Developing an Analysis Procedure and Dispersion Model for Pristine and W-Doped VO2 Thin Films Using Density Functional Theory and Spectroscopic Ellipsometry

Vanadium dioxide (VO2) and its unique phase transition from semiconductor to metal near room temperature (TIMT = 68 °C) offer significant potential for applications in smart materials and advanced technologies. This transition is accompanied by a drastic modulation of VO2's optical properties in the near- and far-infrared regions. Tungsten (W) has been successfully used as a dopant to lower the transition to room temperature. VO2 is highly dependent on the synthesis method, as for each fabrication protocol, the optical properties differ. Therefore, the optical properties of VO2 must be determined frequently. In this work, a universal analysis procedure to accurately determine the optical properties of all pristine VO2 thin films is presented. Density functional theory is employed to create a dispersion model specifically catered to VO2, a novel approach that justifies the oscillator center energies. This dispersion model explicates the four different contributions to the absorption of VO2 between 2 and 5 eV. We showcase the versatility of our dispersion model by applying it to data sets from the literature, correctly fitting each one. We then further illustrate the robustness of the analysis procedure by successfully applying it to W-doped VO2 thin films. This allows for a direct comparison of the optical properties of pristine and W-doped VO2 well below and well above the transition temperature for the first time. We show that W-doping affects the optical properties almost exclusively in the low-temperature phase for near-infrared photon energies. Indeed, the optical absorption of the doped films is higher than that of the pristine films for photon energies below 1 eV, and the onset of optical absorption is lowered to near 0 eV as opposed to 0.4 eV for pristine VO2.