【Member Papers】Effect of argon-oxygen ratios on crystallization and surface morphology of gallium oxide thin films by low-pressure chemical vapor deposition

      Researchers from the Xi’an University of Technology have published a dissertation titled "Effect of argon-oxygen ratios on crystallization and surface morphology of gallium oxide thin films by low-pressure chemical vapor deposition" in 2025 IEEE Workshop on Wide Bandgap Power Devices and Applications in Asia (WiPDA Asia) .

Project Support

      This work was supported by the National Natural Science Foundation of China (Grant No.62474139 and 61904146); Science and technology planning project of Xi'an (No.2023JH-GXRC-0122).

Background

      Boasting an ultra-wide bandgap (4.6-4.9eV) and exceptional Baliga's figure-of-merit (3444), β-Ga₂O₃ has emerged as a promising candidate for next-generation high-power electronic devices. However, this material faces a critical challenge in thermal management due to its relatively low thermal conductivity (27 W/m · K) compared to conventional semiconductors like Si (150 W/m·K), 4H-SiC (490 W/m·K), and GaN (230 W/m·K). To address this limitation, aluminum nitride (AlN) presents an ideal heteroepitaxial substrate solution, offering dual advantages: First, the (-201) β-Ga₂O₃ and (0001) AlN planes exhibit remarkably low lattice mismatch (2.4%), facilitating high-quality epitaxial growth. Second, AlN's superior thermal conductivity (340 W/m·K) effectively compensates for β-Ga₂O₃'s thermal limitations. In this study, we employed low-pressure chemical vapor deposition (LPCVD) for β-Ga₂O₃ film growth on AlN substrates, motivated by its high deposition rate and cost-effectiveness. While process parameters crucially influence film quality, the argon-to-oxygen ratio - a critical yet underexplored growth parameter - remains insufficiently studied. We systematically investigated the effects of varying argon-oxygen ratios on the structural characteristics and surface morphology of β-Ga₂O₃ films. Through comprehensive XRD, AFM, SEM and XPS characterization, our work determines optimal process conditions that balance crystalline quality with surface uniformity.

Abstract

      β-Ga₂O₃ thin films were heteroepitaxially grown on AlN(0001) substrates via low-pressure chemical vapor deposition (LPCVD). A comprehensive investigation was conducted to characterize the crystalline structure, surface morphology, and chemical composition of both the deposited films and heterostructures. X-ray diffraction (XRD) analysis revealed that the β-Ga₂O₃ films exhibited phase purity with a highly preferred (-201) growth orientation. The surface topography evolution under various growth conditions was systematically examined using scanning electron microscopy (SEM) and atomic force microscopy (AFM). X-ray photoelectron spectroscopy (XPS) measurements demonstrated an optimal Ga/O atomic ratio of 1.5, confirming the formation of stoichiometric Ga-O bonding configurations. These findings substantiate the feasibility of fabricating high-quality β-Ga₂O₃ films with superior physical properties through LPCVD heteroepitaxy on AlN substrates, thereby establishing a crucial foundation for developing next-generation  Ga₂O₃-based power electronic devices.

Conclusion

      β-Ga₂O₃ thin films were deposited on AlN substrates via low-pressure chemical vapor deposition (LPCVD) under four Ar/O₂ flow ratios: 200:5, 200:10, 200:15, and 200:20. Characterization results demonstrate that the Ar/O₂ ratio significantly influences the crystal structure, surface morphology, and chemical composition of β-Ga₂O₃ films. X-ray diffraction (XRD) analysis reveals maximum diffraction peak intensity at the 200:5 ratio with a full width at half maximum (FWHM) of 0.151°. Furthermore, the crystalline quality of β-Ga₂O₃ films deteriorates with increasing oxygen proportion. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) results indicate that the film surface becomes progressively smoother with higher oxygen ratios, accompanied by significantly reduced surface roughness. At 200:10 and 200:15 ratios, the root mean square (RMS) roughness values reach 1.79 nm and 4.21 nm, respectively, corresponding to relatively flat surface morphologies. However, excessive oxygen content (200:20) leads to decreased film densification. X-ray photoelectron spectroscopy (XPS) analysis confirms the coexistence of two Ga oxidation states and non-lattice oxygen in the films, evidencing oxygen vacancy formation. The proportion of Ga³⁺ increases steadily with oxygen enrichment, while non-lattice oxygen content decreases correspondingly. These observations suggest gradual quality improvement with oxygen ratio elevation. Notably, the calculated stoichiometric ratio reaches 1.50 at the 200:15 condition, approaching the optimal value. This work provides fundamental guidelines for fabricating high-quality β-Ga₂O₃ films on AlN substrates, laying a foundation for future device applications.