Study On The Ferroelectricity And Memristive Effect In Doped BiFeO₃ Thin Films

BiFeO₃（BFO）is the only lead-free single-phase multiferroic material that exhibits simultaneously ferroelectric,antiferromagnetic and ferroelastic properties at room temperature.It has been widely used in the fields of spintronics,information storage and optoelectronic technology.The ferroelectric Curie temperature of BFO is 1100 K and the antiferromagnetic Neel temperature is 640 K.Compared with other ferroelectric materials,BFO has higher spontaneous polarization intensity（P_s=90-100μC/cm²）and smaller band gap width（2.2-2.8 e V）.These characteristics enable it to be used in non-volatile memory,photovoltaic devices,and microelectron-mechanical systems.However,Bi volatilization and Fe valence change of BFO lead to high leakage current,which may affect the ferroelectric properties and interfacial barrier related characteristics,thus affecting device performance.Since the ion radius of rare earth metal elements is close to that of Bi³⁺,and the radius of transition metal ion is close to that of Fe³⁺,it is easy to be doped.Therefore,doping of rare earth elements at A site,doping of transition metal elements at B site,or co-doping at site A and B sites are favored by researchers.Gd³⁺doping can inhibit the volatilization of Bi³⁺and improve the polarization intensity of BFO,while Ni²⁺doping can inhibit the valence change of Fe³⁺and reduce the oxygen vacancy In this thesis,Gd³⁺and Ni²⁺ions were used to modify BFO thin films combined with the research status and experimental conditions.Bi_（1-x）Gd_xFe O₃（x=0.00,0.02,0.04,0.06,0.08,0.10）films were prepared on Nb:Sr Ti O₃（0.7 wt%）（NSTO）single crystal substrate.The 5%of Ni²⁺element introduced on B sites in the Gd³⁺doped BFO film exhibits the best ferroelectric and memristive properties.It was found this co-doping technique is beneficial to improve the performance of the device.And Bi Fe_0.95Ni_0.05O₃（BFNO）was prepared as reference.Finally,the microstructure,piezoelectric response,ferroelectric properties,storage properties and storage mechanism of thin films are systematically and deeply studied.The main research results are as follows:（1）Combined with AFM,the optimum experimental conditions of BFO film were obtained by changing the temperature and oxygen pressure during the BFO film preparation.Pulsed laser deposition system was used to prepare the Bi_（1-x）Gd_xFe O₃（x=0.00,0.02,0.04,0.06,0.08,0.10）,Bi Fe_0.95Ni_0.05O₃（BFNO）and Bi_0.92Gd_0.08Fe_0.95Ni_0.05O₃（BGFNO）films with above corresponding targets and obtained growth conditions.The films were measured with X-ray diffractometer（XRD）and Scanning Probe Microscope（SPM）,the results showed that the XRD peak position of the film shifts to a higher Angle,and the film quality and ferroelectric properties improved with the increase of the atom doping ratio.（2）The piezoelectric response and ferroelectric properties of the Bi_（1-x）Gd_xFe O₃（x=0.00,0.02,0.04,0.06,0.08,0.10）thin films were measured by piezoelectric force microscope（PFM）and Positive-Up-Negative-Down（PUND）measurement,which showed that the films had good piezoelectric response and highest remanent polarization with 8%Gd doping.And the I-V tests of the Au/Bi_（1-x）Gd_xFe O₃（x=0.00,0.02,0.04,0.06,0.08,0.10）/NSTO devices were carried out with Keithley 2400 Source Meter.The results showed that the Au/Bi_（1-x）Gd_xFe O₃（x=0.00,0.02,0.04,0.06,0.08,0.10）/NSTO devices have bipolar resistance switching behavior,and the devices have maximum storage window（2.8×10³）when the Gd doping ratio is 8%.（3）The remanent polarization of BFO film is further improved by co-doping 8%Gd at site A and 5%Ni at site B,the OFF/ON ratio of the corresponding device is 3×10⁶ at room temperature pulse.The results show that Au/Bi_0.92Gd_0.08Fe_0.95Ni_0.05O₃/NSTO device has stable bipolar resistance switching behavior,multistage storage characteristics,and good retention performance.There is no significant change in ON and OFF states for about 10 h.At the same time,the micro and macro electrical properties of the devices and the analysis of mechanism proved that the cause of resistance switching behavior could be attributed to the modulation effect of ferroelectric polarization on the depletion layer width and barrier height at Bi_0.92Gd_0.08Fe_0.95Ni_0.05O₃/NSTO interface.