Osamu Ishiyama

Atomic Control of the SrTiO3 Crystal Surface

The atomically smooth SrTiO3 (100) with steps one unit cell in height was obtained by treating the crystal surface with a pH-controlled NH4F-HF solution. The homoepitaxy of SrTiO3 film on the crystal surface proceeds in a perfect layer-by-layer mode as verified by reflection high-energy electron diffraction and atomic force microscopy. Ion scattering spectroscopy revealed that the TiO2 atomic plane terminated the as-treated clean surface and that the terminating atomic layer could be tuned to the SrO atomic plane by homooepitaxial growth. This technology provides a well-defined substrate surface for atomically regulated epitaxial growth of such perovskite oxide films as YBa2Cu3O7-δ.

Atomic‐scale formation of ultrasmooth surfaces on sapphire substrates for high‐quality thin‐film fabrication

The atomically ultrasmooth surfaces with atomic steps of sapphire substrates were obtained by annealing in air at temperatures between 1000 and 1400 °C. The terrace width and atomic step height of the ultrasmooth surfaces were controlled on an atomic scale by changing the annealing conditions and the crystallographic surface of substrates. The obtained ultrasmooth surface was stable in air. The topmost atomic structure of the terrace was examined quantitatively by atomic force microscopy and ion scattering spectroscopy as well as a theoretical approach using molecular dynamics simulations.

Topmost surface analysis of SrTiO3 (001) by coaxial impact‐collision ion scattering spectroscopy

The terminating atomic plane of SrTiO3 (001) surface was investigated by means of coaxial impact‐collision ion scattering spectroscopy (CAICISS). CAICISS spectra proved that SrTiO3 (001) surfaces of as‐supplied substrates as well as of O2‐annealed substrates were predominantly terminated with TiO2 atomic plane, while the SrO atomic plane came at the topmost surface of SrTiO3 (001) homoepitaxial film. This indicates the structural conversion of the topmost atomic layer from TiO2 to SrO occurred during the SrTiO3 homoepitaxial growth. The azimuth rotational CAICISS spectra exhibited a fourfold symmetry in the surface atom alignments, showing the square lattice structure of a terminating plane.

Room-Temperature Epitaxial Growth of CeO2 Thin Films on Si(111) Substrates for Fabrication of Sharp Oxide/Silicon Interface

Room-temperature (20° C) epitaxy of high-melting point CeO2 thin films was achieved for the first time on Si(111) substrates. Cross-sectional high-resolution transmission electron microscopy, Rutherford backscattering spectrometry and ion scattering spectroscopy confirmed the formation of a sharp oxide/silicon heterointerface with no boundary amorphous layer and single crystallinity of the present CeO2 films. This was achieved by pulsed laser deposition in an ultrahigh vacuum under optimized oxygen partial pressures and by hydrogen termination of the Si surface.

Optimum Growth Conditions of GaAs(111)B Layers for Good Electrical Properties by Molecular Beam Epitaxy

Growth conditions of GaAs layers grown by molecular beam epitaxy (MBE) on (111)B-oriented GaAs substrates were optimized by surface morphology observation and Hall measurement. Good surface morphology was obtained by use of 1.5° off-oriented (111)B substrates. This paper reports for the first time that electron mobilities as high as those on (100) substrates can be obtained even at growth temperature as low as 530°C if the off-oriented (111)B substrates are used, whereas one higher than 600°C is necessary to obtain the same electron mobility on exactly (111)B-oriented substrates. MESFETs and Schottky diodes fabricated on the layer grown at the optimized condition show high electron mobility and suggest low concentration of defects.

Gate oxide reliability on trapezoid-shaped defects and obtuse triangular defects in 4H-SiC epitaxial wafers

The reliability of the gate oxide on large-area surface defects (trapezoid-shaped and obtuse triangular defects) in 4H-SiC epitaxial wafers is discussed. Time-dependent dielectric breakdown measurements revealed that a reduction in charge-to-breakdown (Qbd) occurred at a MOS capacitor including the downstream line of those defects. The deterioration of Qbd was at the same level in trapezoid-shaped defects and obtuse triangular defects. A cross-sectional transmission electron microscope (TEM) image and the simulation of the electric field distribution for the MOS structure indicated that the local fluctuation of the oxide thickness and/or the roughness of the interface on the downstream lines caused the degradation of the reliability of the MOS capacitors.

High-Resolution Imaging of Recording Marks on Phase-Change Film by Lateral Force Microscopy

Recording marks on a phase-change optical disk were observed by lateral force microscopy (LFM), and the images were compared with those obtained by Kelvin force microscopy (KFM). The spatial resolution of LFM was considerably better than that of KFM. Therefore it is expected that LFM will become one of the effective techniques for evaluating the shapes of recording marks in next-generation phase-change optical disks.

Reduction of Point Defects in GaAs Films Grown on Fluoride/Si Structures by the 2-Step Growth Method

The reason why electrical characteristics of GaAs films grown on CaF2/Si(111) structures at high substrate temperature (600°C) are degraded is studied. Rutherford backscattering spectroscopy (RBS) measurements and secondary ion mass spectroscopy (SIMS) observations revealed that the degradation was due to point defects which resulted from a degraded interface region formed by mutual diffusion between GaAs and CaF2. It was found that the 2-step growth method was effective in reducing the point defects of GaAs film.

Characterization of scraper-shaped defects on 4H-SiC epitaxial film surfaces

We have found undiscovered defects on a 4H-SiC epitaxial layer, the shape of which resembles a scraper in images taken by confocal differential interference contrast optical microscopy. The surface morphological structure and formation mechanism of the scraper-shaped defects were investigated by atomic force microscopy and grazing incidence monochromatic synchrotron X-ray topography, respectively. The scraper-shaped defects were surface morphological defects consisting of surface asperity and were caused by the migration of interfacial dislocations. The evaluation of the thermal oxide reliability of metal–oxide–semiconductor capacitors fabricated on these defects was performed by time-dependent dielectric breakdown measurement. The degradation of thermal oxide occurred only on the downstream line of the scraper-shaped defects. The thickness of the oxide layer on these defects was also investigated using cross-sectional transmission electron microscopy.

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.