Optical properties of NiO thin films grown by using sputtering deposition and studied with spectroscopic ellipsometry

We deposited nickel-oxide thin films on silicon substrates at room temperature and 500 degrees C by using a nickel or a nickel-oxide target and DC and RF magnetron sputtering. We annealed the NiO thin films deposited at room temperature. By using spectroscopic eillipsometry, we obtained the refractive indexes, the extinction coefficients, the thicknesses, the band gap energies and the broadenings of the NiO thin films. In order to estimate the dielectric functions of the NiO thin films, we used the parametric optical constant model for the layer analysis. We obtained the band gap energy and the broadening values by using the standard critical point model for the second derivative spectra of the dielectric functions. We discussed the relations between the optical and the structural properties of NiO thin films as a function of processing parameters such as the thickness, the oxygen flow rate, the growth temperature and the annealing temperature. We compared the dielectric functions of the NiO thin films with that of the bulk NiO single crystal. The NiO films deposited at 500 degrees C had not only larger refractive indexes (n) and extinction coefficients (k) but also better crystallinity than those deposited at room temperature. The peak of the refractive index (n) and the threshold of the extinction coefficient (k) became sharp when the films grown at room temperature were annealed. The changes in n and k as functions of the 02 flow ratio are attributed to both free carrier effects and defects due to excess oxygen. In the case of no oxygen flow, the band gap energy decreased as the annealing temperature increased whereas the band gap energy increased with oxygen flow.