Electrical properties of NiO films by faced target sputtering for the hole-transporting layer

NiO thin films have been deposited on glass substrates by off-axis rf-sputtering system adopting a faced target configuration. The glass substrates were located perpendicular to the centerline of the faced NiO targets and deposition was done at 6 mTorr (O₂ only) and 80 watts for 20 minutes. The thickness of NiO films of ~25 nm at 10 cm away from the centerline increased linearly as the substrates was moved towards the center, at which thickness of ~105 nm was obtained. The resistance of NiO films at the center was ~1 MΩ, meanwhile, lots of fluctuation of resistance from 2.8 to 6.8 MΩ at 10 cm away was observed. At fixed distances of 5 and 0 cm from the center NiO films have been deposited varying only the sputtering pressure in the range of 0.5-6 mTorr. At a distance of 5 cm the resistivity and surface resistance of NiO films decreased with small change of thickness in the range of 85-110 nm, as the sputtering pressure was lowered. The minimum values of 847 mΩ·cm and 63 kΩ/sq were obtained at 0.5 mTorr. However, at the center of 0 cm it was found that the NiO thickness decreased linearly with slow reduction of resistance until the pressure of 1 mTorr. And abrupt change of resistance can be identified at 0.5 mTorr with the values of 53 mΩ·cm and 89 kΩ/sq, which can be attributed to the highly generated defect microstructure of NiO films.