Thanks to the tunnelling electro resistance (TER) effect, i.e. the variation of the tunnelling resistance observed upon reversal of the ferroelectric polarization, tunnelling junctions with ferroelectric barriers (FTJs) are currently under investigation for different applications.
In fact, besides the potential use of FTJs as memory cells with the remanent polarization representing the digital information , the viscous dynamics of ferroelectrics makes possible to use these devices also as memristors with potential application as synapses in neuromorphic computing networks [2,3].
In this work, we compare FTJs with a thin (3 nm thick) ferroelectric barrier of BaTiO3 (BTO) epitaxially grown by Pulsed Laser Deposition either on La1/3Sr2/3MnO3 (LSMO) or Nb:SrTiO3 (Nb:STO) bottom electrodes. In both cases a 5 nm thick Pt layer deposited in-situ is used as top contact, and large area junctions (16-1600 μm2) are defined through optical lithography and ion milling.
We show that in both cases the fabricated devices present clear electroresistance with ON/OFF ratios exceeding 10. We also demonstrate that the application of proper pulse trains allows for the continuous tuning of the junction resistance which can be controlled in a memristive fashion both by the amplitude and the duration of the voltage pulses.
Consistently with previous reports [4,5], we observe that the sign of TER is opposite in the two types of devices here considered: while the high resistance state is found in Pt/BTO/LSMO FTJs for ferroelectric polarization pointing away from Pt this is observed in Pt/BTO/Nb:STO structures for polarization pointing towards Pt.
To investigate how the bottom electrodes determines the TER sign, we performed spectroscopic impedance measurement on the devices. By measuring the junction capacitance as a function of bias voltage we have found evidences that a p-type semiconducting layer is formed at the LSMO/BTO interface whereas the expected n-type character is observed in the case of Nb:STO-based FTJs. Following these observations we propose a simple electrostatic model which could explain the experimental sign of TER and relate it to the different sign of majority carriers in LSMO and Nb:STO electrodes.
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