Makoto Fujiwara

Existence of tetrahedral site symmetry about Ge atoms in a single-crystal film of Ge2Sb2Te5 found by x-ray fluorescence holography

The authors discuss x-ray fluorescence holography (XFH) measurements taken from an epitaxial layer of the digital versatile disk random access memory (DVD-RAM) material Ge2Sb2Te5 grown on a single-crystal GaSb(100) substrate. By using fluorescent photons from the Ge atoms in the matrix, a three-dimensional atomic image was obtained around the Ge atoms in a Ge2Sb2Te5 film; details of the three-dimensional atomic arrangement will aim at clarification of the high-speed writing and erasing mechanism of the laser-induced crystal-amorphous phase transition in this DVD-RAM material. Analysis of the XFH images revealed that the epitaxial layer did not possess a hexagonal structure as in the equilibrium phase of Ge2Sb2Te5, but a cubic structure with tetrahedral site symmetry about Ge atoms, different from the previous powder diffraction result. The present structure may support the umbrella-flip model of the Ge atoms between the octahedral site in the distorted rocksalt crystal and the tetrahedral site in the amor...

Effects of dielectric discontinuity on the ground-state energy of charged Si dots covered with a layer

The one- and two-electron ground-state energies of singly and doubly charged silicon (Si) dots modelled by a sphere covered with a silicon dioxide layer embedded in various dielectric media can be calculated as functions of the sphere size and the thickness of the oxide by extending the work of Allan et al and Babic et al. The electron-self-polarization, electron - electron and electron-polarization energies are treated by first-order perturbation theory, taking the confined free-electron state as the unperturbed state. By changing the thickness of the oxides or the surrounding dielectric medium, the applied voltage required for the tunnelling of one electron when one electron already exists inside the dot is greatly reduced. We discuss the possible consequence of electron tunnelling in a Si dot and the electroluminescence of porous Si.

Effect of frequency sweep direction on motion of excited ions in fourier transform ion cyclotron resonance cells.

Ion motion during frequency sweep excitation was computer simulated to study the effect of the direction of frequency swee on the z motion of ions in a cylindrical Fourier transform ion cyclotron resonance (T-ICR) cell. It is shown that the z motion is more forcefully excited by upsweep than by downsweep; thus at large amplitudes ions are more easily ejected to the trapping electrodes by upsweep and larger cyclotron orbits can be achieved by downsweep. This effect was confirmed by experiment and the results are favorably compared with the calculations. From these results it is concluded that downward sweeping is advantageous for ion detection and upward sweeping is preferable for ion ejection. The simulations clearly explain the effect of the direction of frequency sweep by visualizing the directions of the forces that ions experience immediately after the excitation of their cyclotron motion. It was demonstrated by experiment that the z ejection can be reduced by applying a phase-adjusted ac potential to the trapping electrodes.(847-852)

Simulation for internal energy deposition in sustained off-resonance irradiation collisional activation using a monte carlo method

This paper proposes a novel computational approach employing a Monte Carlo method, aimed at an improved understanding of the dynamics and energetics of activated ions in sustained off-resonance irradiation collisionally activated dissociation (SORI-CAD) experimental events of Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS). In SORI-CAD events, internal energies of activated ions are complicatedly associated with their motion undergoing off-resonance excitation (i.e. alternate accelerations and decelerations) and inherently stochastic ion-neutral collisions. Several types of pseudo-random generators were adapted to probability density functions (PDFs) which characterize the ion-neutral collision process. Simulated ion trajectories involve the realistic feature of pressurized SORI-CAD events, such as a collisional damping and those which have not been illustrated in conventional analytical approaches. The proposed method can simulate the time-varying translational and internal energies of activated ions. The present result suggests that the internal energy of a SORI-activated ion should be inversely proportional to the cube of the SORI excitation frequency offset. Copyright 1999 John Wiley & Sons, Ltd.

Local Structure around Mn Atoms in IV–VI Ferromagnetic Semiconductor Ge0.6Mn0.4Te Investigated by X-ray Fluorescence Holography

The local atomic structure around Mn atoms in Ge1-xMnxTe thin-film single crystal has been investigated by X-ray fluorescence holography (XFH) at room temperature. The obtained atomic image suggests that the Mn atoms replace the Ge atoms in the host GeTe, and the Mn position is stable in the exact positions of the anion fcc sublattice. The XFH result also suggests the fluctuation of the Ge positions or the cation vacancies.

Minibands of eigen-state energies of In 0.53 Ga 0.47 As multi-quantum wells lattice-matched to InP

Nonparabolic subband structure of In0.53Ga0.47As/In0.52Al0.48As multi-quantum wells was studied theoretically and experimentally. In periodic potential of the multi-quantum-well structure, if barrier layers are thin, electron eigen-states become minibands having with energy width. In 10 nm width barrier, the energy width becomes wider at higher energy. When the eigen-state is more than 0.5 eV and near top edge of the quantum well, the energy width is calculated as more than 0.012 eV.

Electron eigen-states in InGaAs/InAlAs multi-quantum wells using photocurrent spectroscopy

Photocurrent spectroscopy in lattice matched In0:53Ga0:47As/In0:52Al0:48As multi-quantum wells structures were measured at room temperatures. Step-like structures accentuated by the exciton peaks of interband transitions were observed on photocurrent spectra. The exciton peaks clearly appeared at lower quantum numbers and do not appear at higher quantum number. It might be caused that the electron eigen-states become subbands having with energy width in the periodic finite potential.

Annealing effects and optical properties of Si: SiO 2 films prepared by radio frequency sputter

We fabricated Si: SiO2 films and assessed its optical characteristics using photoluminescence spectroscopy. The Si: SiO2 films were deposited by sputtering Si tablets (15 mm square) onto a SiO2 target (108 mm diameter). Using excitation by a He-Cd laser, photoluminescence was emitted from the films. The bluish color emission was seen by the naked eye at room temperature. Photoluminescence spectra, that were measured from 400 nm to 1100 nm, were wide and continuous with some peaks. After annealing, that emission of photoluminescence decreased and its color changed to red.

Simulation of FT-ICR mass spectra with a special-purpose computer for many-particle systems

In a previous work, a 3D many-particle model consisting of 1,000 simulation particles interacting with each other has been employed to simulate FT-ICR mass spectra on a single processor PC. However, 1,000 particles are not sufficient. Thus to extend the number of simulation particles, a special-purpose computer board for many-particle systems was combined with the PC. Thereby the number of simulation particles was extended to 10,000. Though the calculation speed is still not sufficient, the speed became 10 times as fast as only CPU in the case of 1,000 simulation particles and it became possible to compute longer ICR signals and simulate higher resolving power spectra. The extended computing configuration was applied to simulation for the influence of magnetic field strength on peak coalescence phenomena.