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Tanaka Group

Exploration of Amorphous Semiconductors, Magnetic Thin Films, Solid-Liquid Interfaces
Kazunobu Tanaka

■Object

This Group is dedicated to the exploration in sprouting technological fields. In six years of Phases I, the Group will strive for studying the dynamic process of structure formation and physical properties at the level of atoms and molecules, and ultimately leading to the creation of new material structure and function, with three different systems: (1) fabrication of semiconductors nano-structure and investigation of defect structure, (2) magnetic thin films and (3) electrical double layer at the solid-liquid interface. In consideration of the progress of research and the availability of human resources, the group configuration will be flexibly adjusted with possible build-up of a new group or amalgamation with another group.

The generic concept of the Group is to look into phenomena from the viewpoints of materials science and materials designing. It is intended to create innovative materials and functions which are not found in nature, through the experimental and theoretical clarification of structure formation process, or surface/interface process in particular, at the atomic and/or molecular level, while aiming at the ultimate goal of materials science, that is, predicting new materials structure for realizing desired properties.

■Results in Fiscal Year

Formation of semiconductor nanostructures and detailed structural analysis

Si microcrystals with a size of 5nm were formed on a graphite surface and their nucleation process was elucidated on the basis of STM and Raman scattering data (Fig. 1).

Light - induced ESR (g = 2.004) in a-Si:H at low temperatures was analyzed using a pulsed ESR technique. Its origin was firstly identified with the electrons trapped at weak Si-Si bond (Fig. 2).

It was also made clear through isotope experiments that the photo-created dangling bond is not directly coupled with impurity atoms such as O and C.

The size of micro-cavities in α-SiO2 was measured by utilizing atomic H as a probe through its ESR hyperfine coupling with 29Si.

Regarding semiconductor nanostructures in a cage of zeolite, EXAFS data suggested that selenium confined in thin channels of cancrinite forms linear chains consisting of dimers (in collaboration with ETL).

Exploration of magnetic nanostructures and new materials

MnSb and ferromagnetic MnGa, both ferromagnetic, were epitaxially grown on GaAs for the first time towards the future photo-induced magnetism in multilayer systems. A regular array of the bilayered Co dots were formed at the elbow sites of the ﾒherringboneﾓ pattern of the reconstructed Au (111) surface, and their Kerr rotation spectra were measured as a function of a dot-layer thickness (Fig. 3). A model was presented to explain the relationship between Kerr rotation spectra and a shape factor of the dots (in collaboration with ETL).

In-situ measurement of solid/liquid interface and exploration of new electrode catalysis

It was confirmed that the highly efficient spectrometer designed here in connection with the ATR - SPP (Surface Plasmon Polariton) method is essential to detect quite weak Raman signal from adsorbates on various metal surfaces. In fact, the sensitivity level was remarkably improved to 103－104 for the CuPc/platinum system.

Fig. 1 STM images (constant current mode) of the surfaces of hydrogenated microcrys-talline silicon films for a variety of film thicknesses. The samples were deposit-ed on graphite substrates (300 ｡C) using PECVD. The formation of microcrystals was confirmed by Raman scatterring measurement for (b), (c), and (d). It is clearly shown that the surface roughness of the films becomes larger when a film thickness is increased.

Fig. 2 Experimentally deconvoluted LESR spectrum in a-Si:H. Shadow region represents part of 29Si hyperfine structure of g=2.004 signal

Fig. 3 Magneto-optical spectra of the Co dots array. The change of the intensity of the negative peak at 2.5eV corresponds to the change of the dot diameter which can be controlled by cobalt coverage.