Point Defects In Metal Oxides And Their Influences On The Resistive Switching

Perfect crystals do not exist in nature;a variety of defects present more or less in crystals.In all kinds of defects,point defects are the most important ones since the scientific issues concerning point defects falls into the category of equilibrium thermodynamics.Defect chemistry is a kind of science that focues on the point defects in materials and their influence on the physical properties of materials.As materials with perovskite structure,strontium titanates(SrTiO3)and barium titanates(BaTiO3)are ideal model systems for the defect chemsity research in metal oxides,owing to their extensive application as electrical ceramics,capability of being structurally stable even after being doped with various dopants with relatively high doping concentrations.In this work,the defect chemistry of metal oxides is intensively investigated based on the model materials of SrTiO3 and BaTiO3.A general model for the defect chemistry of metal oxides is proposed.Based on the model,a suite of software for the numerical simulation of the defect chemistry of metal oxides is developed.In light of the motto that practice is guided by theories,the roles of point defects in the physical properites of metal-oxide-based memristors(Pt/BiFeO3/Pt,Pt/WO3/Pt and Pt/LLTO/Pt)are widely studied,and the significance of the defect chemistry in the improvements of physical properties of metal-oxide-based devcies is disclosed.1.The investigation of the defect chemistry of metal oxides based on SrTiO3 and BaTiO3 model materials.The defect chemistry reactions,mass action laws and electroneutrality condition for accepter and donor doped SrTiO3 and BaTiO3 at different oxygen partial pressures(10-25?1 bar),temperatures(>room temperature),types of dopants(Fe,Al,Ni,Mg,Mn,Nb and La)and dopant concentrations are discussed and studied.The variations of the concentration of the point defects and partial/total electrical conductivity with oxygen partial pressures and temperatures are demonstrated.The general defect chemistry model for metal oxides at different temperatures and oxygen partial pressures is proposed.2.Design and development of a suit of defect chemistry calculation software based on the general defect chemistry model for metal oxides.After intensive investigation of the defect chemistry of metal oxides,a mathematical model is proposed.The functional requirement analysis of the software requires the overall architecture of software to be divided into four modules:the user interface(UI),database,parallel-computing-based core algorithm and data post-processing module.The UI is designed and developed via Qt,a C?? graphical user interface development framework.The implementation of the database is via the structure function in the form of dynamic link library(DLL).Parallism of the core algoritm is realized by the application programming interfaces(APIs)of OpenMP.The corresponding numerical solution algorithm for the model,the improved Brent method,is employed.Double buffering technique is used in the visualization of the calculated results.Tree-like data hierchy is employed for the data storage in the post-processing.For the validation of the software,comparisons of the calculated results with the representive experimental results from literatures are made,which then demonstrate the validaty of the software.3.The fabrication of Pt/BiFeO3/Pt memristors and the study of the resistive switching properties.The Pt/BiFeO3/Pt memristive devices are prepared via sol-gel method and spin-coating.In the investigation of the resistive switching behavior,the coexistence of analog and digital resistive switching is found in a single device,which opens up a way for the design of multi-functional memristors.Moreover,the decisive role of the point defects(oxygen vacancies)in the coexistence of the two types of resistive switching phonenoma is demonstrated.4.The fabrication and resistive switching of Pt/WO3/FTO memristors.The Pt/WO3/FTO devices are fabricated via magnetron sputtering.In the study of the resistive switching behavior of the Pt/WO3/FTO memristors,the abnormal pseudo-electroforming is found.After deep mining of the pseudo-electroforming process,the devices are capable of being used as "intelligent" time-delay switches in neuromorphic computing.Furthermore,the dynamic kinetics of oxygen vacancies at the WO3/FTO heterostructures is proved to be responsible for the appearance of the pseudo-electroforming and the application of the devices in neuromorphic computing.5.The fabrication and resistive switching of Pt/LLTO/Pt memristors.LLTO(lithium lanthanum titanate)is a widely used and promising material for lithium ion batteries.However,the resistive switching behavior of LLTO-based memristors is rarely reported.In this work,the Pt/LLTO/Pt memrisitive devices are implemented via sol-gel method and spin-coating.The resistive switching behavior and the diffusion kinetics of cations(vacancies)are investigated in the Pt/LLTO/Pt structure.Through the modulation of the cations(vacancies),the psychological concept of habituation,which is the fundamental and ubiquitous form of behavioral plasticity,is succefully emulated with the device.In particular,the nine behavioral characteristics of habituation are fully realized.As a kind of "top-down" approach,the realization of habituation inaugurates a way for the building of neuromorphic networks.Moreover,the direct emulation of habituation with physical devices represents an advance in the implementation of neuromorphic devices.