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Mechanical properties of β-Ga2O3 thin films deposited by plasma-assisted molecular beam epitaxy

This study investigates the effect of substrate temperature (500–700°C) on the microstructural, morphological, and nanomechanical properties of β-Ga₂O₃ thin films deposited by plasma-assisted molecular beam epitaxy, revealing that increasing the temperature enhances crystallite size, hardness, Young's modulus, and fracture toughness.

Original authors: Sheng-Rui Jian, Bo-Han Su, Cheng-Wei Liu, Umeshwar Reddy Nallasani, Phuoc Huu Le, Che-Nan Kuo, Yu-Min Hu, Chin-Hau Chia, Wu-Ching Chou, Jenh-Yih Juang

Published 2026-08-05

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CC BY 4.0

Original authors: Sheng-Rui Jian, Bo-Han Su, Cheng-Wei Liu, Umeshwar Reddy Nallasani, Phuoc Huu Le, Che-Nan Kuo, Yu-Min Hu, Chin-Hau Chia, Wu-Ching Chou, Jenh-Yih Juang

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are building a skyscraper out of invisible, super-strong glass. You want this glass to be so tough that it can handle massive electrical storms without shattering, making it perfect for the next generation of lightning-fast computers and power grids. This is the dream behind a material called beta-gallium oxide (β-Ga2O3). It's a "wide bandgap" semiconductor, which is a fancy way of saying it can handle way more voltage and heat than the silicon chips in your phone today. But before engineers can build these skyscrapers, they need to know if the glass is actually strong enough to stand up to the pressure. Just like a real building needs to be tested for how hard you can push on its walls before it cracks, scientists need to measure the "nanomechanical" properties of these thin films. They need to know how hard the material is (hardness), how much it bends before snapping back (elasticity), and how well it resists breaking apart when hit (fracture toughness).

In this study, a team of researchers acted like master architects testing different batches of this super-glass. They grew thin films of β-Ga2O3 on sapphire substrates (think of the sapphire as the foundation) using a high-tech oven called a plasma-assisted molecular beam epitaxy system. The big question they asked was: Does heating the foundation to different temperatures change how strong the glass becomes? They grew films at three different temperatures—500°C, 600°C, and 700°C—and then poked them with tiny, diamond-tipped needles to see what happened.

Here is what they found: The hotter the oven, the stronger the glass. When they grew the films at the highest temperature of 700°C, the tiny crystals inside the material grew larger and more organized, much like how heating sugar makes it form bigger, clearer crystals. Because of this, the material became harder and stiffer. Specifically, the hardness increased from 11.8 GPa at 500°C to 13.4 GPa at 700°C, and its stiffness (Young's modulus) jumped from 262.5 GPa to 305.6 GPa.

The researchers also looked at how the material reacted when they really pressed down on it. They used a technique called nanoindentation, which is like pressing a tiny thumbtack into the surface to see how deep it goes. They found that the films didn't suddenly "pop" or crack immediately under pressure; instead, they deformed smoothly. This suggests that the material handles stress by shifting its tiny grain boundaries rather than snapping apart.

To test how well the films resist breaking, the team also used a heavier "micro-Vickers" indenter to create tiny cracks on the surface. They measured the length of these cracks to calculate something called "fracture toughness," which is a measure of how much energy the material can absorb before it breaks. The results showed that the films grown at 700°C were the toughest, with a fracture toughness of about 3.02 MPa·m¹/², compared to 2.92 MPa·m¹/² for the cooler films. Interestingly, these values were better than some other similar materials made by different methods, suggesting that the specific way these films were grown made them particularly resilient.

In short, the study suggests that if you want the strongest, most crack-resistant β-Ga2O3 thin films for future high-power devices, you should turn up the heat to 700°C during the growth process. The material doesn't just get bigger; it gets significantly tougher, ready to withstand the rigors of the next generation of electronics.

Technical Summary: Mechanical Properties of $\beta$ -Ga $_2$ O $_3$ Thin Films Deposited by Plasma-Assisted Molecular Beam Epitaxy

Problem Statement
While $\beta$ -Ga $_2$ O $_3$ is recognized as a promising wide bandgap (~4.9 eV) semiconductor for next-generation high-power and high-voltage devices due to its high breakdown field and optical transmittance, its practical integration into functional devices requires a comprehensive understanding of its mechanical characteristics. Specifically, there is a need to characterize local contact-induced damage, cracking behavior, and nanomechanical properties (hardness and elastic modulus) to ensure reliability in applications such as solar-blind ultraviolet detectors and field-effect transistors. Furthermore, the correlation between microstructural evolution (driven by growth conditions) and mechanical performance in thin films deposited via plasma-assisted molecular beam epitaxy (PA-MBE) remains a subject of investigation.

Methodology
The study investigated $\beta$ -Ga $_2$ O $_3$ thin films (~120 nm thick) deposited on c-plane sapphire substrates using a PA-MBE system under ultra-high vacuum. Films were grown at three distinct substrate temperatures ( $T_{sub}$ ): 500°C, 600°C, and 700°C. The characterization involved a multi-faceted approach:

Structural Analysis: Grazing incidence X-ray diffraction (GIXRD) was employed to determine crystal structure and calculate crystallite sizes using both Scherrer and Williamson-Hall (WH) analyses.
Surface Morphology: Atomic force microscopy (AFM) was used to examine surface roughness and grain morphology.
Nanomechanical Testing: Nanoindentation using a Berkovich tip was performed to determine hardness ( $H$ ) and Young's modulus ( $E$ ). The Oliver-Pharr method was applied to analyze load-displacement ( $P-h$ ) curves, with penetration depths restricted to <30% of film thickness to minimize substrate effects.
Fracture Analysis: Micro-Vickers indentation (1.96 N load) was conducted to observe cracking patterns (radial, lateral, delamination, and buckling-induced) via scanning electron microscopy (SEM). Fracture toughness ( $K_C$ ) was calculated based on these crack patterns.

Key Results

Microstructure and Morphology: XRD confirmed that all films possessed a monoclinic $\beta$ -phase structure with a dominant (2 $\bar{1}$ 01) orientation. Both Scherrer ( $D_S$ ) and Williamson-Hall ( $D_{WH}$ ) analyses indicated that crystallite sizes increased as $T_{sub}$ rose from 500°C to 700°C. However, microstrain ( $\varepsilon$ ) also increased monotonically from 0.136% to 0.275% with higher temperatures, a trend attributed to complex epitaxial growth mechanisms and domain-matching epitaxy (DME) effects despite the large lattice mismatch with the sapphire substrate. AFM revealed that surface roughness (RMS) and step-bunching phenomena increased with higher $T_{sub}$ .
Nanomechanical Properties: Nanoindentation results showed a direct correlation between crystallite size and mechanical strength. As $T_{sub}$ increased, hardness rose from 11.8 GPa to 13.4 GPa, and Young's modulus increased from 262.5 GPa to 305.6 GPa. The $P-h$ curves were smooth without "pop-in" or "pop-out" events, suggesting deformation remained within the elastic regime or was governed by grain boundary sliding rather than dislocation nucleation or phase transitions. The authors suggest these trends align with an inverse Hall-Petch effect, where grain boundary structures play a primary role in strain compensation.
Fracture Behavior and Toughness: Micro-Vickers indentation induced complex damage morphologies, including radial cracking, lateral cracking, delamination, and buckling-induced cracking. The fracture toughness ( $K_C$ ) values were calculated to be 2.92, 2.97, and 3.02 MPa·m $^{1/2}$ for films grown at 500°C, 600°C, and 700°C, respectively. This indicates that films with larger crystalline sizes exhibit better resistance to delamination and buckling-induced cracking.

Significance and Claims
The paper establishes that increasing the substrate temperature during PA-MBE growth of $\beta$ -Ga $_2$ O $_3$ thin films enhances both the crystalline quality (larger grain size) and the mechanical robustness (higher hardness, modulus, and fracture toughness). The study highlights that the mechanical response of these films is significantly influenced by grain boundary structures, potentially following an inverse Hall-Petch relationship.

The authors note that the fracture toughness values obtained (approx. 3.0 MPa·m $^{1/2}$ ) are comparable to other technologically important oxide materials like ZnO and are substantially higher than values reported for $\beta$ -Ga $_2$ O $_3$ films prepared by plasma-enhanced chemical vapor deposition. The authors attribute these differences to variations in microstructures and grain morphologies resulting from different deposition methods and conditions. This work provides essential baseline mechanical data for the reliable incorporation of PA-MBE grown $\beta$ -Ga $_2$ O $_3$ thin films into functional high-power electronic devices.

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