Research On Resistance Change Characteristics And Mechanism Of Cerium Oxide Memristive Devices Prepared By Magnetron Sputtering

Current requirements for information storage and processing in the era of big data and artificial intelligence have increased exponentially.The explosive growth of information requires ultra-large-capacity storage devices and efficient information processing systems.Memristors can be used for neuromorphic computation and information storage and have great potential in simulating“brain-like”learning and memory.Using memristors as bionic synapses in neuromorphic circuits can achieve storage and computing with high energy efficiency and density.The unstable characteristics of memristors are a huge obstacle that limits their application,and cause errors in neuromorphic circuits and replicating issues.To reveal the factors that influence the performances and their instabilities of memristors from the point of view of the material composition structure,this thesis mainly is dedicated to the following work:（1）The Pt/CeO_x/Pt devices prepared by magnetron sputtering showed rectification in their asymmetrical current-voltage（I-V）curves during voltage sweeps.X-ray photoelectron spectroscopy（XPS）showed that the deposited CeO_x film had an inhomogeneous composition,more oxygen vacancies existed in CeO_x near the top electrode,and fewer oxygen vacancies existed in CeO_x near the bottom electrode.The asymmetric resistance change of the Pt/CeO_x/Pt device can be explained by the presence of more oxygen vacancies in CeO_xnear the top electrode and the Schottky conduction mechanism.（2）In order to further verify whether the CeO_x layer deposited by magnetron sputtering is inhomogeneity,a CeO_x layer was first deposited in an Ar:O₂=2:1atmosphere;then,a CeO_y layer was deposited in an Ar atmosphere to prepare a device with a Pt/CeO_y/CeO_x/Pt structure.There were more oxygen vacancies in the CeO_y layer near the top electrode,and fewer oxygen vacancies in the CeO_x layer near the bottom electrode and the voltage sweep of the device showed a rectification phenomenon which is consistent with the Pt/CeO_x/Pt devices,which further verifies that the asymmetric I-V curves is caused by the inhomogeneous composition of oxygen vacancies in the cerium oxide layer.For Pt/CeO_y/CeO_x/Pt devices,resistance switching behavior was observed when a voltage was applied.However,due to the large number of oxygen vacancies in the CeO_ylayer,excessive migration of oxygen vacancies occurred when a positive voltage was applied,resulting in a negative SET phenomenon（under a negative voltage the oxygen vacancy channel is broken,increasing the negative voltage,the oxygen vacancy channel grows in the reverse direction,repaired the broken oxygen vacancy channel,and realized the process of transition from high resistance state to low resistance state）in the devices.Compared with Pt/CeO_x/Pt devices,Pt/CeO_y/CeO_x/Pt devices could not only realize bipolar and unipolar resistance switching characteristics,but also realize a negative SET process.（3）Memristors with a Pt/Al/CeO_x/Pt structure embedded in a metal Al intercalation layer were prepared,which could achieve complementary memristors（CRS）characteristics,and had a higher ON/OFF ratio,lower working voltage,and the dispersibility of the working voltage was smaller.Compared with Pt/CeO_x/Pt devices,the insertion of metal Al increased the oxygen vacancies in the CeO_x layer and improved the performances of CeO_x devices.After the devices went through Forming process under the voltage sweep of 0～±1～0 V,the complementary resistive switching process could be realized.（4）The resistance switching characteristics of TiN/CeO_x/TiN devices were investigated.Ti ON intermediate layer between the sputtered CeO_xlayer and the bottom electrode TiN was formed.TiN increased the oxygen vacancy content in the CeO_x layer.After the device formed an oxygen vacancy channel,excessive oxygen vacancies caused that the device could not be switched to a high-resistance state（HRS）,and remained in low-resistance states（LRS）.Compared with Pt/CeO_x/Pt devices,TiN/CeO_x/TiN devices were still at LRS after the Forming process,and the bipolar and unipolar resistance switching processes of the devices could not be realized.According to the research results,it can be concluded that due to the redox properties of CeO_x,redox reactions are prone to occur between Ce³⁺and Ce⁴⁺,which are accompanied by the consumption and generation of oxygen vacancies.As a result,the formation of oxygen vacancy channels is very random and difficult to control.Therefore,the SET voltage of the device is unstable and has a large variation.As to the Pt/Al/CeO_x/Pt device embedded with the metal Al intercalation layer,the value and variation of the switching voltage can be decreased.TiN/CeO_x/TiN devices with TiN as the electrode and the oxygen vacancy in CeO_xis further increased,so that the device cannot be switched to the high resistance state.It can be seen that the activity of the electrode has a great influence on the device.Therefore,the amount and distribution of the oxygen vacancies play a key role in the performance of the memristors.