Ellipsometric Study of NbO$_{2}$ Grown by MBE on LSAT from 77 to 800 K

NbO$_{2}$ is a transition metal oxide that has been of interest for several decades. Like other complex oxides it has a metal-insulator transition provoked by external stimuli such as temperature, pressure, and electric fields. Our study shows the dielectric function of NbO$_{2}$ grown by molecular beam epitaxy, optical axis in-plane, on (LaAlO$_{3})_{0.3}$ (Sr$_{2}$AlTaO$_{6})_{0.35}$ (LSAT) substrates. The ellipsometric angles were measured from 0.76 to 6.52 eV using a UV/VIS variable-angle spectroscopic ellipsometer and from 250 to 1200 cm$^{-1}$ using an FTIR ellipsometer. By using regression analysis we modeled our optical spectra with one model over the entire range from the mid-infrared to the near UV. For the LSAT substrate, we used optical constants from a previous study. A sum of Tauc-Lorentz oscillators describes the dielectric function of the NbO$_{2}$ film. We have measured the dielectric function of the sample from 77-800 K. This has allowed us to see that the absorption peaks sharpen/broaden with decreasing/increasing temperature. We have also plotted the direct and indirect band gaps as a function of temperature.