Research On The Resistance Switching Of CoFeB And HfO₂ Containing Multilayer Films(M.S.Dissertation)

With the wide development of micro nano devices,especially after the development of flash memory devices meets the bottleneck,it is urgent to develop a new generation of nonvolatile memory technology.Resistive random access memory(RRAM)uses the change of resistance state in some thin film materials to store information.Compared with flash memory devices,resistive memory has better miniaturization characteristics,because only the arrangement of electronic structure in thin film materials is affected by external magnetic field,electric field,temperature and other conditions.Resistive memory has the characteristics of fast read-write speed,low fabrication cost,low energy consumption,environmental protection and 3D integration.It is a strong competitor for the new generation of nonvolatile memory candidate products.According to the reported resistive layers with resistance switching characteristics,they can be divided into three categories:oxide resistive materials,organic resistive materials and solid electrolyte resistive materials.The devices made of metal oxide resistive materials have attracted the attention of researchers because of their good miniaturization,simple structure and good compatibility with complementary metal oxide semiconductor(CMOS)circuits.This paper mainly studies the characteristics of resistance switch based on CoFeB and HfO₂ multilayer films,and elaborates the preparation of thin film devices,measurement of electrical properties,I-V curve analysis,mechanism interpretation of resistance switch phenomenon and the regulation of light,etc.the main contents and conclusions are as follows:1.This paper introduces the research background,status and characteristics of RRAM,and expounds the classification of resistance switch phenomenon,the working mechanism and mechanism explanation of resistance switch phenomenon.2.The multilayer films containing CoFeB were prepared by magnetron sputtering on ITO.Two kinds of Ag/BiFeO₃/CoFeB/BiFeO₃/ITO thin film devices and Ag/BiFeO₃/CoFeB/ITO thin film devices with different thickness are studied.The I-V curves of three kinds of thin film devices are measured by Keithley2400 system,and the regulation of light on the thin film devices is studied.The conclusions are as follows:(1)The characteristics of bipolar resistance switch are obvious in Ag/BiFeO₃/CoFeB/BiFeO₃/ITO thin film devices under the condition of darkness and light intensity of 2.2mw/cm².In the continuous switching period,the resistance switch is relatively stable.White light has a regulatory effect on the resistance switching phenomenon(current,switching ratio and persistence)of thin film devices.The reason is that the white light irradiates the device,making the photogenerated electrons formed in the device participate in the capture and release of charge by oxygen vacancies and defects.The bipolar resistance switch characteristics of Ag/BiFeO₃/CoFeB/BiFeO₃/ITO devices can be explained by the capture/release mechanism of oxygen vacancy in the Schottky depletion layer between the interface of the devices.(2)The threshold resistance switch characteristics of Ag/BiFeO₃/CoFeB/BiFeO₃/ITO thin film devices with varying thickness show obvious threshold resistance switching characteristics under the conditions of dark and light intensity of 2.2mw/cm².White light has a regulatory effect on the resistance switching phenomenon(switch ratio and persistence)of thin film devices,The reason is that white light produces more Ag ions inside the device and forms conductive filaments.The threshold resistance switch characteristics of Ag/BiFeO₃/CoFeB/BiFeO₃/ITO thin film devices show obvious threshold type resistance switching characteristics.The mechanism of cation migration in the theory of conducting filaments can be used to explain.(3)The characteristics of bipolar resistance switch are shown in Ag/BiFeO₃/CoFeB/ITO thin film devices under the condition of darkness and light intensity of 1.2mw/cm².In the continuous switching period,the resistance switch ratio is relatively stable.White light has a regulatory effect on the resistance switching phenomena(current,switching ratio and durability)of thin film devices.The resistance switch characteristics of Ag/BiFeO₃/CoFeB/ITO thin film devices should be derived from the mechanism of electron capture/release by oxygen vacancy in the sample.3.The multilayer devices containing HfO₂ were prepared by magnetron sputtering in n-type conductive single crystal Si.Three thin film devices,Si/C//HfO₂/C/Ta,Si/C/HfO₂/BiFeO₃/Ta and Si/C/HfO₂/Al₂O₃/Ta,were studied.The I-V curves of three kinds of thin film devices are measured by Keithley2400 system,and the regulation of light on the thin film devices is studied.The conclusions are as follows:(1)Under dark conditions,Si/C//HfO₂/C/Ta thin film devices exhibit bipolar resistance switching characteristics.The mechanism of charge capture and release by oxygen vacancy and defect in the depletion layer of Schottky barrier is used to explain the resistance switch phenomenon of Si/C//HfO₂/C/Ta thin film devices.(2)Under the condition of darkness and illumination intensity of 0.8mw/cm²,Si/C/HfO₂/BiFeO₃/Ta thin film devices show obvious bipolar resistance switching characteristics.White light has a regulatory effect on the resistance switching phenomenon(current,switch ratio and persistence)of thin film devices,The reason is that the white light irradiates the device,making the photogenerated electrons formed in the device participate in the capture and release of charge by oxygen vacancy and defect.The resistance switch of Si/C/HfO₂/BiFeO₃/Ta thin film devices is explained by the mechanism of charge capture and release by oxygen vacancy and defect in Schottky barrier depletion layer.(3)Under the condition of darkness and illumination intensity of 0.8mw/cm²,Si/C/HfO₂/Al₂O₃/Ta thin film devices show obvious bipolar resistance switching characteristics.The white light optimization enhances the resistance switch phenomenon(current size,switch ratio and durability)of thin film devices,The reason is that white light irradiates the device,making more positively charged oxygen vacancies appear in the device and forming conductive filaments.The resistance switch characteristics of Si/C/HfO₂/Al₂O₃/Ta thin film devices can be explained by the theory of anion transfer of conductive filaments.