Y. Kido

Characterization and control of the HfO2/Si(001) interfaces

The HfO2/Si(001) interfaces formed by reactive dc sputter deposition of Hf buffer layer followed by HfO2 stacking were analyzed by high-resolution transmission electron microscopy, medium energy ion scattering (MEIS), and photoelectron spectroscopy using synchrotron-radiation lights. The present MEIS analysis determined the elemental depth profiles and revealed that no Hf buffer layer resulted in growth of SiO2 at the interface, and that the presence of the Hf layer led to the formation of Si-rich silicate-like interlayers. The binding energy shifts of Si-2p3/2 identified the chemical bonds of the interfacial layers and confirmed the formation of SiO2 (no buffer layer) and silicate layers (presence of the buffer layers) at the interfaces. The Hf-buffer layer suppresses the O diffusion toward the interface and thus the thicker the buffer layer, the thinner the Hf-silicate interlayer. The deposition condition of HfO2 (1.3 nm)/Hf (1.3 nm) has achieved the highest permittivity of 28 for HfO2 (3.6 nm) and 8 fo...

A new toroidal electrostatic analyzer and application to surface analysis

A new toroidal electrostatic analyzer (ESA) configuration for medium energy ion scattering has been designed and fabricated for structural analysis of surfaces. The present analyzer has a wide interelectrode distance of 16 mm and an energy range of 10% of the pass energy at a constant applied voltage. The analyzer is mounted horizontally on a turntable and accepts ions scattered within a well defined angular range. To get a good energy resolution, we employed a photon-counting image acquisition system (PIAS: Hamamatsu Photonics) with spatial resolution of 40–50 μm, combined with a three-stage microchannel plate. The electric fields of the toroidal ESA including the fringing fields were calculated by a finite element method. Ion trajectories through the analyzer were calculated using a Monte Carlo method. In this way, the optimum conditions of the geometry and size of the entrance and exit slits were determined. Our toroidal analyzer resolved three surface peaks from the Si isotopes (28Si, 29Si, and 30Si) ...

The source of the Ti 3d defect state in the band gap of rutile titania (110) surfaces

The origin of the Ti 3d defect state seen in the band gap for reduced rutile TiO(2)(110) surfaces has been excitingly debated. The probable candidates are bridging O vacancies (V(O)) and Ti interstitials (Ti-int) condensed near the surfaces. The aim of this study is to give insights into the source of the gap state via photoelectron spectroscopy combined with ion scattering and elastic recoil detection analyses. We have made three important findings: (i) The intensity of the gap state observed is well correlated with the sheet resistance measured with a 4-point probe, inversely proportional to the density of Ti-int. (ii) Sputter∕annealing cycles in ultrahigh vacuum (UHV) lead to efficient V(O) creation and condensation of Ti-int near the surface, while only annealing below 870 K in UHV condenses subsurface Ti-int but does not create V(O) significantly. (iii) The electronic charge to heal a V(O) is almost twice that to create an O adatom adsorbed on the 5-fold Ti row. The results obtained here indicate that both the V(O) and Ti-interstitials condensed near the surface region contribute to the gap state and the contribution to the gap state from the Ti-int becomes comparable to that from V(O) for the substrates with low sheet resistance less than ∼200 Ω∕square.

Superior properties of over thin films

It is demonstrated that (NBCO) thin films, deposited by laser ablation, show extraordinary properties which are unprecedented in the 123 system. These include ease of oxidation, very high crystallinity, surface stability, and a relatively high . The properties of NBCO thin films are compared with those of the best reported films, and the origins of their differences are discussed. The implications of NBCOs superior thin-film quality in the context of device applications are also addressed throughout the paper.

Catalytic Activity of Pt/TaB2(0001) for the Oxygen Reduction Reaction

Proton-exchange-membrane fuel cells (PEMFCs) are a promising power source for automobiles. For their wide application, however, there still remain several problems. 2] One problem is the limited mass activity (reaction rate per mass) of cathode electrocatalysts for the oxygen reduction reaction (ORR). Bulk Pt has a high specific activity (reaction rate per surface area), and the specific activity can be further increased by alloying the subsurfaces with several nonprecious metals, such as Fe, Co, Ni, Cu, Sc, or Y, or by replacing subsurfaces with Pd. However, the specific areas (surface area per mass of the precious metal) of bulk materials are small, and therefore, the mass activities (specific activity multiplied by specific area) are also small. To increase the mass activity, the specific surface area should be increased by decreasing the catalyst size to the nanometer scale. Although Pt nanoparticles supported on carbon (Pt/C) are used practically in PEMFCs, the mass activity is not sufficiently high because the decrease in the size of the catalyst leads to a decrease in the specific activity as a result of the so-called particle-size effect. 9, 10] To avoid the particlesize effect, the specific surface area must be increased while maintaining the extended bulklike surface morphology. This new approach was employed by the company 3M in the development of nanostructured thin-film (NSTF) catalysts, in which Pt films with a thickness of a few tens of nanometers are deposited on organic nanostructured whiskerlike supports. The discovery of these new electrocatalysts inspired a number of studies on the fabrication of electrocatalysts with an extended Pt surface and high specific surface area with the aim of further increasing the mass activity. Herein, we show that a high mass activity of 1890 Ag , which is six times as high as that of Pt/C (299 Ag ), can be attained by the use of an epitaxial Pt thin film with a thickness of 1.5 nm on a TaB2(0001) single-crystal substrate. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of the Pt/TaB2 structure at the 10 10 TaB2 incidence and the 2 1 10 TaB2 incidence are shown in Figure 1. TaB2(0001) was selected because of its strong bonding with Pt, as shown by our DFT calculations, which indicated a cohesive energy of the Pt monolayer on TaB2(0001) terminated with Ta of 6.47 eV, which is much larger than that of a Pt monolayer on a graphene sheet (3.76 eV) or on Pt(111) (5.06 eV). We deposited Pt on the cleaned TaB2(0001) substrate and carried out CO annealing to obtain a flat and uniform Pt surface. Figure 2a shows the cyclic voltammogram (CV) recorded during CO annealing. In the anodic scan of the first cycle, the oxidation current appeared at approximately 0.5 V and then gradually increased (preignition potential region). This oxidation current disappeared in following cycles. The oxidation current in the preignition region is due to CO oxidation at the sites of Pt adatoms and adislands; thus, the disappearance of this current suggests the elimination of the Pt adatoms and adislands. Figure 2b shows the voltammogram for CO stripping in an argon-purged solution. The electrochemical surface area (ECSA) was estimated from the charge of 420 mC cm 2 required for CO oxidation to be 0.21 cm, which corresponds to a roughness factor (ECSA/ geometrical surface area) of 1.06. The ORR activity of the Pt/ TaB2(0001) alloy was evaluated by linear sweep voltammetry with a rotating disk electrode (measurement at 1600 rpm) under oxygen-saturated conditions (Figure 2c). The current on Pt/TaB2(0001) was corrected to compensate for the geometrical conditions of the working electrode (see Figure S1 in the Supporting Information for details). The specific activity of Pt/TaB2(0001) (kinetic current at 0.9 V) was 4961 mAcm , which is more than twice that observed for polycrystalline Pt (1400 mAcm ) and Pt(111) (1867 mAcm ). The mass activity of Pt/TaB2(0001) was 1890 Ag , which is almost six times that of Pt/C (299 Ag ). The CVs recorded under argon-saturated conditions are shown in Figure 2d. The shape of the CV of Pt/ TaB2(0001) is more similar to that of polycrystalline Pt than to Figure 1. HAADF-STEM images of the epitaxial Pt thin film on the TaB2(0001) single-crystal substrate as observed for: a) 10 10 1⁄2 TaB2 incidence; b) 2 1 10 1⁄2 TaB2 incidence.

A novel analysis system of synchrotron-orbital-radiation-light induced photoemission coupled with ion scattering: SORI

Abstract We designed and constructed a novel analysis beam line connected to a 575 MeV electron synchrotron with a super-conducting magnet working at the Ritsumeikan University. It allows synchrotron-orbital-light induced photoemission and medium energy ion scattering spectroscopy under ultrahigh vacuum conditions and thus enables us to determine the electronic states and atomic configurations of surfaces and interfaces for various materials. Samples are prepared by molecular beam epitaxy and can be analyzed in situ by the above spectroscopy without exposing them to the air. The optical system consists of a cylindrical mirror for collection of the synchrotron radiation-light, a couple of plane mirrors for deflection and a varied-space plane grating as a monochromator and a toroidal mirror for focusing the monochromated light on a sample. Exchange of the varied-space plane grating monochromators of different line densities covers the photon energies from 5 up to 700 eV and the resolution ( ΔE E ) is estimated to be better than 6 × 10−4. Photoelectrons are detected with a hemispherical energy analyzer with a radius of 139.7 mm. A new toroidal electrostatic analyzer was designed and fabricated to realize a layer-by-layer analysis using medium energy ion scattering. A wide interelectrode distance of 18 mm covers a wide energy range, W( = 0.1 × E0, where E0 is the energy of ions deflected along a central curvature) at a fixed applied voltage and thus reduces the acquisition time without lowering the statistics. This paper represents the preliminary data obtained so far concerning surface hydrogen detection and surface relaxation of TiO2-terminated SrTiO3 (0 0 1).

Atomic scale characterization of HfO2∕Al2O3 thin films grown on nitrided and oxidized Si substrates

One and three bilayers of HfO2(9A)∕Al2O3(3A) thin films were grown by atomic layer chemical-vapor deposition on Si(001) substrates whose surfaces were nitrided or oxidized. The films as-grown and postannealed in an ultrahigh vacuum were analyzed by atomic force microscopy, photoelectron spectroscopy, and medium energy ion scattering. For the one- and three-bilayer films grown on the nitrided Si substrates, the HfO2 and Al2O3 layers are mixed to form Hf aluminates at temperatures above 600°C. The mixed Hf aluminate layer is partly decomposed into HfO2 and Al2O3 grains and Al2O3 segregates to the surface by postannealing at 900°C. Complete decomposition takes place at 1000°C and the surface is covered with Al2O3. The surfaces are uniform and almost flat up to 900°C but are considerably roughened at 1000°C due to the complete decomposition of the Hf aluminate layer. In contrast, for one- bilayer films stacked on the oxidized Si substrates, Hf silicate layers, including Hf aluminate, are formed by annealing a...

Effect of fluorine ion implantation on the microstructure and microhardness of AISI 440C stainless steel

Abstract AISI 440C stainless steel along with pure iron and chromium have been implanted with 30 keV fluorine ions at doses from 5 × 1016 to 6 × 1017 ions/cm2 and surface hardnesses of the 440C steel have been examined. Implanted regions inside the specimens were studied by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The results of XPS and XRD measurements indicate the formation of FeF2 and suggest small grains of CrF2 and non-stoichiometric chromium fluorides in 440C steel. It may be concluded that an increase of the surface hardness in fluorine implanted 440C steel is related to precipitation of metal fluorides and surface oxidation.

Surface structure of CuSi(111) at high temperature

The epitaxial growth and the structure of Cu on Si(111)7 × 7 deposited at high temperature (< 300–600°C) was investigated mainly by medium energy ion scattering (MEIS) and scanning tunneling microscopy (STM). The domain images whose periodicity is about 5.5 ± 0.2 times of the Si bulk unit were observed at high sample bias (VS = 2.5 V). The periodicity coincides with the ‘5 × 5’ incommensurate structure that was observed by reflection high energy electron diffraction (RHEED). The ratio of dark to bright area of ‘5 × 5’ was estimated to guess the Si surface structure in the ‘5 × 5’ incommensurate layer after counting the number of Si atoms. The ratio was about 0.83 and there are about 1 monolayer of Si atoms in the incommensurate layer. The structure of the Si bulk that is just beneath the incommensurate layer might be the double layer and the first layer of the Si bulk might be relaxed inward by 0.01 nm after the measurements of the blocking profiles by MEIS.

The mechanism of emerging catalytic activity of gold nano-clusters on rutile TiO2(110) in CO oxidation reaction.

This paper reveals the fact that the O adatoms (O(ad)) adsorbed on the 5-fold Ti rows of rutile TiO(2)(110) react with CO to form CO(2) at room temperature and the oxidation reaction is pronouncedly enhanced by Au nano-clusters deposited on the above O-rich TiO(2)(110) surfaces. The optimum activity is obtained for 2D clusters with a lateral size of ∼1.5 nm and two-atomic layer height corresponding to ∼50 Au atoms∕cluster. This strong activity emerging is attributed to an electronic charge transfer from Au clusters to O-rich TiO(2)(110) supports observed clearly by work function measurement, which results in an interface dipole. The interface dipoles lower the potential barrier for dissociative O(2) adsorption on the surface and also enhance the reaction of CO with the O(ad) atoms to form CO(2) owing to the electric field of the interface dipoles, which generate an attractive force upon polar CO molecules and thus prolong the duration time on the Au nano-clusters. This electric field is screened by the valence electrons of Au clusters except near the perimeter interfaces, thereby the activity is diminished for three-dimensional clusters with a larger size.