Optical characterization of transparent nickel oxide films deposited by DC current reactive sputtering

In this paper, we characterize high transparency p-type semiconducting NiO thin films deposited by Direct Current Reactive Magnetron Sputtering from a pure Ni target in a mixture of oxygen and argon gases on Corning glass/SnO2:F substrates at different oxygen contents ranging from 0% at 30%. The influence of the O2/Ar ratio and thickness on transmittance has been examined using ultraviolet–visible spectroscopy. The results show that whatever the oxygen proportion into the discharge, the nickel oxide films exhibit a polycrystalline structure. At low oxygen content, the preferential orientation is (111), for stoichiometric films the XRD diagram is powder-like whereas the preferential orientation is (200) for higher oxygen content. For low and high oxygen content, the transmittance is low. Thanks to plasma method and its ability to tune the oxygen content in the discharge and therefore the film composition, we have been able to explore carefully the intermediate zone and obtain transparent films. The optical absorption coefficient α has been calculated from the transmittance and the variation of (αhν)2 versus the photon energy (hν) for nickel oxide is presented. The optical band gap energy has been evaluated and varies from 3.2 to 3.8 eV.