Tsuyoshi Uda

A two-dimensional device simulator of semiconductor lasers

Abstract A two-dimensional simulator for aid in designing semiconductor lasers is developed. Poissons equation and the current continuity equations for electrons and holes as well as the wave equation and rate equation for photons are numerically solved. Heterojunctions and carrier degeneracy are rigorously treated, and analytical results on channeled-substrate-planar lasers are presented to demonstrate the simulator. Reasonable agreement is found between calculated and experimental results, and calculated results clarify precisely the operation mechanism of semiconductor lasers. The present work enables computer simulation for the first time to be a practical design aid in research and development of various kinds of semiconductor lasers.

A proposal of nanoscale devices based on atom/molecule switching

This paper proposes a very small switching device, called an atom relay, which would supersede present metal‐oxide‐semiconductor devices for the next decade. The basic configuration of an atom relay consists of an atom wire, a switching atom, and a switching gate, with total dimensions below 10 nm, and an operation speed at more than terahertz level. The operation principle of the atom relay is that a switching atom is displaced from the atom wire by the electric field supplied from the switching gate, and the atom relay exhibits an ‘‘off’’ state. The switching characteristics of the atom relay are demonstrated by simulation, and it is shown that the electron propagation is successfully cut if a gap of about 0.4 nm is formed in the atom wire by the displacement of the switching atom. A self‐relay structure, in which the switching atom is displaced by the electric field from the atom wire itself, enables a dynamic memory cell, and the functions are ascertained by simulation. Fundamental logic circuits as N...

Electronic structure and optical properties of hydrogenated silicon clusters

Abstract Optical properties of hydrogenated silicon clusters are investigated using first-principles density-functional pseudopotential calculation. Energy gaps of hydrogenated silicon particles 7.3–15.5 A in diameter are in the range 4.62–2.45 eV. Transitions between band-edge orbitals are allowed with a radiative lifetime of 0.5–500 μs. When a cluster is partially dehydrogenated, lattice distortion enhances the photoluminescence intensity. Further dehydrogenation creates mid-gap states related to dangling bonds decreasing the photoluminescence intensity. These results suggest that the photoluminescence of porous silicon is attributable to hydrogenated silicon particles in the porous layer.

Theoretical study of the structural evolution of small hydrogenated silicon clusters: Si6Hx

Abstract Density functional calculations were performed for the structural properties and energetics of small hydrogenated silicon clusters: Si6Hx (0 ⩽ x ⩽ 14). We find that the structures of Si6Hx can be classified into several distinct families in terms of the arrangement of silicon atoms. In particular, we find a series of structures which are intermediate between compact and tetrahedral atomic arrangements. Based on calculated formation energies we address the relative stability of the Si6Hx clusters.

Structure and electronic property of Si(100)/SiO 2 interface

Abstract Stable structures and electronic states of Si(100) SiO 2 interface have been investigated with the first-principles molecular dynamics method. Quartz, tridymite, and pseudo β-cristobalite are employed as the initial structures of SiO 2 at the interface to find the stable ones by structural optimization. It is found that the optimized tridymite-type SiO 2 on Si is most stable for a thin (about 7 A) SiO 2 layer. For a thicker (about 15 A) layer, however, this structure becomes less stable than the others, and the optimized quartz-type SiO 2 structure becomes most stable. Variation of the band gap perpendicular to the interface has also been investigated. In the SiO 2 region from the structural interface to a point about 1 A away from it, the band gap remains as narrow as that of silicon. The dramatic change of the band gap takes place in the SiO 2 region from about 1 to 4 A away from the interface.

Effects of nitrogen atom doping on dielectric constants of Hf-based gate oxides

We have theoretically shown that doping of N atoms increases the dielectric constant of the Hf silicates, enhancing both the electronic and lattice polarization contributions. The enhancement of the lattice contribution is dominant and is attributed to low-frequency vibration modes induced by the N doping. It is found that Hf and Si ions bonded to N atoms located at O vacancies largely vibrate in the modes and play a crucial role in the enhancement. Doped N atoms are shown to also improve the electric characteristic of the silicates, elevating O vacancy levels appearing in the band gap of the silicates.

Doping Effect in Chalcogenide Glasses

Electronic properties of doped chalcogenide glasses are discussed on the basis of the Street-Mott model by taking a finite width of gap states into account. The conduction activation energy depends strongly on the shape of the gap state energy distribution. A rather gradual change in the activation energy with the increas of impurity concentrations, often observed in experiments, is well reproduced by introducing relatively small values for the distribution width. It is then possible to estimate the fraction of dopant atoms, which actually change conductivity, by comparing the results of the present model with experiments.

Initial oxide-growth process on the Si(100) surface

We investigate the initial growth of oxide on the Si(100) surface by means of first-principles molecular-dynamics calculations. By inserting oxygen atoms successively into the backbonds and the dimer bonds, the variation in surface geometry and in adsorption energy is studied. Once a backbond is oxidized, substitution at the rest backbond of the dimer atom gets much less favorable. The other bonds show a similar reactivity to that on the clean surface. Thus, the oxide growth of the first atomic layer proceeds rather uniformly until one monolayer coverage. This process seems compatible with the recent observation of layer-by-layer oxidation by scanning reflection electron microscopy [H. Watanabe et al., Phys. Rev. Lett. 80 (1998) 345]. However, the energetics of the second-layer oxidation lead to preferential inward growth rather than lateral growth.

Valence band structure of hydrogenated amorphous silicon-carbon alloys

Abstract A systematic study is made of the valence band structure of a-Si x C 1−x :H by in situ photoelectron spectroscopy using synchrotron radiation and the nonlocal pseudopotential method for supercell configuration model.

Atomic and molecular processes on Si(001) and Si(111) surfaces

Abstract Despite the fact that remarkable progress has been made in the determination of the static atomic structures of Si surfaces, the kinetic and dynamic aspects of these surfaces are not yet well understood. A brief review is given on the theoretical studies of kinetic and dynamic properties based on first-principles computer simulations, focusing on the initial stage of homoepitaxial crystal growth on Si(001) and the low-temperature dynamics of Si(111)-7 × 7.