Kenji Shiraishi

A ribonucleotide reductase gene involved in a p53-dependent cell-cycle checkpoint for DNA damage.

The p53 gene is frequently inactivated in human cancers. Here we have isolated a p53-inducible gene, p53R2, by using differential display to examine messenger RNAs in a cancer-derived human cell line carrying a highly regulated wild-type p53 expression system. p53R2 contains a p53-binding sequence in intron 1 and encodes a 351-amino-acid peptide with striking similarity to the ribonucleotide reductase small subunit (R2), which is important in DNA synthesis during cell division. Expression of p53R2, but not R2, was induced by ultraviolet and γ-irradiation and adriamycin treatment in a wild-type p53-dependent manner. Induction of p53R2 in p53-deficient cells caused G2/M arrest and prevented cells from death in response to adriamycin. Inhibition of endogenous p53R2 expression in cells that have an intact p53-dependent DNA damage checkpoint reduced ribonucleotide reductase activity, DNA repair and cell survival after exposure to various genotoxins. Our results indicate that p53R2 encodes a ribonucleotide reductase that is directly involved in the p53 checkpoint for repair of damaged DNA. The discovery of p53R2 clarifies a relationship between a ribonucleotide reductase activity involved in repair of damaged DNA and tumour suppression by p53.

ON-OFF switching mechanism of resistive–random–access–memories based on the formation and disruption of oxygen vacancy conducting channels

We study the ON-OFF switching mechanism of oxide-based resistive–random–access–memories using theoretical calculations. Electron deficient vacancies (VO) up to 1+ charge states would stabilize a cohesive filament, while further electron removal will stabilize the disrupted VO configurations with 2+ charges. The VO cohesion-isolation transition upon carrier injection and removal is shown to be a strong driving force in the ON-OFF switching process. We also propose that bipolar or unipolar behavior is determined by how the carriers are injected into VO. The control of the carrier injection by the electrode material selection is essential for desired bipolar switching.

Spin-polarized electronic structures of La2CuO4

Abstract We have performed the spin-polarized total-energy band-structure calculation for La 2 CuO 4 within the local spin density functional formalism using the semirelativistic normconserving pseudopotentials and the gaussian-orbitals basis set. An antiferromagnetic insulating state is found to be stable agaist a paramagnetic metallic state.

Surface Stability and Growth Kinetics of Compound Semiconductors: An Ab Initio-Based Approach

We review the surface stability and growth kinetics of III-V and III-nitride semiconductors. The theoretical approach used in these studies is based on ab initio calculations and includes gas-phase free energy. With this method, we can investigate the influence of growth conditions, such as partial pressure and temperature, on the surface stability and growth kinetics. First, we examine the feasibility of this approach by comparing calculated surface phase diagrams of GaAs(001) with experimental results. In addition, the Ga diffusion length on GaAs(001) during molecular beam epitaxy is discussed. Next, this approach is systematically applied to the reconstruction, adsorption and incorporation on various nitride semiconductor surfaces. The calculated results for nitride semiconductor surface reconstructions with polar, nonpolar, and semipolar orientations suggest that adlayer reconstructions generally appear on the polar and the semipolar surfaces. However, the stable ideal surface without adsorption is found on the nonpolar surfaces because the ideal surface satisfies the electron counting rule. Finally, the stability of hydrogen and the incorporation mechanisms of Mg and C during metalorganic vapor phase epitaxy are discussed.

GeTe sequences in superlattice phase change memories and their electrical characteristics

We studied GeTe structures in superlattice phase change memories (superlattice PCMs) with a [GeTe/Sb2Te3] stacked structure by X-ray diffraction (XRD) analysis. We examined the electrical characteristics of superlattice PCMs with films deposited at different temperatures. It was found that XRD spectra differed between the films deposited at 200u2009°C and 240u2009°C; the differences corresponded to the differences in the GeTe sequences in the films. We applied first-principles calculations to calculate the total energy of three different GeTe sequences. The results showed the Ge-Te-Ge-Te sequence had the lowest total energy of the three and it was found that with this sequence the superlattice PCMs did not run.

Vacancy Cohesion-Isolation Phase Transition Upon Charge Injection and Removal in Binary Oxide-Based RRAM Filamentary-Type Switching

We report on the electronic roles in filamentary-type switching of binary oxide-based resistive random access memories using ab initio calculations. We show that charge injection and removal determine the thermodynamic stability of the vacancy filament and the diffusion in the memory devices; electron injection induces the vacancy cohesion that stabilizes the filament, whereas removal of these electrons favors the vacancy isolation that destabilizes the filament; electron removal makes the energy barrier of the vacancy diffusion processes small enough to be overcome by joule heating. The vacancy cohesion-isolation processes are induced by charge injection and removal that leads to occupation of the bonding-like electron states, which can be controlled by shifting the system Fermi level via an applied voltage during memory operation. The vacancy cohesion-isolation phase transition upon charge injection and removal is thus one of the main factors that govern resistive switching. Based on the physics, we propose three-layer stack structures for further improvement of the memory characteristics.

Charge-dependent oxygen vacancy diffusion in Al2O3-based resistive-random-access-memories

We theoretically study an oxygen vacancy (VO) diffusion in Al2O3-based resistive-random-access-memories (ReRAMs). We find that the activation energy of VO diffusion in Al2O3 strongly depends on the charge state of VO. In ReRAM, the charge state of VO can be easily changed by applying voltage and the lowest activation energy is observed at q = 2+. The operation voltage on Al2O3-based ReRAM is close to the activation energy at q = 2+, indicating that VO diffuses with doubly positive state. Moreover, the activation energy at q = 0 is close to that observed in bulk Al2O3, which explains the discrepancy between previous experimental and theoretical studies.

Physics in designing desirable ReRAM stack structure — Atomistic recipes based on oxygen chemical potential control and charge injection/removal

We clarify the importance of three-layers ReRAM stack-structures and provide guidelines for further optimization by both charge injection/removal and oxygen chemical potential. We determine atomistic structures corresponding to the ON-OFF switching process of ReRAMs using ab initio calculations. The cohesion-isolation of oxygen vacancies is found to be a strong driving force in the ON-OFF switching observed in oxide-based ReRAMs, and this phase transition can be controlled by injecting/removing charges while altering the oxygen chemical potential. Based on this concept, we propose universal guidelines for designing desirable ReRAM stack structures by introducing an oxygen vacancy barrier layer.

Structural design of AlN/GaN superlattices for deep-ultraviolet light-emitting diodes with high emission efficiency

We demonstrate on the basis of first-principles calculations that a AlN/GaN superlattice with one or two GaN monolayers is efficient for near-band-edge C-plane emission of deep-ultraviolet light-emitting diodes. We find that such superlattices lead to a significant increase of the C-plane components of the optical matrix element up to 57% relative to GaN bulk. At the same time, the energy gap of these superlattices is in the deep-UV region, where the shortest emission wavelength is 224u2009nm. This is remarkably shorter than that in Al-rich AlGaN alloys.

Fundamental origin of excellent low-noise property in 3D Si-MOSFETs ∼ Impact of charge-centroid in the channel due to quantum effect on 1/f noise ∼

We fabricated Si nanowire (NW) nFETs, and used them to experimentally demonstrate the superior noise properties of 3D MOSFETs. By carefully comparing the NWFETs with planar FETs, we found that it was critical to control the location of the centroid of the electron density in the inversion channel in order to obtain a noise spectral density with low magnitude. Self-consistent calculations of the Schrödinger and Poisson equations clearly reveal the advantages of NWFETs in electron distribution due to quantum confinement, specifically in the small gate-overdrive (Vg-Vt) condition. Moreover, by increasing Vd, the range where the NWFET exhibits superior noise properties to a planar FET can be extended to larger Vg-Vt because the effective Vg near the drain is reduced.