Optical properties of laser-melt-quenched Au-Si alloys

Glassy films of Au-Si alloys were produced by nanosecond laser quenching in the composition range from 25-93 at.% Si. Reflection and transmission coefficients as well as electrical conductivity were measured from 1.4 to 5.4 eV. The complex dielectric function, obtained from Kramers-Kronig analysis, is presented. The optical properties of the Au-Si system over the whole phase diagram can be described using a Lorentz-Drude model with a free-electron term and a single-resonance bound-electron term. We conclude that two electronic transitions occur in the system: Si switches between the metallic and the semiconducting state at about 30 at.% Si, whereas Au switches from the metallic to a dielectric state at about 85 at.% Si. These two transitions of the constituents separate three regions of markedly different electronic behavior in the alloy.