Integration of polycrystalline Ga2O3 on diamond for thermal management
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-02-10 |
| Journal | Applied Physics Letters |
| Authors | Zhe Cheng, Virginia D. Wheeler, Tingyu Bai, Jingjing Shi, Marko J. Tadjer |
| Institutions | United States Naval Research Laboratory, Georgia Institute of Technology |
| Citations | 113 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research investigates the heterogeneous integration of Gallium Oxide (Ga2O3) thin films onto high thermal conductivity diamond substrates using Atomic Layer Deposition (ALD) to address critical thermal management challenges in Ga2O3 electronics.
- Core Achievement: Successful integration of Ga2O3 on single crystal diamond, yielding a high Thermal Boundary Conductance (TBC) of up to 179 MW/m2K at the interface.
- Thermal Transport Mechanism: The measured TBC is approximately 10 times larger than that observed in Van der Waals bonded Ga2O3-diamond interfaces, confirming that strong, likely covalent, interfacial bonding significantly enhances heat transfer.
- Film Quality Limitation: The ALD-grown Ga2O3 thin films exhibited extremely low thermal conductivity (~1.5 W/mK), which is close to the amorphous limit. This is attributed to the nanocrystalline structure (10-20 nm grains) causing extensive phonon grain boundary scattering.
- Interface Chemistry Impact: Different in-situ diamond surface pretreatments (Ga-rich and O-rich) resulted in a measurable decrease in TBC (about 20% reduction) compared to the ultra-clean interface, indicating that interface chemistry is a critical factor in thermal transport.
- Value Proposition: This study provides a promising, scalable route for improving heat dissipation from Ga2O3 devices, which is essential for realizing their potential in high-power and high-frequency applications.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Bulk beta-Ga2O3 Thermal Conductivity (k) | 10 - 30 | W/mK | Dependent on crystal orientation |
| Diamond Thermal Conductivity (k) | >2000 | W/mK | Substrate material for heat extraction |
| ALD Ga2O3 Thin Film k (Measured) | 1.50 - 1.76 | W/mK | Low value due to nanocrystalline structure (10-20 nm grains) |
| Amorphous Limit k (Calculated) | 1.15 | W/mK | Cahill model estimate at room temperature |
| Diffuson Minimum k (Calculated) | 0.90 | W/mK | Diffuson model estimate at room temperature |
| Highest TBC (Samp2, Ultra-clean) | 179 | MW/m2K | Achieved with strong interfacial bonding |
| Ga-rich TBC (Samp3) | 136 | MW/m2K | ~20% reduction compared to ultra-clean interface |
| O-rich TBC (Samp4) | 139 | MW/m2K | ~20% reduction compared to ultra-clean interface |
| Van der Waals Bonded TBC (Reference) | ~17 | MW/m2K | TBC of exfoliated Ga2O3 on diamond (10x lower than Samp2) |
| ALD Growth Temperature | 350 | °C | Deposition temperature in Fiji 200 G2 reactor |
| ALD Growth Rate | 0.65 | A/cycle | Angstroms per cycle |
| Ga2O3 Film Thickness (TBC samples) | 28 - 30 | nm | Thin films used for interface sensitivity |
| Ga2O3 Film Thickness (k sample) | 115 | nm | Thick film used for bulk film property measurement |
| Transducer Layer Thickness (Al) | ~84 | nm | Used for Time-domain Thermoreflectance (TDTR) |
Key Methodologies
Section titled âKey MethodologiesâThe study utilized Atomic Layer Deposition (ALD) for film growth and Time-domain Thermoreflectance (TDTR) for thermal characterization, supported by Transmission Electron Microscopy (TEM) for structural analysis.
- Substrate Preparation: Single crystalline (100) thermal grade diamond substrates were subjected to a rigorous cleaning sequence to remove metal and non-diamond carbon contamination:
- Acid treatments (HNO3:HCl and HNO3:H2SO4).
- Ultrasonic cleaning in ethanol.
- Final HF etch.
- ALD Growth Parameters: Ga2O3 thin films were deposited at 350 °C using alternating cycles of precursors:
- Ga Precursor: Trimethylgallium (TMG, STREM PURATREM).
- Oxidizing Source: Remote pure oxygen plasma.
- Chamber Pressure: 8 mTorr during plasma exposure.
- Interface Pretreatment Variations (In-situ): Different surface conditions were tested prior to ALD growth to investigate TBC effects:
- Ultra-clean (Samp2): Reference condition without specific in-situ chemical treatment.
- Ga Flashoff (Samp2 emulation): Dosing with TMG to create a gallium sub-oxide, followed by removal using a hydrogen plasma pulse (emulating MBE cleaning).
- Ga-rich (Samp3): Initiated by 10 consecutive Ga pulses (super saturation).
- O-rich (Samp4): Received 10, 10s O2 plasma pulses.
- Thermal Measurement (TDTR): A layer of Al (~84 nm) was deposited as a transducer. TDTR utilized a modulated pump beam (400 nm) for heating and a delayed probe beam (800 nm) to detect surface temperature variation, allowing inference of film thermal conductivity and TBC.
- Structural Analysis (TEM): TEM was used to confirm film thickness, analyze the nanocrystalline grain structure (10-20 nm), and verify the atomically abrupt, void-free contact at the Ga2O3-diamond interface.
Commercial Applications
Section titled âCommercial ApplicationsâThe successful integration of Ga2O3 with diamond, achieving high TBC through strong interfacial bonding, is critical for advancing wide bandgap semiconductor technology in demanding thermal environments.
- High Power RF and Microwave Electronics: Ga2O3 devices (like FETs) integrated with diamond are essential for minimizing hot-spots and maintaining reliability in high-frequency amplifiers and transmitters where thermal dissipation is paramount.
- Power Switching Devices: Utilization of Ga2O3âs high breakdown electric field (8 MV/cm) in power converters and inverters, where the diamond substrate enables operation at higher current densities and temperatures than conventional substrates.
- Automotive and Aerospace Power Systems: Applications requiring robust, high-efficiency power electronics that must operate reliably under extreme thermal loads.
- Scalable Heterogeneous Integration: The results support the use of scalable bonding techniques, such as room-temperature Surface Activated Bonding (SAB), to integrate high-quality, independently grown beta-Ga2O3 layers onto high thermal conductivity substrates (diamond or SiC).
- Thermal Management Solutions: Providing a proven pathway for designing lateral device architectures that effectively channel heat away from the active device region into the diamond heat spreader.
View Original Abstract
Gallium oxide (Ga2O3) has attracted great attention for electronic device applications due to its ultra-wide bandgap, high breakdown electric field, and large-area affordable substrates grown from the melt. However, its thermal conductivity is significantly lower than that of other wide bandgap semiconductors such as SiC, AlN, and GaN, which will impact its ability to be used in high power density applications. Thermal management in Ga2O3 electronics will be the key for device reliability, especially for high power and high frequency devices. Similar to the method of cooling GaN-based high electron mobility transistors by integrating it with high thermal conductivity diamond substrates, this work studies the possibility of heterogeneous integration of Ga2O3 with diamond for the thermal management of Ga2O3 devices. In this work, Ga2O3 was deposited onto single crystal diamond substrates by atomic layer deposition (ALD), and the thermal properties of ALD-Ga2O3 thin films and Ga2O3-diamond interfaces with different interface pretreatments were measured by Time-domain Thermoreflectance. We observed a very low thermal conductivity of these Ga2O3 thin films (about 1.5 W/m K) due to the extensive phonon grain boundary scattering resulting from the nanocrystalline nature of the Ga2O3 film. However, the measured thermal boundary conductance (TBC) of the Ga2O3-diamond interfaces is about ten times larger than that of the van der Waals bonded Ga2O3-diamond interfaces, which indicates the significant impact of interface bonding on TBC. Furthermore, the TBC of the Ga-rich and O-rich Ga2O3-diamond interfaces is about 20% smaller than that of the clean interface, indicating that interface chemistry affects the interfacial thermal transport. Overall, this study shows that a high TBC can be obtained from strong interfacial bonds across Ga2O3-diamond interfaces, providing a promising route to improving the heat dissipation from Ga2O3 devices with lateral architectures.