Tsuneo Yasue

Model of leakage characteristics of (Ba, Sr)TiO3 thin films

We have investigated the dependence of leakage current and capacitance of Pt/Ba0.5Sr0.5TiO3/Pt capacitors on annealing temperature under high vacuum conditions. It is observed that leakage currents increase asymmetrically for negative and positive bias voltage with increasing annealing temperature. A model of leakage current and capacitance characteristics has been proposed on the assumption of generation of oxygen vacancies by annealing at the interfaces of the dielectric film adjacent to the Pt electrodes. The model predicts the oxygen vacancies of about 1020 cm−3.

Super-High Brightness and High-Spin-Polarization Photocathode

Using a newly developed transmission-type photocathode, an electron beam of super-high brightness [(1.3±0.5)×107 Acm-2sr-1] was achieved. Moreover, the spin-polarization was as high as 90%. We fabricated a transmission-type photocathode based on a GaAs–GaAsP strained superlattice on a GaP substrate in order to enhance the brightness and polarization greatly. In this system, a laser beam is introduced through the transparent GaP substrate. The beam is focused on the superlattice active layer with a short focal length lens. Excited electrons are generated in a small area and extracted from the surface. The shrinkage of the electron generation area improved the brightness. In addition, a GaAs layer was inserted between the GaP substrate and the GaAsP buffer layer to control the strain relaxation process in the GaAsP buffer layer. This design for strain control was key in achieving high polarization (90%) in the transmission-type photocathode.

High brightness and high polarization electron source using transmission photocathode with GaAs-GaAsP superlattice layers

In order to produce a high brightness and high spin polarization electron beam, a pointlike emission mechanism is required for the photocathode of a GaAs polarized electron source. For this purpose, the laser spot size on the photocathode must be minimized, which is realized by changing the direction of the injection laser light from the front side to the back side of the photocathode. Based on this concept, a 20kV gun was constructed with a transmission photocathode including an active layer of a GaAs–GaAsP superlattice layer. This system produces a laser spot diameter as small as 1.3μm for 760–810nm laser wavelength. The brightness of the polarized electron beam was ∼2.0×107Acm−2sr−1, which corresponds to a reduced brightness of ∼1.0×107Am−2sr−1V−1. The peak polarization of 77% was achieved up to now. A charge density lifetime of 1.8×108Ccm−2 was observed for an extracted current of 3μA.

Real Time Magnetic Imaging by Spin-Polarized Low Energy Electron Microscopy with Highly Spin-Polarized and High Brightness Electron Gun

We developed a spin-polarized low energy electron microscopy (SPLEEM) with a highly polarized and high brightness spin electron gun in the present study. Magnetic structures of Co/W(110) were observed with an acquisition time of 0.02 s with a field of view of 6 µm. We carried out a dynamic observation of magnetic structures with the SPLEEM during the growth of Co on W(110).

Formation of size controlled Ge nanocrystals in SiO2 matrix by ion implantation and annealing

Abstract The Ge nanocrystals were formed in a SiO 2 matrix by the multi-energy ion implantation and subsequent annealing. Utilizing the multi-energy ion implantation, the Ge atoms were uniformly introduced in the SiO 2 matrix. Transmission electron microscopy (TEM) observation showed that the nanocrystal size was more uniform than the case when the Ge nanocrystals were formed by the other methods, such as the single-energy ion implantation and the magnetron co-sputtering. This is due to the uniformity of the Ge concentration in a SiO 2 matrix introduced by the multi-energy ion implantation. In addition, TEM observation and Raman scattering spectra showed that the nanocrystal size varied with the annealing temperature and the implantation dose. The uniformity and the size of the Ge nanocrystals were controlled by the ion implantation energy/dose and the annealing temperature. Auger electron spectra showed that the variation of the nanocrystal size was related to the reduction of the Ge oxide. The reduction of the Ge oxide in the SiO 2 matrix might be related to the strengths of the chemical bonds.

Characterization of spectroscopic photoemission and low energy electron microscope using multipolarized soft x rays at BL17SU/SPring-8

Spectroscopic photoemission and low energy electron microscope (SPELEEM) improved its performance after installation at BL17SU/SPring-8, where a multipolarization-mode undulator is employed to produce circularly and linearly polarized soft x rays. This undulator enables us to study the domain structures of ferromagnetic and antiferromagnetic materials by x-ray magnetic circular dichroism and x-ray magnetic linear dichroism. SPELEEM is used to study light elements (C, N, and O), 3d transition-metal elements and 4f rare earth elements, utilizing a wide range of photon energies. The two cylindrical mirrors adopted in front of SPELEEM ensure an illumination area of 14 x 14 microm(2) on the samples. The lateral resolution of a secondary electron photoemission electron microscope image is estimated to be better than 85 nm, whereas the energy resolution of the instrument is better than 0.4 eV.

Photon Stimulated Desorption of Positive Ions from LiF

Photon stimulated desorption (PSD) of positive ions from LiF is observed in the photon energy region between 30 and 70 eV. The creation of the Li+(1s→2s) core exciton leads to the rapid rise of both Li+ and F+ desorption yields. It is found that PSD of positive ions from LiF is due to the interatomic Auger transition. The yield spectrum of F+ ions is different from that of Li+ ions above 60 eV. The F+ yield spectrum has a deep valley at around 63 eV, while the Li+ spectrum shows a plateau in the same region. However the difference in the yield spectra of Li+ and F+ ions can not be explained at present.

Effects of Oxygen Vacancy Diffusion on Leakage Characteristics of Pt/(Ba0.5Sr0.5)TiO3/Pt Capacitor

We investigated leakage characteristics of Pt/(Ba0.5Sr0.5)TiO3(BST)/Pt capacitors by measuring current-voltage characteristics during annealing in vacuum and atmosphere. Built-in electric fields (Ei) generated at Pt/BST interfaces are derived from analysis of the leakage currents based on the Schottky conduction mechanism. It was found that Ei varies in proportion to the square root of annealing time (√ t). In addition, we found that an external electric field applied during annealing in vacuum induces asymmetric degradation in the leakage characteristics at the top and bottom Pt/BST interfaces. Such behavior in the leakage characteristics is interpreted in terms of the diffusion of oxygen vacancies at the interfaces. Diffusivity of oxygen vacancies is determined from Ei-√ t curves.

Macroscopic Single-Domain Graphene Growth on Polycrystalline Nickel Surface

We used low-energy electron microscopy to observe, in situ, the growth of graphene sheets on a polycrystalline Ni substrate through surface segregation phenomena. On a flat Ni surface with roughness of less than 5 nm, macroscopic single-domain graphene grew from an extremely small number of nucleation sites. The single-domain graphene, free of rotational disorder, expanded continuously in a carpet like manner, not only over steps on the substrate surface, but also over its domain boundaries, independent of the Miller index plane. These findings might provide a breakthrough in fabricating expansive single-domain graphene sheets on polycrystalline metal surfaces.

Quantitative Analysis of 90° Closure Domains Occurring by Compressive Stress in Fe3%Si(110) Steels

The 3-D structures of the 90° closure domains in demagnetized Fe3%Si(110) steels occurring by the compressive stress were quantitatively analyzed using domain theory. The dependence of the compressive stress and the thickness in the 90° closure domains were evaluated using the proposed closure domain model with the 90° domain width. It was revealed that the 90° closure domain width decreases with an increase in compressive stress and steeply increases with a decrease in compressive stress. Moreover, our calculations predict that the critical compressive stress for the occurrence of the closure domains increase with the decrease in specimen thickness. The results suggest that when the thickness becomes thinner, enhancement of local compressive stress may be needed to refine 180° basic domains along the rolling direction. The validity of the calculation was confirmed by comparison with the past data and our presented domain observations by photoemission electron microscopy combined with X-ray magnetic circular dichroism.