In-situ transmission electron microscopy (in-situ TEM) was performed to investigate the switching operation of a resistive random access memory (ReRAM) made of copper, tungsten oxide and titanium nitride (Cu/WOx/TiN). In the first Set (Forming) operation to initialize the device, precipitation appeared inside the WOx layer. It was presumed that a Cu conducting filament was formed, lowering the resistance (on-state). The Reset operation induced a higher resistance (the off-state). No change in the microstructure was identified in the TEM images. Only when an additional Reset current was applied after switching to the off-state could erasure of the filament be seen (over-Reset). Therefore, it was concluded that structural change relating to the resistance switch was localized in a very small area around the filament. With repeated switching operations and increasing operational current, the WOx/electrode interfaces became indistinct. At the same time, the resistance of the off-state gradually decreased. This is thought to be caused by Cu condensation at the interfaces because of leakage current through the area other than through the filament. This will lead to device degradation through mechanisms such as endurance failure. This is the first accelerated aging test of ReRAM achieved using in-situ TEM.

Tunnel electroresistance in ferroelectric tunnel junctions (FTJs) has attracted considerable interest, because of a promising application to nonvolatile memories. Development of ferroelectric thin‐film devices requires atomic‐scale band‐structure engineering based on depolarization‐field effects at interfaces. By using FTJs consisting of ultrathin layers of the prototypical ferroelectric BaTiO3, it is demonstrated that the surface termination of the ferroelectric in contact with a simple‐metal electrode critically affects properties of electroresistance. BaTiO3 barrier‐layers with TiO2 or BaO terminations show opposing relationships between the polarization direction and the resistance state. The resistance‐switching ratio in the junctions can be remarkably enhanced up to 105% at room temperature, by artificially controlling the fraction of BaO termination. These results are explained in terms of the termination dependence of the depolarization field that is generated by a dead layer and imperfect charge screening. The findings on the mechanism of tunnel electroresistance should lead to performance improvements in the devices based on nanoscale ferroelectrics.

Owing to the recent discovery of the current-induced metal-insulator transition and unprecedented electronic properties of the concomitant phases of calcium ruthenate Ca2RuO4, it is emerging as an important material. To further explore the properties, the growth of epitaxial thin films of Ca2RuO4 is receiving more attention as high current densities can be applied to thin-film samples and the amount can be precisely controlled in an experimental environment. However, it is difficult to grow high-quality thin films of Ca2RuO4 due to easy formation of crystal defects originating from sublimation of RuO4; therefore, the metal-insulator transition of Ca2RuO4 is typically not observed in the thin films. Herein, stable current-induced metal-insulator transition is achieved in high-quality thin films of Ca2RuO4 grown by solid phase epitaxy under high growth temperatures and pressures. In the Ca2RuO4 thin films grown by ex-situ annealing at >1200 °C and 1.0 atm, continuous changes in the resistance of over two orders of magnitude are induced by currents with a precise dependence of the resistance on the current amplitude. A hysteretic, abrupt resistive transition is also observed in the thin films from the resistance-temperature measurements conducted under constant-voltage (variable-current) conditions with a controllability of the transition temperature. A clear resistive switching by the current-induced transition is demonstrated in the current-electric-field characteristics, and the switching currents and fields are shown to be very stable. These results represent a significant step toward understanding the highcurrent-density properties of Ca2RuO4 and future development of Mott-electronic devices based on electricity-driven transitions.

Ferroelectric resistive switching was artificially induced in a conductive ferroelectric capacitor by inserting a thin dielectric layer at an electrode/ferroelectric interface. Ferroelectric capacitors consisting of semiconducting Bi-deficient Bi1−δFeO3 layers with SrRuO3 electrodes showed no resistive switching, but resistive switching emerged in these ferroelectric capacitors when a thin LaFeO3 dielectric layer was inserted at one of the SrRuO3/Bi1−δFeO3 interfaces. In addition to resistive switching, SrRuO3/LaFeO3/Bi1−δFeO3/SrRuO3 devices showed rectifying current–voltage characteristics, suggesting an asymmetric potential distribution along the stacking direction in the device. The results shed light upon the mechanism of resistive switching in ferroelectric diodes and demonstrate that interface engineering provides a simple but effective approach toward controlling the ferroelectric resistive switching characteristics.

We demonstrate that the inclusion of a Ta interfacial layer is a remarkably effective strategy for forming interfacial oxygen defects at metal/oxide junctions. The insertion of an interfacial layer of a reactive metal, that is, a "scavenging" layer, has been recently proposed as a way to create a high concentration of oxygen defects at an interface in redox-based resistive switching devices, and growing interest has been given to the underlying mechanism. Through structural and chemical analyses of Pt/metal/SrTiO/Pt structures, we reveal that the rate and amount of oxygen scavenging are not directly determined by the formation free energies in the oxidation reactions of the scavenging metal and unveil the important roles of oxygen diffusibility. Active oxygen scavenging and highly uniform oxidation via scavenging are revealed for a Ta interfacial layer with high oxygen diffusibility. In addition, the Ta scavenging layer is shown to exhibit a highly uniform structure and to form a very flat interface with SrTiO, which are advantageous for the fabrication of a steep metal/oxide contact.