Tailoring thermochromic and optical properties of VO2 thin films by pulsed laser deposition using different starting routes

Abstract

The insulator to metal transition (IMT) of vanadium dioxide (VO2) at about 68 °C enables a variety of optical applications, including switching and modulation, and tuning of optical resonators. However, due to the multivalent nature of vanadium, controlling the synthesis of stoichiometric VO2 is still a challenge. Moreoever, the temperature dependent optical properties are strongly influenced by the microstructure of the VO2 thin film. In this paper, we report for the first time the effect on the microstructure and optical properties of two different routes implemented using the same PLD process to obtain VO2 thin films exhibiting an IMT; one route involving ablation of a metallic vanadium (V) target in an oxygen atmosphere, the other ablation of a vanadium oxide V2O5 target in a vacuum, both followed by a similar rapid thermal annealing process. Each route leads to significantly different VO2 thin films, exhibiting an IMT transition temperature of 69 °C with the V2O5 route, and of 50 °C with the V + O2 route. In addition, VO2 films obtained from V + O2 have a narrower hysteresis width, a smoother surface with very low roughness, a slightly higher refractive index, a higher absorption coefficient, and a narrower optical band gap than VO2 film obtained from V2O5. The thermochromic and optical properties of both VO2 are discussed in relation with the nanostructural properties. Our findings highlight the value and versatility of the PLD process in adapting the optical properties of VO2 films to the application concerned.