Ellipsometric and optical reflectivity studies of reactively sputtered NbN thin films

Abstract

The optical parameters of NbN thin films reactively sputtered mainly at room temperature are investigated by in situ ellipsometry using a He-Ne laser source. Systematic correlations between the ellipsometric parameters and the film properties are observed. In particular, the Tc film shows an approximately linear relation with the parameter ψ over the wide range of 7–17 K. Optical reflectivity measurements are carried out in the wavelength range of 0.2–2.5 μm for these films. Each spectrum is well fitted by the curve calculated using the Drude model with a screened plasma frequency of 25 000–30 000 cm−1. The Tc film also shows a stronger correlation with the damping constant of plasma excitation than with the reflectivity. This is considered to be a universal degradation curve for this system, similar to those observed in high -Tc A15 compounds. The relation of Tc to ψ is found to be equivalent to this correlation.