Space-charge Effect on Electroresistance in Metal-Ferroelectric-Metal capacitors

International audienceResistive switching through electroresistance (ER) effect in metal-ferroelectric-metal (MFM) capacitors has attracted increasing interest due to its potential applications as memories and logic devices. However, the detailed electronic mechanisms resulting in large ER when polarisation switching occurs in the ferroelectric barrier are still not well understood. Here, ER effect up to 1000% at room temperature is demonstrated in C-MOS compatible MFM nanocapacitors with a 8.8 nm-thick poly(vinylidene fluoride) (PVDF) homopolymer ferroelectric, which is very promising for silicon industry integration. Most remarkably, using theory developed for metal-semiconductor rectifying contacts, we derive an analytical expression for the variation of interfacial barrier heights due to space-charge effect that can interpret the observed ER response. We extend this space-charge model, related to the release of trapped charges by defects, to MFM structures made of ferroelectric oxides. This space-charge model provides a simple and straightforward tool to understand recent unusual reports. Finally, this work suggests that defect-engineering could be an original and efficient route for tuning the space-charge effect and thus the ER performances in future electronic devices. Research on resistive switching in metal-ferroelectric-metal (MFM) capacitors has been intensified recently due to its potential applications in the next generation of non-volatile memories and logic devices 1–4. The resistance in MFM structures can show large variations depending on the orientation of the ferroelectric polarisation. The polarisation in thin ferroelectric layers induces surface charges which in turn induce a depolarizing field that acts against the polarisation. In an MFM structure, the polarisation charges at the ferroelectric/electrode interface are partially screened by the electrons of the electrodes minimizing the effect of the depolarizing field. This results into an asymmetry in the barrier heights of the electronic band structure as sketched in Fig. 1a. Assuming that electrons dominate the conductance, the barrier height is lowered on the front side of the polarisation (where the positive bound charges are located) while it is increased on the back side. By reversing the polarisation state with an external electric field, the energy profile at MFM interfaces can thus be modulated. The electronic resistance through the MFM structure can be switched between two states allowing either a larger (ON) or smaller (OFF) flow of the current from one electrode to the other. This is the so-called electroresistance (ER) effect 5–