Study Of Electronic Structure And Transportation Behavior Of Defects In Rutile TiO₂

Resistance switching phenomenon is a hot topic in the field of device physics research.It shows that under the excitation of external electric field,the resistivity of insulating dielectric material changes reversibly.For some special structure of resistive switching device,the resistivity can remain at a stable level after the excitation disappears.Based on this property,it will become possible to develop a binary or multi-valued nonvolatile memory device with resistivity as an information storage carrier.At present,a variety of resistance switching mechanism models have been proposed in related research literatures,trying to describe the physical behavior in devices from the aspects of composition,morphology and charge transport of conductive pathways in insulating dielectric materials.However,most of the existing models still remain at the macroscopic phenomenological level.For a better understanding of resistance switching phenomenon and improvement of the devices,a microscopic explanation at atomic level is needed.Aiming at the problems in the research of resistance switching phenomenon,this paper systematically studies the electronic structure and transport properties of defects in rutile titanium dioxide based on density functional theory framework.The main contents are as follows:1.Study of the band structure of rutile with DFT+U correction.The standard DFT method has self-interaction error in the processing of electronic correlation problems,which is modified by DFT+U in this paper.In order to obtain the appropriate parameter U,the influence of different U parameters on the calculation results is evaluated by the“carpet" calculation method.Then the values of the U parameters are determined by combining the linear response method and the empirical parameter setting.The calculation results show that with a 4.5 eV correction for 0-2p orbital and 4.0 eV correction Ti-3d will,the calculated band structure and lattice constant can match the experimental results well.2.Study of the polaronic configurations of the intrinsic defects in the rutile structure.Using the DFT+U correction,we calculated the band structure and carrier polaronic configurations of the intrinsic defects in rutile.It is shown that the two intrinsic defects of oxygen vacancies and titanium interstitial can form electron polarons,while titanium vacancy defects can form hole polarons,and the oxygen interstitial does not exhibit spin polarization characteristics.In addition,the localized position of carrier in polaronic configuration is related to the initial structure of the ion relaxation process.In polaronic structure,it is found that when an electron is trapped at one Ti grid to form a polaron,the six-coordination titanium-oxygen bonds around the titanium ion will elongate Based on this property,we manually adjust the initial atomic structure to guide the excess electrons in the defect structure trapping at different positions.In addition,the oxygen vacancy defect in different supercell size is calculated.It is found that the concentration of oxygen vacancies affects the Coulomb interaction parameter U and further causes the most stable polarization structure to change from small polaronic states to mixed polaronic states with the defect concentration.This calculation is consistent with the spectral experimental data in the literature.3.Study of the transportation properties of intrinsic defects in rutile.Using CI-NEB method,we modeled the process of Frenkel defect formation of Ti and O interstitials.It is found that the formation energy of Ti interstitial(3.97 eV)is smaller than O interstitial(4.8 eV).Then,the transition states of four intrinsic defects along different paths in the crystal lattice is calculated.The results show that Ti diffusion along[001]direction has the lowest energy barrier of 0.5 eV.4.Study of the occupation position and diffusion barrier of doping elements in rutile.By relaxing the structures of dopants at different positions in the rutile lattice,it is found that the most stable occupation site is closely related to the dopants'electronegativity and ionic radii.For metal,with increasing of ionic radii,the occupation position of dopants tends to change from empty oxygen tetrahedral center to octahedral center and finally replace Ti at grid.In addition,the CI-NEB method is used to search the transition state of different dopants diffusing along the[001]direction.The results show that N\Cr\Mn\Tc have the largest barrier,which indicates the energy barrier of the transition state is mainly affected by the number of valence electrons.In addition,the increasing barrier height of alkali metal also reveals an influence of ionic radii.