Study On Resistive Switching Mechanism And Synaptic Characteristics Of Bismuth Ferrite Based Memristors

As a next-generation non-volatile memory that integrates storage and computing,memristor is regarded as an ideal hardware for artificial synapses due to its advantages of high speed,low power consumption,and high-density integration.As a typical ABO₃-type perovskite structure material,Bi Fe O₃（BFO）exhibits resistive-switching feature by applying bias voltage to induce the oxygen vacancies ordered.The performance of BFO memristor is not only affected by preparation method,but also regulated by the device structure（e.g.the bottom electrode materials）.Mn-doped BFO（BFMO）memristors were fabricated on three different bottom electrodes（Ti N,Sr Ru O₃,Nb:Sr Ti O₃）by the technology of magnetron sputtering,lithography,and electron beam evaporation.The effects of bottom electrode materials on resistive-switching characteristics and synaptic plasticity of BFMO-based memristors were investigated by testing the device DC I-V curves,multi-resistance states,long-term synaptic plasticity,STDP learning law with a semiconductor analyzer.Firstly,BFMO memristors with the three bottom electrodes all present distinct resistive-switching characteristics.The Ti N and SRO bottom-electrodes devices show a resistive-switching process with a positive reset and a negative set with sudden change characteristics.However,the NSTO bottom electrode device displays a gradual resistive process of negative reset and positive set and its resistive window is the largest（>1200）.Moreover,the I-V curves of BFMO-based memristors at various temperatures were further fitted to investigate the conduction mechanism.It was found that the low resistance state of Au/BFMO/Ti N devices is dominated by the ohmic and SCLC mechanisms,while the high resistance state is dominated by the Schottky emission mechanism.As for the Au/BFMO/SRO device,it is ohmic conduction at low resistance state,but Schottky emission conduction at high resistance state and high voltage.Interestingly,the Au/BFMO/NSTO device conforms to the Poole-Frank mechanism at low voltage,and conforms to the Schottky emission mechanism at high voltage.Secondly,the multi-resistance states of BFMO memristors with three different bottom electrodes were regulated by changing the limiting current or voltage,it was found that there existed a phenomenon of synaptic weight saturation during the pulse regulation.Furthermore,the conductance of three different bottom electrode devices was tuned by continuous potentiation pulse and depression pulse stimulation,subsequently,the nonlinearity of conductance in each operating cycle was calculated.We surprisingly find that BFMO memristors with three types of device structure not only have the characteristics of long-term potentiation and long-term depression but also show the ultrashort device switching time of less than 50 ns.Finally,according to the impulse response curves of the three devices,pulse waveforms were designed to simulate the STDP synaptic plasticity learning rule,and the time constantτless than 100 ns was obtained.The research on the conduction mechanism and the synaptic plasticity exploration of BFMO memristors with three different bottom electrodes will be of great reference value for the application of BFO materials to neuromorphic devices.