Study On Synaptic Biomic Devices Based On ZnO/ZnS Memristors

Memristor is regarded as the fourth fundamental passive components with two-terminal structure.It has received much attention in biological simulation of artificial synaptic device because of the unique nonlinear electrical characteristics,low consumption,rapid writing/reading speed and high integration.At present,artificial synaptic devices have been implemented by many researchers in material selection,performance improvements,mechanism exploration and synaptic function simulation.But there are still many problems need to be solved,such as the stability should be further optimized,power consumption needs to be further reduced and sensitivity should be improved.In this research,electrochemical metallization memory cells based on oxide?ZnO?and sulfide?ZnS?are found to show excellent cycle stability,excellent retention time,ultra-low operating voltage,low power consumption and strong synaptic plasticity.The following conclusions can be drawn:?1?Both advantages of oxide zinc and sulfide zinc based memristors are combined by comparing their respective characteristics.Cu/annealed ZnS/Pt based on lightly oxidized ZnS films are found to show highly controllable memristive switching comparing pure oxide zinc and sulfide zinc based memristors,including excellent endurance of cycles over 1500,long retention time over 10⁶ s,low switching voltages about+0.2 V.On the basis of the Cu/annealed Zn S/Pt cell,biological synaptic learning and memory function are successfully simulated,such as the nonlinear transmission characteristic,long-term/short-term plasticity?LTP/STP?,pike timing dependent plasticity?STDP?,spike rate dependent plasticity?SRDP?and synaptic voltage dependent plasticity?SVDP?.?2?Cu/annealed ZnS/Pt cell shows high electrical sensitivity.Various existing memristive devices have been used to emulate synaptic functions with stimulation voltages mainly in the range of several hundreds to several thousands of millivolts.Compared to biological synapses,memristive ones apparently show a much lower sensitivity to electrical stimuli since much higher stimulation voltages are needed to induce synaptic plasticity.Cu/annealed ZnS/Pt cell shows an ultra-low SET voltage of several millivolts?about 6 mV?and maintaining the change for a long time.Furthermore,based on such devices,an ultra-sensitive memristive synapse is realized where the synaptic functions of both short-term plasticity and long-term potentiation are emulated by applying electrical stimuli several millivolts in amplitude,whose sensitivity greatly surpasses that of biological synapses.The dynamic processes of memorizing and forgetting are mimicked through a 5×5memristive synapse array.In addition,the ultra-low operating voltage can help to reduce the damage to device internal structure and volatility.It also provides another effective solution to the relatively high energy consumption of synaptic devices besides reducing the operating current and pulse width.?3?Switching mechanism of Cu/annealed ZnS/Pt cell have been proposed by XRD,XPS,PPMS,AFM and TEM detection means.The excellent electrical properties and ultra-low SET voltage likely originates from a two-layer structure of ZnS films,i.e.the lightly oxidized and unoxidized layers,where the filament rupture/rejuvenation is confined to the two-layer interface region several nanometers in thickness due to different ion transport rates in these two layers.Ultra-low voltage can produce strong electric fields,which induce the change of devices resistance state and ensure that the cycle stability of devices.In addition,the internal electric field distribution models have been established through CMOSOL software simulator and verify the reason of the ultra-low opening voltage.In the end,to verify the universality of above mechanism model,the Cu/ZnO/ZnS/Pt structure was fabricated,in which a ZnO thin film is employed as a substitute for the lightly oxidized ZnS layer.