Kenju Horii

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.

Dielectric confinement effects on the impurity and exciton binding energies of silicon dots covered with a silicon dioxide layer

The binding energies of hydrogenic impurities and excitons in silicon (Si) dots modelled by a sphere covered with a silicon dioxide (SiO2) layer and embedded in various dielectric media, are calculated as a function of the sphere size and the thickness of the oxide. The strong-confinement limit was considered, so that all of the electrostatic interactions are treated as first-order perturbations considering the confined free-electron state as the unperturbed state. A recently proposed size correction for the dielectric constant of the Si dot is also taken into account. These calculations demonstrate the importance of the image-charge effect due to dielectric discontinuity. In addition, it is shown that the binding energies of the impurities and the excitons inside the silicon dots are greatly affected by alteration of the thickness of the oxides or the surrounding dielectric media.

Three-Dimensional Atomic Image around Mn Atoms in Diluted Magnetic Semiconductor Zn0.4Mn0.6Te Obtained by X-Ray Fluorescence Holography

X-ray fluorescence holography (XFH) was applied at the beamline BL37XU of SPring-8 to visualize the three-dimensional local structure around the Mn atoms in a diluted magnetic semiconductor Zn0.4Mn0.6Te. The sample was irradiated by X-rays of certain energies beyond the Mn K absorption edge in order to obtain the Mn Kα fluorescence hologram. A three-dimensional atomic image around the Mn central atoms was derived from the hologram using Bartons algorithm. The nearest- and third-nearest-neighbor Te atoms were clearly visualized. However, the second-nearest-neighbor Zn or Mn atoms are barely visible in this image due to the small scattering efficiencies of these atoms.

Self‐Affine Fractal of Porous Silicon Surfaces before and after Natural Oxidization

Atomic force microscopy was used to measure the morphological changes of the porous silicon surfaces in ambient atmosphere. The rough surfaces were quantitatively analyzed using the fractal theory. The morphology of the porous silicon surface oxidized in the environment for three days was found to retain the same self-affine fractal character as the fresh samples. The roughness exponent of aged samples, which characterizes the spatial scale of the surface roughness, was α = 0.64 ± 0.04 and fractal dimensions D = 2.36 ± 0.04, while those of fresh samples were α = 0.53–0.64 and D = 2.36–2.47. However, a growth exponent β, which is another fractal exponent and characterizes the time-dependent dynamics of the roughening process, was lost in the aged samples.

Novel Operation Scheme for Realizing Combined Linear-Logarithmic Response in Photodiode-Type Active Pixel Sensor Cells

A novel operation scheme that realizes a combined linear-logarithmic response in conventional photodiode-type (PD) three-transistor (3-Tr) active pixel sensor (APS) cells is presented. It has been revealed that, by setting a reset transistor in weak inversion through properly selected bias voltages, conventional PD 3-Tr APS cells can be operated with a seamlessly combined linear-logarithmic response. A modification in the conventional APS cells that enhances their dynamic ranges in a linear response mode is also described.

Temperature-Dependent Surface Tension of Critical Nucleus in Crystal-Melt Nucleation.

Using the previously developed order-parameter model of liquid-solid nucleation of bcc elements [J. Phys. Soc. Jpn. 65 (1996) 2311], we calculate various thermodynamic functions of liquid-solid interface and the temperature dependence of the effective surface tension of critical nuclei in supercooled liquid metals. The results are compared with the diffuse interface model of Spaepen [Solid State Physics 47 (1994) 1] and Granasy [J. Non-Cryst. Solids 162 (1993) 301]. It is shown that the assumption made in the diffuse interface model is qualitatively correct in the order-parameter model. However, the temperature coefficient of the effective surface tension becomes positive in the diffuse interface model, while it becomes negative in the order-parameter model. It is suggested that the temperature dependence is determined from a delicate balance of the interfacial entropy and the entropy of fusion.

A Wide-Dynamic-Range Photodiode-Type Active Pixel Sensor Cell with Seamlessly Combined Logarithmic–Linear–Logarithmic Response

A wide-dynamic-range photodiode-type (PD) active pixel sensor (APS) cell with seamlessly combined logarithmic–linear–logarithmic response is proposed for the first time and its operation is verified by circuit simulation. It has two additional metal–oxide–semiconductor field-effect transistors (MOSFETs) stacked in parallel to the reset MOSFET of conventional three-transistor (3-Tr) PD APS cells. By utilizing our previously proposed operation scheme that achieves automatic linear–logarithmic response switching, the proposed 5-Tr APS cell realizes seamlessly combined logarithmic–linear–logarithmic response.

Temperature Dependence of Liquid-Vapor Nucleation Rate for the Yukawa Model Fluid

ABSTRACT The nonclassical nucleation theory based on the density functional method for the Yukawa model fluid is studied. The integral equation that determines the density profile and the work of formation of a nucleus is transformed into a simple differential equation of the Ginzburg-Landau (or Cahn-Hilliard) model of phase transition such that it can be solved numerically. The temperature dependence of the nucleation rates of liquid drops and of vapor bubbles of this Yukawa model fluid is compared with the predictions from the classical nucleation theory. The resultant temperature dependence and deviation from the classical prediction for liquid drops is compatible with the observed discrepancies between the experimental results and the classical predictions. † A part of this work was presented at the Ninth International Conference on Liquid and Amorphous Metals, Chicago 1995.

X‐Ray Fluorescence Holographic Study on a Single‐Crystal Thin Film of a Rewritable Optical Media

In this article, we discuss X‐ray fluorescence holography (XFH) using a third‐generation synchrotron radiation facility through an application to a DVD‐RAM material thin film. Three‐dimensional atomic images were obtained at 100 K around the Ge atoms in a Ge2Sb2Te5 single‐crystal thin film by means of XFH technique at the beamline BL37XU of the SPring‐8 to clarify the high‐speed writing and erasing mechanism of this DVD material. From the obtained XFH images, it was concluded that the single‐crystal thin film has a mixture of rocksalt and zinc‐blende structures. In addition, the images indicate large distortions associated with the existence of vacancies of the Ge(Sb) site. The present XFH results are in good agreement with the previous XAFS results, which has predicted a phase transition due to an umbrella flip motion of the Ge atoms.