Koh-ichi Nittoh

Chemical coating of large-area Au nanoparticle two-dimensional arrays as plasmon-resonant optics

Innovative nanophotonic applications require a technique for generating not a nanometer-scale point but a large-area (mm2−m2) near-field light source. We succeeded in developing a large-area near-field light source that is densely constructed of uniform-size gold nanoparticles (AuNPs) two-dimensionally arrayed with regular interparticle gaps, which has tunable localized surface plasmon resonance bands (600–1100 nm). The near-field excitation properties based on the optical tunability of the AuNP two-dimensional arrays demonstrate that our chemical coating of large-area near-field light sources is widely applicable such as for high-sensitivity optical sensors and high-efficiency solar cells.

High-density G-centers, light-emitting point defects in silicon crystal

We propose a new method of creating light-emitting point defects, or G-centers, by modifying a silicon surface with hexamethyldisilazane followed by laser annealing of the surface region. This laser annealing process has two advantages: creation of highly dense G-centers by incorporating carbon atoms into the silicon during heating; freezing in the created G-centers during rapid cooling. The method provides a surface region of up to 200 nm with highly dense carbon atoms of up to 4 × 1019 cm−3 to create G-centers, above the solubility limit of carbon atoms in silicon crystal (3 × 1017 cm−3). Photoluminescence measurement reveals that the higher-speed laser annealing produces stronger G-center luminescence. We demonstrate electrically-driven emission from the G-centers in samples made using our new method.

Plasmon-Resonant Optics on an Indium–Tin-Oxide Film for Exciting a Two-Photon Photochromic Reaction

Two-dimensional arrays of gold nanoparticles (AuNPs) as localized surface plasmon-resonant (LSPR) optics on a transparent conductive layer of indium–tin-oxide (ITO) were successfully fabricated. The typical surface coverage of the 10 nm sized AuNPs is over 87% for on ITO film roughness of ±0.2 nm. The LSPR wavelength is tunable in the range of 622–905 nm. By adjusting the LSPR wavelength to 905 nm at peak, two-photon photochromic reaction of diarylethene derivative in solution phase was demonstrated with irradiation by an incoherent near-infrared light in the range of 0.017–0.033 W/cm2 instead of a laser, thanks to the AuNP two-dimensional array.

Hybrid Laser Activation of Highly Concentrated Bi Donors in Wire-δ-Doped Silicon

Hybrid laser annealing, i.e., a serial combination of laser exposure and furnace annealing, is demonstrated to activate Bi donors that are wire-δ-doped in Si. The photoluminescence reveals that the dense Bi atoms are activated so efficiently that an impurity band develops upon rapid radiation heating of the focused area close to the melting point of Si. The unintentional defects that are created thereby can be totally eliminated by subsequent furnace annealing at 390 °C. As a result, we attained a record concentration of active Bi donors >1018 cm-3 in excess of the predicted solubility limit.

Dopant activation mechanism of Bi wire- δ -doping into Si crystal, investigated with wavelength dispersive fluorescence x-ray absorption fine structure and density functional theory

We successfully characterized the local structures of Bi atoms in a wire-δ-doped layer (1/8 ML) in a Si crystal, using wavelength dispersive fluorescence x-ray absorption fine structure at the beamline BL37XU, in SPring-8, with the help of density functional theory calculations. It was found that the burial of Bi nanolines on the Si(0 0 1) surface, via growth of Si capping layer at 400 °C by molecular beam epitaxy, reduced the Bi-Si bond length from [Formula: see text] to [Formula: see text] Å. We infer that following epitaxial growth the Bi-Bi dimers of the nanoline are broken, and the Bi atoms are located at substitutional sites within the Si crystal, leading to the shorter Bi-Si bond lengths.

Characterization of Highly Concentrated Bi Donors Wire-δ-Doped in Si

We studied the Bi wire-δ-doping process to achieve a high concentration of Bi donors in Si. Our process has two steps: (i) burial of Bi nanowires in Si by molecular beam epitaxy, and (ii) activation of Bi atoms in the δ-doped layer by laser annealing. The peak concentration of Bi atoms in the δ-doped layer is controlled by two parameters: the coverage of surfactant layer, and the growth temperature during the Si cap-layer growth, whose maximum concentration is larger than 1020 cm-3. Photoluminescence and electrical carrier transport measurements reveal that dense Bi atoms are activated upon heating the area at close to the melting point of Si. As a result, our doping process results in Bi donors in the wire-δ-doped layer with concentration of >1018 cm-3. This will be useful for establishing next-generation, quantum information processing platform.