Tunnel electroresistance in BiFeO3 junctions: size does matter

Abstract

In ferroelectric tunnel junctions, the tunnel resistance depends on the polarization orientation of the ferroelectric tunnel barrier, giving rise to tunnel electroresistance. These devices are promising to be used as memristors in neuromorphic architectures and as non-volatile memory elements. For both applications device scalability is essential, which requires a clear understanding of the relationship between polarization reversal and resistance change as junction size shrinks. Here we show robust tunnel electroresistance in BiFeO 3 -based junctions with diameters ranging from 1200 to 180 nm. We demonstrate that the tunnel electroresistance and the corresponding fraction of reversed ferroelectric domains change 1 Electronic mail: vincent.garcia@thalesgroup.com 2 drastically with the junction diameter: while micron-size junctions display reversal in less than 10% of the area, the smallest junctions show an almost complete polarization reversal.Modeling the electric-field distribution, we highlight the critical role of the bottom electrode resistance which significantly diminishes the actual electric field applied to the ferroelectric barrier in the mixed polarization state. A polarization-dependent critical electric field below which further reversal is prohibited is found to explain the large differences between the ferroelectric switchability of nano-and micron-size junctions. Our results indicate that ferroelectric junctions are downscalable and suggest that specific junction shapes facilitate complete polarization reversal.Ferroelectric materials possess a spontaneous electrical polarization that is switchable by an external electric field. This enables the use of thin ferroelectric films sandwiched between electrodes as non-volatile memories.1 In such ferroelectric memories, the information is encoded by the polarization orientation and recovered in a destructive capacitive readout.When the thickness of the ferroelectric films is of the order of a few nanometers, electron tunneling becomes possible. In these ferroelectric tunnel junctions, 2,3 the tunnel resistance varies depending on the orientation of the polarization; this tunnel electroresistance effect enables a non-destructive information readout. These interfacial magnetoelectric coupling phenomena can be probed by tunnel magnetoresistance experiments, resulting in a non-volatile control of the spin-polarization. 26-29Moreover, selecting oxide electrodes subject to field-induced electronic phase transitions upon polarization reversal may result in enhanced tunnel electroresistance. 29,30 Hence, ferroelectric tunnel junctions offer a fantastic playground to explore electric-field-driven modifications at the nanoscale. 31 Top electrodes of Pt (10 nm) / Co (10 nm) with diameters ranging from 180 nm to 1200 nm ( Fig. 1(a)) are defined by electron-beam lithography, sputtering, and lift-off. 18 We use the conductive tip of an atomic force microscope (AFM) to connect individual top electrodes and perform electric transport measurements under a constant v...

Quasiparticle tunnel electroresistance in superconducting junctions

“…In the OFF state, the conductance is directly proportional to A (note that the hollow triangles in Fig. 3a, b display constant G 100 /A values), while in the ON state it is not, as already observed in ferroelectric tunnel junctions 31,32 . In fact, in the ON state the conductance is directly proportional to the junctions' perimeter P (note that solid triangles in Fig.…”

“…The scaling observed in the ON state implies that the resistance switching occurs only over the junction's periphery. As shown earlier 21,31 , this can be understood by considering that the electric field produced upon application of V pol is stronger over the edges than in the central area of the junction, favouring the local activation of the switching mechanism. Note that this explanation applies to any electric-field activated switching mechanism 21,31 .…”

“…As shown earlier 21,31 , this can be understood by considering that the electric field produced upon application of V pol is stronger over the edges than in the central area of the junction, favouring the local activation of the switching mechanism. Note that this explanation applies to any electric-field activated switching mechanism 21,31 . For instance, if the mechanism is based on the switching of the ferroelectric polarisation, the argument is that the polarisation reverses only over the junction' periphery, but remains pinned everywhere else 31 .…”

Resonant tunneling across a ferroelectric domain wall

“…The tunneling resistance of a FTJ is modulated by electric field induced ferroelectric polarization switching-the effect known as tunneling electroresistance (TER). Following the theoretical predictions [9,10], there have been a number of successful experimental demonstrations of the TER effect in trilayer junctions [11][12][13][14][15][16], showing the potential of FTJs for nonvolatile memory applications [17,18]. The structural and/or electronic asymmetry of the FTJ plays a decisive role for the TER effect.…”

Tunneling Anisotropic Magnetoresistance in Ferroelectric Tunnel Junctions

“…Following the theoretical predictions, 4 , 5 there have been a number of experimental demonstrations of the TER effect in trilayer junctions. [6][7][8][9][10] It was shown that the sizable TER effect can be achieved by using dissimilar electrodes [11][12][13] , interface engineering [14][15][16] , applied bias 17,18 , or defect control. 19 Contrary to FE capacitors where leakage currents are detrimental to the device performance, the conductance of a FTJ is the functional characteristic of the device.…”