Low Voltage Resistive Switching Characteristics Of Pseudo-flexible HfO₂/NiO Nano-arrays

With the coming of the era of big data,the rapid development of the internet of things and artificial intelligence technology urgently need a new generation of information storage devices with faster reading and writing speed,higher storage density,lower storage power consumption,longer data retention and smaller size.Among new types of non-volatile memory devices,resistive random access memory（RRAM）is simple in structure and compatible with the fabrication process of current CMOS.On the other hand,with the appearance of foldable phones,curved televisions and wearable electronic devices,the flexible RRAM devices have become a new research hotspot.Traditional fully flexible organic RRAM devices exhibit an excellent flexibility,but their thermal stability and endurance are far lower than those of traditional inorganic RRAM devices.On the contrary,inorganic RRAM devices are stable but their plasticity is limited.In this thesis,NiO nano-arrays were designed and fabricated on flexible mica substrates,and resistive switching characteristics and physical mechanism of RRAM devices based on NiO nano-arrays in flat and bending states were studied.Nano-bowl-like NiO（nb-NiO）arrays on flexible Pt-coated mica substrates were fabricated by electro-deposition and magnetron sputtering with the assistance of the monolayer colloidal sphere template.The nano-bowl array structure not only increases the contact area between the resistive switching layer and the electrodes,but also makes the polycrystalline NiO film thickness change periodically in the plane of parallel substrate,which results in the locally ordered spatial distribution of conductive filaments.Therefore,swtiching voltages of device reduce to below 0.6 V.Next,systematic comparative experiments determine that active Ag top electrode is more beneficial to improve the ratio of the high resistance to the low resistance（R_H/R_L）of the device than the inert Pt electrode.Covering the nb-NiO with a 20 nm-thick HfO₂ film can greatly depress the local leakage current in the high resistance state,and significantly improve the stability of the resistive switching performance.The optimized Ag/HfO₂（20 nm）/nb-NiO/Pt cell shows excellent resistive switching characteristics in the flat state:a R_H/R_L higher than 10⁴,an endurance more than 1200 cycles,a retention time longer than 10⁴s,and switching time less than 8.7μs.Because the nano-bowl array structure can release the tensile or compressive stress in the bend cell in time,the anti-tensile property of the cell is improved.Under the bending radius of 2.5 cm,the bent cell almost keeps the initial resistive switching performance even after switching between the bending and the straight state for nearly 500 times,showing a good flexibility.By fitting and analyzing the I-V curves in different voltage regions,it is found that oxygen vacancy conductive filaments in the nb-NiO bulk and F-N tunneling of electrons at the HfO₂/nb-NiO interface are responsible for the resistive switching characteristics.Our research provides a new research path for the design and fabrication of a new generation of low power consumption flexible RRAM device.