Metallic-mode reactive sputtering of nickel oxide thin films and characterization of their electrochromic properties

Nickel oxide is a promising electrochromic (EC) material, and it is a material that changes color upon undergoing electrochemical oxidation and reduction. In this study, we prepared nickel oxide thin films by sputtering a nickel metal target using water vapor (H2O) as a reactive gas, and investigated the effects of substrate cooling and water vapor flow ratio in the atmosphere of argon (Ar) gas, R H2 O = H2O/(Ar + H2O). High and constant target voltage and plasma emission peaks of Ni atoms were obtained up to R H2 O = 75% at ‒80 °C, indicating that the Ni target maintained a metallic state. The deposition rate increased with increasing R H2 O when the substrates were cooled, and a maximum deposition rate of 34.6 nm min−1 was obtained at R H2 O = 50%. The nickel oxide thin films that were formed in the metallic target mode showed high transmittance in the as-deposited state and good EC properties.