Naoyuki Maejima

Analysis of electronic structure of amorphous InGaZnO/SiO2 interface by angle-resolved X-ray photoelectron spectroscopy

The electronic structures of amorphous indium gallium zinc oxide (a-IGZO) on a SiO2 layers before and after annealing were observed by constant final state X-ray photoelectron spectroscopy (CFS-XPS) and X-ray adsorption near-edge structure spectroscopy (XANES). From the results of angle-resolved CFS-XPS, the change in the electronic state was clearly observed in the a-IGZO bulk rather than in the a-IGZO/SiO2 interface. This suggests that the electronic structures of the a-IGZO bulk strongly affected the thin-film transistor characteristics. The results of XANES indicated an increase in the number of tail states upon atmospheric annealing (AT). We consider that the increase in the number of tail states decreased the channel mobility of AT samples.

Negative Photoelectron Diffraction Replica in Secondary Electron Angular Distribution

Photoelectron diffraction (PD) is an element selective local structure analysis method. Atomic arrangements around photoelectron emitter atom can be deduced from the position of forward focusing peaks (FFPs) and diffraction rings around them. We have measured 2π-steradian Ge 3p and a series of secondary electron patterns from the Ge(111) surface at a kinetic energy of 600 eV by varying photon energy. The destruction of PD patterns due to the defocusing effect by inelastic plasmon excitations was observed at 40 eV off from the elastic peak. Moreover, we found that the secondary electron patterns at over 160 eV off from the elastic peak are a complete negative contrast replica of 600-eV PD. The contrast was about one-fourth of that of the original PD. The mechanism of this new phenomenon can be explained by the decrease in isotropic secondary electron beam intensity caused by the absorption process of time- and space-reversal PD.

Site-Specific Stereograph of SiC(0001) Surface by Inverse Matrix Method

The 2π-steradian (full hemisphere) Si 2p and C 1s photoelectron intensity angular distributions (PIADs) of the 6H-SiC(0001) surface 4° off towards the [1\bar100] direction were measured. In a bulk crystal, pairs of mirrored local atomic sites with respect to the 1\bar100 planes exist. Thus, a sixfold symmetry is expected for PIADs from the bulk. However, all the measured PIADs showed a threefold symmetry owing to the preferential appearance of terraces with one type of local atomic site caused by anisotropic step bunching along the [11\bar20] direction. Taking the finite inelastic mean free path of photoelectrons into account, PIADs for one kind of Si and C atomic sites were successfully derived by solving an inverse matrix. Three strong forward focusing peaks due to nearby Si and C atoms have been separated from those formed by farther atoms. They showed a circular dichroism of rotational shift around the incident-light axis, which corresponds to the parallax in stereo viewing.

Operando 3D Visualization of Migration and Degradation of a Platinum Cathode Catalyst in a Polymer Electrolyte Fuel Cell

The three-dimensional (3D) distribution and oxidation state of a Pt cathode catalyst in a practical membrane electrode assembly (MEA) were visualized in a practical polymer electrolyte fuel cell (PEFC) under fuel-cell operating conditions. Operando 3D computed-tomography imaging with X-ray absorption near edge structure (XANES) spectroscopy (CT-XANES) clearly revealed the heterogeneous migration and degradation of Pt cathode catalyst in an MEA during accelerated degradation test (ADT) of PEFC. The degradative Pt migration proceeded over the entire cathode catalyst layer and spread to MEA depth direction into the Nafion membrane.

Strain Engineering for Anion Arrangement in Perovskite Oxynitrides

Mixed-anion perovskites such as oxynitrides, oxyfluorides, and oxyhydrides have flexibility in their anion arrangements, which potentially enables functional material design based on coordination chemistry. However, difficulty in the control of the anion arrangement has prevented the realization of this concept. In this study, we demonstrate strain engineering of the anion arrangement in epitaxial thin films of the Ca1-xSrxTaO2N perovskite oxynitrides. Under compressive epitaxial strain, the axial sites in TaO4N2 octahedra tend to be occupied by nitrogen rather than oxygen, which was revealed by N and O K-edge linearly polarized X-ray absorption near-edge structure (LP-XANES) and scanning transmission electron microscopy combined with electron energy loss spectroscopy. Furthermore, detailed analysis of the LP-XANES indicated that the high occupancy of nitrogen at the axial sites is due to the partial formation of a metastable trans-type anion configuration. These results are expected to serve as a guide for the material design of mixed-anion compounds based on their anion arrangements.

Site-Specific Atomic and Electronic Structure Analysis of Epitaxial Silicon Oxynitride Thin Film on SiC(0001) by Photoelectron and Auger Electron Diffractions

The film and interface structures of epitaxial silicon oxynitride (SiON) thin film grown on a SiC(0001) surface were investigated by photoelectron diffraction. Forward focusing peaks (FFPs) corresp...

Circular Dichroism in Cu Resonant Auger Electron Diffraction

Abstract Upon a core level excitation by circularly polarized light (CPL), the angular momentum of light, i.e. helicity, is transferred to the emitted photoelectron. This phenomenon can be confirmed by the parallax shift measurement of the forward focusing peak (FFP) direction in a stereograph of the atomic arrangement. The angular momentum of the emitted photoelectron is the sum of CPL helicity and the magnetic quantum number (MQN) of the initial state that define the quantum number of the core hole final state. The core hole may decay via Auger electron emission, where in this two electron process the angular momentum has to be conserved as well. Starting from a given core hole, different Auger decay channels with different final state energies and angular momenta of the emitted Auger electrons may be populated. Here we report the observation and formulation of the angular momentum transfer of light to Auger electrons, instead of photoelectrons. We measured photoelectron and Auger electron intensity angular distributions from Cu(111) and Cu(001) surfaces as a function of photon energy and photoelectron kinetic energy. By combining Auger electron spectroscopy with the FFP shift measurements at absorption threshold, element- and MQN-specific hole states can be generated in the valence band.

Interfacial atomic site characterization by photoelectron diffraction for 4H-AlN/4H-SiC() heterojunction

The interfacial atomic structure of an AlN thin film on a nonpolar 4H-SiC() substrate grown by atomic Al and N plasma deposition was studied by photoelectron diffraction and spectroscopy. The epitaxial growth of the thin film was confirmed by the comparison of element-specific photoelectron intensity angular distributions (PIADs). Depth profiles were analyzed by angle-resolved constant-final-state-mode X-ray photoelectron spectroscopy (AR-XPS). No polar angular dependence was observed in Al 2p spectra, while an additional intermixing component was found in interface-sensitive N 1s spectra. The site-specific N 1s PIADs for the AlN film and an intermixing component were derived from two N 1s PIADs with different binding energies. We attributed the intermixing component to SiN interfacial layer sites. In order to prevent SiN growth at the interface, we deposited Al on the SiC() substrate prior to the AlN growth. A significant reduction in the amount of intermixing components at the AlN/SiC interface was confirmed by AR-XPS.