Theoretical Study On The Effect Of Doping On The Properties Of Ferroic Materials And Devices

With the miniaturization and miniaturization of devices and the progress of nano synthesis and characterization technology,people can operate materials and devices on the atomic scale;Thanks to the development of nanotechnology,people can more easily adjust the properties and functions of nanomaterials and devices by various ways,such as doping,defects,changing the chemical ratio,and applying stress.Among them,the modulation of nanomaterials and devices by doping is not only introduced in various ways,including through ion implantation,thermal diffusion or in situ doping in the process of material growth,but also has a significant impact on the electrical,magnetic and transport properties of materials and devices.This paper focuses on advanced ferromagnetic（ferromagnetic,ferroelectric）functional materials and devices,analyzes its mechanism through theory and calculation,and explores the influence of doping on magnetism,ferroelectricity,structural stability and charge transport properties.The research was carried out from the following aspects:1.Band bending and resonant tunneling effect produced by doping in ferroelectric tunnel junctionThe general ferroelectric tunnel junction（FTJ）consists of a nano-thick ferroelectric thin film sandwiched between the electrodes and two electrodes.The reversal of iron electrode in ferroelectric thin films leads to the transition of FTJ between the low resistance state and the high resistance state,and leads to the tunneling electroresistance（TER）effect.FTJ is expected to be used in electronic storage devices,and has attracted wide attention due to its huge application potential.In this work,we use the quantum mechanical model to discuss four doping cases to introduce the resonance effect of ferroelectric polarization into the ferroelectric tunnel junction（FTJ）:oneδ Doped small bandgap dielectric layer,interface-doped dielectric layer with small dielectric constant,interface-doped dielectric layer with high potential barrier,and interface-doped pinned dipole layer.We demonstrate that the energy band offset,the dielectric layer with small dielectric constant,and the pinned dipole layer can cause a significant impact on the morphology of the tunneling barrier as ferroelectric polarization is reversed,thereby realizing the conversion between direct tunneling and resonant tunneling when the appropriate materials are used.This greatly improves the switch conductivity ratio GON/GOFF of ferroelectric tunnel junction,and the resonant tunneling effect also significantly reduces the resistance of the low resistance state（ON）.It provides a new idea for integrating FTJ with complementary metal oxide semiconductor（CMOS）electronic devices.2.Effect of oxygen vacancy and charge doping on the stability of ferroelectric tunnel junctionOxygen vacancy（VO）is a kind of widespread and easily migrating point defect,which controls many physical phenomena in perovskite oxide（ABO3）,such as metal-insulator transition,optical properties,magnetic properties,ferroelectric fatigue and imprinting.In the electrode-perovskite oxide-electrode structure,the electromigration of VO can produce resistance switching phenomenon,and the existence of VO is conducive to the formation of charged domain walls.In this work,we use the first-principle calculation to study the relationship between the formation energy of oxygen vacancies and the phase decomposition of ferroelectric oxides,explained the fatigue effect of ferroelectric tunnel junctions observed in experiments.Further,we study the effect of charge doping and the work function of the electrode on the VO formation energy in the[electrode/perovskite oxide/electrode]heterojunction.Because the electrode causes inevitable electron accumulation in the oxide near the interface,its size depends on the work function of the electrode,and ultimately affects the stability of the oxygen vacancy in the oxide near the interface.We found that the greater the work function of the metal electrode,the lower the VO formation energy at the metal/oxide interface.Our results are helpful to understand the fatigue effect in ferroelectric tunnel junctions,and provide ideas for improving the fatigue resistance of devices by using electrodes with low work function or perovskite oxide electrodes with strengthened electrical bonds.Our research results provide a comprehensive perspective for VO research,and we hope it can promote further research in this field.3.Effect of charge doping on magnetic properties of NiPS₃In this work,we use first-principles calculations to theoretically study the transition of NiPS₃ structure from antiferromagnetic to ferrimagnetic by electron doping.We argue that this is caused by the competition between the Stoner-exchange-dominated intermolecular ferromagnetic order and the super-exchange-dominated AFM order at different doping levels.Our research provides a feasible way to regulate the electronic structure and magnetic order of magnetic materials through doping,thereby achieving magnetoelectric effects.This paper is divided into five parts:The first part is the introduction of various means and ways to achieve doping;The control of doping on material properties and the application of physical phenomena caused by doping in device design;The second part discusses the interface doping in ferroelectric tunnel junctions and δ doping.The influence of doping on the transport properties is mainly discussed;The third part introduces the influence of doping on the stability of ferroelectric tunnel junctions;The fourth part discusses the influence of doping on the magnetic properties of NiPS₃ materials;The fifth part is summary and prospect.