Electrical and thermal characterisation of liquid metal thin-film Ga$$_2$$O$$_3$$–SiO$$_2$$ heterostructures
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2023-03-01 |
| Journal | Scientific Reports |
| Authors | Alexander Petkov, Abhishek Mishra, Mattia Cattelan, D. Field, James W. Pomeroy |
| Institutions | University of Bristol |
| Citations | 7 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This study investigates the electrical and thermal properties of thin-film Ga2O3 deposited on Si/SiO2 heterostructures, addressing key challenges (low thermal conductivity and lack of p-type doping) for Gallium Oxide (Ga2O3) power electronics.
- Enhanced Thermal Conductivity: The out-of-plane thermal conductivity (k) of the thin-film Ga2O3 (30 nm thickness) was measured at 3 ± 0.5 W/mK, which is twice as high as previously reported values for comparable polycrystalline Ga2O3 films (e.g., ALD films on diamond, 1.5 W/mK).
- Negligible Thermal Resistance: Thermal simulations confirm that this thin Ga2O3 layer provides negligible thermal resistance for typical thin-channel transistor heat sources, making it a viable thermal management approach.
- Band Alignment Measured: The Valence Band Offset (VBO) for the Ga2O3/SiO2 interface was measured via XPS at 0.1 eV.
- Band Alignment Predicted: The VBO for Ga2O3/Diamond was predicted to be -2.3 eV, suggesting a significant energetic barrier (blocking interface) for minority carriers in potential p-n Ga2O3-diamond superjunctions.
- Deposition Method: The films (8-30 nm thick) were fabricated using a novel, low-cost method based on the oxidized liquid gallium layer delamination (exfoliation) technique.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Ga2O3 Film Thickness Range | 8 to 30 | nm | Deposited via liquid metal exfoliation |
| Out-of-Plane Thermal Conductivity (k) | 3 ± 0.5 | W/mK | For 30 nm Ga2O3 film on SiO2/Si |
| Predicted k Range (Non-uniform film) | 1.7 to 4.8 | W/mK | For thicknesses between 20 and 40 nm |
| Ga2O3 (beta phase) Band Gap (Eg) | 4.8 | eV | Reference value used for calculations |
| Ga2O3/SiO2 Valence Band Offset (VBO) | 0.1 | eV | Measured via XPS |
| Ga2O3/SiO2 Conduction Band Offset (CBO) | -4.0 | eV | Calculated; implies Type II alignment to Si |
| Ga2O3/Diamond Predicted VBO | -2.3 | eV | Calculated |
| Ga2O3/Diamond Predicted CBO | -2.85 | eV | Calculated |
| Ga2O3 Breakdown Electric Field (Predicted) | ~8 | MV/cm | For beta polymorph |
| SiC Thermal Conductivity (Reference) | 420 | W/mK | Bulk reference value |
| GaN Thermal Conductivity (Reference) | 160 | W/mK | Bulk reference value |
| Si Thermal Conductivity (Doped) | 80 | W/mK | Used in TTR fitting |
| SiO2 Thermal Conductivity (Thin Film) | 1.2 | W/mK | Used in TTR fitting |
| TTR Pump Laser Wavelength | 355 | nm | Frequency tripled Nd:YAG |
| TTR Pump Laser Pulse Rate | 30 | kHz | Repetition rate |
Key Methodologies
Section titled “Key Methodologies”The Ga2O3 thin films were fabricated using liquid metal exfoliation, and characterized using advanced spectroscopic and thermal techniques:
- Liquid Gallium Preparation: A gallium pellet was heated to 50 °C (above its 29 °C melting point). The surface spontaneously oxidized, forming a passivating Ga2O3 oxide skin (a few nanometers thick).
- Exfoliation and Transfer: A liquid gallium droplet, including the oxide skin, was picked up with a pipette tip and placed on a glass slide. The oxide skin was then transferred onto a B-doped Si substrate with a thermal oxide (SiO2) layer.
- Post-Deposition Processing: Excess gallium was removed by rinsing the sample in heated ethanol. The sample was subsequently annealed in oxygen at 250 °C for 1 hour to stabilize the Ga2O3 stoichiometry.
- Thickness Verification: Atomic Force Microscopy (AFM) in tapping mode was used to confirm film thicknesses between 8 nm and 30 nm.
- Electrical Characterization (XPS): High resolution X-ray Photoelectron Spectroscopy (XPS) was used to measure the core levels (Ga 3d, Si 2p) and Valence Band Maximum (VBM) to determine the band alignment (VBO) across the Ga2O3-SiO2 interface.
- Thermal Characterization (TTR): Transient Thermoreflectance (TTR) was employed to measure the out-of-plane thermal conductivity. A 10 nm Cr adhesion layer and a 100 nm Au transducer layer were evaporated onto the Ga2O3 surface prior to measurement.
- Data Fitting: An analytical model was used to fit the TTR transients, allowing for the extraction of the thermal conductivities of the individual layers, including the thin-film Ga2O3.
Commercial Applications
Section titled “Commercial Applications”The findings support the development of next-generation power electronic devices leveraging the ultra-wide band gap properties of Ga2O3, particularly where thermal management is critical.
- Ultra-High Voltage Power Devices: Ga2O3 is a leading candidate for devices exceeding 10 kV due to its predicted 8 MV/cm breakdown field, suitable for high-power rectifiers and MOSFETs.
- Efficient Power Conversion Systems: Applicable in high-voltage applications such as power conversion modules, electric vehicles (EVs), and data centers, where minimizing thermal dissipation is essential for reliability.
- Heterostructure Design: The measured band offsets are crucial for engineering efficient heterojunctions, particularly for integrating n-type Ga2O3 with p-type materials (like p-doped NiO or p-type Diamond) to form p-n junctions or superjunctions.
- Thermal Management Substrates: The high thermal conductivity achieved in the thin Ga2O3 film validates the strategy of integrating Ga2O3 with high-k substrates (SiC or Diamond) to prevent device failure due to poor heat dissipation.
- 2D Material Applications: The liquid metal exfoliation method provides a pathway for low-cost, room-temperature synthesis of thin-film oxides, relevant for 2D material-based applications like gas sensing and wearable electronics.