Optical Constants and Optical Anisotropy of Ultrathin Gold Films

Continuous and homogeneous thin gold films are fabricated on Si-SiO2 substrates using an additional seed layer based on Cu oxide or oxidized Cu. The optical properties of these gold films with thicknesses from 3 to 50 nm are studied with the help of variable angle spectroscopic ellipsometry. The optical constants and the dielectric functions of thin Au films in the wavelength range of 240-1700 nm are extracted by fitting measured ellipsometric data using the Fresnel isotropic and anisotropic models. By applying the Drude approximation to the infrared range, values of the plasma frequency omega p and relaxation time tau are obtained as a function of film thickness. It is found that plasma frequency omega p of the gold films is mostly constant in the thickness range of 3-50 nm with an average being close to the bulk value omega p approximate to 8.45 eV. The relaxation time decreases dramatically from the bulk values of tau approximate to 14 fs to tau approximate to 2 fs for gold films of 3 nm in an agreement with the confinement effect. It is established that the thinnest gold films (approximate to 3 nm) are described better by an anisotropic layer and the underlying reasons for such optical anisotropy are discussed. Ultrathin and ultraflat gold films are fabricated on composite substrates. Optical properties of these gold films are measured using ellipsometry. The extracted optical constants are fitted by Drude model. There is no change in plasma frequency while scattering time decreases at small thicknesses. The thinnest films demonstrate anisotropic optical behavior with different in-plane and out-of-plane optical constants.image (c) 2023 WILEY-VCH GmbH