, exhibiting a uniform grain size of approximately 240 nm.
6CCVD Relevance: This research validates the critical role of high-quality, thin-film MPCVD diamond for device-level thermal management, a core specialization of 6CCVD.

Technical Specifications

The following hard data points were extracted from the characterization of the NCD-passivated GaN HEMTs:

Parameter	Value	Unit	Context
NCD Film Thickness	250-383	nm	Grown NCD passivation layer
NCD Grain Size	~240	nm	Uniform morphology
NCD Growth Temperature	650	°C	Low-temperature MPCVD process
Maximum Current Density (I_Dmax)	555	mA/mm	With NCD passivation (24% increase)
On-Resistance (R_on)	13.2	Ω·mm	With NCD passivation (34% reduction)
Heat Dissipation Improvement	36	%	Compared to non-passivated device
Thermal Slope (w/ NCD)	16.38	°C/W·mm	Junction temperature vs. output power density
Thermal Slope (w/o NCD)	25.88	°C/W·mm	Baseline device performance
Off-State Breakdown Voltage (V_br)	500	V	With NCD passivation
Peak Transconductance (G_m,max)	97.0	mS/mm	With NCD passivation
Diamond Raman Peak	1334.7	cm^-1	Confirms successful NCD growth

Key Methodologies

The NCD passivation layer was fabricated using a two-step MPCVD process, preceded by the deposition of a protective SiN_x layer.

Wafer Structure: Al_0.21Ga_0.79N (20 nm barrier) / GaN (190 nm channel) / GaN buffer (4.49 µm) on a Si substrate.
Protection Layer Deposition: A 50 nm-thick SiN_x layer was deposited via ICP-CVD at 130 °C to protect the AlGaN/GaN material during subsequent high-temperature NCD growth.
Diamond Seeding: Nanocrystalline diamond seed suspension was spin-coated onto the sample surface (2000 rpm for 30 s).
NCD Growth (MPCVD - Two-Step Strategy):
- Equipment: MPCVD system (Worldiray company).
- Constant Gas Flows: H₂ (300 sccm), CH₄ (12 sccm), N₂ (0.05 sccm).
- Step 1 (Nucleation): Duration 120 s, Power 2.0 kW, Pressure 90 mbar. Goal: Form a thin membrane to prevent hydrogen plasma etching.
- Step 2 (Main Growth): Duration 15 min, Power 3.2 kW, Pressure 135 mbar. Goal: Thicken the NCD film and enlarge grains.
Ohmic Contact Formation: Ti/Al/Ni/Au multilayer stack (20/150/50/100 nm) deposited and annealed via RTA at 835 °C for 30 s in pure N₂.
Gate Metalization: Ni/Au (20/200 nm) deposited after selective etching of the NCD and SiN_x layers to expose the gate area.

6CCVD Solutions & Capabilities

The successful integration of MPCVD NCD films for high-efficiency thermal management in GaN HEMTs directly aligns with 6CCVD’s core expertise. We provide the high-quality diamond materials and customization services necessary to replicate, optimize, and scale this critical research.

Applicable Materials

To achieve the high thermal conductivity and low-temperature growth required for GaN device passivation, 6CCVD recommends the following materials, grown via our proprietary MPCVD systems:

Thermal Grade Polycrystalline Diamond (PCD/NCD): We offer highly uniform NCD films optimized for thermal management applications. Our process control allows for precise tuning of grain size and thickness to maximize thermal boundary conductance (TBC) while minimizing non-diamond phases (as noted in the paper, optimizing CH₄ flow is critical, a service our team provides).
Custom PCD Wafers: For scaling up GaN-on-Diamond technology, 6CCVD provides PCD plates/wafers up to 125 mm in diameter, suitable for large-scale production of high-power devices.

Customization Potential

The research highlights the necessity of precise layer control, specific thicknesses, and complex metal stacks. 6CCVD is uniquely positioned to meet these requirements:

Research Requirement	6CCVD Capability	Benefit to Client
NCD Thickness Control	SCD/PCD thickness control from 0.1 µm to 500 µm.	Exact replication of the 250-383 nm NCD passivation layer.
Large Area Processing	PCD wafers up to 125 mm diameter.	Seamless transition from R&D (1.5 x 1.5 cm² pieces) to pilot production scale.
Custom Metalization	Internal capability for deposition of Au, Pt, Pd, Ti, W, Cu.	Direct fabrication of complex ohmic (Ti/Al/Ni/Au) and gate (Ni/Au) stacks onto diamond films or substrates.
Surface Finish	Polishing capability for PCD surfaces to Ra < 5 nm (inch-size).	Ensures optimal surface quality for subsequent layer deposition (e.g., SiN_x interlayers) and bonding applications.
Global Logistics	Global shipping via DDU default, with DDP options available.	Reliable, secure delivery of sensitive materials worldwide.

Engineering Support

The paper noted that future studies require optimizing growth conditions (e.g., reducing CH₄ flow) to improve diamond quality and thermal conductivity.

6CCVD’s in-house team of PhD material scientists specializes in tuning MPCVD recipes for specific electronic and thermal applications. We offer consultation services to assist engineers and researchers in:

Recipe Optimization: Fine-tuning gas ratios (H₂/CH₄/N₂) and power parameters to maximize the thermal conductivity of NCD films for GaN HEMT thermal management projects.
Interface Engineering: Selecting and integrating appropriate interlayers (like the 50 nm SiN_x layer used here) to minimize thermal boundary resistance (TBR) between the diamond and the GaN heterostructure.
Material Selection: Advising on the optimal diamond grade (SCD vs. PCD) based on the required thermal performance and device geometry.

Call to Action: For custom specifications or material consultation regarding high-power GaN thermal solutions, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Thermal accumulation under high output power densities is one of the most significant challenges for GaN power devices. Diamond, with its ultra-high thermal conductivity, offers great potential for improving heat dissipation in high-power GaN devices. In this study, nanocrystalline diamond (NCD) passivated high-electron mobility transistors (HEMTs) based on AlGaN/GaN-on-Si heterostructures were fabricated with a gate length of 2 μm. The NCD film has a thickness of 250-383 nm and a uniform morphology with a grain size of mostly ~240 nm. Compared to the devices without NCD passivation, those devices with the NCD passivation layer show an increase in current density from 447 mA/mm to 555 mA/mm, a reduction in on-resistance from 20 Ω·mm to 13 Ω·mm, and a noticeable suppression of current degradation at high-drain voltages. Junction temperature measurements under varied output power densities reveal a 36% improvement in heat dissipation efficiency with the NCD passivation. These results fully demonstrate the promising potential of NCD for enhancing heat dissipation in high-power GaN devices.

Tech Support

Original Source

DOI: https://doi.org/10.3390/cryst15030242

References

2020 - Prospects for Wide Bandgap and Ultrawide Bandgap CMOS Devices [Crossref]
2022 - Improved Stability of GaN MIS-HEMT With 5-nm Plasma-Enhanced Atomic Layer Deposition SiN Gate Dielectric [Crossref]
2023 - Overview of Wide/Ultrawide Bandgap Power Semiconductor Devices for Distributed Energy Resources [Crossref]
2016 - Analysis of the modulation mechanisms of the electric field and breakdown performance in AlGaN/GaN HEMT with a T-shaped field-plate [Crossref]
2021 - Fabrication and characterization of GaN HEMTs grown on SiC substrates with different orientations [Crossref]
2022 - A 32-A, 5-V-Input, 94.2% Peak Efficiency High-Frequency Power Converter Module Featuring Package-Integrated Low-Voltage GaN nMOS Power Transistors [Crossref]
2021 - A GaN BCM AC-DC Converter for Sub-1 V Electromagnetic Energy Harvesting With Enhanced Output Power [Crossref]
2023 - Cooling future system-on-chips with diamond inter-tiers [Crossref]
2022 - GaN MMICs on a diamond heat spreader with through-substrate vias fabricated by deep dry etching process [Crossref]