Optical and microstructural characterisation of Au–Sn and Cu–Sn diffusive layers

Phase composition, crystallinity, optical and electrical properties were determined for Au–Sn and Cu–Sn ultra-thin films produced by sequential evaporating and co-depositing of metals on glass plates in a vacuum. Thickness of Sn films grown on top of Au(Cu) nanolayers (dAu(Cu) = 20 nm) was varied to obtain different atomic concentration ratios of Au(Cu)-rich diffusive samples up to 1:1. The samples were characterised using the XRD, SEM, spectroscopic ellipsometry and transmittance measurements. The XRD patterns indicated creation of AuSn and AuSn2 intermetallic phases at room temperature in both types of Au–Sn samples, formation of Cu6Sn5 compound in bilayer Sn/Cu samples and Cu10Sn3 intermetallic in the co-deposited Sn–Cu film. There was observed a substantial influence of morphology and phase composition on the effective complex dielectric functions and optical conductivity of the multiphase films, determined using the transmittance and variable angle spectroscopic ellipsometry measurements in the photon energy range of 0.6–6.5 eV. Adopting the Drude–Lorentz parameterisation approach to optical spectra enabled to extract contributions related to the free-carriers, interband transitions and plasmonic effects. The optical resistivity agreed reasonably with the dc-resistivity results, which changed approximately from 17.5 μΩ cm to 26 μΩ cm and from 24 μΩ cm to 96 μΩ cm for investigated Au–Sn and Cu–Sn systems, respectively.