Temperature-Dependent Thermal Resistance of GaN-on-Diamond HEMT Wafers

At a Glance

Metadata	Details
Publication Date	2016-03-09
Journal	IEEE Electron Device Letters
Authors	Huarui Sun, James W. Pomeroy, Roland B. Simon, Daniel Francis, Firooz Faili
Institutions	Element Six (United States), University of Bristol
Citations	68
Analysis	Full AI Review Included

Technical Documentation & Analysis: GaN-on-Diamond HEMT Wafers

Executive Summary

This research confirms the significant thermal advantages of GaN-on-Diamond HEMT wafers, demonstrating properties highly favorable for high-power, high-reliability RF applications.

Thermal Runaway Mitigation: The effective Thermal Boundary Resistance (TBR_GaN-dia) exhibits a beneficial negative temperature dependence, decreasing by up to 30% between 25 °C and 250 °C. This counteracts the risk of thermal runaway observed in GaN-on-SiC and GaN-on-Si devices.
High Substrate Conductivity: The 100 µm MPCVD Polycrystalline Diamond (PCD) substrate maintains a high effective thermal conductivity ($K_{dia}$), approximated as 1300 (T/300K)^-0.9 W/mK, significantly enhancing heat spreading.
Interface Engineering: The temperature dependence is primarily driven by the amorphous SiN_x interlayer ($\sim$40 nm), whose thermal conductivity increases with temperature.
Nucleation Layer Impact: Minimizing the thickness of the Nanocrystalline Diamond (NCD) nucleation region (< 10 nm, Sample A) is critical for achieving the lowest overall TBR_GaN-dia and maximizing heat dissipation.
Measurement Validation: Thermal properties were accurately characterized using Nanosecond Transient Thermoreflectance (TTR) and validated via Finite Element (FE) modeling, providing essential input for device reliability assessment.
Commercial Relevance: These findings strongly favor the use of high-quality MPCVD PCD substrates for next-generation GaN HEMTs operating at high power densities (up to 12 W/mm modeled).

Technical Specifications

The following hard data points were extracted from the analysis of the GaN-on-Diamond HEMT wafers:

Parameter	Value	Unit	Context
Measurement Temperature Range	25 to 250	°C	Used for Transient Thermoreflectance (TTR)
Diamond Substrate Thickness	100	µm	MPCVD Polycrystalline Diamond (PCD)
GaN Epilayer Thickness	0.7	µm	AlGaN/GaN HEMT structure
Amorphous SiN_x Interlayer Thickness	$\sim$40	nm	Deposited via Low-Pressure CVD
Effective Diamond Thermal Conductivity (K_dia)	$\sim$1300 (T/300K)^-0.9	W/mK	Spatially weighted average
TBR_GaN-dia Reduction (Sample A)	$\sim$30	%	Reduction over the 25 °C to 250 °C range
Effective K_SiNx (Room Temp)	1.6 $\pm$ 0.1	W/mK	Thermal conductivity of the SiN_x interlayer
NCD Thermal Conductance (K_NCD/d_NCD)	$\sim$0.1	GW/m²K	Contribution from the Nanocrystalline Diamond layer
Metalization Stack	Cr (10 nm) / Au (150 nm)	-	Used as transducer layer for TTR
Modeled HEMT Power Dissipation	Up to 12	W/mm	Used to calculate channel peak temperature rise ($\Delta$T_channel)

Key Methodologies

The GaN-on-Diamond wafers were fabricated and characterized using advanced CVD and thermal measurement techniques:

Epitaxial Starting Material: AlGaN/GaN epilayers were initially grown on a Silicon (Si) substrate (0.7 µm thick GaN buffer layer).
Si Substrate Removal: The Si substrate was removed to expose the GaN surface.
Interlayer Deposition: A $\sim$40 nm thick amorphous SiN_x layer was subsequently deposited onto the exposed GaN surface using Low-Pressure Chemical Vapor Deposition (CVD).
Diamond Growth: A 100 µm thick Polycrystalline Diamond (PCD) layer was grown via Microwave Plasma CVD (MPCVD).
Interface Control: Two samples (A and B) were created using different seeding methods to control the thickness of the diamond nucleation/transition region (Sample A: < 10 nm; Sample B: 50-100 nm).
Metalization for Measurement: The samples were coated with a 10 nm Cr adhesion layer followed by a 150 nm Au film to serve as the optical transducer layer.
Thermal Measurement: Nanosecond Transient Thermoreflectance (TTR) was employed, using a 10 ns pulsed 355 nm laser for heating and a continuous 532 nm laser for monitoring the transient temperature change.
Data Extraction: A verified Finite Element (FE) transient thermal model was used to fit the measured transients and extract the temperature-dependent TBR_GaN-dia and the effective diamond thermal conductivity ($K_{dia}$).

6CCVD Solutions & Capabilities

6CCVD specializes in providing the high-quality MPCVD diamond materials and precision engineering services required to replicate and advance the GaN-on-Diamond technology demonstrated in this research.

Applicable Materials

To achieve the high $K_{dia}$ and low TBR demonstrated in this paper, researchers require high-purity, high-thermal-conductivity Polycrystalline Diamond (PCD).

High Thermal Conductivity Polycrystalline Diamond (PCD) Wafers: 6CCVD offers MPCVD PCD substrates up to 125mm in diameter, suitable for large-scale HEMT fabrication. We guarantee the necessary high thermal conductivity (K > 1000 W/mK) required for effective heat spreading in high-power GaN devices.
Custom Thickness PCD: The paper utilized 100 µm thick diamond. 6CCVD provides PCD wafers with custom thicknesses ranging from 0.1 µm up to 500 µm, allowing engineers to optimize the thermal path for specific device architectures.

Customization Potential

The success of GaN-on-Diamond relies heavily on precise interface engineering and metalization, areas where 6CCVD provides comprehensive support:

Precision Thickness Control: We offer PCD substrates tailored to the exact 100 µm thickness used in this study, or customized thicknesses up to 500 µm for enhanced mechanical stability or thermal mass.
Advanced Metalization Services: The TTR measurement required a Cr/Au stack (10 nm Cr, 150 nm Au). 6CCVD offers in-house metalization capabilities, including the deposition of Au, Pt, Pd, Ti, W, and Cu, enabling researchers to integrate custom transducer or contact layers directly onto the diamond surface.
Surface Finish Optimization: Achieving low TBR is dependent on the diamond nucleation surface quality. 6CCVD provides high-quality polishing services, ensuring surface roughness (Ra) < 5 nm for inch-size PCD wafers, critical for minimizing interfacial defects and maximizing thermal coupling.

Engineering Support

The research highlights that the thermal performance is highly sensitive to the SiN_x interlayer and the Nanocrystalline Diamond (NCD) nucleation region thickness.

Interface Optimization Consultation: 6CCVD’s in-house PhD team specializes in MPCVD growth parameters and material science. We offer consultation services to assist researchers in optimizing the diamond seeding density and growth recipe to minimize the NCD transition layer thickness, thereby achieving the lowest possible TBR_GaN-dia (replicating the superior performance of Sample A).
Thermal Modeling Assistance: We provide material specifications and engineering data necessary for accurate Finite Element (FE) thermal modeling, supporting the assessment of GaN-on-Diamond transistor thermal resistance for high-power RF reliability tests.

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

View Original Abstract

The thermal properties of GaN-on-diamond high-electron mobility transistor (HEMT) wafers from 25 °C to 250 °C are reported. The effective thermal boundary resistance between GaN and diamond decreases at elevated temperatures due to the increasing thermal conductivity of the amorphous SiNx interlayer, therefore potentially counteracting thermal runaway of devices. The results demonstrate the thermal benefit of GaN-on-diamond for HEMT high-power operations, and provide valuable information for assessing the thermal resistance and reliability of devices.

Tech Support

Original Source

DOI: https://doi.org/10.1109/led.2016.2537835

References

1997 - Heat capacity, conductivity, and the thermal coefficient of expansion