Controlling the optical parameters of self-assembled silver films with wetting layers and annealing

Abstract

We investigated the influence of presence of Ni and Ge wetting layers as well as annealing on the permittivity of Ag films with thicknesses of 20, 35 and 65nm. Most of the research on thin silver films deals with very small (<20nm) or relatively large (≥50nm) thicknesses. We studied the transition region (around 30nm) from charge percolation pathways to fully continuous films and compared the values of optical parameters among silver layers with at least one fixed attribute (thickness, wetting and capping material, post-process annealing). Our study, based on atomic force microscopy, ellipsometric and X-ray photoelectron spectroscopy measurements, shows that utilizing a wetting layer is comparable to increasing the thickness of the silver film. Both operations decrease the roughness-to-thickness ratio, thus decreasing the scattering losses and both narrow the Lorentz-shaped interband transition peak. However, while increasing silver thickness increases absorption on the free carriers, the use of wetting layers influences the self-assembled internal structure of silver films in such a way, that the free carrier absorption decreases. Wetting layers also introduce additional contributions from effects like segregation or diffusion, which evolve in time and due to annealing.