First-Principles Study Of Oxygen Defect Behaviors In Rutile TiO₂ For Resistive Memory

Resistive memory technology employs electrical transition characteristics which happen in many oxides,solid electrolyte materials and organic materials.It is expected to open up a new generation of non-volatile information storage.It can be used in large-scale integrated circuits with both low power consumption and high speed.According to previous studies on the resistance switching mechanism,it was found that the working process of resistive random-access memory?RRAM?devices is closely related to the formation,connection and rupture of conducting filaments in dielectric materials.However,most of these studies have been inferred from experimental phenomena,and the microscopic physical mechanisms underlying RRAM need further exploration.Rutile TiO₂ as one of the binary metal oxides used in the dielectric layer of RRAM is also the material used to study resistive mechanism in the early stage.It was reported that oxygen vacancies in TiO₂ are strongly related to conducting filaments.Moreover,conducting nanofilaments were composed of an anoxic phase.Therefore,in order to in-depth understand switching mechanism from a microscopic perspective,we research behaviors of oxygen-related defects in rutile TiO₂,including the distribution,the stable structures,the property and the dynamic evolution,with the help of first-principles calculations.The specific research contents and conclusions are as follows:1.The distribution trend of oxygen vacancies in TiO₂.At first,multiple defect models with different numbers and distributions of oxygen vacancies were constructed,and the formation energies of these models were calculated under different experimental conditions.We proposed that results of vacancy formation energies were affected by exchange correlation function.Then,we verified conclusions used higher precision hybrid function.It was confirmed that oxygen vacancies are dispersed with non-neighboring lattices in rutile TiO₂.In addition,regardless of whether the shape and volume of cells are changed during structural relaxations,vacancy formation energies were not affected through comparison.We give the recommended exchange correlation function for calculating oxygen defects of TiO₂ based on the above studies.PBE is more suitable after considering the accuracy and cost for the related calculations.2.Oxygen defect behaviors and dynamic processes in rutile TiO₂.We calculated the formation energies of structures with oxygen intrinsic defects.The stable defect structures of rutile TiO₂ were summarized.Oxygen interstitials in TiO₂ are stable by pushing away adjacent oxygen atoms.According to the sensitivity of Frenkel defects?which coexisted oxygen interstitial and vacancy?to vacancies,they are classified into V_O-independent Frenkel defect and V_O-dependent Frenkel defect.Selecting V_O-independent Frenkel defect for molecular dynamics?MD?simulations,oxygen interstitials move by“kick-off”behavior?see Chapter 4 in paper for more details?which is a method of pushing the adjacent oxygen atom out its position and moving one by one.We reveal the dynamic image of oxygen interstitial in TiO₂:the movement of oxygen interstitial is divided into the electric field driving process whose motion direction is determined by electric field and the vacancy-nearby recombination process that oxygen interstitial and vacancy recombines when the oxygen interstitial atom comes near the oxygen vacancy.When the oxygen interstitials are far from the oxygen vacancies in the electric field,the direction of their movement is controlled by the external electric field.When the oxygen interstitials come near the oxygen vacancies,they will be captured by the low energy valleys of oxygen vacancies and the Frenkel defects recombine even without electric field.Furthermore,it is verified from the atomic scale that the annihilation process of oxygen vacancies in rutile TiO₂,that is,the reset process of TiO₂-RRAM is easier than the set process.To summarize,we have studied the oxygen-related defects in the prototypical resistive-switching material TiO₂ by first-principles calculations from the atomic scale in order to further reveal the switching mechanisms.In this paper,the revealed dynamic processes of oxygen interstitials and the relationship between defect recombination and resistive switching of devices can benefit to understand the microscopic mechanism in RRAM.We also expect that the studies here can offer knowledges to design other high-performance RRAM materials in the near future.