Toyotsugu Ishibashi

Nanocrystal Formation of Metals in Thermally Grown Thin Silicon Dioxide Layer by Ion Implantation and Thermal Diffusion of Implanted Atoms in Heat Treatment

Ion implantation technique is useful method to dope atoms in a thin layer to make nanoparticles. However, the thermal annealing is required for recovering ion-induced damages and growing NPs. Therefore, the redistribution of implanted atoms in the layer is very important in the formation process of nanoparticles in size and position. We have investigated the redistribution of Ag atoms implanted in a thermally grown 25-nm-thick SiO2 layer on silicon substrate. Ag negative ions were implanted at 10 keV in the thin oxide layer, where the projected ranges are calculated 12 nm. Samples were annealed at a temperature of 800 degrees or less for 1 h in vacuum with Ar gas flow (50 ml/m). Depth profiles of implanted atoms were investigated by high-resolution Rutherford backscattering spectrometry (HRBS). The formed nanoparticles in the layer were studied by high-resolution cross-sectional transmission electron microscope (XTEM). The samples implanted with Ag negative ions at 10 keV with 5 ? 1015 ions/cm2 were measured by HRBS at scattering angle of 80 degree with He ions at 400 keV. Ag atoms distributed at the surface and at a depth corresponded to the calculated profile after annealing at a temperature below 500 ?C. It is expected that the surface accumulation of Ag atoms resulted from thermal diffusion of implanted atoms during implantation. At 500 ?C, the very small peak in concentration was observed at a depth of 22 nm. This means that a diffusion barrier for Ag atoms exits in this depth. The diffused atoms accumulated at this depth. At 700 ?C, the main peak of concentration was appeared at 20 nm in depth, in which FWHM was 7 nm. The XTEM observation showed that the Ag NPs aligned at the same depth of 20 nm along the interface of SiO2/Si, and that they were nanocrystals (NCs). This mono-layered Ag NCs well corresponded to the HRBS spectra. Thus, we have formed almost mono-layered nanocrystals for Ag implantation in thin silicon dioxide layer.

Light Transmission and Texture Changes of Mixed Liquid Crystals by Temperature Variations

Abstract The changes in liquid-crystal light transmission caused by temperature variations are investigated in connection with texture changes. Materials under investigation are the nematic-cholesteric and the smectic-cholesteric mixed liquid crystals. In the nematic-cholesteric mixed liquid crystal, the light scattering centers produced by the temperature variation from the liquid phase to the cholesteric phase cannot disappear with the applied voltage of 1 kHz. On the contrary, the light scattering centers produced in the dynamic scattering mode can disappear with the applied voltage of 1 kHz. In the cholesteric phase of a smectic-cholesteric mixed liquid crystal, the planar texture appears in a heating process but the focal conic texture in a cooling process. This texture difference causes the difference in the light transmission between the heating and the cooling processes. Further, the difference in the light transmission of a planar texture depends on the mesh area: as the area is larger, the trans...

Optic Effects of Smectic-Cholesteric Mixed Liquid-Crystals

Abstract Light transmission characteristics in optic effects of smectic-cholesteric mixed liquid-crystals are experimentally investigated. Cholesteric addition to a smectic liquid crystal changes greatly transmission characteristics in a cholesteric phase. Although a smectic liquid crystal alone is transparent, the mixed liquid crystals become opaque. However, this opaque state isconverted to the transparent state with applied fields. After the fields are removed, the transparent state is stored in case of then n-type mixture, e.g., BBBA:CN = 90:10 in weight, but returns to the original opaque state in case of thep-type mixture, e.g., C0B:CN = 90:10 in weight. In the smectic phase, the n-type mixture hardly responds to applied fields. but thep-type mixtureeasilyresponds. The response is dependent on a cell temperature: higher field is required in lower temperature. This temperature characteristic can be used for display application of liquid-crystal light valve.

Germanium nanoparticles formed in silicon dioxide layer by multi-energy implantation and oxidation state of Ge atoms

Ge nanoparticles (NPs) embedded silicon oxide is expected to be promising light emission source, especially, UV – blue light region. We have tried to form Ge NPs in a 100- nm-thick SiO2 layer on Si substrate by multi-energy implantation of Ge negative ions with energies of 50, 20 and 10 keV and doses of 1.4 × 1016, 3.2 × 1015 and 2.2 × 1015 ions/cm2, respectively. Samples were annealed for 1 h at a temperature less than 900oC. By this implantation, Formations of Ge nanoparticles in a surface 50-nm depth region were expected. The depth distribution of implanted Ge atoms in the oxide was measured by XPS (Ge 2p, O 1s, Si 2p) with monochromatic Al K.. and Ar etching at 4 keV. The depth profiles were well agreed with the cross-sectional TEM image. But some extent of Ge atoms diffused to the SiO2/Si interface at 900 oC. The chemical sifted spectra of Ge 2p3/2 showed about 60 % of the oxidation of Ge atom around the end of the range (EOR) even in the as-implanted sample. This oxidation was considered to be due to the excess oxygen atoms near EOR by forward of sputtered oxygen atoms from SiO2 layer. Raman spectra supported this oxidation. In a preliminary investigation of cathode luminescence, the Ge-implanted sample with annealing at 600oC showed CL peak at 3.12 eV (397 nm in wavelength) in UV-blue region at room temperature. This means the Ge-implanted sample has a possibility for light emission in the UV-blue region.

Liquid-Crystal Electrothermo-Optic Effects and Their Application to Display

Liquid-crystal electrothermo-optic effects are studied with changing temperature of the liquid-crystal cell subjected to an applied field, 30 kV/cm and 35 kV/cm, respectively. To each case of the field intensity, the frequency of the applied field is taken as 150, 250, and 350 Hz. A liquid crystal becomes opaque in a higher temperature region and transparent in a lower temperature region below the transition temperature from the mesophase to the isotropic phase. By this contrast, a thermally addressed display is demonstrated for dynamic figures. It is also demonstrated that the display can be changeably used for dynamic or static figures.

Cathode and Photo Luminescence of Silicon Dioxide Layer Implanted with Ge Negative Ions at Multi-Energy

Ge- ions were implanted into SiO2 layer three times by changing the energies of 50, 20 and 10 keV to form Germanium nanoparticles at a relatively wide-depth region. Then, the samples were annealed at 600–900°C for 1 h. Although Ge-nanoparticle formation was confirmed by cross-sectional TEM observation, XPS analysis showed about 30–60 % of the Ge atoms in SiO2 on average were oxidized. In cathode and Photo luminescence measurements, the emissions of around 400 nm in wavelength from the samples were observed. The position of cathode luminescence peak was independent of Ge fluence in the implantation, and temperature in the measurement. These results suggest that the luminescence mechanism is not due to quantum confinement effect of Ge nanoparticles, it is due to the oxygen defect center of oxidized germanium. The luminescence intensity changed dramatically with varying Ge fluence in the implantation.

Thermal Diffusion Barrier for Ag Atoms Implanted in Silicon Dioxide Layer on Silicon Substrate and Monolayer Formation of Nanoparticles

We have investigated thermal diffusion behavior of implanted Ag atoms in SiO2 by using a high‐resolution RBS method in the formation process of monolayered Ag nanoparticles. Ag atoms were implanted by negative ion implantation at 10 keV with 5×1015 ions/cm2 into the 25 nm‐thick SiO2/Si. Samples were annealed at 500 – 800°C for 1 h under Ar gas flow. At annealing temperature of 500°C, implanted Ag atoms distributed at the surface and at a depth corresponded to the calculated profile. It is expected that the surface accumulation of Ag atoms resulted from thermal diffusion of implanted atoms during implantation. At 500°C, the very small peak in concentration was observed at a depth of 22 nm. This means that a diffusion barrier for Ag atoms exits in this depth. The diffused atoms accumulated at this depth. At 700°C, the main peak of concentration was appeared at 20 nm in depth, where FWHM was 7 nm. These results well corresponded to the mono‐layered Ag nanocrystals observed by HR‐TEM.

Germanium Nanoparticle Formation into Thin SiO2 Films by Negative Ion Implantation and Their Electric Characteristics

Germanium nanoparticles in a thin SiO2 film on Si have been formed by negative ion implantation for the development of very low power consumption electron devices using nanoparticles. Their electrical properties of 25‐nm‐SiO2/Si films including Ge nanoparticles were investigated with CV method after subsequent annealing at various temperatures. Ge atoms were implanted at 10 keV with fluencies of 1×1015 and 5×1015 ions/cm2. Samples were annealed at 300, 500, 700 and 900°C for 1 h. Depth profiles of implanted Ge atoms in the SiO2 films were measured by using a high‐resolution RBS technique. The formed Ge nanoparticles were studied by cross‐sectional TEM observation. After annealing at less than 700°C, Ge nanoparticles were confirmed in the film. After 300°C‐annealing, a CV curve had so small hysteresis that could not be applied to memory devices. After 500°C‐annealing, both samples with 1×1015 ions/cm2 and with 5×1015 ions/cm2 had obvious hysteresis curves. Calculations of charge and nanoparticle intensity ...