Temperature-Dependent Residual Stress and Optical Properties of Asymmetric W-Doped VO2-Based Trilayer Thin Films

This study aims to reduce the phase transition temperature (PTT) of W-doped vanadium dioxide (VO2) multilayer thin films. We designed and fabricated two asymmetric multilayer thin film structures; namely, TiO2/VO2-5%W/ITO and ITO/VO2-5%W/TiO2. The W-doped VO2-based Trilayer thin films were deposited using an electron beam evaporation combined with the ion-assisted deposition (IAD) technique. An experimental study was conducted on the temperature-dependent residual stress and optical properties of the two asymmetric VO2-based three-layer structures. The VO2-based thin films were characterized using UV–Vis–NIR spectrophotometry, Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and an improved Twyman–Green interferometer combined with fast Fourier transform (FFT) analysis for residual stress measurement. The trilayer structures incorporated a ~60 nm thick W-doped VO2 middle layer, which plays a critical role in modulating thermochromic behavior and residual stress evolution. The results show that both trilayer thin films demonstrated excellent optical performance in transmission spectra. Raman spectral analysis revealed a blue shift in the characteristic W-doped VO2 peaks, accompanied by a decrease in peak intensity as the temperature increased. Heating experiments on asymmetric W-doped VO2 trilayer thin films revealed that the critical transition temperature of the ITO/VO2-5%W/TiO2/B270 trilayer film structure was significantly reduced to 45 °C. This demonstrates the effectiveness of our proposed multilayer film design in improving the PTT of W-doped VO2 thin films. Analysis of the changes in residual stress of the trilayer thin films during heating experiments revealed that the residual stress shifted from compressive to tensile in the temperature range of 40 °C to 50 °C. The thermal expansion coefficient and biaxial modulus of the TiO2/VO2-5%W/ITO trilayer film structure were 5.37 × 10−6 °C−1 and 295.7 GPa, respectively. In addition, the thermal expansion coefficient and biaxial modulus of the ITO/VO2-5%W/TiO2 trilayer film structure were 6.65 × 10−6 °C−1 and 745.0 GPa.