Developing an Analysis\nProcedure and Dispersion Model\nfor Pristine and W‑Doped VO<sub>2</sub> Thin Films Using Density\nFunctional Theory and Spectroscopic Ellipsometry

Vanadium dioxide (VO<sub>2</sub>) and its unique phase\ntransition\nfrom semiconductor to metal near room temperature (<i>T</i><sub>IMT</sub> = 68 °C) offer significant potential for applications\nin smart materials and advanced technologies. This transition is accompanied\nby a drastic modulation of VO<sub>2</sub>’s optical properties\nin the near- and far-infrared regions. Tungsten (W) has been successfully\nused as a dopant to lower the transition to room temperature. VO<sub>2</sub> is highly dependent on the synthesis method, as for each\nfabrication protocol, the optical properties differ. Therefore, the\noptical properties of VO<sub>2</sub> must be determined frequently.\nIn this work, a universal analysis procedure to accurately determine\nthe optical properties of all pristine VO<sub>2</sub> thin films is\npresented. Density functional theory is employed to create a dispersion\nmodel specifically catered to VO<sub>2</sub>, a novel approach that\njustifies the oscillator center energies. This dispersion model explicates\nthe four different contributions to the absorption of VO<sub>2</sub> between 2 and 5 eV. We showcase the versatility of our dispersion\nmodel by applying it to data sets from the literature, correctly fitting\neach one. We then further illustrate the robustness of the analysis\nprocedure by successfully applying it to W-doped VO<sub>2</sub> thin\nfilms. This allows for a direct comparison of the optical properties\nof pristine and W-doped VO<sub>2</sub> well below and well above the\ntransition temperature for the first time. We show that W-doping affects\nthe optical properties almost exclusively in the low-temperature phase\nfor near-infrared photon energies. Indeed, the optical absorption\nof the doped films is higher than that of the pristine films for photon\nenergies below 1 eV, and the onset of optical absorption is lowered\nto near 0 eV as opposed to 0.4 eV for pristine VO<sub>2</sub>.