Techniques

Keywords in Next-Generation Materials Science

green

Green technology is an enveloping term that encompasses the underlying technologies for solving two main sets of challenges - firstly, overarching themes associated with energy and resource conservation, rare element substitution, and RoHS compliance; and secondly, those relating to the development of green functional materials and devices, such as environmental sensors, high-performance catalysts, and systems for the generation and storage of energy. Our research focuses on the interfaces inside these functional materials and devices which lend them their unique properties, and our overall goal is to understand and control these interfacial phenomena. In the interfacial region between two dissimilar solids, local anisotropic structural strain and compositional changes (which are not observed in the bulk) can generate unique local electronic states in the material, and give rise to functional properties such as polarization, rectification, light emission, photothermoelectric effects, interface magnetization, and electronic conductivity. Establishing atomic site-specific characterization methods and atomic scale techniques for the design and construction of interfaces is of utmost importance for the development of functional materials and devices which depend on the properties generated by interfacial phenomena.

green

We aim to elucidate the mechanisms behind the functionality of key materials in green innovation.

Background

While developing our 2D photoelectron spectroscopy and diffraction methods, we came up with the idea of combining Auger electron diffraction and X-ray absorption spectroscopy to form a novel technique - diffraction spectroscopy. When using Auger electron diffraction to study crystal surfaces, forward focusing peaks and diffraction rings appear, directed from the emitter atoms to the scattering atoms, which makes it possible to calculate information on the atomic structure; for example, the intensity of the peaks and rings allow us to identify the emitter atoms. A spectrum can be obtained at each local site by using the characteristic forward focusing peaks and diffraction patterns of each site as local atomic probes. The following measurement combinations have been found to be effective for materials analysis:

o Core level photoelectron spectroscopy × photoelectron diffraction: depth- and site-resolved compositional and chemical state analysis
o Valence band photoelectron spectroscopy × photoelectron diffraction: depth- and site-resolved analysis of electronic states and atomic trajectories
o X-ray magnetic circular dichroism spectroscopy × Auger electron diffraction: depth- and site-resolved analysis of conduction bands and magnetic structures

Laboratory Goals

Our research group relies on the synergy between the device fabrication team, which identifies problems, and the analysis development team, which addresses the challenges of solving them. We aim to establish new analysis and design techniques with a focus on interfacial regions, and to elucidate the atomic structure and the electronic state of each atomic site in interfaces relating to specific devices.
Specifically, we focus on the following systems and topics.
A) Heterojunction field-effect transistors based on nonpolar surfaces, e.g. AlN/SiC(112 ?0), and insulating films free of interfacial defects, e.g. SiOxNy/SiC(0001). We analyze the interfacial atomic structure and electronic state of these SiC-based power devices, and provide feedback on the heterostructure fabrication process.
B) Graphene-substrate interactions, including the effect of the substrate's surface states on the electronic states of graphene, e.g. in exfoliated graphene on SiO2 and SiC(0001), and the control of Dirac cones through laminate heterostructures, e.g. graphene/BN/Cu(111). We conduct layer-by-layer analyses of the structure and electronic states of graphene laminates, and identify the interactions which are most advantageous for device performance.
C) Amorphous thin films and polycrystalline surfaces. Examples include depth-resolved compositional analysis in the interfacial regions of InGaZnO/SiO2 heterojunctions, and microscopic photoelectron diffraction spectroscopy of microdomains using a microfocus X-ray source and a display-type ellipsoidal mesh analyzer (DELMA), which enables the study of oxide film formation on each surface domain of a polycrystalline Si solar cell.
D) Magnetic thin films, including research on perpendicular magnetization induced by spin reorientation transitions, e.g. in Ni/Cu(001), and interface magnetization induced by ferromagnetic substrates, e.g. in Gd/polycrystalline-Fe. We have developed magnetic structure analysis methods with atomic layer-resolution, and we aim to control the perpendicular magnetic switching that occurs with phase transitions.

助成金・支援

本研究室の活動は下記の支援をいただいています。この場を借りて感謝申し上げます。

  • 科学研究費 基盤B
  • 奈良先端大 支援財団
  • 奈良先端大 国際イニシアチブ
  • 奈良先端大 物質創成科学研究科