Microstructurally Optimizing the Mid-Infrared Optical Modulation Properties of Vanadium Oxide Thin Films via Magentron Sputtering and Subsequent Annealing

Vanadium dioxide (VO₂) has emerged as a promising candidate for mid-infrared optical modulation owing to its reversible metal-insulator transition property. This study proposed and evaluated an efficient and stable process for fabricating VO₂ thin films with enhanced optical limiting performance through crystallinity control and microstructural optimization. This process couples the magnetron sputtering technique with gradient annealing treatment. The influences of annealing temperature on the microstructural evolution and optical properties of vanadium oxide films were systematically investigated through X-ray diffraction, X-ray spectroscopy, and scanning electron microscope observations. The results indicated that annealing the sputtered film at 550 °C successfully produced a high-quality monoclinic VO₂(M1) film with a low defect density and excellent crystallinity. The optimized film exhibited uniformly distributed island-like grains 250–300 nm in diameter, a significantly reduced oxygen vacancy concentration (17.3%), and a substantially increased V⁴⁺ content. Optical characterizations demonstrated remarkable thermal switching performance: the mid-infrared transmittance decreased sharply from 85% at 25 °C to 35% at 80 °C, achieving a 50% thermally induced modulation depth corresponding to a 12.5-fold enhancement over that of a non-annealed film. Furthermore, the optimal film exhibited a three-fold increase in modulation depth under 3.8 μm laser irradiation over that of its non-annealed equivalent. The annealing process effectively addressed the challenges of phase purity control and defect state regulation by promoting preferential grain growth and oxygen vacancy repair. This study accordingly provides critical insights into the structure–defect–property relationships of phase-change optical materials and establishes a practical pathway for the scalable production of functional oxide thin films.