Optical study of InZnO/Ag/lnZnO thin films grown on glass substrates by using spectroscopic ellipsometry: Plasmon and antireflection effects

InZnO (IZO)/Ag/InZnO/glass thin films were grown using continuous roll-to-roll direct current sputtering deposition. The thickness of the resulting IZO was nominally 40 nm, and the thickness of the Ag varied between 3.6 nm and 37.9 nm. The ellipsometric angles (Ψ, Δ) of the IZO/Ag/IZO/glass thin films were measured using spectroscopic ellipsometry. The thickness and the dielectric function of the Ag films were estimated as functions of the Ag film thickness by using a multi-layer model analysis with the parametric optical constant and Drude models. Measurements of the optical transmittance of the IZO(40nm)/Ag/IZO(40nm)/glass thin films showed that the transmittance increased compared to that of IZO(80nm)/glass, increasing up to 13% for 9.3 nm-thick Ag films. By comparing the measured and simulated transmittances, we attributed the enhancement of the transmittance to the antirefiection coating of the symmetric IZO/Ag/IZO multilayer. Sheet resistance data showed that the t = 3.6 nm film was near or below percolation threshold, suggesting that the thinnest film was an aggregation of Ag nanoparticles that were not well-connected. The thicker films were above percolation. From the Drude model, the plasmon frequency, w_P, and the electron relaxation time, τ, were estimated. The second derivatives of the dielectric function spectra were obtained, resulting in double peaks, near 4.0 and 4.2 eV, for all films, which were attributed to interband transitions. For t = 3.6 nm, we found other peaks at 1.49, 3.47, and 4.60 eV. The 1.49 eV peak was assigned to be a Lorentzian peak arising from a localized plasmon in the nanoparticles. The physical origins of the 3.47 and 4.60 eV peaks are unknown.