Modification of optical properties in ultrathin ALD-grown NbN layers using oxygen plasma treatment

Overview & Synopsis

NbN thin films with a nominal thickness of 10 nm were grown by atomic layer deposition (ALD) on Al$_2$O$_3$-buffered Si substrates and treated with oxygen plasma for 0 s (untreated), 20 s, 40 s, and 60 s to investigate oxygen-induced changes in their optical properties. Spectroscopic ellipsometry (SE) measurements were performed before and after oxygen plasma treatment over the ultraviolet-visible-near-infrared (1.24-6.20 eV) and infrared (0.041-0.827 eV) spectral ranges. Film thickness determined by X-ray reflectivity (XRR) was used as a fixed parameter in the optical modeling to extract the dielectric function, optical constants, and effective free-carrier concentration. We observed the emergence of a distinct absorption feature near 4.9 eV following oxygen plasma treatment, consistent with the oxidation of the NbN surface and the resulting change in its optical properties. Increasing the treatment time from 0 to 60 s reduced the effective carrier concentration from $1.20 \times 10^{22}$ to $1.03 \times 10^{22}$ cm$^{-3}$ and the effective carrier mobility from 2.44 to 2.27 cm$^2$ V$^{-1}$ s$^{-1}$. The obtained results provide a basis for understanding the effects of oxygen plasma treatment on NbN thin films.