Takahiro Yamasaki

Direct observation of oxygen movement during resistance switching in NiO/Pt film

We demonstrate that both a low resistance state and a high resistance state can be written by bipolar voltage application in a local region of NiO/Pt films by using conducting atomic force microscopy. To investigate how oxygen played a role in the resistance switching phenomenon, a local writing process in O18 tracer gas atmosphere was carried out and the composition change was examined by time-of-flight secondary ion mass spectroscopy. As a result, it was revealed that oxygen moves to the anode side, and the composition of the NiO surface might change thereby causing the change in resistance.

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.

Hydration effect on the optical property of a DNA fiber: First-principles and molecular dynamics studies

We have studied the theoretical hydration effect on the optical property of a deoxyribonucleic acid (DNA) double helix fiber. First-principles electronic structure and molecular dynamics simulations reveal that the electronic structure of the DNA fiber varies according to the amount of hydration or the relative humidity. We show that ultraviolet optical conductivity is influenced by hydration structure and DNA deformation, and our findings agree with other theoretical results and experimental observations. Infrared (IR) optical conductivity is estimated by the molecular dynamics approach. The humidity dependence of optical conductivity due to dipole relaxation of water is in close agreement with experimental observations. The theoretical IR absorption spectrum due to DNA vibrations agrees with the experimental spectrum. We discuss deformation and screening effects of the DNA fiber on humidity dependence of the optical spectra.

Two-Component Density Functional Calculations of Positron Lifetimes for Band-Gap Crystals

Lifetimes of positrons in perfect crystals having band gaps are calculated using the two-component density functional theory. For electron–positron correlation, we use the local density approximation (LDA) and the correction of the LDA using a high-frequency dielectric constant. While the LDA is found to slightly underestimate positron lifetimes, the correction of the LDA well reproduces experimental lifetimes. We discuss the quality of crystals used in a positron annihilation experiment by comparing theoretical and experimental positron lifetimes.

Oxygen Vacancies in Amorphous HfO 2 and SiO 2

Formation energies and electronic properties of oxygen vacancies in amorphous HfO 2 gate dielectrics are investigated by employing the first-principles method based on the density functional theory. We have found that the formation energy of neutral oxygen vacancy in amorphous HfO 2 distributes from 4.7 to 6.1 eV, most of which is lower than the value for cubic HfO 2 , 6.0 eV. We also investigated the stabilities of the Vo pairs in various charged state and compared with those in amorphous SiO 2 . We found that Vo ++ is stabilized in the vicinity of Vo in SiO 2 . In HfO 2 , however, this does not happen. This suggests the difference of defect propagation mechanism in HfO 2 and SiO 2 .

Molecular Dynamics Study of Oxidation Process with SiO emission and incorporation into the Si/SiO 2 System

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Defect States Due to Silicon Dangling Bonds at the Si(100)/SiO2 Interface and the Passivation by Hydrogen Atoms

The defect states due to the Si dangling-bonds at the Si(100)/SiO 2 interface is investigated by employing the first-principles method based on the density functional theory. Two prototypes of the defects at the interface are considered. One exists on one end of a Si-Si dimer. On the other hand, the other exists on an edge of a Si-O-Si bridge. The electronic structures for these systems were calculated to investigate the interface states. For the former, two defect states strongly localizing on the silicon dangling bond at the interface appear in the band gap. The latter defect also generates two defect states. But the upper level is in the conduction band, while the lower level is in the band gap. It is also shown that the interface states completely disappear by introducing a H atom into the interface and terminating the dangling bonds. Our results suggest the silicon dangling-bond on a Si-Si dimer with no adjacent O atoms as a candidate for the P b1 center.

Theory of initial oxidation stages on Si(100) surfaces by spin-polarized generalized gradient calculation

The chemical reaction of an O{sub 2} molecule on the Si(001) surfaces is studied by the ab initio molecular dynamics with the spin-polarized generalized gradient approximation. The dissociative adsorption of an O{sub 2} molecule has been found to occur over two adjacent Si(001) dimers with a spin flip-flop transition. The spin-state transition could cause a substantial retardation for dissociative adsorption of an O{sub 2} molecule. The C-type defect after chemisorption of a dissociated O{sub 2} molecule, however, shows a spin-triplet state, indicating a barrier-less dissociative chemisorption. These results explain well the overall aspects including preferential oxidation of C-type defects found by recent experimental reports.

Growth mechanism of Si dimer rows on Si(001)

Initial processes of Si dimer row growth on Si(001) surface is studied by the first principles molecular dynamics method. The authors optimize several different ad-Si clusters composed of one to four atoms on the surface and estimate activation energies for some important growth processes. At lower temperatures, a metastable ad-Si dimer in the trough between substrate dimer rows attracts monomers and tends to grow into a short diluted-dimer row in the perpendicular direction to the substrate dimer rows. In high temperatures as ad-Si dimers can diffuse, a direct dimer condensation process is possible to elongate the dense-dimer rows also in the perpendicular direction.

MOLECULAR-DYNAMICS SIMULATION OF MOLECULAR-BEAM-EPITAXY USING LENNARD-JONES AND EMPIRICAL Si POTENTIALS

In a range of – 40% to + 40% lattice mismatch, molecular-beam epitaxial growth is simulated for Lennard-Jones system. Various growth patterns are obtained according to the extent of the lattice mismatch and they are shown stable from the viewpoint of energy. Results obtained agree well with experiments for metal heterostructure. Defects generated are found to move as growth proceeds. Empirical Si potentials are examined in respect of (100) surface reconstruction; dimerization energy is calculated and surface is reconstructed dynamically. The Stillinger-Weber potential is found best and successfully applied to the study of the interaction between a source of Si atoms and a reconstructed surface.