Effect of thickness on infrared optical property of VO₂ film deposited by magnetron sputtering

A series of VO₂ films with different thicknesses (from about 25 nm to 250 nm) were prepared on sapphire substrates by radio frequency magnetron sputtering. The deposition times varied from 15 min to 150 min. The metal to insulator transitions (MIT) of the films were studied. The optical transmittance of the films to infrared light (with a wavelength of 4.0 µm) at room temperature (30°C) varied significantly with film thickness, ranging from 86.53% to 41.01%. The modulation property also changes with thickness, decreasing from 22.89% to 14.74%. The phase transition temperature remained approximately 70°C during heating, and approximately 53°C during cooling, despite a tenfold increase in the deposition time, and the resulting thickness of the films. Raman spectroscopy of the films indicated that the intensities of the characteristic peaks corresponding to V₂O₅ increase with the increasing of film thickness. Temperature-dependent Raman spectroscopy indicated that the peaks corresponding to VO₂ undergo reversible changes during heating and cooling of the films, while the peaks corresponding to V₂O₅ remain unchanged throughout. Careful control of the V₂O₅ content of the films (by varying the duration of the deposition process) allows control over their transmittance and optical modulation properties without changes in the phase transition temperature. This provides a new method of controlling the optical properties of these materials and shows promise for their potential applications in thermochromic windows.