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Description
Gallium oxide (Ga₂O₃) has emerged as a promising ultra-wide bandgap semiconductor with potential applications in power electronics, solar-blind deep-ultraviolet optoelectronics, gas sensors, and X-ray detectors. Depending on the polymorph (α, β, γ, and κ), it features a bandgap of 4.4 - 5.6 eV, a high breakdown electric field of ~8 MV/cm, and a figure of merit that overcomes that of SiC and GaN. Among these polymorphs, the monoclinic β-Ga₂O₃ phase is thermodynamically the most stable and extensively investigated, and the methods for large-size single-crystal wafer fabrication are well-established. Nevertheless, high bulk production costs motivate intensive development of heteroepitaxial and non-epitaxial thin films.
Understanding optical transition characteristics, surface electronic structure, and local bond configurations of Ga₂O₃ thin films is crucial for controlling their electronic properties and evaluating overall film quality. The local bond configurations can be investigated by theoretical modeling (e.g., DFT calculations) and experimental techniques such as XPS, FTIR, Raman spectroscopy, and TEM analysis. In this work, Raman and FTIR spectroscopy were employed to investigate the evolution of these local configurations and their effect on optical transmittance, refractive index dispersion, and optical bandgap of Ga₂O₃ thin films. For this purpose, the films with different Si-doping levels (0.01 and 0.1 mol.%) were grown by MOCVD to a thickness of 135-250 nm on sapphire, Si, and SiO₂ substrates using Ga(acac)3 as the gallium source and TEOS as the silicon dopant precursor, followed by thermal annealing at 550°C in a forming gas atmosphere. XRD and XPS analyses confirmed successful growth of the β-Ga₂O₃ phase with a near-stoichiometric Ga/O atomic ratio of 0.69. Raman and FTIR spectra clearly demonstrated coexistence of octahedral (GaO₆) and tetrahedral (GaO₄) Ga coordination environments. While these local configurations remained unaffected by Si doping, thermal annealing induced noticeable structural rearrangement, marked by an increase in tetrahedral GaO₄ bonds. Conversely, optical properties, including high transparency exceeding 95% and an optical bandgap of 4.9-5.3 eV, were primarily defined by the Si-doping level, showing no sensitivity to annealing. Multi-angle spectroscopic ellipsometry (250–2100 nm) was utilized to extract complex refractive index dispersion and film thickness, implementing a two-layer model accounting for a 14–22 nm surface roughness layer. The refractive index at 633 nm was found to range within 1.91–1.99, showing negligible sensitivity to thermal annealing. The most pronounced variation of the refractive index arose from the choice of substrate, where low doped films on SiO₂ demonstrated a sufficient decrease to ~1.91.
Overall, the findings of the work demonstrate the ability to independently control local bond configurations and optical properties of Ga₂O₃ thin films through annealing and doping. In particular, annealing in forming gas goes beyond simple hydrogen passivation of oxygen vacancies by actively promoting formation of new Ga-O bonds.
Acknowledgment. This work was supported by the Slovak Research and Development Agency (project No. SK-UA-25-0079) and by the Ministry of Education and Science of Ukraine (project No. M/61-2026).