Grain-boundary structures and their impact on the electrical properties of NiO films deposited by reactive sputtering

Nickel oxide (NiO) is an extensively studied oxide for use as a resistive switching material. In this study, NiO films were deposited by reactive sputtering under various oxygen (O-2) flow rates (0.9-1.3 sccm). The structures of the NiO films were characterized by X-ray diffraction and transmission electron microscopy. The analyses revealed that the NiO films had a < 111 >-oriented columnar structure with a grain diameter of 20-40 nm and Ni segregation preferentially appeared at grain boundaries (GBs) in the NiO films deposited under an O-2 flow rate of 1.0 sccm or less. Conductive-atomic force microscopy measurements demonstrated that Ni segregation served as dominant electrical conducing paths, which caused a decrease in the resistivity of the NiO films. Furthermore, a resistance decrease similar to analog set behavior, even without a current limit, was observed in Pt/NiO/Pt capacitor structures with NiO films comprising Ni segregation. The resulting resistance decreased in the range of two orders of magnitude, while the applied voltage increased. After the resistance decrease, an increase in the temperature coefficient of resistivity was observed. It is suggested that these changes enacted on the voltage applications were linked to voltage-induced oxygen migration in the GBs including Ni segregation. The GB structures are also discussed in terms of the resistive switching behavior.