Study On The Resistive Switching And Magnetic Properties Of Ionic Liquid Pre-treated TiO₂ And TiO₂-Graphene Hybird Thin Films

Compared with conventional memory,resistive random access memory(RRAM)has the advantages of small size,low power consumption and simple structure.It is considered as one of the most competitive candidates for future non-volatile memory and has attracted lots of attention.Due to the easy preparation,good stability,large switching ratio,low cost and good compatibility with traditional CMOS processes,the metal oxide material has attracted more attention among various materials in the field of RRAM devices.In the past two decades,the room temperature ferromagnetism in metal oxides have been investigated.The latest research suggests that the magnetic properties of metal oxide films could be controlled by applied electric field,which provides new possibilities of multidimensional storage in RRAM application.However,there are still problems of metal oxide-based RRAM devices need to be solved,such as large operating voltage,requiring forming process,poor endurance properties and so on.In this dissertation,the ionic liquid(IL)pretreated TiO₂ film(IL-TiO₂)and graphene quantum dots(GQDs)doped TiO₂ film(TiO₂-GQDs)are fabricated as the dielectric layers respectively.The resistive switching behaviors and the accompanying magnetism change of the two TiO₂-based RRAM devices are systematically investigated,and the mechanism are discussed.The main results are as follows:(1)TiO₂ films with a thickness of about 170 nm were prepared by sol-gel spin-coating method.After that,the films were pretreated by ionic liquid under the electric field.Then the devices of Al/IL-TiO₂/Pt structure were fabricated with Al as the top electrode.Compared to the devices based on no IL pretreated TiO₂ films(Al/TiO₂/Pt),the Al/IL-TiO₂/Pt devices demonstrate forming free resistive switching behavior,and the advantages in low operating voltage,stable resistive switching behavior,and enhanced saturation magnetization.We found that the enhanced performance of Al/IL-TiO₂/Pt devices is ascribed to the large number of oxygen vacancies in the TiO₂ dielectric layer generated by ionic liquid pretreatment.(2)TiO₂ films implanted graphene quantum dots(TiO₂-GQDs)were fabricated on Pt substrates by sol-gel spin-coating method.The Al/TiO₂-GQDs/Pt devices were obtained by using Al as the top electrode.The device exhibits stable bipolar resistive switching behavior.After various measurements,we found the Al/TiO₂-GQDs/Pt devices have the following characteristics:forming free;1000 consecutive cycles with excellent stability at room temperature,and at least 300 consecutive stable cycles at 80°C.The magnetism properties of Al/TiO₂-GQDs/Pt devices in the low-resistance state and high-resistance state were measured.We found that the saturation magnetization in the low-resistance state was twice higher than that in the high-resistance state.After the XPS analysis of the film,we concluded that the graphene quantum dots implanted in the dielectric layer have played an important role in the improvement of device performance.The graphene quantum dots in the dielectric layer can enhance the local electric field under the applied of bias voltage.The enhanced local electric field can guide the formation and rupture of oxygen vacancy conducting filaments,resulting in better endurance properties of the device.(3)We had found that the magnetic properties could be controlled by applied electric field in both Al/TiO₂-GQDs/Pt and Al/IL-TiO₂/Pt devices.We believe that the change of magnetism under electric field is attributed to the increase of the oxygen vacancy concentration in the TiO₂film.According to the d⁰ ferromagnetism theory,the room-temperature ferromagnetism of metal oxide films originates from the oxygen vacancies in the films.Both implanting graphene quantum dot and ionic liquid pretreatment can generate more the oxygen vacancy in the initial state of the film.The increase of oxygen vacancy concentration enhances the room temperature ferromagnetism of the film.When the device is in low resistance state,more oxygen vacancies generated to form the oxygen vacancy conducting filaments,which resulting in the further increase in the magnetism.