Takeshi Yanagida

Resistive switching multistate nonvolatile memory effects in a single cobalt oxide nanowire.

A multistate nonvolatile memory operated at sublithographic scale has been strongly desired since other nonvolatile memories have confronted the fundamental size limits owing to their working principles. Resistive switching (RS) in metal-oxide-metal junctions, so-called ReRAM, is promising for next generation high-density nonvolatile memory. Self-assembled oxide nanowire-based RS offers an attractive solution not only to reduce the device size beyond the limitation of current lithographic length scales but also to extract the underlying nanoscale RS mechanisms. Here we demonstrate the multistate bipolar RS of a single Co(3)O(4) nanowire (10 nm scale) with the endurance up to 10(8). In addition, we succeeded to extract a voltage-induced nanoscale RS mechanism rather than current-induced RS. These findings would open up opportunities to explore not only for the intrinsic nanoscale RS mechanisms with the ultimate size limit but also for next generation multistate three-dimensional ReRAM.

Nonvolatile bipolar resistive memory switching in single crystalline NiO heterostructured nanowires.

We have demonstrated the nonvolatile bipolar resistive memory switching in single crystalline NiO heterostructured nanowires for the first time. The self-assembled NiO nanowires are expected to open up opportunities to explore not only the detailed nanoscale mechanisms in NiO resistive memory switching but also next-generation nanoscale nonvolatile memory devices with the potential for high-density device integration and improved memory characteristics.

Resistive-switching memory effects of NiO nanowire/metal junctions.

We have demonstrated the construction of highly stable resistive switching (RS) junctions with a metal/NiO nanowire/metal structure and used them to elucidate the crucial role of redox events in the nanoscale bipolar RS. The presented approaches utilizing oxide nanowire/metal junctions offer an important system and platform for investigating nanoscale RS mechanisms of various oxide materials.

Intrinsic Mechanisms of Memristive Switching

Resistive switching (RS) memory effect in metal-oxide-metal junctions is a fascinating phenomenon toward next-generation universal nonvolatile memories. However the lack of understanding the electrical nature of RS has held back the applications. Here we demonstrate the electrical nature of bipolar RS in cobalt oxides, such as the conduction mechanism and the switching location, by utilizing a planar single oxide nanowire device. Experiments utilizing field effect devices and multiprobe measurements have shown that the nanoscale RS in cobalt oxides originates from redox events near the cathode with p-type conduction paths, which is in contrast with the prevailing oxygen vacancy based model.

Unipolar resistive switching characteristics of room temperature grown SnO2 thin films

The resistive switching characteristics of room temperature grown SnO2 films were investigated by fabricating the metal-oxide-metal sandwich structures. The unipolar operation was found in all devices. Experiments, including the size and material dependencies of the top electrodes and the three terminal device structures, demonstrated the rupture and formation of conducting filaments near the anode. The Ohmic behavior was observed in both on- and off-states when using Au and Ti top electrodes, whereas the Schottky behavior was only found in the off-state for Pt. The analysis on the transport properties indicates the presence of insulative crystalline SnO2 near the anode in the off-state.

Cellulose Nanofiber Paper as an Ultra Flexible Nonvolatile Memory

On the development of flexible electronics, a highly flexible nonvolatile memory, which is an important circuit component for the portability, is necessary. However, the flexibility of existing nonvolatile memory has been limited, e.g. the smallest radius into which can be bent has been millimeters range, due to the difficulty in maintaining memory properties while bending. Here we propose the ultra flexible resistive nonvolatile memory using Ag-decorated cellulose nanofiber paper (CNP). The Ag-decorated CNP devices showed the stable nonvolatile memory effects with 6 orders of ON/OFF resistance ratio and the small standard deviation of switching voltage distribution. The memory performance of CNP devices can be maintained without any degradation when being bent down to the radius of 350 μm, which is the smallest value compared to those of existing any flexible nonvolatile memories. Thus the present device using abundant and mechanically flexible CNP offers a highly flexible nonvolatile memory for portable flexible electronics.

Epitaxial growth of MgO nanowires by pulsed laser deposition

We fabricated single-crystalline MgO nanowires epitaxially grown on MgO single crystal substrate using the Au catalyst-assisted pulsed laser deposition (PLD). Controlling appropriately the amount of Au catalyst and the substrate temperature was found to be crucial for the MgO nanowire growth using the catalyst-assisted PLD. In addition, (100) oriented MgO nanowires were epitaxially grown on (100), (110), and (111) oriented substrates, allowing the limited growth directions. The possible growth mechanism of MgO nanowires in PLD is discussed. This feasibility of PLD for fabricating MgO nanowires would contribute to incorporating the rich functionalities of various transition metal oxides into nanowires via in situ construction of heterostructures in oxide nanowires.

Scaling Effect on Unipolar and Bipolar Resistive Switching of Metal Oxides

Electrically driven resistance change in metal oxides opens up an interdisciplinary research field for next-generation non-volatile memory. Resistive switching exhibits an electrical polarity dependent “bipolar-switching” and a polarity independent “unipolar-switching”, however tailoring the electrical polarity has been a challenging issue. Here we demonstrate a scaling effect on the emergence of the electrical polarity by examining the resistive switching behaviors of Pt/oxide/Pt junctions over 8 orders of magnitudes in the areas. We show that the emergence of two electrical polarities can be categorised as a diagram of an electric field and a cell area. This trend is qualitatively common for various oxides including NiOx, CoOx, and TiO2-x. We reveal the intrinsic difference between unipolar switching and bipolar switching on the area dependence, which causes a diversity of an electrical polarity for various resistive switching devices with different geometries. This will provide a foundation for tailoring resistive switching behaviors of metal oxides.

Influence of ambient atmosphere on metal-insulator transition of strained vanadium dioxide ultrathin films

The effect of ambient atmosphere on metal-to-insulator transition (MIT) in strained vanadium dioxide (VO2) ultrathin films (7–8nm) grown epitaxially on TiO2 (001) single crystal substrate by pulsed laser deposition was investigated by varying the ambient oxygen pressure and substrate temperature with the intention being to control arbitrarily the MIT events of strained VO2 ultrathin films, including the MIT temperature and the resistivity change. When reducing the ambient oxygen pressure, the MIT temperature remained almost constant, whereas the change of resistivity during MIT tended to be smaller. Contrary, varying the substrate temperature resulted in the variation of the MIT temperature with keeping the abruptness of transition. The lower the substrate temperature, the lower the MIT temperature, and in addition the MIT temperature ∼290K with keeping the abrupt change of resistivity is found to be the lowest compared with previously reported values. The reduction of the MIT temperature was found to cor...

Effect of the heterointerface on transport properties of in situ formed MgO/titanate heterostructured nanowires.

Heterostructured transition metal oxide nanowires are potential candidates to incorporate rich functionalities into nanowire-based devices. Although the oxide heterointerface plays a crucial role in determining the physical properties, the effects of the heterointerface on the oxide nanowires properties have not been clarified. Here we investigate for the first time the significant role of the heterointerface in determining the transport properties of well-defined MgO/titanate heterostructured nanowires by combining a technique for in situ formation of a oxide heterointerface and microwave conductivity measurement. Variation of the heterointerface strongly affects the nanowires transport properties due to the crystallinity and the atomic interdiffusion at the oxide heterointerface. Thus, the precise in situ formation of a well-defined heterointerface is crucial to create oxide heterostructured nanowires with the desired transport properties.