Localized plasmon resonances on grains in smooth Ag films

Surface roughness of polycrystalline Ag thin films frequently used in plasmonic applications can be considerably reduced with a Ge wetting interlayer, that inhibits island growth due to large Ag-Ge adhesion. However, segregation of Ge to the Ag free surface and grain boundaries leads to an increase of the specific resistivity and of the imaginary part of Ag permittivity in the 500 - 800 nm spectral range. Here we demonstrate that this increase in losses is due to localized plasmon resonances on silver grain boundaries. The investigated samples are deposited on SiO₂ substrates using e-beam physical vapour deposition. The samples are characterized using atomic force microscopy (AFM), scanning electron microscopy (SEM), spectroscopic ellipsometry (SE), X-ray diffraction (XRD), X-ray reflection (XRR) and X-ray photoelectron spectroscopy (XPS).