Characterization of niobium oxide films prepared by reactive DC magnetron sputtering

Niobium oxide films have been prepared on glass and Si(100) substrates by reactive dc magnetron sputtering, of a metallic Nb target in an argon-oxygen atmosphere. The properties of the films as a function of oxygen partial pressure have been studied systematically by Rutherford backscattering spectroscopy. X-ray diffraction, X-ray reflectometry and optical spectroscopy. Rutherford backscattering studies show that the films are substoichiometric at lower oxygen flow while stoichiometric films can be formed above 7.5 sccm O-2 flow. Grazing incidence X-ray diffraction studies show that the films are amorphous. X-ray reflectometry was used to determine the sputter rate, density and surface roughness of the films. The films prepared at lower oxygen flow rates had a higher density than those prepared at higher oxygen flow rates. For appropriate oxygen flow rates fully transparent niobium oxide films can be grown with rates up to 0.5 nm/s for a constant cathode current of 900 mA. From optical spectroscopy measurements of reflectance and transmittance we have determined the optical constants such as the refractive index (n), extinction coefficient (k) and band gap (E.) as well as the film thickness. For the fully transparent films prepared above 7.5 sccm O-2 flow rate a band gap of 3.5 to 3.6 eV is obtained. Optical spectroscopy measurements show that the refractive index of the films decreases upon increasing oxygen flow rate and this finding is related to a decrease of density upon increasing oxygen flow.