Ken Ichi Ikeda

Epitaxial Chemical Vapor Deposition Growth of Single-Layer Graphene over Cobalt Film Crystallized on Sapphire

Epitaxial chemical vapor deposition (CVD) growth of uniform single-layer graphene is demonstrated over Co film crystallized on c-plane sapphire. The single crystalline Co film is realized on the sapphire substrate by optimized high-temperature sputtering and successive H(2) annealing. This crystalline Co film enables the formation of uniform single-layer graphene, while a polycrystalline Co film deposited on a SiO(2)/Si substrate gives a number of graphene flakes with various thicknesses. Moreover, an epitaxial relationship between the as-grown graphene and Co lattice is observed when synthesis occurs at 1000 °C; the direction of the hexagonal lattice of the single-layer graphene completely matches with that of the underneath Co/sapphire substrate. The orientation of graphene depends on the growth temperature and, at 900 °C, the graphene lattice is rotated at 22 ± 8° with respect to the Co lattice direction. Our work expands a possibility of synthesizing single-layer graphene over various metal catalysts. Moreover, our CVD growth gives a graphene film with predefined orientation, and thus can be applied to graphene engineering, such as cutting along a specific crystallographic direction, for future electronics applications.

Low-temperature fabrication of polycrystalline Si thin film using Al-induced crystallization without native Al oxide at amorphous Si/Al interface

Low-temperature fabrication of polycrystalline silicon (poly-Si) thin film has been performed by Al-induced crystallization (AIC), and the structural properties have been investigated. In our experiments, to prevent native oxidation of Al film, an amorphous silicon (a-Si)/Al bilayer was formed on the SiO2/Si substrate by electron beam evaporation without breaking the vacuum. The a-Si/Al/SiO2/Si structure was then heated at a low temperature of 400°C to induce AIC. It was confirmed that layer exchange of the a-Si/Al bilayer is induced even though there is no native oxidation of Al film, which was demonstrated by scanning transmission electron microscopy and energy dispersive X-ray analysis. The mechanism for layer exchange of the a-Si/Al bilayer has been discussed. Furthermore, it was verified by scanning electron microscopy and spectroscopic ellipsometry that the a-Si/Al thickness ratio of roughly 1:1 is suitable to achieve a flat surface morphology of poly-Si. In addition, it was found, by X-ray diffraction and orientation imaging microscopy, that the Si(111)-oriented grain becomes dominant with decreasing thickness of the a-Si/Al bilayer.

Competition and cooperation between lattice-oriented growth and step-templated growth of aligned carbon nanotubes on sapphire

The authors study the growth mechanism of single-walled carbon nanotubes (SWNTs) horizontally aligned on A-plane single crystal sapphire (112¯0) by making the controlled step/terrace structure. SWNT growth direction was sensitive to the surface geometry of the sapphire, and there was competition between two growth modes, lattice-oriented growth and step-templated growth. On the substrate with single-atomic steps, SWNTs aligned parallel to the [11¯00] direction as dominated by the lattice-oriented growth mode, while SWNTs aligned along the step edges when the double-atomic or higher steps were formed. Our findings have potential for the fabrication of designed architecture of SWNTs by combining these two growth modes.

Graphite intercalation compounds prepared in solutions of alkali metals in 2-methyltetrahydrofuran and 2,5-dimethyltetrahydrofuran

Abstract The intercalation of alkali metals (Li, K, Rb and Cs) into natural graphite flakes in the solutions of 2-methyltetrahydrofuran (MeTHF) and 2, 5-dimethyltetrahydrofuran (diMeTHF) containing naphthalene has been studied by X-ray diffraction (XRD), where naphthalene is used as a dissolving agent for alkali metals. When MeTHF is used as a solvent, binary Li- and K-graphite intercalation compounds (GICs) have been obtained, in contrast to the case of Rb and Cs, where both binary alkali metal GICs and ternary alkali metal-MeTHF-GICs are obtained. Using diMeTHF instead of MeTHF, a selective preparation of binary GICs for all the alkali metals has been established.

Patterned Growth of Graphene over Epitaxial Catalyst

Rectangle- and triangle-shaped microscale graphene films are grown on epitaxial Co films deposited on single-crystal MgO substrates with (001) and (111) planes, respectively. A thin film of Co or Ni metal is epitaxially deposited on a MgO substrate by sputtering while heating the substrate. Thermal decomposition of polystyrene over this epitaxial metal film in vacuum gives rectangular or triangular pit structures whose orientation and shape are strongly dependent on the crystallographic orientation of the MgO substrate. Raman mapping measurements indicate preferential formation of few-layer graphene films inside these pits. The rectangular graphene films are transferred onto a SiO(2)/Si substrate while maintaining the original shape and field-effect transistors are fabricated using the transferred films. These findings on the formation of rectangular/triangular graphene give new insights on the formation mechanism of graphene and can be applied for more advanced/controlled graphene growth.

Influence of top surface passivation on bottom-channel hole mobility of ultrathin SiGe- and Ge-on-insulator

Bottom-channel hole mobility was examined by a pseudo-metal-oxide-semiconductor field-effect transistors method for ultrathin SiGe-on-insulator (SGOI) and Ge-on-insulator (GOI), which were fabricated using Ge condensation by dry oxidation. By comparing samples with and without a top SiO2 layer, we investigated the influence of top surface passivation on bottom-channel hole mobility. Mobility degradation was found in an ultrathin SGOI/GOI layer without top SiO2 and became more serious with a decrease in the thickness of the SGOI/GOI layer, which strongly suggested that top surface passivation is necessary to evaluate accurate channel mobility. A 13-nm-thick GOI with passivation showed a high mobility value of 440 cm2/V s.

Unidirectional growth of single-walled carbon nanotubes.

Unidirectional growth of single-walled carbon nanotubes (SWNTs) was achieved using the patterned Co-Mo salt catalyst on the r-plane sapphire substrate. This is in marked contrast with the SWNTs grown on an a-plane sapphire and ST-cut quartz, on which the SWNTs grew bidirectionally. This new growth mode is not dependent on the gas flow and attributed to the asymmetric surface atomic arrangement of the sapphire surface.

Annealing Behavior and Recrystallized Texture in ARB Processed Copper

The recrystallization behavior and texture development in copper accumulative roll-bonding (ARB) processed by various cycles (2, 4 and 6 cycle) were studied by differential scanning calorimetry (DSC) analysis and SEM/EBSP method. The exothermic peaks caused by recrystallization appeared at 210 ~ 253 in each sample. The peak positions shifted to lower temperature as the number of ARB cycles increased. This result indicated that the evolution of finer microstructure with increasing number of the ARB cycles enhanced the occurrence of recrystallization at lower temperature. The stored energy calculated from the DSC curve of the ARB processed copper increased with the increasing strains. During an annealing, the preferential growth of cube-oriented grains ({100}<001>) occurred in each sample. The recystallization behavior of ARB processed copper having low stacking fault energies was distinguished from that of so-called “recovery type” materials, i.e. aluminum and low carbon steels, which shows rather continuous changes in microstructure during annealing. The accumulated strains provided the driving force for the preferential growth, which was the same mechanism as the preferential growth in normally rolled copper. The sharpest cube texture developed in ARB processed copper by 4 cycles. The difference of cube texture development between 2 cycles and 4 cycles was caused by the distribution of cube-oriented regions which corresponded to the nucleation sites of recrystallized grains before annealing. More nanocystalline layers in the vicinity of bonded interfaces were distributed in ARB processed copper by 6 cycles than 4cycles. The nanocystalline structure could grow faster than the cube-oriented grains and led to the inhibition of sharp cube texture in the ARB processed copper by 6 cycles.

Organic mediated synthesis of highly luminescent Li+ ion compensated Gd2O3:Eu3+ nanophosphors and their Judd–Ofelt analysis

Highly luminescent red emitting Gd2O3:Eu3+, Li+ nanophosphor has been synthesized by the solvothermal combustion of the metal–citrate complex in diethylene glycol medium. The morphology and luminescence properties of these nanophosphors are found to be highly sensitive to the extent of lithium ion compensation. It is found that lithium ions promote grain growth and alter the morphology of the Gd2O3:Eu3+ nanophosphor from nearly spherical to cobblestone like. A significant enhancement in intensity of luminescence and quantum efficiency is observed in lithium compensated nanophosphors. The highest emission intensity is observed for the Gd1.75Eu0.1Li0.15O3 nanophosphor, about 1.83 times that of Gd1.9Eu0.1O3 and is attributed to the enhanced intra 4f–4f emission transitions arising from the modifications of the crystal field and distortion of the local symmetry around the europium ions. The luminescence decay profiles are found to be single exponential in nature and the lifetime measured was 1.36 ms for the Gd1.75Eu0.1Li0.15O3 nanophosphor. The chromaticity coordinates of these nanophosphors indicated high colour purity. Judd–Ofelt intensity parameters indicated that lithium compensation increases the polarization of the local environment and an increase of covalency and asymmetry around the europium ions.

Application of superconducting striplines to traveling-wave type LiNbO/sub 3/ optical modulator

The performance of a traveling-wave-type LiNbO/sub 3/ optical modulator with superconducting electrodes has been studied theoretically and experimentally. In the case of velocity matching between signal and optical waves using a shielding plane on top of the coplanar stripline, numerical calculations of the attenuation constants of both superconducting and normal-conducting striplines indicate that the performance of the optical modulator is expected to be much superior to that using normal metals with respect to the figure of merit of bandwidth/driving power. Low-temperature operation of the optical modulator with superconductor electrodes has been demonstrated in the temperature range between 4.2 K and 300 K for DC signals as well as microwave signals at frequencies from 8 GHz to 12 GHz. It was shown that the modulation voltage was not very sensitive to temperature variation.<<ETX>>