Kensuke Nishioka

Coulomb-staircase observed in silicon-nanodisk structures fabricated by low-energy chlorine neutral beams

A silicon-nanodisk structure, a nanometer-scale silicon disk on extremely thin SiO2 film, was fabricated by etching a 3.5–4-nm-thick polycrystalline silicon (poly-Si) thin film/1.4–3-nm-thick underlying oxide layer/Si substrate structure with a 7-nm-diameter ferritin iron-core mask and Cl neutral beam etching (NBE). The degree of etching was precisely controlled by detecting its depth using x-ray photoelectron spectroscopy. Cross-sectional scanning transmission electron microscopy (STEM) with elemental mapping by electron energy-loss spectroscopy (EELS) revealed that the underlying oxide layer remained while the Si layer was accurately etched. The STEM-EELS observation also revealed that there was an Si layer about 1–2-nm thick even in the nanodisk, while the nanodisk’s surface region was covered by native oxide. Removing the surface oxide layer prior to the NBE process could decrease the nanodisk diameter. Irradiation by Cl NB of the underlying 1.4-nm-thick SiO2 film increased the thickness of the SiO2 f...

Charging and Coulomb staircase effects in silicon nanodisk structures fabricated by defect-free Cl neutral beam etching process

A defect-free nanometer-scale silicon disk (nanodisk) on thin SiO2 film was precisely fabricated by using Cl neutral beam etching of a 3.5–4-nm-thick polycrystalline silicon on 1.4–3-nm-thick underlying SiO2 with a 7-nm-diameter ferritin iron core mask. Kelvin force microscope observations revealed that nanodisks could maintain injected positive and negative charges. Additionally, Coulomb staircases were observed by I-V measurement of a nanodisk at a temperature of 25K. These results indicate that the nanodisk fabricated in this research had a precise quantum effect structure and attained the single electron property. This process has great potential in the development of future quantum effect devices.

A New Silicon Quantum-Well Structure with Controlled Diameter and Thickness Fabricated with Ferritin Iron Core Mask and Chlorine Neutral Beam Etching

A disk-shaped silicon nanostructure, which works as a quantum well, was fabricated by etching a polycrystalline silicon/SiO2/silicon wafer with a chlorine neutral beam. The diameter (about 8 to 10 nm) and thickness (about 2 to 4 nm) of the nanodisk were controlled by changing the surface-oxide-removal conditions and deposition thickness of poly-silicon. Current–voltage measurements of the nanodisk showed staircase characteristics at room temperature (RT). The staircase width does not depend strongly on nanodisk diameter, but it strongly depends on nanodisk thickness. This result suggests that the nanodisk works as a quantum-well at RT.

Fabrication of Crystallized Si Film Deposited on a Polycrystalline YSZ Film/Glass Substrate at 500°C

We used a polycrystalline YSZ film deposited on the quartz substrate as a seed layer to enhance crystallization of the Si film deposited on it. The YSZ film was deposited by reactive sputtering at less than 50°C and the preferential orientation was (111). The 60-nm-thick Si film was deposited by e-beam evaporation method on the substrate with and without the YSZ film. It was found that the crystallization of the Si film deposited on the YSZ film occurs but not without the YSZ film at the temperature of 515°C. The SEM image of the Secco-etched Si film showed that the crystallized Si film consisted of ~50-nm-size grains and, from the TEM image, it is supposed that the Si grains are grown directly from the YSZ film.

Surface modification of an amorphous Si thin film crystallized by a linearly polarized Nd:YAG pulse laser beam

Amorphous Si films of 60 and 10nm thick on glass substrates were irradiated by a linearly polarized Nd:YAG pulse laser with the wavelength λ=532nm at the incident angle θi=0. The surface of the irradiated 60-nm-thick film had both periodic ridges perpendicular to the electric field vector E and aperiodic ridges roughly parallel to E, where the spatial period of the periodic ridges was almost λ. From the continuous 10-nm-thick film, the separate rectangular Si islands were formed with a periodic distance of λ, with the edges parallel or perpendicular to E. When θi was increased from normal incidence of the s-polarized beam for a 60-nm-thick film, the aperiodic ridges were reduced while the periodic ridges were still formed. For a 10-nm-thick film, the Si stripes were formed perpendicular to E, using the s-polarized beam at θi=12°. In order to investigate the mechanisms of the surface modifications of, in particular, aperiodic ridges, islands, and stripes, we improved the previous theoretical model of the p...

Thin film p-i-n poly-Si solar cells directly converted from p-i-n a-Si structures by a single shot of flash lamp

We propose a novel production method to fabricate high-efficiency thin-film poly-Si solar cells using flash lamp annealing (FLA) for crystallization of micrometer-order-thick p-i-n amorphous silicon (a-Si) structure, prepared by catalytic chemical vapor deposition (Cat-CVD, Hot-Wire CVD) on low-temperature glass substrates and following high-pressure water vapor annealing for defect passivation. The FLA enables us to crystallize a-Si films with only one pulse of less than 10 ms duration, and use of Cat-CVD provides a-Si cost-effectively because of high deposition rate of a-Si over 10 nm/s. Secondary ion mass spectroscopy (SIMS) profiles reveal that diffusion of dopants in p- or n-type layers is sufficiently suppressed after FLA, indicating possibility of simultaneous crystallization of p-i-n stacked a-Si films. High-pressure water vapor annealing (HPWVA) enhances the minority carrier lifetime of the poly-Si up to about 10 μs and drastically improves diode properties of the p-i-n poly-Si structure. No light-induced degradation is observed in the solar cell property of the poly-Si solar cell after 24-hour 1-sun light soaking.

Novel Stacked Nanodisk with Quantum Effect Fabricated by Defect-free Chlorine Neutral Beam Etching

A stacked nanodisk with two nanodisks connected in series by coupling three tunnel junctions was successfully fabricated by using a bio-nano-process that comprises NF3 treatment and defect-free neutral beam (NB) etching as etchers and a 7-nm-diameter ferritin iron-oxide core as a mask. Precise control of the etching process to etch such an alternative structure was achieved by combining NF3 treatment and three-step NB etching with X-ray photoelectron spectroscopy (XPS) analysis. The transmission electron microscope (TEM) image clearly revealed an alternative structure for nanodisks and tunnel junctions in series. A Coulomb staircase was observed using conductive atomic force microscope (AFM) at room temperature. The results suggest that stacked nanodisks with the quantum effect could be promising candidates for quantum effect devices.

Fabrication of Periodic Arrays of Nano-sized Si and Ni dots on SiO2 Using Linearly Polarized Nd:YAG Pulsed Laser

Periodic arrays of nano-sized Si and Ni dots were fabricated by only irradiating a linearly polarized Nd:YAG pulsed laser beam to Si and Ni thin films deposited on silicon dioxide (SiO 2 ) film. The interference between an incident beam and a scattered surface wave leads to the spatial periodicity of beam energy density distribution on the surface of the irradiated samples. A thin film was melted using a laser beam, and the molten film was split and condensed owing to its surface tensile according to the periodic energy density distribution. Then, the fine lines (line and space structure) were formed periodically. After the formation of fine lines, the sample was rotated by 90°, and the laser beam was irradiated. The periodic energy density distribution was generated on the fine lines, and the lines split and condensed according to the periodic energy density distribution. Eventually, the periodically aligned nano-sized dots were fabricated on the SiO 2 film.