Minimizing optical loss in ultrathin Ag films based on Ge wetting layer: Insights on Ge-mediated Ag growth

Abstract

The wettability of Ag deposited on oxide substrates is generally improved by adding a Ge seed layer. However, the realization of optically transparent Ag electrodes is still a crucial challenge owing to the lack of an effective method for circumventing the catastrophic optical losses caused by the Ge layer. Herein, a strategy is proposed to minimize the optical losses in the Ag-on-Ge configuration grown on ZnO substrates; this provides new insights into the utilization of Ge as a wetting inducer. The evolution of ultrathin films from Ag nanoparticles provided clear evidence for the strong out-diffusion of a substantial amount of Ge atoms toward the top surface of evolving Ag nanostructures. The effective improvement in Ag wetting, even with minimal concentrations of Ge, could be explained by the ability of Ge to reduce the thermodynamic free energy at both the top and bottom boundaries of evolving Ag regions. It was also observed that immiscible and mostly removable Ge atoms from Ag lattices were subsequently oxidized at the boundaries contacting oxides. These unique features of Ge allowed for the fabrication of an extremely smooth, ultrathin (4.5 nm) Ag film on an atomically thin (0.3 nm) Ge layer, with successful circumvention of optical losses.