Resistive Switching Behavior In BiFeO₃ Epitaxial Thin Films

The ferroelectric resistive random access memory combines the advantages of ferroelectric memory and resistive random access memory.It attracts widespread attentions not only because of the advantages of simple structure,fast operation,low power consumption,but also the non-volatile properties in nanometer size,high density and stable storage ability.In this thesis,room temperature single-phase multiferroic BiFeO3(BFO)thin films were prepared by pulse laser deposition(PLD).The resistive switching behaviors of BFO thin films under different electrodes,polarization directions and strain states were studied,and the resistive switching mechanism was also investegated.The main contents are as follows:1.BFO thin films were prepared on(001)-oriented single crystal SrTiO3(STO)substrate by PLD.We designed three heterostructures with various bottom electrodes:Au/BFO/Nb-SrTi03(NSTO),Au/BFO/La2/3Sr1/3MnO3(LSMO)/STO,and Au/BFO/SrRu03(SRO)/STO.The XRD and RSM results show that all the films are epitaxially grown on the single crystal substrates,and fully strained by the substrates.The PFM results show that the initial polarization directions of BFO thin films are different in the three heterostructures:in Au/BFO/SRO/STO,the polarization point downward to bottom electrode,while in Au/BFO/NSTO,Au/BFO/LSMO/STO,the polarization point upward to top electrode.The I-V curves of BFO films indicate that the three heterostructures exhibit unipolar and bipolar resistive switching behaviors,respectively.And the I-V characteristics can be consistently stable after 10 times scan,the RoN/ROFF rate was also consistently steady after 200 times continuous testing.The analysis shows that the resistive switching of Au/BFO/SRO/STO mainly was affected by the conductive filaments mechanism,and the resistive switching behavior of Au/BFO/LSMO/STO heterostructures mainly be modulated by the ferroelectric polarization,while both of the mechanisms may work together in Au/BFO/NSTO.2.High-quality ultra-thin BFO thin films were prepared by PLD on(001)oriented STO substrate with LSMO as the bottom electrode.The XRD and RSM analysis show that all the films were epitaxially grown and fully strained along the orientation of substrate.The AFM result shows that the surface of BFO film is smooth and the BFO thin film has good ferroelectric properties from the PFM measurements.The TEM results show that the thickness of BFO ultra-thin film is 6 nm,and the interface is flat and clear.No defects can be detected.At different polarization states,the I-V curves show different characteristics:when the polarization is pointing downward,the I-V curves show the negative diode characteristic,and the diode rectification characteristics weaken as the voltage increase;when the polarization is pointing upward,the I-V curves show the positive diodes behavior and the diode rectification characteristics increase with the voltage increasing.The analysis of the electrical transport mechanism shows that the conduction mechanism in BFO film is Schottky thermal emission mode.3.By changing the strain states of BFO thin film,the effect of strain on the resistive switching behavior of BFO films were studied.BFO epitaxial films with thicknesses of 50 nm,100 nm,and 150 nm were fabricated by PLD on LSMO buffered(001)-oriented STO single crystal substrate.The XRD results show that all BFO and LSMO films grow along the substrate orientation.As the thickness of the BFO film increases,the out-of-plane tensile strain of BFO thin films change from 2.78%to 0.76%.The ferroelectric hysteresis loops show that the remnant polarizations of BFO films increase and the coercive fields decrease as the film thickness increase.The dielectric constants increase significantly with the thickness of BFO thin films increase,while the dielectric loss decrease with the increase of thickness.As the strain decreases,the leakage current of BFO thin films gradually decreases,the asymmetric rectifier of I-V curves disappear,and the RON/ROFF of BFO films decrease from 30 to 12.However,under the external stress,the resistive switching behaviors of BFO thin film have a good stability.