Investigation Of The Properties And Mechanism Of BIFEO₃ Thin Films For Resistive Random Access Memory

Resistive random access memory?RRAM?has been recognized as one of the next generation non-volatile memory due to its high storage density,low power consumption,simplicity of the structure and compatibility with the CMOS process etc.As one of the resistive effect material,Bi FeO₃ has attracted worldwide attention due to its high ratio of resistance and volatile,and with ferroelectric,pyroelectric,piezoelectric,dielectric electro-optic properties of magnetoelectric coupling properties at the same time.However,the underlying physical mechanism of resistive effect in BiFeO₃ thin film is not clear presently,which restrict the commercial use of RRAM.The resistive effect of BiFeO₃ has been studied a lot from experiment and mechanism respectively by researchers around the world.In this thesis,the resistive properties of BiFeO₃ thin film is researched,and the mechanism of resistiance and conduction are analyzed profoundly.Furtherly,the effect of ferroelectric polarization on the resistance is studied to explain the transport mechanism.The researches above lay a foundation for the commercialization of RRAM.The main works are as follows:?1?The experimental sample is Au/BiFeO₃/SrRuO₃ sandwich structure as basic memory cell of the variable memory.The results of XRD test show that the sample is a single crystal with good crystallinity and the crystal orientation is?100?.According to the calculation results of the degree of lattice distortin in XRD test,the existence of residual tensile stress in the sample is verified.Hysteresis loop test results show that the samples have good ferroelectric properties,high remanent polarization.But the imprint problem exists due to the asymmetric coercive voltage.The results of remanent polarization under different times show that the sample has good retention characteristics,namely non-volatile.?2?The I-V curves were measured to characterize the change of resistance state of sample under different electric fields.The results show that resistance changes from high resistance to low resistance under positive voltage and from low resistance to high resistance under negative voltage,and the ratio of resistance can reach 10³.The space charge limited current conductive mechanism of sample controlled by traps is confirmed by fitting the different conductive mechanism of I-V curves.The resistance mechanism of the sample is trapping/detrapping,which means the change of trap filling degree leading to the change of barrier height of Bi FeO₃ between electrode,thus the film resistance alters.Furthermore,it is found that the charge filling in the shallow traps can not maintain after removing the power,and sample returns to high resistance state.And the sample can be kept in a low resistance state by the filling of deep level traps after power failure.?3?The original sample without polarization is compared with the sample under different polarization degrees in order to investigate the effect of polarization on the performance of resistance.The comparison of the resistance curve shows that the full circle polarization will increase the transition voltage V_SCL and the ratio of high and low resistivity.Furtherly,thermally stimulated current?TSC?experiment has been done,the trapped charge is inspired out of the traps and the sample in low resistance state returns to high resistance state after TSC experiment.The thermally stimulated current has not been detected in the samples in high resistance state without polarization after TSC experiment.The comparison results proved that the mechanism of resistive effect is trapping/detrapping,and polarization in a complete circle can increase the number of traps in the sample.It is found that the samples in different polarization states can exhibit the same low resistance state,which provides a method for achieving high resistance at low voltage.Furthermore,the measurement for the resistance retention ability of samples has been done,the resistance retention ability of samples is different,which shows that the polarization has an influence on the resistance retention ability.