Memristive Heterojunction Devices Based On Transition Metal Oxides:Fabrication And Resistive Switching Mechanism

Since the Hewlett-Packard Laboratories verified the memristor model with a single device in 2008,the research of memristors has attracted extensive attention.Memristors have shown great potential for developing novel nonvolatile memories,logic operation or the simulation of brain function.Devices that can simulate the learning,forgetting and cognitive functions of the human brain are particularly important for the development of the artificial intelligence,while the brain functions depend on a complex neural network in the brain.Therefore,to simulate different neural functions with memristors have become a hotspot in the field.Materials,fabrication and structures are all important for controlling various physical variables during the resistive switching（RS）for simulating complex neural function.Our research works are mainly focused on the following two points:first,the effects of electroforming methods and electrode materials on RS were studied;second,the RS properties of memristive devices with pn and nn junctions were investigated and the mechanisms were discussed.Specific research contents are as follows:CuO is a low-cost semiconducting material.Using reactive RF magnetron sputtering method,copper oxide was fabricated under different oxygen contents.Then after annealing at 400oC,Pt/CuO/Pt device was prepared for electrical performance test,and the coexistence of unipolar and bipolar RS was found in the device with eight percent oxygen content.Moreover,analog and digital RESET processes were observed under different V_SET polarities.Plausible resistive switching mechanism was discussed,it is reasonable to assume that the migration of oxygen vacancies under the external field dominates the BRS process,while the Joule heating effect controls the URS process.ZnO and Ta₂O₅ are two semiconducting materials.Using DC magnetron sputtering method,single layer devices with different electrode materials were prepared to verify the effects of active electrode Ag,inert electrode Pt,and easily oxidized electrodes Ti,Al.Based on p-type CuO,n-type ZnO,Ta₂O₅,two kinds of bilayer devices Ag/Ta₂O₅/CuO/Pt and Pt/Ti/Ta₂O₅/ZnO/Pt were designed.Continuous nonvolatile RS properties were implemented in Ag/Ta₂O₅/CuO/Pt devices.The device conductance can be incrementally increased or decreased by consecutive potentiating or depressing pulses,which is important to mimic Long term synaptic plasticity LTP and LTD.What’s more,this memristor could reflect brain’s learning and forgetting process.The resistive switching was explained with the consideration of pn junction and space charge limited current（SCLC）.Tunable quantized conductance RS characteristics were found in the Pt/Ti/Ta₂O₅/ZnO/Pt devices,and the spike-timing-dependent plasticity in biology was demonstrated.Through a series of comparative experiments,it was proved that Ta₂O₅mainly played a role of a series of resistance by controlling the voltage and current compliance to affect the forming process,which led to the quantized conductance.