Takaharu Nishihara

ATOMICALLY DEFINED EPITAXY AND PHYSICAL PROPERTIES OF STRAINED LA0.6SR0.4MNO3 FILMS

La0.6Sr0.4MnO3 thin films were fabricated on SrTiO3 (001) substrates using pulsed laser deposition with observing persistent intensity oscillation of reflection high-energy electron diffraction. By atomic force microscopy, the surface of resulting films was confirmed to be extremely flat, showing atomically smooth terraces and 0.4 nm high steps corresponding to a unit cell height of perovskite. The surface terminating atomic layer was unambiguously assigned to the MnO2 layer by coaxial impact collision ion scattering spectroscopy. Crystal symmetry of the films is distorted into a tetragonal one due to the strain to fulfill perfect in-plane matching with the substrate even for films as thick as 100 nm. Even for films as thin as 4 nm (10 unit cells), ferromagnetic transition takes place to induce a metallic state and large negative magnetoresistance is observed as well.

Computer Simulation for Analysis of Lattice Polarity of Wurtzite GaN{0001} Film by Coaxial Impact Collision Ion Scattering Spectroscopy

Signal intensities of coaxial impact collision ion scattering spectra have been computed for c-axis-oriented GaN films at various incident angles in order to analyse the lattice polarity based on the three-dimensional two-atom triple-scattering model. It was found that Ga and N signal intensities show specific incident angle dependences on (0001) and (000) surfaces. The physical image of each characteristic feature in the spectra was also clarified.

High Resolution Electron Microscopy of Crystal and Defect Structures of the High-Tc Superconductor Ba2YCu3O7-δ

Structural study of the superconducting oxide Ba2YCu3O7-δ has been made at room temperature by high resolution electron microscopy. The orthorhombic structure was confirmed by the direct imaging of metal atoms. Planar defects with stacking sequence of metal-oxygen planes in the La2CuO4 type of structure were observed. Depending on the selected arca in electron diffraction, the axial ratio b/a was found to vary in a range from 1.003 to 1.018. Black and white contrasts appearing alternately along the twin boundaries are discussed in relation to the lattice distortion and the oxygen deficiency in the Cu-O layer.

Terminating Structure of Plasma-Assisted Molecular Beam Epitaxial GaN{0001} Film Surface Identified by Coaxial Impact Collision Ion Scattering Spectroscopy

Terminating structures of the GaN{0001} films grown on nitrided sapphire(0001) substrates by plasma-assisted molecular beam epitaxy (MBE) have been investigated by coaxial impact collision ion scattering spectroscopy (CAICISS). The analyses of incidence angle dependences of time of flight (TOF) spectra have shown that the surfaces of GaN films grown under both N-rich and Ga-rich conditions are (0001) N-planes terminated with Ga atoms. This implies that (0001), N-terminated surfaces of GaN films grown under these conditions are unstable and a Ga-rich condition is required to avoid the N-deficiency in the grown GaN film.

Electron Diffraction and Microscopy of the Structure of La-Ba(Sr)-Cu Oxides at Room Temperature

Structural study of the La-Ba(Sr)-Cu oxides was made by means of electron diffraction and microscopy at room temperature. In the orthorhombic La1.9Ba0.1CuO4 weak extra reflections are found and the space group was determined as Pccm. On the other hand, no extra reflections are found for the tetragonal La1.8Ba0.2CuO4 with onset Tc=28 K and La1.8Sr0.2CuO4 with onset Tc=36 K. High resolution reveals the metal atom arrangement in these compounds.

Rf Power Dependence of AC Josephson Current in Point-Contacts of BaY(Tm)CuO Ceramics

Using BaYCuO and BaTmCuO ceramics, point-contact Josephson junctions were fabricated to observe ac Josephson effect. Clear Shapiro steps were observed at 4.2 K. The rf power dependence of Josephson currents were qualitatively fitted to Bessel functions.

Surface Structure of ZnO Single Crystals Analysed by Ion Scattering Spectroscopy

A ZnO crystal of wurtzite-type structure has polar surfaces of (0001) and (0001), which are terminated by Zn and O ions, respectively. Observation conducted by atomic force microscopy showed that step/terrace structures were formed on the (0001) and (0001) surfaces, whose step height was predominantly half of lattice parameter, c0. The ionic arrangement of the surfaces was determined by coaxial impact-collision ion scattering spectroscopy (CAICISS); the CAICISS spectra had a periodicity of 60° for azimuth angle dependence. From the analyses of CAICISS spectra it was shown that the (0001) and (0001) surfaces had a single dangling bond, and no ionic distortion occurred.

AC Josephson effect in point-contacts of Ba-Y-Cu-O ceramics

Clear Shapiro-steps were observed in I-V curves of point-contact Josephson junctions fabricated by BaYCuO ceramics. The observed voltage step is just equal to applied frequency times (h/2e), where h and e are, respectively, the Planck constant and electron charge.

Identification of Surface Atoms of LiGaO2(001) Substrate for Hexagonal GaN Film by Coaxial Impact Collision Ion Scattering Spectroscopy

Terminating surface atoms of LiGaO2(001) substrate for GaN thin film growth have been directly identified by coaxial impact collision ion scattering spectroscopy (CAICISS). We found that the terminating atoms of the easily etched surface of the substrate are oxygen and that those of the hardly etched surface are metal (Li and Ga). The relation is explained by the surface bonding model of the wurtzite-type AII-BIV compound. GaN thin films grew epitaxially only on the metal surface of the substrate.

Identification of the terminating structure of 6H–SiC(0001) by coaxial impact collision ion scattering spectroscopy

The terminating structure of 6H–SiC(0001) substrates fabricated by the Acheson method was directly identified by means of coaxial impact collision ion scattering spectroscopy (CAICISS). The CAICISS spectra showed that the topmost surfaces of the samples were Si-terminated planes for both the front and rear faces. It was also shown that the (0001)Si face was composed of Si-terminated flat terraces and steps, the height of which corresponded to one-half the unit cell length along the 6H–SiC c axis.