Takafumi Ishibe

Influence of nanometer-sized interface on reaction of iron nanocrystals epitaxially grown on silicon substrates with oxygen gas

Iron (Fe) nanocrystals (NCs) were epitaxially grown on silicon (Si) substrates, where interfacial alloying of Fe and Si (silicidation) was prevented using an ultrathin SiO2 film. Nanowindows (NWs) composed of Si and germanium (Ge) were introduced into this SiO2 layer. The crystallographic arrangement of the Si substrates was conveyed though the NWs, while Fe and Si atoms were not intermixed. Reactions between the epitaxial Fe NCs and Si substrate in the presence of oxygen gas were also investigated. Oxygen atoms facilitated the diffusion of Fe from NCs to Si substrates mainly through Si NWs. As a result, increase of oxygen concentration led to Si oxidation near the interface. This means Fe NCs played a role like a catalysis for Si oxidation. The interfacial reaction was changed drastically by control of nanometer-sized interfaces using Ge NWs in the ultrathin SiO2 films.

Control of epitaxial growth of Fe-based nanocrystals on Si substrates using well-controlled nanometer-sized interface

We have developed an epitaxial growth technique for Fe-based nanocrystals (NCs) on Si substrates with high selectivity of their crystal structure. Ge NCs with controlled shape and strain were initially epitaxially grown on Si substrates covered with an ultrathin SiO2 film. Using these well-controlled Ge NCs as nucleation sites, Fe-based NCs could be formed with crystal-structure selectivity. In Fe deposition on the Ge NCs at room temperature, bcc-Fe NCs were formed, where epitaxial growth was influenced by the Ge NC shapes related to surface coverage. For Fe deposition at 250–300 °C, Fe-Ge alloying occurred without intermixing with Si. The epitaxially grown crystal structures were determined by the strain state of the Ge NCs: Fe1.7Ge NCs with a B82 structure for spherical strain-relaxed Ge NCs with a lattice constant close to that of bulk Ge, and e-FeGe NCs with a B20 structure for flattened strained Ge NCs with a lattice constant close to that of bulk Si. All the NCs had sharp interfaces, where interfaci...

Epitaxial iron oxide nanocrystals with memory function grown on Si substrates

High-density Fe3O4−δ nanocrystals (NCs) were epitaxially grown on Si substrates by molecular beam epitaxy with epitaxial Ge NCs being used as nucleation sites. Scanning tunneling spectroscopy measurements showed that the surface bandgap of the as-grown Fe3O4−δ NCs was ~0.2 eV, consistent with that reported for Fe3O4−δ films. Conductive atomic force microscopy measurements of the NCs revealed hysteresis in the voltage–current curves, indicating bipolar resistive switching behavior. The measurement results established the superiority of the NCs to thin conventional polycrystalline Fe3O4−δ films/Si in terms of resistive switching characteristics. This demonstrated the possibility of developing resistance random access memory devices composed of ubiquitous Fe3O4−δ NC materials.

Effect of Fe coating of nucleation sites on epitaxial growth of Fe oxide nanocrystals on Si substrates

We studied the effect of Fe coating on the epitaxial growth of Fe3O4 nanocrystals (NCs) over Fe-coated Ge epitaxial nuclei on Si(111). To completely cover Ge nuclei with Fe, some amount of Fe (>8 monolayers) must be deposited. Such covering is a key to epitaxial growth because an Fe coating layer prevents the oxidation of Ge surfaces during Fe3O4 formation, resulting in the epitaxial growth of Fe3O4 on them. This study demonstrates that an appropriate Fe coating of nucleation sites leads to the epitaxial growth of Fe3O4 NCs on Si substrates, indicating the realization of environmentally friendly and low-cost Fe3O4 NCs as the resistance random access memory material.

Resistive switching characteristics of isolated core-shell iron oxide/germanium nanocrystals epitaxially grown on Si substrates

The core-shell nanostructure of epitaxial Fe3O4 nanocrystals over Ge nuclei showed a large Off/On resistance ratio (∼100), which was the largest value in Fe3O4 materials. The nanocrystals with an average diameter of ∼20 nm were grown epitaxially on Si substrates, whose areal density was high (∼1011 cm−2), and each nanocrystal was isolated from each other. The electrical measurement of the individual isolated nanocrystals by conductive-atomic force microscopy showed the bipolar-type resistive switching in local voltage-current curves, depending on the Fe-O composition. It was also revealed that activation sites for resistive switching were the Fe3O4/Ge interfaces, where electric-field-induced compositional variation caused large resistive changes. This demonstrated the possibility of developing resistance random access memory devices based on ubiquitous materials.

Embedded-ZnO Nanowire Structure for High-Performance Transparent Thermoelectric Materials

We present the structure of ZnO nanowires (NWs) embedded in ZnO films for high-performance transparent thermoelectric materials. The design concept is that the ZnO NWs exhibit high power factor and work as phonon scatterers to reduce the thermal conductivity. Here, we form an embedded-ZnO NWs structure on Si(111) substrates using physical vapor transport for ZnO NW formation and pulsed laser deposition for embedding NWs with ZnO. The NWs grew along the c-axis orientation vertically on the ZnO buffer/Si(111) substrates. Nanoscale voids near NWs were also observed in filling ZnO. The electrical measurements of films including NWs exhibited the reduction of electrical conductivity from that of bulk ZnO to a similar extent to the reduction in the case of ZnO films without NWs. This indicates that there was small electron scattering by ZnO NWs and the voids. However, considering that the mean free path of electron becomes lower by increasing carrier concentration, the electron scattering effect by nanostructuring can be found to be even weaker under the high doping condition compared with phonon scattering with large mean free path. Therefore, our study develops embedded-ZnO NWs structures promising for high-performance thermoelectric material with high electrical conductivity and low thermal conductivity.

Thermoelectric performances in transparent ZnO films including nanowires as phonon scatterers

We report the fabrication technique and electrical propertis of embedded-ZnO nanowires (NWs) structure for high-performance transparent thermoelectric materials. In fabrication technique, we revealed that the length of NW is less than 2 μm for embedding NWs. Our embedded-ZnO NWs structures showed a little smaller electrical conductivities than ZnO films (without ZnO NWs) at the carrier concentration of ~ 1018 cm-3. However, at high carrier concentration, our embedded-ZnO NWs structures are expected to show high thermoelectric performances due to the drastic reduction of κ and smaller σ degradation, because there is the difference of MFP between electron and phonon. Our work showed the design guide to enhance the performance of transparent thermoelectric materials.

Resistive switching at the high quality metal/insulator interface in Fe3O4/SiO2/α-FeSi2/Si stacking structure

Fe3O4-based films composed of ubiquitous elements are promising for resistive switching. In general, the disadvantage of this film is the low Off/On resistance ratio. We achieved the highest resistance ratio in a Fe3O4-based stacking structure including a thin SiO2 layer with a high quality interface. For fabrication of the stacking structure, Fe oxide films were epitaxially grown on the intentionally formed α-FeSi2 layers on Si substrates, where the high quality epitaxial interfaces were formed owing to the α-FeSi2 layer role: blocking of Si atom diffusion from the substrate through the interface. The high quality Fe3O4/α-FeSi2 interfaces were oxidized by the low O2 pressure annealing process to succeed in inserting thin SiO2 layers at the interfaces. The resulting stacking structure of the Fe3O4 film/SiO2 layer/α-FeSi2 layer showed the resistive switching behavior with the resistance ratio of ∼140 which is the highest value of Fe3O4 materials. This high value comes from much higher resistance in the high resistive state because the stacking structure has a thin SiO2 insulator layer with high quality interfaces without defects working as leakage sites. This means overcoming the disadvantage of conventional Fe3O4-based films, low resistance ratio, and demonstrates the possibility of realization for rare-metal-free resistance random access memory.Fe3O4-based films composed of ubiquitous elements are promising for resistive switching. In general, the disadvantage of this film is the low Off/On resistance ratio. We achieved the highest resistance ratio in a Fe3O4-based stacking structure including a thin SiO2 layer with a high quality interface. For fabrication of the stacking structure, Fe oxide films were epitaxially grown on the intentionally formed α-FeSi2 layers on Si substrates, where the high quality epitaxial interfaces were formed owing to the α-FeSi2 layer role: blocking of Si atom diffusion from the substrate through the interface. The high quality Fe3O4/α-FeSi2 interfaces were oxidized by the low O2 pressure annealing process to succeed in inserting thin SiO2 layers at the interfaces. The resulting stacking structure of the Fe3O4 film/SiO2 layer/α-FeSi2 layer showed the resistive switching behavior with the resistance ratio of ∼140 which is the highest value of Fe3O4 materials. This high value comes from much higher resistance in the hig...