Hysteresis loop variations induced by nitrogen flow rate in solution-based VO₂ films

Due to the special semiconductor-to-metal transition (SMT) properties, vanadium dioxide (VO₂) film has attracted tremendous interest for both the fundamental correlated electronic structure studies and the phase-change devices applications. It is known that the SMT features of VO₂ are always closely associated with the film preparation, which becomes an important issue for the research in this field. In the current study, we explored the preparation of VO₂/Al₂O₃ films by adopting the sol-gel method in conjunction with a subsequent annealing process. The effects of nitrogen flow rate on the structure, composition, morphology, and electrical properties of the VO₂ films were investigated. Results showed that V₂O₅ film was formed without nitrogen protection. As the nitrogen flow rate increased from 100 mL/min to 500 mL/min, the films exhibited a polycrystalline structure with VO₂ (020) preferred orientation. Simultaneously, the V⁴⁺ content within the film remained largely constant, while the surface grains gradually enlarged, accompanied by a more dispersed grain size distribution. Moreover, the increased nitrogen flow rate led to a consistent reduction in both the transition temperature during the heating process and the hysteresis width of the film. The asymmetry of the hysteresis loop of the thin film was also enhanced, which was closely related to the grain size distribution on the surface of the film. These findings not only elucidated the influence of nitrogen flow rate on the phase transition properties of VO₂ during annealing, but also offered a novel insight into the regulation of VO₂ phase transitions.