| In recent years,as a kind of emerging nonvolatile resistive memories,ferroelectric tunnel junction（FTJs）have attracted considerable attention.FTJs are composed of two conductive electrodes separated by a nanometer-thick ferroelectric barrier,in which the data are stored in non-volatile resistance states and can be read nondestructively,overcoming a main drawback of commercial ferroelectric random access memories.In addition,the two-terminal device structure makes the integration of FTJs into cross-point arrays possible and the state-of-the-art feature size of 4F2could be expected.In principle,FTJs are functioned by the polarization-modulation on barrier profile of ferroelectric layer,giving rise to the switching of device conductance between a high（OFF）and a low（ON）resistance state,and thus achieving the electroresistance phenomenon.The electroresistance mechanism,however,has been reported not only from the polarization-modulation of barrier profiles.Electrical migration of charged defects has also been observed as a possible origin for the resistive switching.However,the electroresistance in actual FTJ devices is much more complicated due to the presence of oxygen vacancies.In this paper,oxygen vacancies are introduced into the Pb（Zr,Ti）O3 Barrier by adjusting the growth temperature,and two kinds of electroresistive phenomena are found in Pt/Pb（Zr,Ti）O3/（La,Sr）Mn O3 tunnel junction,which are regulated by ferroelectric and ion migrated respectively.The oxygen vacancies are observed by X-ray photoelectron spectroscopy and their effects on the widely-adopted piezoresponse force microscopy characterizations of ultrathin ferroelectric films have been analyzed by AC voltage-dependent hysteresis loops.For the Pt/Pb（Zr,Ti）O3/（La,Sr）Mn O3 device that is modulated by the polarization reversal,a counterclockwise resistance-voltage（R-V）relationship is observed due to the tunneling between a high and a low barrier,whereas the R-V hysteresis loop is changed to clockwise with the existence of oxygen vacancies,in which conductive filaments are formed in the Pb（Zr,Ti）O3 barrier.However,such an ionic electroresistance isn’t stable during repetitive switching.Further investigation on memristive behaviors is thus performed on the ferroelectric-controlled Pt/Pb（Zr,Ti）O3/（La,Sr）Mn O3 tunnel junctions.An excellent linearity is achieved in continuous resistance change owing to the nucleation-limited-switching mode of domain switching in the Pb（Zr,Ti）O3 barrier,giving rise to spike-timing-dependent plasticity behaviors for the Hebbian rule of learning and memory.On this basis,Pt/Pb（Zr,Ti）O3/（La,Sr）MnO3 ferroelectric tunnel junctions with different polarization orientations were epitaxial grown on Sr Ti O3 substrates with different growth orientations.The phase composition and morphology of the tunnel junctions were characterized by X-ray diffraction and atomic force microscopy,respectively.It is proved that T phase and R phase Pb（Zr,Ti）O3 ferroelectric tunnel junctions with the same thickness exhibit both of them have higher electroresistive phenomena when the spontaneous polarization orientation with growth orientation are consistent,and the electroresistance is obviously proportional to ferroelectric polarization intensity.These results provide insights into the distinguishing of ferroelectric and ionic contributions in electroresistance of FTJ devices,the relationship between the electroresistance and the polarization intensity of ferroelectric tunnel junction devices with different orientations is proved,and facilitating deep understanding of nonvolatile resistive memories. |
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