Nobuo Otsuka

Crystal structure and semiconductor-metal transition of the quasi-two-dimensional transition metal oxide, La2NiO4

Mesures de diffraction RX, calorimetrie differentielle a balayage, RPE, susceptibilite magnetique et conductivite electrique, entre 10 et 300 K, sur des poudres et monocristaux La 2 NiO 4

Direct observation of MgCl2‐supported Ziegler catalysts by high resolution transmission electron microscopy

The surface atomic structure of MgCl 2 crystalline particles and MgCl 2 -supported Ziegler catalysts was observed by means of high resolution transmission electron microscopy. Step-terrace surface structures, characteristic of the structure of the MgCl 2 crystal, are found in the observed images of MgCl 2 particles. The observation of the structure of MgCl 2 -suuported Ziegler catalysts shows that the MgCl 2 crystals are severely deformed by the processes of catalyst preparation. Due to the preparation procedure used the structure of the catalyst changes from crystalline to amorphous

Study of Luminescent Region in Anodized Porous Silicons by Photoluminescence Imaging and Their Microstructures

The luminescent region in anodized porous silicon was examined by means of a photoluminescence imaging technique. It was found for the first time that the luminescence in the visible region originates from the topmost surface layer. This was confirmed not only by the surface photoexcitation but also by the excitation of the cleaved edge. This topmost surface layer does not show any diffraction spots with transmission-electron microscopy and is regarded as an amorphous layer. The microstructure of this layer observed by means of secondary-electron microscope (SEM) consisted of microparticles with the dimensions of 5~30 nm, where the lower dimension is limited by the SEM resolution.

Vacancy ordering of Ga2Se3 films by molecular beam epitaxy

Ga2Se3 films have been grown by molecular beam epitaxy. Single‐crystal Ga2Se3 films were obtained for the first time on (001) GaP substrates at a substrate temperature of 550 °C and at a VI/III flux ratio greater than 15. Extra diffraction spots with weak intensity were observed in reflection high‐energy electron diffraction patterns of the Ga2Se3 films grown at a VI/III ratio above 150. These extra spots were also observed in the transmission electron diffraction pattern. The extra points are attributed to the ordering of native Ga vacancies in the defect zinc blende structure.

Surface atomic process of incorporation of excess arsenic in molecular-beam epitaxy of GaAs

The surface atomic process that leads to the incorporation of extremely high concentrations of excess arsenic into GaAs layers during growth by moelcular-beam epitaxy at low temperatures is investigated. A model of the surface atomic process is derived on the basis of the results of growth experiments where the dependence of the concentration of excess arsenic in the GaAs layer on the flux condition and growth temperature has been examined. In the model, arsenic atoms that are chemisorbed on the arsenic-terminated GaAs(100) surface serve as precursors of excess arsenic, and, hence, the concentration of excess arsenic depends directly on the steady-state coverage of the chemisorbed arsenic atoms. The model is developed quantitatively on the basis of the Langmuir adsorption model, which explains the majority of experimental results but leads to an extremely low activation energy for desorption of adsorbed arsenic atoms.

Observation of the verwey transition in Fe3O4 by high-resolution electron microscopy

Abstract High resolution electron microscopy (HREM) of Fe 3 O 4 has been carried out at temperatures near the Verwey transition (∼120 K) with a point resolution of 3 A. Lattice fringes of both the high- and the low-temperature phases have been observed at these temperatures. The crystal symmetry of the low-temperature phase indicated by the lattice images is consistent with the result obtained by earlier diffraction studies. A series of lattice images showing the transition from the low-temperature phase (to the high-temperature phase) has been obtained. The transformation to the high-temperature phase occurs through the penetration of the high-temperature phase into areas of the low-temperature phase. A quick motion of domain boundaries in the low-temperature phase, which is consistent with almost instantaneous rearrangements of charge ordering, has been observed. The possibility of determining the ordered arrangement of Fe 2+ and Fe 3+ ions directly by HREM is discussed.

Arsenic flux dependence of incorporation of excess arsenic in molecular beam epitaxy of GaAs at low temperature

Incorporation of excess As in GaAs layers grown by molecular beam epitaxy was studied by varying As fluxes for four different substrate temperatures, 210, 240, 270, and 290 °C. Concentrations of excess As in GaAs layers were estimated by measuring increases of lattice spacings with x-ray diffraction, and the substrate surface temperature was monitored by using a quartz rod connected to an infrared pyrometer with its end placed in the vicinity of the substrate surface. Nearly stoichiometric GaAs layers without any detectable increase of the lattice spacing are grown at all substrate temperatures under the As atom flux equal to the Ga atom flux. With a slight increase of the As flux from the above stoichiometric condition, the concentration of excess As sharply increases for all substrate temperatures. For the substrate temperature of 210 °C, the concentration of excess As is saturated in the range of As atom fluxes more than three times the Ga atom flux, while similar tendencies are observed for other subs...

Growth of single phase, single crystals of V9O17☆

Abstract Single crystals of Magneli phase V 9 O 17 were successfully grown for the first time by a vapor transport technique utilizing TeCl 4 as a transport agent. No trace of syntactically intergrown V 8 O 15 could be detected in these crystals. The structure is triclinic and is homologous to other Magneli phase compounds V n O 2 n −1 ( n = 3 ∼ 8). The lattice parameters (of the reduced cell) measured are a = 7.009 Ȧ b = 14.518 Ȧ c = 5.418 Ȧ α = 95.5 ° β = 108.9 ° γ = 83.1 ° This compound manifests a metal-insulator transition as in other Magneli phase compounds. The transition temperature has been determined to be 79 K. At and below the transition point down to 16 K, at which an anomalous peak in the resistivity exists, a time dependence of resistivity at fixed temperatures is found. The existence of V 10 O 19 was also confirmed by transmission electron microscopy in a material produced under highly nonequilibrium conditions, intergrown in the matrix of V 9 O 17 in a microsyntactoc fashion. The existence of V 10 O 19 has never been reported.

Photoluminescence of confined excitons in nanoscale C60 clusters

Abstract Formation of C60 aggregates has been found in toluene, benzene and carbon disulfide (CS2) solutions by means of photoluminescence spectroscopy. From the detailed investigation of the temperature dependence of luminescence, it has been confirmed that the C60 aggregates are formed at freezing temperature of these solvents in the cooling process. The 0 0 transition energy of their luminescence spectrum has been found to be blue-shifted in comparison with that of C60 crystal. It has been also found that the irradiation of ultraviolet light upon the C60 aggregates in benzene at 265 K transforms them to stable substances under atmosphere, which look like round-shaped nanoscale particles in the high-resolution TEM images.

ELECTRICAL AND STRUCTURAL PROPERTIES OF BE- AND SI-DOPED LOW-TEMPERATURE-GROWN GAAS

Excess As is incorporated in GaAs grown at low substrate temperatures by molecular beam epitaxy. Excess As is distributed in the epilayer as defects and the material exhibits considerable strain. When annealed to moderate temperatures, the strain is seen to disappear and the excess As is now in the form of semimetallic clusters. It has been proposed that these As clusters form buried Schottky barriers with the GaAs matrix and are surrounded by spherical depletion regions. In this article, we examine the effects of doping on the material properties and compare our results to the buried Schottky barrier mode. Si‐doped GaAs epilayers grown at 250 °C, with doping densities between 5×1017 and 5×1018 cm−3, were annealed to temperatures between 700 and 1000 °C for 30 s. Be‐doped GaAs epilayers grown at 250 °C, with doping densities between 5×1017 and 5×1019 cm−3, were annealed to temperatures between 700 and 900 °C for 30 s. Using extensive Hall measurements and transmission electron microscopy, we observe that ...