Resistive Switching And Synaptic Behaviors Of N-TiO₂ Nanorod Arrays Based Memristors

Memristors exhibit widely prospects of applications of data memory,neuromorphic computing,and in-memory computing.The structure and composition of materials and electrode configuration are essential factors which effect behaviors of memristor devices.In this work,the popular used TiO₂ based materials is studied.The effects of growth morphology,nitrogen doping,and electrode types of TiO₂nanorod arrays（TiO₂ NRAs）on resistive switching behaviors are investigated.Finally,synaptic behaviors have been realized using optimized nitrogen doped TiO₂ nanorod arrays（N-TiO₂ NRAs）device.This research aims to prepare durable and stable memristors and accelerate the application of neuromorphic computing for memristors.First,TiO₂ NRAs are synthesized on the surface of FTO substrate.The effects of hydrochloric acid molarity,growth time and temperature during hydrothermal processes on morphology of N-TiO₂ NRAs are investigated.Memristors with different dimensions of TiO₂ NRAs are prepared using different hydrothermal temperature.The results show that,as the temperature increasing from 140℃to 200℃,the dense of TiO₂ NRAs first decreases slightly and then increases,which has a minimum value of24%at 160℃.The contents of oxygen defects in TiO₂ NRAs first decreases and then increases with increasing synthesized temperature,and there is a minimum value of 26.11%at 180℃.The fabricated memristors with the dense of TiO₂ NRAs less than 27%exhibit rectification and bipolar resistive switching behaviors.While the memristor with the dense of 37%exhibits only bipolar resistive switching.In addition,the lower the contents of oxygen defects in TiO₂ NRAs,the higher the reset voltage requested and the smaller the current value for LRS.The switching behaviors of Au/TiO₂ NRAs/FTO memristor are fitted with vacancy-trapped space charge limited current（SCLC）mechanism.In brief,rectification and non-rectification effects can be realized by tunning the growth morphology of TiO₂ NRAs.Second,two-step/one-step methods of N doping are designed to fabricate N-TiO₂ NRAs based memristors.The results of cycle-to-cycle test show the variable coefficient（σ/μ）of HRS and LRS for device fabricated by two-step method are 14%and 12%respectively,while the values for device fabricated by one-step method are 6%and 8%respectively.The stability of devices using one-step method are well than that of devices using two-step method.The analysis of element contents indicates that the distribution of defects of nitrogen doped devices are‘funnel’shape.By adjusting N composition,the devices can exhibit volatile or non-volatile behaviors.When the molarity of nitrogen ranges from 0.01665 to 0.02498 mol/L,the fabricated devices exhibit non-volatile property.When the molarity of nitrogen ranges from 0.03330 to0.05827 mol/L,the devices exhibit volatile property.Volatile memristors perform similar behaviors to Leaky Integrate-and-Fire function of neuron.The response of current of volatile devices could gradually increase with sequential pulses and a decay process of LRS follows withdrawing those pulses.Then,the effects of electrode of Au,Cu,and Al on resistive switching behaviors are studied.The results show the devices with Au top electrode present rectification,negative differential effects,bipolar resistive switching behavior and retention of 3500 s.The devices with Cu top electrode present bipolar/unipolar resistive switching,and retention of over 10000 s.The devices with Al top electrode present bipolar resistive switching,negative differential effects,and retention of over 18000 s.The variable coefficient of cycle-to-cycle of memristors with Au,Cu,and Al electrode is 24.35%,18.08%,and 11.74%,respectively.Finally,the synaptic behaviors of Al/N-TiO₂ NRAs/FTO device are studied.Al/N-TiO₂ NRAs/FTO memristor presents gradually changed conductance,bipolar resistive switching behavior,endurance of over8000 times,and HRS/LRS ratio of over 16.The memristor has realized synaptic behaviors,such as potentiation,depression,paired pulse facilitation（PPF）,short term memory（STM）,long term memory（LTM）and spiking timing dependent plasticity（STDP）.These synaptic behaviors are induced by creation/accumulation and diffusion effects of vacancies.Furthermore,Al/N-TiO₂ NRAs/FTO memristor has been used to successfully mimic learning and forgetting processes of synapse for neuromorphic computing.The stimulating processes are dependent on the amplitude,duration,and interval of input pulses.The memristor can response with pulses of minimum width of 20μs.The memory lifetime（MLT）of memristor can be tuned from few seconds to tens of thousands of seconds.All these synaptic and learning/forgetting behaviors of Al/N-TiO₂ NRAs/FTO memristor have verified huge prospect of application of neuromorphic computing.