Changeable electroresistance in Pt/Pb(Zr,Ti)O3/(La,Sr)MnO3 tunnel junctions and memristive properties for synaptic plasticity emulation

Ferroelectric tunnel junctions (FTJs) are promising candidates for the next-generation memory technologies. 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. Here, we achieve two kinds of electroresistance behaviors in Pt/Pb(Zr,Ti)O3/(La,Sr)MnO3 tunnel junctions by introducing oxygen vacancies in the Pb(Zr,Ti)O3 barrier. 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)MnO3 device that is modulated by the polarization reversal, a counterclockwise resistance–voltage (R–V) relationship is observed due to the tunneling between high and low barriers, 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 is not stable during repetitive switching. Further investigation on memristive behaviors is, thus, performed on the ferroelectric-controlled Pt/Pb(Zr,Ti)O3/(La,Sr)MnO3 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. These results provide insight into the distinguishing of ferroelectric and ionic contributions in electroresistance of FTJ devices, facilitating deep understanding of nonvolatile resistive memories.