Shunichi Hishita

Passivation of active recombination centers in ZnO by hydrogen doping

The effect of hydrogen doping on luminescence properties of ZnO was investigated. Hydrogen was incorporated in the ZnO crystal by irradiation with an inductively coupled plasma (ICP), in particular, the pulse modulated mode operation of ICP, and the luminescence spectra and hydrogen concentration of the resultant samples were analyzed. A hydrogenated region of 20–100 nm was formed at the sample surface by the irradiation and the concentration of hydrogen was 1017–1018 cm−3. Hydrogen doping improved the ultraviolet emission efficiency of all the samples, and the degree of improvement depended on the initial state (impurity concentration) of the original samples. The most significant improvements were recorded for the sample lightly contaminated with Cu, Al, and Li. The correlation between impurity concentration and hydrogen doping effects is discussed from the viewpoint of charge transfer between hydrogen and the other impurities.

Suspended Single‐Crystal Diamond Nanowires for High‐Performance Nanoelectromechanical Switches

Nanoor microelectromechanical systems (N/MEMS) have become an important mainstream technology for a wide range of applications such as chemical, biological, and mass sensors; actuators; radio-frequency N/MEMS; micro-optoelectromechanical systems; and many others. [ 1 ] Among these applications, the nanoor microelectromechanical (N/MEM) switch is one of the most attractive fi elds due to its zero-current leakage, lowpower consumption, and no necessity for the formation of an p-n junction, and can be applied in various fi elds such as digital logic circuits, memory, and radio-frequency wireless communication. According to the International Technology Roadmap for Semiconductors, the N/MEM switch has been considered as one of the potential strategies beyond complementary metaloxide-semiconductor (CMOS). The current N/MEM switches based on silicon technologies are suffering from problems of stiction, abrasion, and poor mechanical and tribological properties, leading to the poor reliability of the electric contacts. [ 2 – 4 ]

Structural characterization of epitaxial BaTiO3 thin films grown by sputter deposition on MgO(100)

Epitaxial thin films of BaTiO3 have been prepared using radio‐frequency magnetron sputter deposition on MgO(100) substrates. Various x‐ray‐diffraction techniques were employed to characterize the crystal structure of the films. The films were fully tetragonal and consisted of c domains. Their tetragonality has been shown to be 1.013, which is almost the same as the bulk value of BaTiO3. However, the cross‐sectional transmission electron microscopic micrograph showed that a very small amount of a domains coexists forming 90° domain boundaries in the matrix of c domains. In spite of the negligible strain caused by the phase transformation, it seems to be inevitable to form a certain small amount of a domains in the BaTiO3 film on MgO system.

Nanoporous Carbon Sensor with Cage-in-Fiber Structure: Highly Selective Aniline Adsorbent toward Cancer Risk Management

Carbon nanocage-embedded nanofibrous film works as a highly selective adsorbent of carcinogen aromatic amines. By using quartz crystal microbalance techniques, even ppm levels of aniline can be repetitively detected, while other chemical compounds such as water, ammonia, and benzene give negligible responses. This technique should be applicable for high-throughput cancer risk management.

Low‐Temperature Remediation of NO Catalyzed by Interleaved CuO Nanoplates

A copper(II)-oxide-based exhaust catalyst exhibits better activity than Pt- and Rh-nanoparticle catalysts in NO remediation at 175 °C. Following theoretical design, the CuO catalyst is rationally prepared; CuO nanoplates bearing a maximized amount of the active {001} facet are arranged in interleaved layers. A field test using a commercial gasoline engine demonstrates the ability of the catalyst to remove NO from the exhaust of small vehicles.

Dephasing of coherent phonons by lattice defects in bismuth films

We have studied the effect of point defects on coherent optical phonons in ion-implanted bismuth polycrystalline films. Ultrafast dynamics of coherent phonons and photogenerated carriers in the femtosecond time domain have been investigated by means of pump-probe reflectivity measurements. The dephasing rate of the A1g phonon increases linearly with increasing ion dose, which is explained by the additional dephasing process of the coherent phonon originated from scattering of phonons by the defects. Carrier dynamics are also found to be affected by additional scattering process mediated by point defects.

Fabrication of ZnO Microstructures by Anisotropic Wet-Chemical Etching

ZnO microstructures were fabricated by wet-chemical etching with aqueous solutions of HCl and CH 3 COOH and patterned resist masks prepared by electron-beam lithography. Anisotropy of the etching rate caused the formation of microstructures corresponding to the crystal habit. In fact, all the pit structures made on (0001), (1010), and (1120) surfaces of ZnO single crystal were characterized by {101n} (n = integer) faces, and all the pyramidal structures made on the (0001) surface were characterized by {112n} facets forming hexagonal projections. Regarding etchant dependence, CH 3 COOH[aq] enabled us to create structures with sharp edges and comers, while etching with HCl[aq] resulted in structures with rounded shapes. The coalescence of pits during chemical etching was also investigated using mask patterns having symmetrically aligned windows that were close to each other.

Ultrafast Dynamics of Surface-Enhanced Raman Scattering Due to Au Nanostructures

Ultrafast dynamics of surface-enhanced Raman scattering (SERS) was investigated at cleaved graphite surfaces bearing deposited gold (Au) nanostructures (∼10 nm in diameter) by using sensitive pump-probe reflectivity spectroscopy with ultrashort (7.5 fs) laser pulses. We observed enhancement of phonon amplitudes (C═C stretching modes) in the femtosecond time domain, considered to be due to the enhanced electromagnetic (EM) field around the Au nanostructures. Finite-difference time-domain (FDTD) calculations confirmed the EM enhancement. The enhancement causes drastic increase of coherent D-mode (40 THz) phonon amplitude and nanostructure-dependent changes in the amplitude and dephasing time of coherent G-mode (47 THz) phonons. This methodology should be suitable to study the basic mechanism of SERS and may also find application in nanofabrication.

Non-equilibrium defects in aluminum-doped zinc oxide thin films grown with a pulsed laser deposition method

Zinc oxide (ZnO) films doped with aluminum (Al) were deposited with a pulsed laser deposition technique to characterize the charge compensation phenomena in ZnO. In particular, oxygen radical (O*) irradiation during film deposition was used to modify the oxygen stoichiometry. Irradiation with O* decreased electron concentration in Al-doped ZnO. The lattice parameter of the resultant films also varied with the growth conditions. However, no obvious correlation between electron concentration and lattice parameter was found. The self-diffusion coefficients indicated the presence of non-equilibrium defects. The properties of the films are discussed from the viewpoint of non-equilibrium compensated defects detected in the diffusion measurements.

Growing BaTiO3 thin films on Si(100) with MgO-buffer layers by sputtering

Abstract Epitaxial thin films of MgO were grown on Si(100) with a cube-on-cube relationship as a buffer layer for the growth of ferroelectric BaTiO 3 thin films by RF magnetron sputter deposition. Partially (001) or (100) textured BaTiO 3 thin films were obtained on Si(100) with the MgO buffer layer. On the other hand, randomly oriented BaTiO 3 thin films with large-scale cracks were prepared without the MgO layer. The comparison of the crystallographic orientation, morphology and dielectric properties of the BaTiO 3 thin films on Si(100) with and without the MgO layer revealed the favourable influence of the MgO-buffer layer on the growth of BaTiO 3 thin films on Si(100) substrates.