Ga₂O₃ exhibits remarkable properties, including broad bandgap, high breakdown field strength, high Baliga quality factor, robust thermal stability, and favorable optical absorption characteristics. These distinctive attributes render it highly suitable for power devices, solar-blind ultraviolet (UV) detection, and UV light-emitting applications. To date, reports concerning Ga₂O₃ thin film synthesis via solution-based methods are comparatively scarce. Pulsed laser deposition and radio frequency sputtering entail costly equipment, while chemical vapor deposition exhibits a relatively low deposition rate. Conversely, solution-based approaches offer benefits encompassing minimal environmental impact and cost, procedural simplicity, and the capacity to yield large and uniformly coated surfaces. However, challenges endure the Ga₂O₃ film synthesized via the solution method, manifesting as reduced crystallinity and inadequate absorption of solar-blind UV light. The content of impurities in precursor solutions, Ga³⁺ concentration, film drying and annealing temperatures, as well as the ambient atmosphere, collectively influence the impurity content, crystallinity, and surface morphology of Ga₂O₃ films, consequently shaping their optical and electrical traits. To mitigate impurities in the solution, we employ 1,2-diaminopropane containing two amino groups as a stabilizer and devise precursor solutions featuring elevated Ga³⁺ concentrations to curtail the usage of reagents beyond gallium sources. Elevated drying and annealing temperatures are harnessed to amplify film crystallinity. Furthermore, we delve into the crystalline attributes, surface morphology, optical characteristics, bandgap properties, and electrical performance disparities among Ga₂O₃ films subjected to annealing within nitrogen, air, and oxygen environments, simultaneously probing the defect energy levels within these films.

In this investigation, 2-methoxyethanol, gallium nitrate hydrate, and 1,2-diaminopropane function as a protective solvent, a metal precursor, and a stabilizer respectively to maintain sol stability. This results in a precursor solution with a Ga³⁺ molar concentration of 2.0 mol/L. The precursor solution is applied via spin-coating onto quartz glass substrates, followed by being dried at 200 ℃. Subsequently, annealing at 1100 ℃ for 1 h takes place under nitrogen, air, and oxygen atmospheres, utilizing a tube furnace. X-ray diffraction analysis is performed to assess the crystal structure of the film across different annealing atmospheres. Scanning electron microscopy is employed to characterize film morphology. UV-Vis spectrophotometry is utilized to gather transmittance and reflectance spectra. Photoluminescence spectroscopy is applied to scrutinize the luminescent properties of the films under varying annealing conditions. Small-area ion sputtering is utilized to establish Au electrode contacts on the surface of the Ga₂O₃ film, and the I-V characteristics of the Ga₂O₃ film are subjected to testing.

After being annealed in nitrogen, air, and oxygen atmospheres, the Ga₂O₃ films display pronounced crystallinity, featuring average crystal sizes of 23.5 nm, 24.9 nm, and 28.1 nm, respectively (Fig. 1). Nitrogen-annealed Ga₂O₃ films show the highest density, the most uniform morphology, and the greatest thickness of 222 nm (Fig. 2). The films exhibit transmittance exceeding 80% within the visible light spectrum while allowing less than 10% transmission of 190 nm UV light, indicating favorable selective absorption of solar blind UV light [Fig. 3(b)]. In addition, the bandgap widths of Ga₂O₃ films annealed in nitrogen, air, and oxygen atmospheres are 5.10 eV, 5.07 eV, and 5.18 eV, respectively [Fig. 3(d)]. Photoluminescence spectra disclose that nitrogen-annealed Ga₂O₃ films emit the most intense luminescence, suggesting an abundance of radiative recombination defects. The defects potentially stem from heightened gallium or oxygen vacancies due to reduced crystallinity. In contrast, oxygen-annealed Ga₂O₃ films exhibit feeble luminescence, signifying diminished radiative recombination defects and effective suppression of UV-blue light emission (Fig. 4). Defect energy levels in Ga₂O₃ films are scrutinized via photoluminescence spectra (Fig. 5). Comparable analyses for other samples unveil a correlation between positions of donor energy levels and bandgap width (Table 1). It can be obtained from the I-V characteristic curve that the resistance of the Ga₂O₃ films annealed in nitrogen, air, and oxygen atmospheres is 5.32×10⁹ Ω, 9.19×10⁹ Ω, and 5.83×10¹⁰ Ω, respectively. By comparing the resistances of diverse samples, a link between resistance alterations and photoluminescence intensities is revealed (Fig. 6). When the Ga₂O₃ film demonstrates favorable density, its resistance primarily hinges on internal radiative recombination defects. Consequently, nitrogen-annealed Ga₂O₃ films exhibit augmented conductivity.

We employ a solution-based approach to deposit Ga₂O₃ films on quartz glass substrates and conduct high-temperature annealing at 1100 °C within nitrogen, air, and oxygen atmospheres. The outcomes from XRD, SEM, UV-Vis, PL, and I-V assessments are as follows. 1) Films annealed in nitrogen, air, and oxygen atmospheres exhibit elevated crystallinity, featuring average crystal sizes of 23.5 nm, 24.9 nm, and 28.1 nm, respectively. They demonstrate preferable absorption of selective solar-blind UV light with bandgap widths of 5.10 eV, 5.07 eV, and 5.18 eV, respectively. Moreover, these films annealed in nitrogen, air, and oxygen atmospheres manifest elevated resistances attributed to the participation of deep energy levels D₂, D₃, and D₄ as donors, yielding resistances of 5.32×10⁹ Ω, 9.19×10⁹ Ω, and 5.83×10¹⁰ Ω, respectively. 2) Films annealed in nitrogen show superior density, uniformity, and thickness (222 nm), concurrently presenting heightened radiative recombination defects, which in turn result in diminished resistance. 3) Under the annealing condition of 1100 ℃ for 1 h, nitrogen-annealed Ga₂O₃ films evince superior morphology, optical properties, and electrical performance, unveiling their potential for electronic and optoelectronic applications.