Optical constants of electrodeposited mixed molybdenum-tungsten oxide films determined by variable-angle spectroscopic ellipsometry

Mixed molybdenum-tungsten oxides of varying stoichiometry (MoxW1-O-x(3), 0 < x < 1) prepared by cathodic electrodeposition from aqueous peroxo-polymolybdotungstate solutions on indium tin oxide (ITO) coated glass substrates were evaluated by variable-angle spectroscopic ellipsometry (VASE) and transmission measurements from 200 to 1000 nm (1.24-6.2 eV). A Tauc-Lorentz dispersion model was used to determine the real and imaginary components of the complex refractive index for MoxW1-O-x(3) films as a function of Mo fraction. The optical band gaps were also estimated from Tauc plots. The refractive index increased (2.07-2.20 at 800 nm) while the optical band gap decreased (3.38-2.95 eV) in a linear fashion for MoxW1-O-x(3) films with increasing Mo fraction. These trends correlate chiefly with Mo-doping-induced changes in film structure and grain size as supported by X-ray diffraction measurements.