Study On The Non-volatile Memory Performance Of Pt/BaTiO₃/Nb:SrTiO₃ Ferroelectric Tunnel Junction

Ferroelectric tunnel junctions(FTJs)have attracted great attention for potential applications in next-generation nonvolatile memories,owing to attractive advantages such as high-density integration,fast write/read speed,low energy consumption and nondestructive readout of data storage.However,the reliabilities for nonvolatile storage of FTJs have been rarely studied,hence the mechanism of retention failure and resistance switching fatigue of FTJs are not clear yet.Therefore,we performed below research on Pt/BaTiO₃/Nb:SrTiO₃ junctions,to investigate nonvolatile storage abilities of FTJs.Firstly,three kinds of tunnel junctions with four-unit cell SrTiO₃,La Al O₃ and BaTiO₃barriers were fabricated,to reveal the role of ferroelectric polarization in resistive memory properties of metal/insulator/semiconductor tunnel junctions.As expected,compared with another two kinds of tunnel junctions,the FTJs with BaTiO₃ barrier show three orders of magnitude higher current ratio,better resistance retention and switching endurance abilities,on account of the ferroelectric polarization's modulations on the barrier.By means of Fick's second law,the fact of electrons'diffusion behavior near the insulator/Nb:SrTiO₃ interface are described,which will lead to serious resistance state decay in tunnel junction device.Then resistance switching endurance measurement on Pt/BaTiO₃/Nb:SrTiO₃ FTJs were performed,and fatigue behavior is observed after a finite number of write/erase cycles.By performing scanning transmission electron microscopy,it is found that the concentration of oxygen vacancies in BTO thin films grows gradually and eventually induces a lattice collapse of perovskite structure.While the lost of ferroelectricity in BaTiO₃ barrier is verified by PFM.Finally,through the analyses of barrier profile and switching dynamic of FTJs,with the aid of equivalent circuit analyses and first principles calculations,the physical mechanism of switching fatigue in FTJs is revealed at atomic-scale,which will lead to the lost of current ratio and the rise of coercive voltage.Besides,the(La_0.67,Sr_0.33)Mn O₃/BaTiO₃/Co Fe₂O₄/Nb:SrTiO₃multiferroic tunnel junctions(MFTJs)are designed and fabricated,while four nonvolatile resistance states found in the devices.The ferroelectric polarization switching in BaTiO₃ also alters the band alignment of Co Fe₂O₄ barrier relative to the(La_0.67,Sr_0.33)Mn O₃electrode,giving rise to the change of tunneling magnetoresistance ratio by about 10 times and even the reversal of its sign depending upon the resistance states.Continuous conductance change with a ON/OFF current ratio of?54400%and long-term memory with the spike-timing-dependent plasticity of synaptic weight for Hebbian learning are achieved by controlling the polarization switching of BaTiO₃,showing the MFTJs'great potential in nonvolatile information storage and neuromorphic computing.