Study On The Electric Field Regulation Of Resistive Switching Behaviors Of Several Metal Oxide Nanostructures

Memristors are regarded as one of the most competitive candidates for next-generation nonvolatile memory due to their advantages of simple structures,excellent scalability,low power consumption,fast switching speed and compatibility with the current CMOS process,etc.However,so far,the resistive switching process and microscopic mechanism are different due to different materials,and without a consistent and universal physical model.Moreover,the problems of large dispersion and poor repeatability of their operating parameters also limit memristors’practical application and commercialization.This master dissertation reviews the research progress of oxide-based memristors,and at the same time puts forward some research ideas for the above problems.And on this basis,the author summarizes the research work during the master’s degree under the title of"Study on the Electric Field Regulation of Resistive Switching Behaviors of Several Metal Oxide Nanostructures".It includes the following contents and results.Firstly,in order to explore the regulation of the external electric field on the resistive switching mechanism of memristors,we have designed and prepared the memristor devices based on Al/TiO₂/FTO nanostructures by sol spin coating and vacuum evaporation process.The test results show that the obtained samples exhibit electronic resistive switching（eRS）in the lower voltage range,but the samples of this structure exhibit ionic resistive switching（iRS）when the test voltage is enough large.Based on the experimental data and modeling analysis,it is found that there is a critical electric field to induce the transformation of the above two resistive switching behaviors in this sample.When the applied electric field is smaller than the critical field,the oxygen vacancies in the structure can hardly migrate,instead them act as a trap centers.It is only possible to change the resistance state by trapping and detrapping carriers,resulting in eRS in the Al/TiO₂/FTO structure.When the applied electric field is greater than or equal to the critical field,oxygen vacancies can migrate and condense into conductive filaments,which lead to the resistive switching behavior in the Al/TiO₂/FTO structure transform into the iRS dominated by the formation and rupture of the conductive filaments.In addition,the size of the critical field is estimated by adjusting the thickness of the TiO₂ layer in the structure and through the optimization of the thickness parameter and the applied test voltage range,the reversible transformation between the two resistive switching behaviors can be achieved in the above structure.The discovery of the device characteristics has potential application value.Secondly,in order to further explore the controlling mechanism of the thin film memristor interface transition layer on its different resistive switching behavior,the Cu/Al/FTO nanostructure memristor devices have been prepared by vacuum thermal evaporation process.X-ray photoelectron spectroscopy（XPS）depth profile scan confirms that there is an interface transition layer consisting of Al,Al₂O₃,SnO and SnO₂ between the Al/FTO interfaces.The experimental results show that the adjustment of the microstructure of the interface layer by the electric field can be reflected in the evolution of the resistive switching behavior of the sample,that is,from irreversible write-once-read-many-times（WORM）characteristics to reversible bipolar resistive switching（BRS）behaviors with negative differential resistance（NDR）effect and subsequent disappearance and recovery of the BRS.Based on the analysis of relevant experimental data,it is found that the above-mentioned resistive switching behavior in this structure is caused by the permanent rupture of the pre-existing Al conductive filament in the interface transition layer and the evolution of its local microstructure under the action of an electric field.Therefore,a model for explaining the evolution of this interface-driven resistive switching behavior is proposed,in which the synergistic effect of trapping and detrapping of carriers induced by Al³⁺defects and the built-in electric field caused by the migration of the Al³⁺defects themselves under the electric field would be considered to be responsible for the observed NDR effect.On the basis of the above work,the ordered Cu/TiO₂/Ti nanopore array membrane memristor devices have been prepared by the following steps in order to explore the influence of local electric field on the stability of the devices’resistive switching behaviors.Firstly,Ti bottom electrodes that retain the nanopores mark are obtained by removing the oxide layer grew during the first-step anodization.Then,as-prepared Ti electrodes are subjected to second-step oxidation for different time series.Finally,Cu nano-cone top electrodes are deposited on Ti/TiO₂ samples by thermal vacuum evaporation method,with the help of a mask plate,to prepare Cu/TiO₂/Ti nano-ordered array structure.Preliminary performance tests show that this novel ordered Cu/TiO₂/Ti nanopore array membrane structure can limit the growth path of conductive filaments by regulating the distribution of local electric fields,which thereby greatly improves the stability and yield of the devices.In addition,it was found that lateral（pore diameter）and longitudinal（pore depth）dimensions of the nanopores can be effectively controlled by adjusting the process parameters such as voltage and electrolyte temperature for further optimizing device performance.This research work would have important scientific significance and application value for improving the stability of the memristors.