Research On Resistive Switching And Synapse-Like Characteristics Of Single-Crystalline Oxide

Memristor is the most promising device for memory and information processing because its potential application in new generation non-volatile memory and neuromorphic computing.The related research mainly focused on oxide thin films,and oxygen vacancies play an important role in the properties of memristor.Therefore,in order to tune the perporties of resistive switching behavior,oxygen vacancies need to be tailored.However,there are still some problems need to be solved regarding the tunning of oxygen vacancies in oxide-based memristor.And the effects of tunning oxygen vacancies on the resistive switching are not clear.Most oxide thin films investigated are polycrystalline thin films which are fabricated by deposition process.There are grains and grain bounderies.It is well known that oxygen vacancies tend to aggregate near grain boundaries,which will lead to inhomogeneous distribution of oxygen vacancies.That induces large deviations of forming voltages and the subsequent resistive switchings among different memristor-cells on same thin film,which,in aspect of research,makes the effects of tunning oxygen vacancies become difficult to be evaluated,and in aspect of future application,is a disadvantage for high density devices.In addition,randomly induced oxygen vacancies during the deposition cannot be avoided,which makes the tuning of properties by post-processing become complicated and uncontrollable.Single-crystalline oxide thin film can provide an ideal environment without OVs generated in the deposition process and the coexistence of grain and grain boundaries for the tunning of oxygen vacancies and related research.Compared with single crystal bulk materials,single crystalline oxide thin films are suitable for vertical capacitor-like structure of practical devices.Therefore,in present work,based on single crystalline LiNbO₃ thin films,the investigation regarding the tunning of oxygen vacancies and the effects of oxygen vacancies on resistive switching and synaptic-like characteristics of memristor was conducted.?1?In order to conduct post-processing on single-crystalline oxide thin film to enable the memristor based on the thin film show the resistive switching behavior,we need to conduct research on the generation mechanism of resistive switching in single crystalline oxide materials.Vacuum annealing process was conducted on SrTiO₃ single crystal.The effect of vacuum annealing process on SrTiO₃ single crystal was investigated,which indicated the generation of oxygen vacancies in SrTiO₃ single crystal during the vacuum annealing process.The resistive switching behavior with self-compliance property was observed,which can be ascribed to the swichable diode effect.Such effect was caused by the mechanism that redistribution of oxygen vacancies under the external electric field results in the formation of n–n⁺junction or n⁺–n junction.And in order to investigate the influence of amount of oxygen vacancies on the resistive switching behavior,SrTiO₃ single crystal was annealed by laser with different fluences.The XPS measurement indicates that amount of oxygen vacancies increases with the increase of laser fluence.Resistive switching behavior is only observed in the sample annealed by laser with relatively high fluence after an electro-forming process,which indicates that resistive switching appears when enough oxygen vacancies can be generated to form conductive filaments under an external electric field.?2?Single-crystalline LiNbO₃ was fabricated by crystal ion slicing technique for the investigation regarding the resistive switching on single-crystalline oxide thin film.After annealing in Ar or in vacuum,electro-forming has been observed on the thin films.On each thin film,electro-forming processes of 12-14 memristor-cells show high uniformity.After electro-forming process,I-V curves,retention,and endurance were tested,and rectifying filamentary resistive switching was observed.The thin film was measured by TEM?EDS and so on,and the analysis of conduction mechanism was conducted.Based on the results,such resistive switching was interpreted by a model that the local filament consisted of oxygen vacancies does not penetrate throughout the LiNbO₃ thin film resulting in asymmetric contact barriers at the two interfaces.Based on the results mentioned above,in order to control the resistive switching properties further,local post-processing needs to be conducted on the thin film.Thus,Ar⁺irradiation was used.Firstly,Ar⁺irradiation conducted on the entire thin film to verify the effects of Ar⁺irradiation on the thin film.The measurement of I-V curves was conducted,which sets the time for the Ar⁺irradiation later by comparision.Conductive-AFM measurements are performed by using a conductive tip as a top electrode,I-V curves recorded at different 8 dots show high uniformity,even through the diameter of the contacting dots between the conductive tip and LNO thin films is only several nanometers.The investigation on the effect of Ar⁺irradiation indicates that it generated oxygen vacancies in the single crystalline LiNbO₃ thin film.Based on the Ar⁺irradiation conducted on the entire thin film,local Ar⁺irradiation was conducted.Memristor cells with same local Ar⁺irradiated zone but different size of top electrode were fabricated and measured.The results indicated that the property of conductive filament was determined by the oxygen vacancies in local Ar⁺irradiated zone.And memristor-cells with different number of local Ar⁺irradiated zones were fabricated and measured.The resistive switching behavior can be tuned by changing the number of local Ar⁺irradiated zones within the memristor-cell to tune the oxygen vacancies in the thin film.?3?The synaptic plasticity and STDP characteristics were measured.The synaptic plasticity can be tuned by changing the number of local Ar⁺irradiated zones within the memristor-cell.Memristors with different synaptic plasticity were respectively used in neuromorphic computing as one weight in the artificial neural network to realize a simple function.The effect of synaptic plasticity on the training process of artificial neural network in neuromorphic computing was investigated.In order to try to use memristors to realize edge computing of the artificial neural network at sensor side,the feasibility of combination of memristor and sensor was tested.The function that the resistance of memristor can be changed when corresponding sensor accepted stimuli has been realized.