Effect of copper content on the electrical stability of nickel oxide films

• By introducing Cu into the NiO films, the electrical stabilities can be improved.
• Thermal stability of resistivity (ρ): highest temperature limit was 200 °C.
• The aging stability of ρ remained fairly constant for at least 53 days.
• Various mechanisms, affecting stability of the NiO–Cu films, are discussed.

In this study, NiO films with various Cu contents of 0–23.3 at.% were deposited on low alkali glass substrates, respectively, by using radio frequency magnetron sputtering at room temperature. In order to investigate thermal stability, the NiO and NiO–Cu films were post-annealed at temperatures of 50–400 °C in various atmospheres (air, O2 and vacuum). For the pure NiO film (with no Cu content) electrical resistivity (ρ) did not change, if it was annealed at temperatures below 150 °C both in air and O2. However, ρ of the NiO–Cu films (with Cu content ≧ 18.3 at.%) remains unchanged, even if they were annealed up to 200 °C (in air and O2). In a vacuum atmosphere, ρ of the NiO–Cu films (containing 18.3 at.% Cu) stayed constant only up to 150 °C. After this point, ρ decreases significantly. We believe that post-annealing the Cu-doped NiO films at higher temperatures in vacuum could result in the Cu precipitating at the grain boundaries, which leads to reduction in ρ. In addition, we also investigated the aging effect on the NiO and NiO–Cu films under the air atmosphere of 1 atm, 25 °C and 70% relative humidity condition. ρ of pure NiO films increased only slightly after aging time of 53 days. However, carrier mobility (μ) increased and carrier concentration (n) dropped significantly after 15 days, For the NiO–Cu films (with Cu contents ≧ 13.2 at.%), after 53-day aging time, both ρ and μ rose slowly, while n decreased slightly.