Akira Sugimura

Band-to-band Auger effect in GaSb and InAs lasers

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.

Optical actuation of micromirrors fabricated by the micro-origami technique

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.

Band‐to‐band Auger recombination in InGaAsP lasers

Band‐to‐band Auger recombination in InGaAsP lasers is studied theoretically. An approximation method for the calculation is derived and the Auger coefficient is given explicitly as a function of injected carrier density. Auger lifetime and the threshold current density for the InGaAsP laser are calculated with good agreement with reported experimental results. This shows that Auger recombination is one of the dominant factors which causes the poor temperature characteristics of InGaAsP laser threshold current.

Nonparabolic subband structure of Ga0.47In0.53As‐InP quantum wells

Nonparabolic subband structure of GaInAs‐InP quantum wells is studied theoretically. The dispersion relations for electron, light hole, and split‐off hole subbands are obtained using the envelope function approximation taking into account band nonparabolicity. Reported experimental electron effective mass values and photoluminescence energies for wide quantum wells are resonably explained by the present theory. The experimental photoluminescence energies, for the well width less than 50 A, differ significantly from the calculated results.

Effects of thermal processes on photoluminescence of silicon nanocrystallites prepared by pulsed laser ablation

The temperature dependence of the photoluminescence (PL) spectra of Si nanocrystallites prepared using an inert-gas-ambient pulsed laser ablation technique was characterized. Although the PL intensity of as-deposited Si nanocrystallites was very weak at room temperature, it increased with annealing in N2 or O2 gas. The PL intensity of the sample annealed in O2 gas was the largest at room temperature since the reduction in intensity with increased temperature was small. The nonradiative recombination process is discussed in terms of the temperature dependence of the PL intensity. Our results suggest that annealing of Si nanocrystallites in O2 gas reduces the nonradiative recombination of electron–hole pairs.

Electron correlation in the FeSe superconductor studied by bulk-sensitive photoemission spectroscopy

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.

Effect of structure on radiative recombination processes in amorphous silicon suboxide prepared by rf sputtering

We studied the relationship between the structure and photoluminescence (PL) mechanism of amorphous silicon suboxide (a-SiOx) thin films prepared by the cosputtering method. The microscopic structure of the film was estimated by x-ray photoemission spectroscopy and infrared absorption spectroscopy. Electronic states were investigated by optical absorption and photothermal deflection spectroscopy. The results indicate that the film is separated into two types of regions: Si-rich cluster regions and amorphous SiO2-rich regions. The size of the Si-rich cluster is estimated, according to the quantum confinement model in which no other effects are assumed to exist, to be less than several nm, when the oxygen fraction x is larger than 1.0. The PL peak energy increased monotonically with the x value, whereas it showed different temperature dependences between the larger x value samples and the smaller ones. The PL characteristics observed can consistently be explained by assuming that there are two origins for P...

Structure‐dependent threshold current density in InGaAsP quantum well lasers

Threshold current density is studied theoretically for InGaAsP multiple quantum well lasers by including the Auger recombination process. All the possible transitions between quantized subbands of two‐dimensional carriers are taken into account in evaluating radiative and Auger processes. The Auger recombination current depends strongly on the quantum well structure, resulting in the necessity for an elaborate structure design. A design procedure is elucidated for the structure which gives the lowest threshold current density for InGaAsP multiple quantum well lasers.

Structural and optical properties of surface-hydrogenated silicon nanocrystallites prepared by reactive pulsed laser ablation

Pulsed laser ablation (PLA) in an inert background gas is a promising technique for preparing Si nanoparticles. Although an inert gas is appropriate for preparing pure material, a reactive background gas can be used to prepare compound nanoparticles. We performed PLA in hydrogen gas to prepare hydrogenated silicon nanoparticles. The mean diameter of the primary particles measured using transmission electron microscopy was approximately 5 nm. The hydrogen content in the deposits was very high and estimated to be about 20%. The infrared absorption corresponding to Si–Hn (n = 1, 2, 3) bonds on the surface were observed at around 2100 cm−1. The Raman scattering peak corresponding to crystalline Si was observed, and that corresponding to amorphous Si was negligibly small. These results indicate that the Si nanoparticles were not an alloy of Si and hydrogen but Si nanocrystallite (nc-Si) covered by hydrogen or hydrogenated amorphous silicon. This means that PLA in reactive H2 gas is a promising technique for preparing surface passivated nc-Si. The deposition mechanism and optical properties of the surface passivated silicon nanocrystallites are discussed.

Photoluminescence from Excited Energy Bands in Au25 Nanoclusters

The photoluminescence of Au25 nanoclusters was studied to elucidate its mechanism. We observed luminescence from the excited band at 2.8 eV in addition to the well-known 1.8 eV emission. The excited band luminescence showed rapid decay described by a stretched exponential function with the mean relaxation time of 2.2 ns. The 1.8 eV luminescence indicated double exponential function decay with the relaxation times of 1.9 and 0.7 µs. We compared the results with a model and found that the 1.8 eV emission corresponds to the transition between quantum levels of 6sp electronic states, while the excited band luminescence corresponds to that between the 6sp and 5d states.