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 | Institute of Structure of Matter, Institute of Materials for Electronics and Magnetism |
| Citations | 7 |
| Analysis | Full AI Review Included |
Technical Documentation: Hybrid $\kappa$-Ga${2}$O${3}$/Polycrystalline Diamond Heterostructures
Section titled “Technical Documentation: Hybrid $\kappa$-Ga${2}$O${3}$/Polycrystalline Diamond Heterostructures”Executive Summary
Section titled “Executive Summary”This documentation analyzes the successful heteroepitaxial growth of orthorhombic $\kappa$-Ga${2}$O${3}$ thin films on polycrystalline diamond (PCD) substrates via Metal-Organic Vapor Phase Epitaxy (MOVPE), targeting advanced UV and ionizing radiation detectors.
- Novel Architecture: First reported growth of the highly sensitive $\kappa$-Ga${2}$O${3}$ polymorph on PCD, creating a hybrid wide-bandgap semiconductor platform.
- Critical Process Optimization: A mandatory, controlled slow-cooling (SC) step (650 °C down to 300 °C over 3.5 hours) was identified as essential to mitigate thermal mismatch stress between the Ga${2}$O${3}$ film and the diamond substrate.
- Performance Improvement: The slow-cooled sample (SC) exhibited superior electrical properties, including a dark resistivity ($\rho_{d}$) of $1.25 \times 10^{10}$ $\Omega$ cm and a significantly higher mobility-lifetime product ($\mu\tau$) of $3.43 \times 10^{-5}$ cm2 V-1.
- Defect Reduction: Slow cooling minimized interface defects and reduced the density of electrically active shallow traps, leading to lower dark current and improved charge collection efficiency.
- Structural Quality: XRD confirmed the c-oriented orthorhombic $\kappa$-Ga${2}$O${3}$ phase, with the SC sample showing improved crystalline quality (FWHM of 0.16° for the 004 peak) compared to the naturally cooled sample (0.34°).
- Target Applications: The resulting heterostructures are promising prototypes for deep-UV photodetectors operating in harsh environments and real-time, direct-reading X-ray dosimeters, leveraging diamond’s high thermal conductivity and tissue equivalence.
Technical Specifications
Section titled “Technical Specifications”The following hard data points were extracted from the optimized (Slow-Cooled) growth process and resulting material characterization:
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Substrate Material | Polycrystalline Diamond (PCD) | N/A | Thermal Management Grade |
| Substrate Dimensions | $10 \times 10$ | mm2 | Lateral size used in experiment |
| Substrate Thickness | 300 | µm | Standard thickness |
| Growth Method | MOVPE | N/A | Metal-Organic Vapor Phase Epitaxy |
| Growth Temperature | 650 | °C | Optimal deposition temperature |
| Chamber Pressure (Growth/Cooling) | 100 | mbar | Constant pressure maintained |
| $\kappa$-Ga${2}$O${3}$ Film Thickness (Avg) | 580 | nm | Average thickness of the film |
| Dark Resistivity ($\rho_{d}$) | $1.25 \times 10^{10}$ | $\Omega$ cm | Measured on the Slow-Cooled (SC) sample |
| Mobility-Lifetime Product ($\mu\tau$) | $3.43 \times 10^{-5}$ | cm2 V-1 | SC sample, measured at $\lambda = 270$ nm |
| XRD FWHM (004 peak) | 0.16 | ° | Full Width Half Maximum, indicating structural quality (SC sample) |
| Ohmic Contact Range | Up to 200 | V | Confirmed linearity for Au contacts (SC sample) |
Key Methodologies
Section titled “Key Methodologies”The successful fabrication of high-quality $\kappa$-Ga${2}$O${3}$/PCD heterostructures relied on precise substrate preparation and a highly controlled post-deposition cooling sequence.
- Diamond Substrate Cleaning (Pre-Deposition):
- Acid cleaning in $\text{HClO}{4}:\text{H}{2}\text{SO}{4}:\text{HNO}{3}$ (1:1:1) at boiling point for 20 min to remove non-diamond phases.
- Acid cleaning in aqua regia ($\text{HCl}:\text{HNO}_{3}$, 3:1) at boiling point for 5 min to remove metallic contaminants.
- Ultrasound sonication in hot acetone for 5 min to remove organic contaminants.
- Rinsing in deionized water and drying in pure $\text{N}_{2}$ flow.
- MOVPE Growth Parameters:
- Reactor Geometry: Horizontal, without substrate rotation.
- Temperature: 650 °C.
- Pressure: 100 mbar.
- Precursors: Trimethylgallium (TMG) and ultrapure $\text{H}_{2}\text{O}$.
- Ratio: $\text{H}_{2}\text{O}/\text{TMG}$ ratio of approximately 200.
- Gas Flow: $\text{H}_{2}\text{O}$ flow-rate of 200 sccm; Helium (He) carrier gas total flow-rate of 400 sccm.
- Deposition Time: 15 min.
- Optimized Slow Cooling (SC) Procedure (Critical Step):
- Cooling performed under constant He flow and 100 mbar pressure.
- Step 1: Cooling from 650 °C to 500 °C over 2 hours (Average rate: 1.25 °C/min).
- Step 2: Cooling from 500 °C to 300 °C over 1.5 hours (Average rate: 2.22 °C/min).
- Step 3: “Natural” cooling from 300 °C down to room temperature by turning off the furnace.
- Metal Contact Fabrication:
- RF sputtering deposition of 300 nm thick Au contacts (99.999% purity).
- Contact geometry: Two rectangular $4.2 \times 1.6$ mm2 pads separated by a 1 mm wide gap.
6CCVD Solutions & Capabilities
Section titled “6CCVD Solutions & Capabilities”The research highlights the potential of hybrid diamond architectures for high-performance radiation detection, but also underscores the extreme material challenges posed by thermal mismatch. 6CCVD is uniquely positioned to supply the necessary high-quality diamond substrates and advanced processing required to replicate and scale this research.
Applicable Materials
Section titled “Applicable Materials”To replicate or extend this research, the following 6CCVD materials are recommended:
- Polycrystalline Diamond (PCD) Substrates: The paper utilized a thermal management grade PCD. 6CCVD offers high-purity, low-defect PCD wafers.
- Recommendation: Optical Grade PCD (Thickness: 300 µm to 10 mm) is recommended for improved surface quality (Ra < 5 nm for inch-size wafers), which can enhance the initial epitaxial quality of the $\kappa$-Ga${2}$O${3}$ layer and minimize interface defects.
- Single Crystal Diamond (SCD) Substrates: For applications requiring the absolute highest structural quality and thermal performance, SCD substrates (up to 500 µm thick) can be supplied, though lattice mismatch challenges would be more pronounced.
Customization Potential
Section titled “Customization Potential”The success of this hybrid device relies on precise material dimensions and contact definition. 6CCVD’s in-house capabilities directly address the requirements of this study:
| Research Requirement | 6CCVD Capability | Value Proposition |
|---|---|---|
| Substrate Size | Custom plates/wafers up to 125 mm (PCD) | Scale-up from $10 \times 10$ mm2 prototypes to production-ready, large-area detectors. |
| Thickness Control | SCD/PCD thickness from 0.1 µm to 500 µm | Ability to supply substrates up to 10 mm thick, or thin films (e.g., 300 µm) with high precision. |
| Metalization | Internal capability for Au, Pt, Pd, Ti, W, Cu | Direct fabrication of the required Au interdigital Schottky contacts via sputtering or e-beam deposition, streamlining the device manufacturing process. |
| Surface Finish | Polishing to Ra < 5 nm (PCD) | Providing ultra-smooth surfaces critical for high-quality heteroepitaxial growth (MOVPE) and minimizing interface defect density. |
Engineering Support
Section titled “Engineering Support”The core challenge identified in this paper—managing the massive thermal mismatch between Ga${2}$O${3}$ and diamond (thermal expansion coefficients differ by $15\times$)—is a common issue in hybrid Wide Bandgap Semiconductor (WBS) architectures.
6CCVD’s in-house PhD team specializes in MPCVD growth and WBS integration. We offer consultation services to assist researchers and engineers in:
- Material Selection: Choosing the optimal diamond grade (PCD vs. SCD) and surface orientation to minimize lattice strain and thermal stress effects.
- Interface Engineering: Developing custom diamond surface treatments (e.g., specific termination or polishing) to improve adhesion and reduce the formation of voids and defects at the $\kappa$-Ga${2}$O${3}$/Diamond interface.
- Custom Dimensions: Providing laser cutting and shaping services to meet unique detector geometries required for specific UV or X-ray dosimetry applications.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
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]
- 2005 - Wide bandgap semiconductor detectors for harsh radiation environments [Crossref]
- 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]