Study On The Resistance Switching By The Ferroelectric Polarization Of The La_xBi_1-xFeO₃

In the current age of information technology,memories play an indispensable role.Compared with several other memories,ferroelectric non-volatile memories have significant advantages in terms of switching speed,resistance ratio,retentivity and cyclicity.Because of the in-depth research of various ferroelectric materials and the growing maturity of ferroelectric heterojunction technology in recent years,ferroelectric memory has become one of the most non-volatile memories with application value.As a typical multi-ferrous material,the Curie temperature and Nair temperature of BiFeO₃?BFO?are Tc?1100 K and TN?640 K,respectively.Moreover,BFO is one of the most concerned ferroelectric materials due to its excellent ferroelectric properties,structural tunability and photovoltaic characteristics.However,the narrow band gap of BFO will cause higher leakage current,which may reduce the performance of its ferroelectricity and interface barrier-related characteristics,and then affect the performance of the device.So far,many methods have been used to overcome the high leakage current problem of BFO ceramics,among which the ion substitution method is favored by researchers.Adding rare earth elements in BFO ceramics can change the phase structure,which can effectively improve its electrical properties.Since La³⁺ and Bi³⁺ have similar radii of 1.032 ? and 1.030 ?,it has been confirmed that doping of La at the Bi site can not only effectively avoid the formation of Bi vacancies,but also improve crystallization and stabilize crystal structure.The substitution of La³⁺ for Bi³⁺ improves the ferroelectric properties and structure of BFO,such as coercive voltage,domain size and fatigue behavior.Therefore,LBFO is selected as ferroelectric layer to construct devices with more advantages.In this thesis,heterostructures with BFO as the ferroelectric layer was prepared,and the experimental results show that its performance is not excellent enough.Then,the properties of the BFO film are improved by doping La elements with different concentrations to achieve the purpose of improving device performance.In the experiments,we used micro and macro testing methods to study the surface morphology and ferroelectricity of the thin film,and measured the switching resistance behavior of devices composed of thin films.The main research contents are as follows:1.BiFeO₃ targets and La_xBi_1-xFeO₃?x = 0.05,x = 0.10,x = 0.15,and x = 0.20?targets were successfully prepared by using the solid-phase reaction method.The effects of solution etching time,annealing temperature and annealing time on the surface morphology of the substrate during the etching process were discussed.The NSTO substrate with Ti O2 layer as the termination surface was obtained.2.The surface morphology and ferroelectric properties of the BFO film were studied by using atomic force microscopy and piezoelectric force microscopy,and BFO film with flat and dense surface and good ferroelectricity was obtained.Then,Keithley 2400 was used to measure the I-V,R-V and retention cycle characteristics of the Pt/BFO/NSTO heterojunction.The heterostructure shows stable bipolar resistance switching behavior,multi-level storage characteristics,excellent retention characteristics,and the maximum resistance OFF/ON ratio of room temperature pulse reading is 8.8×103.It is also proved that the resistance change effect is due to the modulation effect of the ferroelectric polarization reversal on the width of depletion region and the height of potential barrier of the BiFeO₃/Nb-doped Sr Ti O3 interface.3.Since the Pt/BFO/NSTO heterojunction switching ratio is not large enough,we improved the properties of the BFO film by doping La elements with different concentrations.Scanning probe microscope proved that La0.10Bi0.90 Fe O3 film has strong ferroelectricity and low coercive field.By optimizing the thickness of La0.10Bi0.90 Fe O3,the Pt/LBFO?6.2 nm?/NSTO heterostructure not only shows stable bipolar resistance switching behavior,multi-level storage characteristics,excellent retention characteristics,but also the maximum resistance at room temperature pulse reading OFF/ON ratio is as large as 2.8 × 105.At the same time,through the micro and macro electrical properties of the heterojunction and the analysis of its internal conduction mechanism,it is proved that the observed resistance switching behavior is attributed to the modulation effect of the ferroelectric polarization reversal on the width of depletion region and the height of potential barrier of the La BiFeO₃/Nb-doped SrTiO₃ interface.