Spectroscopic ellipsometry study of the dielectric response of Au–In and Ag–Sn thin-film couples

Optical properties and phase composition of In–Au and Sn–Ag ultra-thin films grown by sequential evaporating and co-depositing of metals in a vacuum were investigated combining X-ray diffraction and spectroscopic ellipsometry methods. The atomic concentration ratios of bilayer and co-deposited samples were the same, i.e. In(Sn):Au(Ag) = 1:2. The XRD patterns indicated creation of AuIn, AuIn2, Au3In2, Au9In4 and Ag3Sn intermetallic compounds at room temperature. The effective complex dielectric functions of the composite layers, image, were determined from ellipsometric quantities Ψ and Δ measured in a photon energy range of 0.6–6.5 eV. The free-carrier parameters (unscreened plasma frequency and free-carrier damping) and optical resistivity were evaluated using a semiclassical Drude–Lorentz model of the effective dielectric function. There was noticed a distinct influence of phase composition and surface morphology on the optical constants and conductivity of the samples: ρop changed from approximately 15 μΩ cm to 37 μΩ cm for Ag–Sn structures, composed of β-Sn and Ag3Sn phases, and from 21 μΩ cm to 83 μΩ cm for Au–In multiphase system. Lower resistivity demonstrated diffusive layers formed after deposition of an In(Sn) thin film on the noble metal underlayer.