Permittivity, specific resistivity and surface roughness of silver nanolayers for plasmonic applications

We report on measurements of optical, morphological and electrical properties of silver nanolayers. The Ag films of thickness from 10 to 500 nm are deposited in e-beam evaporator. Fused silica and sapphire substrates are used with nominal root-mean-square (RMS) roughness equal 0.3 and 0.2 nm, respectively. Silver is deposited either directly on substrates or on Ge, Ni, or Ti wetting interlayer. The refractive index n and the extinction coefficient κ of Ag films are derived from spectroscopic ellipsometry and reflectance measurements carried in air in the spectral range from 0.6 to 6.5 eV (2200 – 193 nm) using a rotating analyzer ellipsometer (V-VASE, J.A. Woollam Co.). Surface roughness is measured using AFM (Ntegra NT-MDT) under tapping mode in air with sharp etalon probes and 5:1 aspect ratio. Ag layers of 10 and 30 nm thickness have nearly the same RMS roughness when deposited at temperatures from 180 to 350 K. The lowest RMS=0.2 nm is achieved for 10 nm film Ag/Ge evaporated at 295 K. The sheet resistance of the Ag films is measured using two methods: the van der Pauw method with the electrical contacts located on perimeters of the samples and four probes contacting the samples at points lying in a straight line. Specific resistivity of Ag films on fused silica change from <109 to 1.80 [μΩ∙cm] when thickness increases from 10 to 500 nm. Specific resistivity of 10, 30 and 50 nm thick Ag films on 1 nm Ge wetting layer are equal 14.01, 7.89, and 5.58 [μΩ∙cm], respectively, and are about twice higher than those of Ag films on Ti or Ni interlayers.