Takanori Koshikawa

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.

Ion scattering analysis programs for studying surface and interface structures

We have synthesized ion scattering analysis programs for studying surface and interface structures. The probing ion energies can be selected in a wide energy range from 10 keV up to several MeV. One of the present programs allows the simulation of both random and aligned spectra from perfect or partially disordered heteroepitaxial films. For defect profiling, the following types are available: (1) asymmetric Gaussian, (2) exponential reduction shape, and (3) step function. Another is used to analyze the structures of islands formed on the top surfaces of substrates. The targets have basically multielemental and multilayered structures comprising the units of amorphous/single crystal. These simulation programs are applied to disorder profiling of Ar+‐implanted LiNbO3, mechanically polished Mn‐Zn ferrite, and GaAs/Si heteroepitaxial films, and to characterizing the Cu‐Si islands on Si substrates.

Ion radiation-enhanced diffusion and segregation in an Au0.56Cu0.44 alloy between − 120°C and room temperature

Using a series of Auger depth profiles, we have studied ion radiation-enhanced diffusion (IRED) coefficients and segregation (IRES) effects at the surface of a sputtered Au in0.56 Cu 0.44 alloy in the temperature range between − 120°C and room temperature. The IRED coefficients yielded the relation D = 9.2×10−14exp(−0.06 eVkT)cm2s at an Ar ion beam density of 40 μAcm2 and an ion energy of 2 keV. These values are greater than those for normal interdiffusion by factors of at least 106, while the activation energy is 20–30 times smaller than the normal one. In the view of enrichment of gold in the top layer, Gibbsian segregation is also considered. It is suggested that, during sputterng, Gibbsian segregation assisted by ion bombardment will result in enrichments of gold at the top layer.

Preferential sputtering of Cu-Ni alloys at low temperature using lower energy auger electron spectra∗☆

Abstract The surface composition of Cu-Ni alloys sputtered sufficiently by Ar + ions are evaluated by AES at room temperature, and also at low (~-150°C) temperature at which the thermal diffusion of the constituent elements to the surface is ignored. The small difference of the surface concentration at each temperature shows that the diffusion enhancement by ion bombardment is small in the preferential sputtering of Cu-Ni alloys at room temperature. The depth of the altered layers of Cu-Ni alloys caused by preferential sputtering was estimated using lower (~100 eV) and higher (700–1000 eV) energy Auger spectra which have different escape dephts. The surface concentration calibrated by the lower energy Auger peaks, which overlap each other, are determined using a calibration curve obtained by the coevaporated Cu-Ni standard samples. The difference between the surface compositions determined by lower and higher energy Auger spectra is very small. This suggests that the depth of the altered layer is larger than the escape depth of Auger electrons (~15 A).

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.

Measurement of structure in the energy distribution of slow secondary electrons from aluminum

The number of secondary electrons per unit energy, N (E), and the derivative dN/dE have been measured carefully for clean and contaminated aluminum using a four‐grid hemispherical retarding‐field energy analyzer. At primary beam energies of 0.5, 1.0, and 1.5 keV, a broadened curve of N (E) vs energy E was obtained for clean aluminum; the more normal narrow curve was obtained when the surface was contaminated with oxygen. This structure is believed due to plasmon decay excitation of electrons in the solid.

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.

Changes in gold concentration at the surface of a AuCu alloy sputtered at low temperature

Abstract A series of depth profiles in Au 0.56 Cu 0.44 produced by bombarding at − 120°C with an argon ion beam of 2 keV energy and current densities of 4, 6, 12 and 24 μA/cm 2 , respectively, are presented. Gold is enriched in the top layer but the concentration rapidly decreases to a minimum at an average depth of about 2 A, then increases slowly and saturates at about 30 A. The change in the composition at this point is called a dip. The magnitude of the dip depends on the ion current density, but the depth is nearly independent of it. Segregation and diffusion effects in establishing the surface composition are stressed.

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.