The Resistive Switching Effect And Charge Trapping Phenomena Of BiFeO₃ Thin Films

The random access memory based on resistive switching effect has received noticeable attention because it possesses high performance with low operating voltage,fast reading and writing speed, which is a promising nonvolatile memory for a new generation. Currently, the questions such as the physical mechanisms and storage mechanisms of the resistive switching are still hot issues. BiFeO₃（BFO） is a typical multiferroic material at room temperature, whose resistive switching effect has been found in both bulk and thin films. In this work, we investigated the properties of epitaxial and polycrystalline BiFeO₃ thin films prepared by pulsed laser deposition（PLD） respectively. The microstructure was studied by X-ray diffraction（XRD） and atomic force microscopy（AFM）, the simultaneous imaging method of current atomic force microscopy（CAFM） and pizeoresponse force microscopy（PFM） was discussed.The metal/BFO/metal resistance switching structure was fabricated, and the low temperature resistive switching effect and charge trapping phenomena of epitaxial and polycrystalline BiFeO3 thin films were discussed by variable temperature, variable frequency, and pulse voltage trains. The resistive switching was explored when the graphene worked as top electrode on BFO. The following contents are included:1. The epitaxial and polycrystalline BFO thin films were grown on different substrates by PLD. The microstructures were analysed by means of XRD and AFM,epitaxial thin film with c-axis orientation has atomic level smoothness, while polycrystalline thin film has multiple orientations of（100） and（110） and the average grain size is about several hundred nanometers. The ferroelectric properties were examined by PFM, representing that the as grown domains are randomly distributed and can be switched by external electric filed. The simultaneous PFM and CAFM images indicate that the polarization and conductance of epitaxial BFO thin film are spatially correlated and that the conductance of polycrystalline BFO thin film has nothing to do with polarization in space, which has a random distribution.2. The low temperature resistive switching effect and charge trapping phenomena of BFO/SRO epitaxial structure were discussed. The results exhibit stable rectifying effect and resistive switching behavior in Au/BFO/SRO structure, the device shows endurance with the ON/OFF ratio up to two orders of magnitude of 104 cycles andretention of 105 s at room temperature. Electrical pulse can trigger the device into multilevel resistance state at a certain pulse voltage. The resistive switching and additional capacitance peaks of the device disappear when the temperature is blow 140 K. By fitting the low temperature conduction mechanism and theoretical analysis, the resistive switching of Au/BFO/SRO is ascribed to the combined effects of ferroelectric polarization, electromigration of ionized oxygen vacancies and the occupation of interface traps. The additional capacitance peaks origin from the trapping or detrapping effect of the trap levels at the metal/ferroelectric interface.3. The BFO/Pt polycrystalline structure also shows the rectifying effect and resistive switching behavior, however, whose current hysteresis curves are quite different with BFO/SRO structure and have much smaller resistance switching ON/OFF ratio. The structure has reliable resistance switching performance at room temperature,the device remains stable of 700 cycles and of 105 s retention. By analysis, the rectifying effect and resistive switching characteristics of the polycrystalline device can be ascribed to the Schottky junction at interface. The disappearance of hysteresis in I-V and C-V curves at low temperature suggests the combined influence of Schottky junction and nature in ferroelectricity.4. The resistance switching was explored when few layer graphene（FLG） was adopted as top electrode. The resistance switching of FLG/BFO/SRO is quite different with Au/BFO/SRO structure, the experiments show that the current is about pA magnitude and the coercive fields for polarization reversal are about ±4 V when the AFM tip is located on BFO directly; while the current is about nA magnitude and the coercive fields for polarization reversal are about +2 V and-0.5 V when the AFM tip is located on FLG, what’s more, the piezoelectric response on FLG is detected much stronger than on BFO. However, the ON/OFF ratio of FLG/BFO/SRO is only about 5,which is much smaller than M/BFO/M structure with the value of dozens or hundreds.