Masaaki Araidai

Field emission mechanisms of graphitic nanostructures

Field emission (FE) and the electronic-states origin of graphitic nanostructures were investigated by first-principles calculations based on time-dependent density-functional theory. We find that the FE current from graphitic ribbons changes remarkably depending on the hydrogen termination and the direction of the applied electric field. Also, the FE current from graphene sheets shows a dramatic increase around vacancy defects. We verified, through the analysis of local electronic structures and energy distributions of emitted electrons, that the dangling-bond (or

Composition dependence of magnetoresistance effect and its annealing endurance in tunnel junctions having Mn-Ga electrode with high perpendicular magnetic anisotropy

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GeTe sequences in superlattice phase change memories and their electrical characteristics

) character is responsible for these results and governs the nature of the FE of graphitic nanostructures.

First principles study of SiC/SiO 2 interfaces towards future power devices

The composition dependence of the tunnel magnetoresistance (TMR) effect in Mn-Ga/MgO/CoFe magnetic tunnel junctions (MTJs) for Mn54Ga46, Mn62Ga38, and Mn71Ga29 (at. %) electrodes was investigated. An MTJ with a Mn62Ga38 electrode showed a maximum TMR ratio of 23% at 10 K and high annealing endurance up to 375 °C. The bias voltage dependence of the TMR ratio was distinct among MTJs with different Mn-Ga compositions. Here, we discuss this dependence on the basis of the difference in the Δ1 band dispersions for Mn-Ga alloys calculated by first principles.The composition dependence of the tunnel magnetoresistance (TMR) effect in Mn-Ga/MgO/CoFe magnetic tunnel junctions (MTJs) for Mn54Ga46, Mn62Ga38, and Mn71Ga29 (at. %) electrodes was investigated. An MTJ with a Mn62Ga38 electrode showed a maximum TMR ratio of 23% at 10 K and high annealing endurance up to 375 °C. The bias voltage dependence of the TMR ratio was distinct among MTJs with different Mn-Ga compositions. Here, we discuss this dependence on the basis of the difference in the Δ1 band dispersions for Mn-Ga alloys calculated by first principles.

Surface-segregated Si and Ge ultrathin films formed by Ag-induced layer exchange process

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 200 °C and 240 °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.

Density functional study for crystalline structures and electronic properties of Si1− x Sn x binary alloys

We clarify the intrinsic problems of SiC/SiO2 interfaces by the first principles calculations. The unique nearly free electron like characteristics of SiC conduction band bottom causes unexpected formation of interface states near the conduction band bottoms by process induced strain. These results indicate that strain free process is necessary for fabricating high quality NMOSFET. Another proposal is developing PMOSFET instead of presently popular NMOSFET. Moreover, we also discuss the Vth instability caused by proton diffusion.

Electronic states origin of field emission of silicon clusters

We have developed a new method of growing Si or Ge ultrathin films on a Ag(111) surface by using a Ag-induced layer exchange (ALEX) process toward the creation of 2D honeycomb sheets of Si and Ge, known as silicene and germanene, respectively. In the present paper, we clarify ALEX features, specifically the surface segregation of Si (or Ge) atoms from the underlying substrate, focusing on the annealing temperature and time. Hard X-ray photoelectron spectroscopy analyses demonstrate that surface-segregated Si (or Ge) exists on the Ag surfaces after the epitaxial growth of the Ag layer on Si(111) [or Ge(111)] substrates; the amount of segregated Si (or Ge) can be controlled by a subsequent annealing. Also, we find that the segregation of an ultrathin Si or Ge layer proceeds at an interface between Ag and the AlO x capping layer.

Field Emission of Diamond Surfaces by Time-Dependent Density-Functional Calculations

We have carried out density functional theory (DFT) calculation for Si1− x Sn x alloy and investigated the effect of the displacement of Si and Sn atoms with strain relaxation on the lattice constant and E– k dispersion. We calculated the formation probabilities for all atomic configurations of Si1− x Sn x according to the Boltzmann distribution. The average lattice constant and E– k dispersion were weighted by the formation probability of each configuration of Si1− x Sn x . We estimated the displacement of Si and Sn atoms from the initial tetrahedral site in the Si1− x Sn x unit cell considering structural relaxation under hydrostatic pressure, and we found that the breaking of the degenerated electronic levels of the valence band edge could be caused by the breaking of the tetrahedral symmetry. We also calculated the E– k dispersion of the Si1− x Sn x alloy by the DFT+U method and found that a Sn content above 50% would be required for the indirect–direct transition.

Edge states of hydrogen terminated monolayer materials: Silicene, Germanene and Stanene ribbons

The electronic states origin of the field emission of silicon clusters as a model system of covalent-bond nanostructures is elucidated by time-dependent density-functional calculations. The local electronic properties, σ- or π-bonding states of silicon clusters, are crucial for understanding the field emission characteristics. The findings in this study provide a theoretical basis for understanding and designing Si- and C-based nanostructured field emitters.

55-µA Ge x Te 1−x /Sb 2 Te 3 superlattice topological-switching random access memory (TRAM) and study of atomic arrangement in Ge-Te and Sb-Te structures

The electron field emission (FE) of diamond C(100) 2×1 surfaces has been investigated using time-dependent density-functional calculations. Dangling-bond (DB) states are found to be the main source of FE current of a clean surface. Notwithstanding the disappearance of the DB states by hydrogen (H) termination of a surface, FE current increases compared with that from the clean surface. An important finding from a comparison with the FE of graphitic ribbons is that the electronic structures being influenced by either carbon atom geometry or H termination govern the microscopic mechanism of the FE of carbon materials.