Structural and Photoelectronic Properties of κ-Ga2O3 Thin Films Grown on Polycrystalline Diamond Substrates
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2024-01-22 |
| Journal | Materials |
| Authors | M. Girolami, Matteo Bosi, Sara Pettinato, C. Ferrari, Riccardo Lolli |
| Institutions | University of Ferrara, University Niccolò Cusano |
| Citations | 7 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This analysis focuses on the first successful growth and characterization of orthorhombic kappa-Gallium Oxide (κ-Ga2O3) thin films on polycrystalline diamond (p-Diamond) substrates via Metal-Organic Vapor Phase Epitaxy (MOVPE).
- Core Achievement: Successful heteroepitaxial growth of κ-Ga2O3 on p-Diamond, creating a novel wide-bandgap hybrid architecture for advanced optoelectronics.
- Critical Process Parameter: A controlled, three-step slow cooling (SC) process (totaling 3.5 hours under He flow) was found to be mandatory. This step mitigates thermal mismatch stress, preventing film delamination and cracking observed in naturally cooled (NC) samples.
- Electrical Performance Improvement: The SC process significantly reduced electrically active defects, resulting in a dark resistivity (ρd) of 1.25 x 1010 Ω cm, which is 2x higher than the NC sample (6.01 x 109 Ω cm).
- Charge Transport Efficiency: The SC sample exhibited superior charge collection efficiency, demonstrated by a mobility-lifetime product (µτ) of 3.43 x 10-5 cm2V-1, nearly seven times higher than the NC sample.
- Structural Confirmation: X-ray Diffraction (XRD) confirmed the presence of the c-oriented orthorhombic κ-Ga2O3 phase, although the film structure was noted as nearly polycrystalline due to the substrate quality.
- Application Potential: This work establishes the foundation for developing high-performance deep-UV and ionizing radiation detectors that leverage κ-Ga2O3’s sensitivity and diamond’s exceptional thermal conductivity (2200 W m-1K-1) and tissue equivalence.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Substrate Material | Polycrystalline CVD Diamond (TM180) | N/A | Thermal management grade |
| Film Material | Orthorhombic κ-Ga2O3 | N/A | Epitaxial thin film |
| Growth Method | MOVPE | N/A | Metal-Organic Vapor Phase Epitaxy |
| Growth Temperature | 650 | °C | Constant deposition temperature |
| Chamber Pressure | 100 | mbar | During growth and cooling |
| H2O/TMG Ratio | ~200 | N/A | Precursor ratio |
| κ-Ga2O3 Bandgap (Eg) | ~4.6 | eV | Corresponds to 270 nm photoresponse peak |
| Dark Resistivity (SC Sample) | 1.25 x 1010 | Ω cm | Measured in Ohmic regime (up to 200 V) |
| Dark Resistivity (NC Sample) | 6.01 x 109 | Ω cm | Measured in Ohmic regime (up to 40 V) |
| Mobility-Lifetime Product (SC) | 3.43 x 10-5 | cm2V-1 | Charge transport efficiency at λ = 270 nm |
| Mobility-Lifetime Product (NC) | 5.20 x 10-6 | cm2V-1 | Charge transport efficiency at λ = 270 nm |
| Diamond Thermal Conductivity | ~2200 | W m-1K-1 | Substrate property |
| Diamond CTE (300 K) | 1.0 x 10-6 | K-1 | Coefficient of Thermal Expansion |
| Ga2O3 CTE (300 K) | 1.5 x 10-5 | K-1 | Coefficient of Thermal Expansion (Mismatch driver) |
| SC Sample FWHM (004 peak) | 0.16 | ° | Measure of lattice strain/mosaic spread |
| NC Sample FWHM (004 peak) | 0.34 | ° | Measure of lattice strain/mosaic spread |
Key Methodologies
Section titled “Key Methodologies”The experiment relied on a highly controlled MOVPE process, with the post-deposition cooling step being the most critical differentiator between the two samples (NC and SC).
- Substrate Cleaning: Polycrystalline CVD diamond substrates (300 µm thick) underwent rigorous cleaning:
- Boiling acid mixture (HClO4:H2SO4:HNO3) for 20 min to remove non-diamond phases.
- Boiling aqua regia (HCl:HNO3) for 5 min to remove metallic contaminants.
- Hot acetone sonication for 5 min to remove organic contaminants.
- MOVPE Deposition:
- Growth performed at 650 °C and 100 mbar pressure.
- Precursors: Trimethylgallium (TMG) and ultrapure H2O (H2O flow: 200 sccm; He carrier flow: 400 sccm).
- Deposition time: 15 min.
- Natural Cooling (NC Sample): Furnace power was simply turned off. This resulted in a rapid initial cooling rate (~15 °C/min), leading to high thermal stress, cracking, and delamination.
- Slow Cooling (SC Sample): A controlled, three-step cooling procedure was implemented under 100 mbar He flow to minimize thermal mismatch effects:
- Step 1: 650 °C to 500 °C over 2 hours.
- Step 2: 500 °C to 300 °C over 1.5 hours.
- Step 3: Natural cooling from 300 °C to room temperature.
- Contact Fabrication: Au metal contacts (300 nm thick) were deposited via RF sputtering in a two-pad geometry (1 mm gap) for electrical characterization.
- Photoelectronic Measurement: Spectral responsivity (R) was measured using a modulated light source (200-1000 nm) and a lock-in amplifier setup. Charge transport properties were analyzed using the Hecht equation to derive the mobility-lifetime product (µτ).
Commercial Applications
Section titled “Commercial Applications”The κ-Ga2O3/Diamond heterostructure is designed to overcome the thermal limitations of pure Ga2O3 devices, making it highly relevant for applications requiring simultaneous high power/sensitivity and thermal robustness.
- Deep-UV (UV-C) Photodetection:
- High-responsivity, solar-blind photodetectors (sensitive to λ < 300 nm) for space applications, missile plume detection, and flame sensing.
- Ionizing Radiation Detection and Dosimetry:
- Real-time, direct-reading X-ray dosimeters for radiotherapy, leveraging diamond’s tissue equivalence and κ-Ga2O3’s high sensitivity and stability.
- Fast-sensitive detectors for X-ray imaging in harsh environments.
- High-Power Electronics and RF Devices:
- Active materials for high-power, high-frequency electronics where efficient heat dissipation is critical. Diamond acts as a high-performance heat spreader, preventing detrimental thermal runaway in the Ga2O3 layer.
- Harsh Environment Sensing:
- Devices designed for operation at high temperatures and high voltages, where conventional silicon or GaAs devices fail.
View Original Abstract
Orthorhombic κ-Ga2O3 thin films were grown for the first time on polycrystalline diamond free-standing substrates by metal-organic vapor phase epitaxy at a temperature of 650 °C. Structural, morphological, electrical, and photoelectronic properties of the obtained heterostructures were evaluated by optical microscopy, X-ray diffraction, current-voltage measurements, and spectral photoconductivity, respectively. Results show that a very slow cooling, performed at low pressure (100 mbar) under a controlled He flow soon after the growth process, is mandatory to improve the quality of the κ-Ga2O3 epitaxial thin film, ensuring a good adhesion to the diamond substrate, an optimal morphology, and a lower density of electrically active defects. This paves the way for the future development of novel hybrid architectures for UV and ionizing radiation detection, exploiting the unique features of gallium oxide and diamond as wide-bandgap semiconductors.
Tech Support
Section titled “Tech Support”Original Source
Section titled “Original Source”References
Section titled “References”- 2023 - Wide bandgap semiconductor-based integrated circuits [Crossref]
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- 2021 - Review of polymorphous Ga2O3 materials and their solar-blind photodetector applications [Crossref]
- 2022 - Self-powered solar-blind ultrafast UV-C diamond detectors with asymmetric Schottky contacts [Crossref]
- 2020 - Fabrication of ε-Ga2O3 solar-blind photodetector with symmetric interdigital Schottky contacts responding to low intensity light signal [Crossref]
- 2013 - Resistant and sensitive single-crystal diamond dosimeters for ionizing radiation [Crossref]