Spectroscopic ellipsometry study of dielectric functions of Ag films and chemically deposited layers of Ag nanoparticles on silicon

Abstract

The optical properties of thin films and layers of Ag nanoparticles on a c-Si substrate are modeled by calculating the complex pseudo-dielectric function <ε>. The evolution of the imaginary part <ε2> with an increase in the thickness of continuous layers from the dielectric function ε of the substrate to ε of bulk silver is studied, which demonstrates the appearance of optical characteristics of Ag and the suppression of c-Si features in the form of critical points. The data measured on the spectroscopic ellipsometer for Ag films produced by magnetron sputtering for several nominal thicknesses from 10 to 300 nm, and layers of Ag nanoparticles chemically deposited on a c-Si substrate, were interpreted using a pseudo-dielect ric function and by calculating the complex dielectric functions ε using the Drude-Lorentz model. Morphological parameters obtained by atomic force microscopy and scanning electron microscopy were also used in the calculations. Since the Ag layers and films were deposited on the c-Si substrate without any additional layers (except for the native layer), the determined pseudo-dielectric functions for thicknesses in the range of 7−50 nm turned out to be close to the calculations of the dielectric functions using the Drude-Lorentz model. It was found that in the calculated and experimental spectra in the imaginary part of ε2 and <ε2> a peak appears at E ∼ 3.9 eV, which is interpreted as a plasmon resonance in the longitudinal mode at oblique incidence of light. In the ε2 spectrum of a layer of disordered Ag nanoparticles produced by chemical deposition, there is a broader localized surface plasmon resonance peak at E ∼ 2.5 eV, in contrast to thin films. Whereas, in a layer of larger nanoparticles, the peak is absent, and its ε dependence is close to ε for bulk Ag.