Spectroscopic ellipsometry characterization of electrochromic tungsten oxide and nickel oxide thin films made by sputter deposition

Electrochromic films of tungsten oxide and nickel oxide were made by reactive dc magnetron sputtering and were characterized by X-ray diffraction, Rutherford backscattering spectrometry, scanning electron microscopy, and atomic force microscopy. The optical properties were investigated in detail by spectroscopic ellipsometry and spectrophotometry, using a multiple-sample approach. The W-oxide film was modeled as a homogeneous isotropic layer, whereas the Ni-oxide film was modeled as an anisotropic layer with the optical axis perpendicular to the surface. Parametric models of the two layers were then used to derive complex refractive index in the 300–1700 nm range, film thickness, and surface roughness. A band gap of 3.15 eV was found for the W-oxide film, using a Tauc–Lorentz parameterization. For the Ni-oxide film, taken to have direct optical transitions, band gaps along the optical axis, perpendicular to it, and in an isotropic intermediate layer at the bottom of the film were found to be 3.95, 3.97, and 3.63 eV, respectively. Parameterization for the Ni oxide was made by use of the Lorentz model.