Optical properties of MoO3/Ag/MoO3 multilayer structures determined using spectroscopic ellipsometry

The authors investigated the optical and electrical properties of MoO3/Ag/MoO3 multilayer structures grown using thermal evaporation on glass. For the top and bottom MoO3 layers, they found that thicknesses of 35 and 20 nm, respectively, gave the highest transmittance in the visible spectral range. Thicknesses of 5, 7, 10, 12, and 15 nm were used for the Ag layer. The MoO3 and Ag layers were amorphous and crystalline, respectively, according to transmission electron microscopy (TEM). An ultrathin, 12-nm-thick Ag layer enhances the transmittance in the visible range relative to that of a 55-nm-thick MoO3 layer (i.e., no Ag layer). The structural and morphological properties of all samples were studied using x-ray diffraction, scanning electron microscopy, and atomic force microscopy. The optical constants were obtained from the measured ellipsometric angles, Ψ and Δ, using a parametric optical constant model. The optical properties (dielectric functions and bandgap energies) of amorphous MoO3 layers were compared to literature values. The authors estimated the optical gap energy values of amorphous MoO3 layers using both the Tauc extrapolation method (Eg = 3.380 eV) and the standard critical point model (Eg = 4.044 eV). The refractive indexes (n, k) and sheet resistances of Ag ultrathin films were significantly different for layer thicknesses of 5 and 7 nm from those of thicker films. This is explained by the percolation effect, based on TEM cross-sectional images.