Study On Defects In Perovskite Ferroelectric Oxides

Ferroelectric materials have spontaneous polarization,and the direction of polarization can be reversed with the change of the applied electric field.Based on this property,ferroelectric materials can be applied to sensors,capacitors,drivers,waveguide modulators,electro-optical devices and non-volatile ferroelectric memories,so it has great application value.Among them,perovskite oxide ferroelectric material is a typical ferroelectric material with rich structure,stability and excellent performance,which is widely used in the design of new electronic devices.However,various defects,such as vacancies,substitutions,doping,etc.,are unavoidable in perovskite ferroelectrics oxides.The generation of defects will have an important impact on the performance of the material,and the modulation of the material performance can be achieved through the control of defects.Therefore,research on defects is crucial for the development of the field of perovskite ferroelectric oxides and the application of the materials.However,there are still many questions about the effects of de fects in ferroelectric materials and devices that deserve to be explored.Oxygen vacancies,as one of the most common intrinsic defects in perovskite ferroelectric materials.have an important influence on the properties of perovskite oxides.BaTiO3 and PbTiO3,as two typical perovskite ferroelectric oxides,have certain similarities,while there are also obvious differences in the origin of polarization and electronic structure.For example,not only the displacement of Pb atoms is the main cause of polarization in PbTiO3,but more importantly.the lone pair of Pb atoms also plays an important role,yet no lone pair exists in BaTiO3.In this paper,a systematic computational study of oxygen vacancy defects in BaTiO3 and PbTiO3,including electron distribution,electron structure,and density of states,is carried out by means of first-principles calculations.The presence of oxygen vacancies can lead to electron doping in the oxide system.It is found that due to the presence of lone pair in PbTiO3,the distribution of doping electrons caused by oxygen vacancies can be more localized,creating localized energy levels in the band gap.In addition to the oxygen vacancies that can lead to electron doping in ferroelectric oxides,the presence of Nb plasma also leads to doping of free electrons in the system.Therefore,we also performed calculations for Nb-doped BaTiO3 and Nb-doped PbTiO3,and made a comparative study with the electron doping due to oxygen vacancies.In electronic devices based on ferroelectric oxides,the presence of oxygen vacancies can also have an important effect on the device performance.Therefore,the study of the distribution of oxygen vacancies in electronic devices and the effect of oxygen vacancies on the electronic structure and transport properties is of practical importance for the design and development of ferroelectric oxide electronic devices.In this paper,the influence of oxygen vacancy in ferroelectric tunnel junction on transport properties is further discussed through the method of first-principles calculation,and the effect of oxygen vacancy in Pt/BaTiO3/Pt and Pt/PbTiO3/Pt on transport properties is compared and analyzed.Oxygen vacancy is easy to form at the interface.When oxygen vacancy defects exist at the interface of Pt/PbTiO3/Pt ferroelectric tunnel junction,the doped electron distribution is more localized than that in the case of oxygen vacancy in the ferroelectric barrier region where BaTiO3 is used,thus affecting the polarization reversal and transport properties of the ferroelectric barrier region more significantly.In addition,when there is oxygen vacancy in the ferroelectric tunnel junction,the doped electrons caused by oxygen vacancy will be conducive to the resonant tunneling of the ferroelectric tunnel junction,so as to significantly improve the transmission rate of the ferroelectric tunnel junction.Through this study,the understanding of the effect of oxygen vacancy defects in perovskite ferroelectric oxides on the electronic properties of materials has been deepened,and the influence of oxygen vacancy on the transport properties of ferroelectric tunnel junction has been analyzed,which provides a reference for improving the transport properties of ferroelectric devices in experiments.