We have investigated the electronic structures of recently discovered superconductor FeSe by soft-x-ray and hard-x-ray photoemission spectroscopy with high bulk sensitivity. The large Fe 3d spectral weight is located in the vicinity of the Fermi level (EF ), which is demonstrated to be a coherent quasi-particle peak. Compared with the results of the band structure calculation with local-density approximation, Fe 3d band narrowing and the energy shift of the band toward EF are found, suggesting an importance of the electron correlation effect in FeSe. The self energy correction provides the larger mass enhancement value (Z −1 ≃3.6) than in Fe-As superconductors and enables us to separate a incoherent part from the spectrum. These features are quite consistent with the results of recent dynamical mean-field calculations, in which the incoherent part is attributed to the lower Hubbard band.

The formation of nanoscale fine structures during pulsed laser ablation of a silicon target in a hydrogen atmosphere has been studied by analyzing the deposited silicon fine structures prepared under different conditions. Transmission electron microscopy, scanning electron microscopy ͑SEM͒, Raman scattering, and infrared absorption studies on the deposited samples indicate that silicon nanocrystallites are produced when the background gas pressure is higher than a critical value. The deposited substance is found to show hierarchical structure having surface hydrogenated silicon nanocrystallites as the primary structure and aggregates of the nanocrystallites as the secondary structure. The secondary structure depends on the hydrogen background gas pressure, while the size of the primary nanocrystallites is 4 -5 nm independent of the pressure. These results suggest that the fine structure is formed in two steps; the silicon nanocrystallites having a stable surface are initially formed and they are subsequently aggregated to form the secondary structure. Analysis of surface free energy suggests that the stability is acquired by termination of the surface by creation of Siu H bonds. We carried out fractal analysis of the SEM image of the deposits and found that the secondary structure shows good self-similar structure when deposited at higher background gas pressure. The fractal dimension of aggregated secondary structure varies from 1.7 to more than 2.0 with decreasing background gas pressure. Comparison of these values with reported results for the fractal growth simulation indicates that the region at which aggregation of the nanocrystallites takes place changes from in the plume to on the substrate with decreasing background gas pressure. Effects of the hydrogen background gas on the nanocrystallization process and spatial distribution of formed nanocrystallites in the plume are discussed. The formation of surface stabilized Si nanocrystallites and their spatial confinement by background gas in the first and second steps determine the hierarchical structure of deposited substance.

Abstract. We performed pulsed laser ablation (PLA) of a silicon target in liquid environment to prepare a silicon colloid solution. The nanoparticles were observed by SEM and TEM measurements. The result of Raman scattering indicates that this particle is mainly composed of silicon nanocrystallites. The optical gap energies of the colloid solutions varied by changing the solvents; 2.9 and 3.5 eV for colloids prepared in water and hexane, respectively. These colloid solutions showed efficient PL intensity. Since Si-(CH 3 ) n related bonds were observed for the specimen prepared in hexane, surface effects other than the quantum confinement effect should be taken into account for the origin of the PL. Our results indicate that new kinds of Sibased colloid solutions can be prepared by PLA in solvent. Since the PL peak energies were sensitive to the surface conditions, these colloid solutions are promising for biological applications such as bio-sensors.

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Natural oxidation processes of surface hydrogenated silicon nanocrystallites prepared by pulsed laser ablation under various hydrogen gas pressures are discussed by measuring the vibrational frequency of Si-H n units on the surface and intensity of Si-O-Si stretching vibration. The surfaces of nanocrystallites are predominantly composed of Si-H bonds and oxidation starts from backbonds of these bonds. The deposited nanocrystal films have a porous secondary structure which depends on the background gas pressure. The oxidation rate observed by infrared absorption measurements depended on this porous secondary structure. The oxidation process is discussed by the correlation between oxidation rate and porous structure of nanocrystal film. We found that Si-O bond density increases with covering the surface of the nanocrystallites during the diffusion of oxygen-related molecules through the void spaces in the porous structure. The surface oxidation of each nanocrystallite is not homogeneous; after the coverage of easy-to-oxidize sites, oxidation continues to gradually progress at the post-coverage stage. We point out that the oxidation process at coverage and post-coverage stages result in different photoluminescence ͑PL͒ wavelengths. Adsorption of the water molecule before oxidation also affects the PL wavelength. Defect PL centers which have light emission around 600 and 400 nm are generated during the coverage and post-coverage oxidation processes, respectively.

Band-to-band Auger recombination for a semiconductor laser is studied theoretically. Auger recombination calculation method is formulated, which may be applicable to a semiconductor with any injected carrier density. Using the formulated equation, injected carrier density dependence, and the temperature dependence of the Auger lifetime for GaSb and InAs are calculated. The temperature dependence of Auger lifetime and the upper limit of the quantum efficiency under laser threshold condition are also discussed. Calculated result shows that Auger recombination affects cw room-temperature operation for an InAs laser considerably.

Micromirrors were fabricated by the micro-origami technique. This technique allows the fabrication of simple and robust hinges for movable parts, and it can be applied to any pair of lattice mismatched epitaxial layers, in semiconductors or metals. A multilayer structure, including AlGaAs/GaAs component layers and an InGaAs strained layer, was grown by molecular beam epitaxy on a GaAs substrate. After definition of the hinge and mirror’s shape by photolithography, the micromirrors were released from the substrate by selective etching. They moved to their final position powered by the strain release in the InGaAs layer. Optical actuation was achieved by irradiation with the 488 nm line of an argon laser, and the mirror’s position was measured by sensing the reflection of a He–Ne laser. Continuous wave irradiation with a power density of 450 mW/mm2 produced an angular deflection of the mirror of around 0.5°. The frequency response of the mirrors shows a resonance at 25 kHz.

Natural oxidation processes of hydrogenated Si nanocrystallites were investigated to clarify effects of surface oxidation on photoluminescence wavelength. Hydrogenated Si nanocrystallites were prepared by pulsed laser ablation in hydrogen gas ambient. The Si–H bonds on the surface of the nanocrystallites enable us to estimate the local configuration of Si–O bonds using infrared frequency shifts. The natural oxidation process was investigated by measuring the density and local configuration of Si–O bonds. The oxidation process can be classified into first and second stages. The first stage is due to the diffusion of oxygen molecules in the nanocrystallites through voids in the porous structure, and the second stage is due to the oxidation of each nanocrystallite from the top surface to the sub-surface. The configurations of Si–O bonds in the first and second stages are silicon-rich and oxygen-rich compositions, respectively. The photoluminescence wavelength was blue-shifted with increasing Si–O bond density. This PL peak shift was not continuous, but three PL peak regions at 800, 600–700, and 400–500 nm were observed. This result indicates that the origin of this PL peak shift is not due to quantum confinement because of decreased diameter of Si nanocrystallites, but is due to the existence of surface oxide. A photoluminescence peak at 800 nm was observed in fresh specimens, and those at 600–700 and 400–500 nm were observed from the first and second stages of oxidation, respectively.