Dielectric function from ellipsometry measurements of niobium films

Niobium is a promising material for high-temperature photonic applications due to its high melting point and chemical stability. Despite this, there is insufficient information about its dielectric function as a function of thickness. In this study, the thickness-dependent dielectric function of niobium films (8, 38, and 283)-nm-thick deposited by sputtering was analyzed with an oxide/film/substrate model over a range of 0.7 eV–4.5 eV. Notably, our films were deposited to have an oxide-free bulk ensuring that the optical properties reflect clean niobium with surface oxide. The Drude model was applied to describe the intraband transitions, while Lorentz oscillators were used for interband transtions. As expected for metals, the real part of the dielectric function was negative for 283 nm-thick films with a slight reduction in magnitude for 38 nm-thick films. However, it became positive for the thinnest film, which was attributed to crossing the percolation threshold, where metal islands touch each other forming conductive paths but with gaps still not covered. The imaginary part of the dielectric function shows nonsystematic variations with changes in thickness, attributed to morphological changes in-plane and out-of-plane of the films, affecting the surface distribution of electron density and electron scattering rate. Dielectric properties show good agreement with the electrical resistivity from Van der Pauw measurements and conductivity maps from scanning tunneling microscopy.