Facile Fabrication of High-Performance Thermochromic VO₂-Based Films on Si for Application in Phase-Change Devices

This work reports on an alternative and advantageous procedure to attain VO₂-based thermochromic coatings on silicon substrates. It involves the sputtering of vanadium thin films at glancing angles and their subsequent fast annealing in an air atmosphere. By adjusting thickness and porosity of films as well as the thermal treatment parameters, high VO₂(M) yields were achieved for 100, 200, and 300 nm thick layers treated at 475 and 550 °C for reaction times below 120 s. Comprehensive structural and compositional characterization by Raman spectroscopy, X-ray diffraction, and scanning-transmission electron microscopies combined with analytical techniques such as electron energy-loss spectroscopy bring to the fore the successful synthesis of VO₂(M) + V₂O₃/V₆O₁₃/V₂O₅ mixtures. Likewise, a 200 nm thick coating consisting exclusively of VO₂(M) is also achieved. Conversely, the functional characterization of these samples is addressed by variable temperature spectral reflectance and resistivity measurements. The best results are obtained for the VO₂/Si sample with changes in reflectance of 30-65% in the near-infrared at temperatures between 25 and 110 °C. Similarly, it is also proven that the achieved mixtures of vanadium oxides can be advantageous for certain optical applications in specific infrared windows. Finally, the features of the different structural, optical, and electrical hysteresis loops associated with the metal-insulator transition of the VO₂/Si sample are disclosed and compared. These remarkable thermochromic performances hereby accomplished highlight the suitability of these VO₂-based coatings for applications in a wide range of optical, optoelectronic, and/or electronic smart devices.