GaN-on-diamond electronic device reliability - Mechanical and thermo-mechanical integrity

At a Glance

Metadata	Details
Publication Date	2015-12-21
Journal	Applied Physics Letters
Authors	Dong Liu, Huarui Sun, James W. Pomeroy, Daniel Francis, Firooz Faili
Institutions	Element Six (United States), Bristol Robotics Laboratory
Citations	30
Analysis	Full AI Review Included

Technical Analysis & Documentation: GaN-on-Diamond Reliability

Reference: Liu, D. et al. (2015). GaN-on-Diamond Electronic Device Reliability: Mechanical and Thermo-Mechanical Integrity. Applied Physics Letters, 107(25), Article 251902.

Executive Summary

This research validates the exceptional structural integrity and thermal stability of Microwave Plasma Chemical Vapor Deposition (MPCVD) diamond substrates integrated with GaN heterostructures, confirming their suitability for next-generation Ultra-High Power Microwave Electronics.

Superior Reliability: The study confirms the high thermo-mechanical stability of the GaN-on-Diamond (GoD) interface, addressing the primary reliability concern related to thermal expansion mismatch.
Interface Strength Quantification: In situ micro-pillar mechanical testing established a lower bound for the GaN/diamond interface strength exceeding 3.3 GPa.
Failure Mode: Under maximum load (360 µN), fracture occurred through the thickness of the brittle GaN layer, demonstrating that the diamond interface remained intact and was significantly stronger than the GaN film itself.
Thermal Stability: The effective Thermal Boundary Resistance (TBR_eff) remained constant (33 ± 4 m²K/GW) after high-temperature annealing up to 950 °C in a nitrogen atmosphere.
Material Specification: The diamond layer used was approximately 100 µm thick, grown via MW Plasma CVD onto a 40 nm dielectric seeding layer.
Application Impact: These findings provide the fundamental basis for reliable GaN HEMTs capable of operating at power densities three times higher than traditional GaN-on-SiC devices.

Technical Specifications

The following hard data points were extracted from the mechanical, thermal, and finite element analysis (FEA) performed in the study.

Parameter	Value	Unit	Context
Diamond Growth Method	MW Plasma CVD	N/A	Used to grow the ~100 µm diamond layer
Diamond Layer Thickness	~100	µm	Grown after Si substrate removal
Dielectric Seeding Layer Thickness	40	nm	Amorphous layer deposited by low-pressure CVD
Maximum Applied Load (Probe Limit)	360	µN	Limit of the customized piezoelectric force measurement system
Load at GaN Fracture	300	µN	Applied to the reduced 0.7 x 3 µm GaN pillar
Interface Strength (Lower Bound)	3.3	GPa	Maximum principal stress calculated at the notch tip prior to GaN failure
Room Temperature Stress (GaN)	0.35	GPa	Compressive stress due to thermal lattice mismatch
Maximum Compressive Stress (GaN)	1.5	GPa	Measured at 900 °C
Maximum Annealing Temperature	950	°C	Used for thermal stability testing (10 mins in N₂)
Thermal Boundary Resistance (TBR_eff)	33 ± 4	m²K/GW	Measured at room temperature for as-grown and annealed samples
GaN Young’s Modulus (FEA Input)	181	GPa	Used in multi-layer numerical model
Diamond Young’s Modulus (FEA Input)	105	GPa	Used in multi-layer numerical model

Key Methodologies

The structural integrity and thermal reliability were evaluated using a combination of advanced fabrication and in situ characterization techniques.

Substrate Preparation: AlGaN/GaN heterostructures, initially grown on Si via MOCVD, had the Si substrate and strain relief layer removed.
Diamond Deposition: A 40 nm amorphous dielectric layer was deposited onto the GaN via low-pressure CVD, followed by the growth of the ~100 µm diamond layer using Microwave Plasma CVD.
Micro-Pillar Fabrication: Focused Ion Beam (FIB) milling (using a 30 kV Ga⁺ beam) was employed to create micro-pillars comprising the GaN, dielectric, and diamond layers.
Stress Concentration: To ensure failure initiation at the interface region, the GaN layer was reduced (0.7 x 3 µm) and a sharp notch (70 nm diameter) was created at the GaN/diamond interface using low-current line milling (26 pA).
Mechanical Testing: A customized piezoelectric force measurement Si probe was used in situ within a Scanning Electron Microscope (SEM) to apply a load (up to 360 µN) parallel to the interface, inducing shear stress.
Thermal Stress Measurement: Raman spectroscopy (488 nm laser, 1 µm spot size) was used to monitor the E₂ GaN phonon peak shift in situ as the sample was heated up to 950 °C in a nitrogen atmosphere, quantifying biaxial compressive stress.
Thermal Boundary Resistance (TBR) Measurement: Transient Thermoreflectance (TTR) was used, employing a 355 nm pulsed Nd:YAG laser (above GaN bandgap) for heating and a 532 nm CW laser for monitoring surface reflectance changes, allowing extraction of TBR_eff.

6CCVD Solutions & Capabilities

The successful replication and advancement of GaN-on-Diamond technology rely on high-quality, customized MPCVD diamond substrates. 6CCVD is uniquely positioned to supply the materials required for reliable, high-power electronic devices.

Applicable Materials for High-Reliability GaN Integration

Research Requirement	6CCVD Material Recommendation	Technical Rationale
High Thermal Conductivity Substrate (Required to replace SiC)	Optical Grade SCD (Single Crystal Diamond)	Provides the highest thermal conductivity (> 2000 W/mK), minimizing device thermal resistance and maximizing power density capability.
Large Area Substrates (For commercial scaling)	High Purity PCD (Polycrystalline Diamond)	Available in large formats up to 125 mm diameter, offering excellent thermal performance for cost-effective, high-volume manufacturing of GaN HEMTs.
Seeding/Interface Layer (Need for precise interface control)	Custom Thickness SCD/PCD:	We offer SCD and PCD layers with thickness control from 0.1 µm up to 500 µm, allowing precise replication or optimization of the required ~100 µm layer used in this study.

Customization Potential for Advanced Research

The mechanical testing methodology employed in this paper highlights the need for precise material specifications and post-processing capabilities, which 6CCVD provides:

Precision Polishing for Low TBR_eff: Achieving the stable, low TBR_eff reported in the paper requires an ultra-smooth interface. 6CCVD guarantees surface roughness of Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD, ensuring optimal thermal contact for subsequent GaN epitaxy or bonding.
Custom Dimensions and Machining: While the paper used FIB for micro-pillar creation, 6CCVD offers advanced laser cutting and shaping services to provide custom plate and wafer dimensions required for specific device layouts or testing methodologies.
Integrated Metalization Services: For researchers developing device contacts or bonding layers (e.g., the Ti/Pt/Au systems often used in GaN HEMTs), 6CCVD offers in-house metalization capabilities, including deposition of Au, Pt, Pd, Ti, W, and Cu, delivering ready-to-use substrates.

Engineering Support & Logistics

6CCVD’s in-house PhD team specializes in material selection and optimization for high-power semiconductor applications. We can assist researchers and engineers in selecting the optimal diamond grade (SCD vs. PCD) and specification (thickness, doping, surface finish) required to replicate or extend this research into reliable GaN-on-Diamond High Electron Mobility Transistors (HEMTs).

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. We offer global shipping (DDU default, DDP available) to ensure timely delivery of critical materials worldwide.

View Original Abstract

The mechanical and thermo-mechanical integrity of GaN-on-diamond wafers used for ultra-high power microwave electronic devices was studied using a micro-pillar based in situ mechanical testing approach combined with an optical investigation of the stress and heat transfer across interfaces. We find the GaN/diamond interface to be thermo-mechanically stable, illustrating the potential for this material for reliable GaN electronic devices.

Tech Support

Original Source

DOI: https://doi.org/10.1063/1.4938002

References

2013 - A new high power GaN-on-diamond HEMT with low-temperature bonded substrate technology