Shigeki Otani

Quantitative Surface Atomic Geometry and Two-Dimensional Surface Electron Distribution Analysis by a New Technique in Low-Energy Ion Scattering

A new technique for analyzing the quantitative surface atomic geometry and the two-dimensional surface electron distribution is reported. The remarkable effectiveness of this new technique is demonstrated for TiC(111).

Temperature dependence of thermal expansion and elastic constants of single crystals of ZrB2 and the suitability of ZrB2 as a substrate for GaN film

Coefficients of thermal expansion (CTE) and elastic constants of single crystals of ZrB2 have been determined in the temperature ranges from room temperature to 1073 K and from room temperature to 1373 K, respectively. The elastic constants of ZrB2 are best characterized by the large value of the Young modulus (as high as 500 GPa) and the small values of the Poisson ratio (0.13–0.15), indicating the high stiffness and hardness and the brittleness, respectively. The values of CTE along the a- and c-axis directions are 6.66×10−6 and 6.93×10−6 K−1, respectively, when averaged over the temperature range from room temperature to 1073 K. The CTE value along the a-axis direction of ZrB2 is only moderately larger than the corresponding value for GaN. This together with the small lattice mismatch along the a-axis direction between ZrB2 and GaN in the heteroepitaxial orientation relationship of (0001)GaN//(0001)ZrB2 and 〈1120〉GaN//〈1120〉ZrB2 indicate that only a small compressive stress develops in the GaN thin-f...

Electronic states of monolayer graphite formed on TiC(111) surface

Abstract Electronic states of a monolayer graphite formed on TiC(111) surface have been investigated by means of XPS, UPS, and work-function measurement. The chemical shift of C1s peaks in XPS spectra has inhibited a large electron transfer from the substrate to the monolayer graphite. On the other hand, the band structure of the graphite overlayer has altered from that of bulk graphite. The work-function measurement has suggested an electron redistribution in the graphite layer. These results indicate that the electronic states of the graphite monolayer are modified not by the charge transfer, but mainly by the orbital hybridization between the graphite monolayer and the substrate, which differs from graphite intercalation compounds.

Atomic chemical composition and reactivity of the TiC(111) surface

Abstract The properties of both the clean and oxygen covered TiC(111) surfaces have been studied by electron spectroscopy and ion scattering spectroscopy. It has been confirmed that the topmost layer of the clean surface consists of Ti atoms, which are arranged in a 1 × 1 surface unit cell. The work function has been measured to be 4.7 ± 0.1 eV, which is much higher than those of the neutral surfaces such as (100). This may be attributed to the compensational charge, which is induced in front of the Ti topmost layer so as to eliminate the electric field of the polar surface.

Zirconium Diboride (0001) as an Electrically Conductive Lattice-Matched Substrate for Gallium Nitride.

Zirconium diboride (ZrB2) has a hexagonal crystal structure with an in-plane lattice constant of 3.168 A, very close to that of GaN (3.189 A). It is a metalloid compound and a very good electrical conductor, with a resistivity of 4.6 µΩcm. We propose ZrB2 (0001) as an electrically conductive lattice-matched substrate for GaN. Firstly, bulk crystal growth of ZrB2 using a radio frequency (rf)-heated floating zone (FZ) method is presented. Relatively large crystals (10 mm×60 mm) were obtained using this method. The thermal expansion coefficient and thermal conductivity of ZrB2 were evaluated using this crystal. Characterization of ZrB2 (0001) mirror-polished substrate was carried out by optical microscopic examination and X-ray diffraction. Finally, the results of heteroepitaxial growth are briefly mentioned.

Mechanism of electron exchange between low energy He+ and solid surfaces

Abstract A general picture on the mechanism of electron exchange between low energy He + ions and solid surfaces is proposed on the basis of experiments on three-dimensional angle resolving ion scattering spectroscopy in which not only He + but neutral He is used as a projectile.

Structural refinement and thermal expansion of hexaborides

Abstract In the boron-framework of hexaborides, CaB 6 , SrB 6 , BaB 6 , YB 6 and LaB 6 , the boron position was found to be adjusted so as to compensate the interaction between the cations. It was estimated from the boron position that a boron–boron bond between the boron octahedra has a 1.5 times higher force constant than that in the octahedron, which is consistent with the results of the neutron and Raman scattering experiments. In addition, the adjustment of the boron position well corresponded with the thermal expansion.

Preparation of TiCx single crystals with maximum carbon content by a floating zone technique

TiCx single crystals with a maximum carbon content (x = 0.96–0.97) were prepared by controlling the compositions of the initial molten zone and the feed rod (a modified zone leveling method). The homogeneity of the composition was checked along the growth and radial directions. The lattice constants, densities and impurities in the crystal, starting material, feed rod and evaporation product, were measured and reported. All impurities except W, whose carbide has a low vapor pressure, were found to be refined by evaporation. The effect of the helium ambient gas pressure was examined. It was found that good quality crystals could be prepared under relatively low pressure (4–5 atm). The good quality of the crystals was confirmed by X-ray transmission topography.

Interaction Potential between He+ and Ti in a keV Range as Revealed by a Specialized Technique in Ion Scattering Spectroscopy

The interaction potential between He+ and Ti in a keV range has been studied by a specialized technique in low-energy ion scattering spectroscopy. The interaction potential is described well by the Thomas-Fermi-Moliere potential for the interaction between neutral He and Ti with a scaled screening radius.

Preparation of ZrB2 single crystals by the floating zone method

Abstract High quality ZrB2 crystals, free of subgrain boundaries, were prepared in 0.8 MPa of helium ambient gas by the RF heated floating zone method. The optimum growth rate was 2.5−3 cm/h. The composition of the molten zone, which influenced the growth rate, was determined by the difference in vapor pressure between Zr and B elements. Helium ambient gas decreased the inclusions in the grown crystals and consequently increased the optimum growth rate, compared with the growth in argon ambient gas.