Resistive Switching Of CeO₂-based Nanoparticle Thin Films

There are various categories of resistive random access memory(RRAM)materials,such as solid electrolyte materials,carbon resistance materials,organic materials,complex oxide materials and binary metallic oxide materials.Recently,based rare earth oxide RRAM has also been widely researched for these rare earth elements have a wealth of electronic structure,unique 4f electronic shell structure,electron energy and speical light,electrical and magnetic properties.Given on that base of nanocrystallization the rare earte material,it can maintain original nature properties,besides can have more excellent characteristics of nanomaterials,such as small size effect,surface interface effect,quantum size effect and strong photo electromagnetic acoustic properties.CeO2 has been widely used in industrial application for its speical properties.The oxygen vacancies during the CeO2 can easily formation and dissipation due to the valence reversible transformation between Ce3+ and Ce4+,and the Ce ions have strong conductivity as same as the oxygen vacancies.The switching mechanism of the metallic oxides RRAM can largely be ascribed to the formation/rupture of filamentary psths due to the filed-induced migration of oxygen vacancies and oxygen ions.In this paper,we fabricated the CeO2 thin film by sol-gel and the resistance characteristics were investugated through different annealed temperatures.The CeO2 nanoparticles,CeO2-ZrO2 and CeO2-ZrO2 composite nanoparticles were prepared using hydrothermal method.The influence of hydrothermal condition,composite ratios and interface microstructure on the resistive characteristics was investigated.The specific contents of the dissertation are as follows:First,the CeO2 thin films prepared by sol-gel method correspond to cubic phase.The crystallinity getting better with the annealed temperature increased and particle size of 7-50 nm.I-V characteristics of these films show stable bipolar resistance switching phenomenon but the ON/OFF ratio is decreased to about 10 and the set voltage increased with the annealed temperature.Then,the CeO2 nanoparticles correspond to cubic phase with different pH prepared by hydrothemal through accommodating different hydrothermal conditions.X-ray diffraction peak intensity increased with the pH.The film of pH=8 not only has high ON/OFF ratio but aslo has low threshold voltage within all films.We found the baking temperature has no effect on the CeO2 nanoparticals with pH=8,but with the baking temperature increased,the CeO2 peak intensity has also increased.The ON/OFF ratio firstly increased and then droped with the baking temperature increased.For the CeO2-ZrO2 composite materials,the composite nanoparticals have solid solution reaction with different pH at the ratio of Ce/Zn=1:1.With the pH from 2 to 12,the reaction increased with Ce0.5Zr0.5O2 and Ce0.2Zr0.8O2.Among these composite nanoparticals thin films,the fil with pH=8 not only have higher ON/OFF ratio but also has the lower threshold voltage.The slope indicates that transport mechanism change from TCLC to SCLC,the switching mechanism can be attributed to SCLC and oxygen vacancies.We found the baking temperature has no effect on the CeO2 nanoparticals with this pH,but with the baking temperature increased,the CZO peak intensity has also increased.The switching mechanism can be attributed to the conductive filaments.For the CeO2-ZnO composite materials,considering the best hydrothermal conditions above,we study the influences of layers about switch characteristic.I-V characteristics of these films show stable bipolar resistance switching phenomenon of different Ce/Zn ratio.The threshold voltage change little and the ON/OFF ratio is about 6000.XRD shows the composite nanoparticles structure has not change lot with the Zn proportions increased.The slope indicates the transport mechanism conform to the SCLC and has no effect on the layers.The switching mechanism can be attributed to the interface and oxygen vacanciesIn summary,reproducible resistive switching is observed in the as-fabricated CeO2 nanostructures.The compound of ZrO2 or ZnO can significantly decrease the set voltage of CeO2 nanoparticles.This research shows that the switching properties of CeO2 can be improved by controlling hydrothermal conditions,optimizing device structure and the size of device etc.This paper can provide useful information for optimizing CeO2 memory devices.