Microstructurally optimizing the mid-infrared optical modulation properties of vanadium oxide thin films via magnetron sputtering and subsequent annealing

Abstract

Vanadium dioxide (VO₂) is a promising material for mid-infrared optical modulation due to its reversible metal-insulator transition. This study presents an efficient and stable method for fabricating VO₂ thin films with enhanced optical limiting performance via crystallinity control and microstructural optimization. The process combines magnetron sputtering with gradient annealing, and the effects of annealing temperature on film structure and optical properties were analyzed using X-ray diffraction, X-ray spectroscopy, and SEM. Annealing at 550 °C yielded high-quality monoclinic VO₂(M₁) films with excellent crystallinity, low defect density, and island-like grains (250–300 nm). The optimized film showed reduced oxygen vacancies (17.3%) and increased V⁴⁺ content. Optical measurements revealed strong thermal switching: mid-infrared transmittance dropped from 85% at 25 °C to 35% at 80 °C, achieving a 50% modulation depth—12.5-fold higher than that of unannealed films. Under 3.8 µm laser irradiation, modulation depth tripled. The annealing process effectively improved phase purity and reduced defects by encouraging grain growth and oxygen vacancy repair. This work provides key insights into the structure–defect–property relationships in VO₂ and offers a scalable route for producing high-performance phase-change oxide thin films.