Masaharu Oshima

Theoretical characterization of X-ray absorption, emission, and photoelectron spectra of nitrogen doped along graphene edges.

K-edge X-ray absorption (XAS), emission (XES), and photoelectron (XPS) spectra of nitrogen doped along graphene edges are systematically investigated by using first-principles methods. In this study we considered pyridinium-like, pyridine-like, cyanide-like, and amine-like nitrogens at armchair and zigzag edges and pyrrole-like nitrogen at armchair edge as well as graphite-like nitrogen at graphene interior site. Our results indicate that nitrogen configuration and its location (armchair or zigzag edge) in nitrogen-doped graphene can be identified via the spectral analysis. Furthermore, some controversial spectral features observed in experiment for N-doped graphene-like materials are unambiguously assigned. The present analysis gives an explanation to the reason why the peak assignment is usually made differently between XPS and XAS.

Direct observation of charge transfer region at interfaces in graphene devices

Nanoscale spectromicroscopic characterizing technique is indispensable for realization of future high-speed graphene transistors. Highly spatially resolved soft X-ray photoelectron microscopy measurements have been performed using our “3D nano-ESCA” (three-dimensional nanoscale electron spectroscopy for chemical analysis) equipment in order to investigate the local electronic states at interfaces in a graphene device structure. We have succeeded in detecting a charge transfer region at the graphene/metal-electrode interface, which extends over ∼500u2009nm with the energy difference of 60u2009meV. Moreover, a nondestructive depth profiling reveals the chemical properties of the graphene/SiO2-substrate interface.

Observation of rebirth of metallic paths during resistance switching of metal nanowire

To clarify the mechanism of resistance-switching phenomena, we have investigated the change in the electronic structure of a Ni nanowire device during resistance-switching operations using scanning photoelectron microscopy techniques. We directly observed the disappearance of density of state (DOS) at the Fermi level (EF) in a high-resistance state and recovery of a finite DOS at EF in a low-resistance state. These results are direct evidence that the Ni nanowire is fully oxidized after switching to the high-resistance state and that Ni-metal conductive paths in the oxidized nanowire are recovered in the low-resistance state.

Band Configuration of SiO2/m-Plane ZnO Heterointerface Correlated with Electrical Properties of Al/SiO2/ZnO Structures

SiO2 films have been deposited on m-plane ZnO(1100) substrates by atomic layer deposition method at 200 °C and the interface has been investigated. Band structures of amorphous SiO2/m-plane ZnO(1100) heterointerface have been characterized by X-ray photoelectron spectroscopy, showing type I band configuration with valence and conduction band offsets of 1.7±0.2 and 3.6±0.2 eV, respectively. Capacitance–voltage (C–V) measurements of Al/SiO2/m-plane ZnO(1100) have been performed. When gate bias was swept from positive to negative, a ledge appeared in the C–V curve of the Al/SiO2/m-plane ZnO structure, which is probably due to the emission of electrons trapped at the near-midgap deep levels. However, it is found that the surface treatment for ZnO using HCl yielding automatically flat stepped and terraced surface improves the C–V curve without the ledge.

Structural Properties of m-Plane InAlN Films Grown on ZnO Substrates with Room-Temperature GaN Buffer Layers

The growth of m-plane InxA1-xN (0.34 < x < 0.89) films was achieved using ZnO(1100) substrates and pulsed-laser-deposited room-temperature-commensurate GaN buffer layers. Atomic force microscopy images of m-plane In0.39Al0.61N exhibited an atomically flat stepped-and-terraced structure. The growth temperature leading to compositional instability of m-plane In-rich InAlN was as low as 300 ?C, which indicates that the low-temperature growth technique is necessary for the epitaxial growth of m-plane InAlN. Although 100-nm-thick m-plane InxAl1-xN (x = 0.39 and 0.36) films were coherently grown, InAlN films with an In composition greater than 50% were partially relaxed. Furthermore, it was determined that the strain relaxation preferentially takes place along the c-axis. The full width at half maximum values for the X-ray rocking curves of a coherently grown m-plane In0.39Al0.61N film were as small as ?250 arcsec, while the crystalline quality of lattice-relaxed In-rich InAlN films was poor. These results indicate that the selection of substrates with appropriate lattice constants is essential for obtaining high-quality m-plane InAlN.

Capability of insulator study by photoemission electron microscopy at SPring-8

A photoemission electron microscopy measurement system on insulating samples was established at the BL17SU beamline of SPring-8 by utilizing an Au pattern evaporation technique combined with photon-induced surface modification.