Analysis of the temperature dependent optical properties of V1−xWxO2 thin films

Abstract

Vanadium dioxide is one of the most promising materials that can be used to control and tune in real time the optical properties of nanoscale devices due to its ability to perform a Metal-Insulator Transition (MIT). In this paper, we report the optical and structural properties of undoped and W-doped V1-xWxO2 vanadium dioxide thin films deposited on oxidized silicon substrates using magnetron sputtering. The structural properties determined by X-ray diffraction showed not only the transition from a monoclinic to a rutile phase when crossing the metal–insulator transition temperature (TMIT) but also a coexistence of phases for the tungsten-doped VO2 even at room temperature. It is also shown that TMIT decreases from 69.6°C to 41.6°C as x increases from 0 to 3%. A point-by-point fitting method is proposed to reliably extract the refractive index and extinction coefficient from spectroscopic ellipsometry measurements as a function of temperature and tungsten concentration. This approach avoids all the correlations between the thickness and the refractive index well known in absorbent films calling into question the uniqueness of the results. The dielectric function is then calculated from the point by point refractive index. A model based on Tauc-Lorentz oscillators in the insulating phase to which a Drude component is added in the metallic phase provides information on the levels of the energy bands and the plasma frequency of the films. The calculation of reflectance and transmittance of V1-xWxO2 thin films highlights the effect of W doping on thermochromic performance.