Optical constant of thin gold films: Structural morphology determined optical response

The performance of metal-based devices is limited by ohmic losses in the metal, which are determined by electron scattering. The structural properties of gold thin films also play an important role in the film quality, which may affect its’ optical properties and the overall capability of the device. At the same time, metal films of different thicknesses are needed for different applications and, since these films are polycrystalline, their internal properties and surface roughness can greatly vary from one thickness to another. In this work, we study, using atomic force microscopy and spectroscopic ellipsometry, the structural morphology and optical properties of polycrystalline gold thin films (fabricated by e-beam deposition at a low sputtering rate smooth gold) in the thickness range of 20 - 200 nm. By extracting the real and imaginary dielectric function and the Drude parameter of electron relaxation rate we demonstrate that the optical losses increase mainly when the film thickness gets lower than 66 nm. Our results reveal that the thickness-dependent contribution is inversely proportional to the film thickness. The experimental results are confirmed by the theoretical model, which predicts optical losses based on structure of the gold films.