Chioko Kaneta

Selective Graphene Formation on Copper Twin Crystals

Selective graphene growth on copper twin crystals by chemical vapor deposition has been achieved. Graphene ribbons can be formed only on narrow twin crystal regions with a (001) or high-index surface sandwiched between Cu crystals having (111) surfaces by tuning the growth conditions, especially by controlling the partial pressure of CH(4) in Ar/H(2) carrier gas. At a relatively low CH(4) pressure, graphene nucleation at steps on Cu (111) surfaces is suppressed, and graphene is preferentially nucleated and formed on twin crystal regions. Graphene ribbons as narrow as ~100 nm have been obtained in experiments. The preferential graphene nucleation and formation seem to be caused primarily by a difference in surface-dependent adsorption energies of reactants, which has been estimated by first principles calculations. Concentrations of reactants on a Cu surface have also been analyzed by solving a diffusion equation that qualitatively explains our experimental observations of the preferential graphene nucleation. Our findings may lead to self-organizing formation of graphene nanoribbons without reliance on top-down approaches in the future.

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.

Charge-transfer interatomic potential for investigation of the thermal-oxidation growth process of silicon

A charge-transfer interatomic potential, based on the hybrid-Tersoff potential that incorporates a covalent-ionic mixed-bond nature, was developed to reproduce the growth process of the thermal oxidation of silicon. A fitting process was employed with various reference structures sampled by MD. Actively exploring and learning the wide-range of phase space enabled us to develop a robust interatomic potential. Our interatomic potential reproduced the bulk properties of Si and SiO2 polymorphs well, in addition to the radial distribution function and bond angle distribution of amorphous SiO2. The covalent-ionic mixed-bond nature of the interatomic potential well reproduced the dissociation process of an oxygen molecule on the Si/SiO2 interface. The initial oxidation simulation was performed on the silicon surface. We grew the amorphous SiO2 layer by incorporating the oxygen molecules into the silicon network at the interface. The density of the SiO2 layer and the charge distribution at the interface showed go...

Molecular dynamics study of Si(100)-oxidation: SiO and Si emissions from Si/SiO2 interfaces and their incorporation into SiO2

Dynamics of Si(100)-oxidation processes at the Si/SiO2 interface and in the SiO2 region are investigated focusing on SiO and Si emissions from the interface and the following incorporation into the SiO2 and/or substrate. Classical molecular dynamics (MD) simulations with variable charge interatomic potentials are performed to clarify these atomic processes. By incorporating oxygen atoms, two-folded Si atoms are formed after structural relaxation at the interface and are emitted as SiO molecules into SiO2. The energy barrier of the SiO emission is estimated to be 1.20 eV on the basis of the enthalpy change in an MD simulation. The emitted SiO molecule is incorporated into the SiO2 network through a Si-O rebonding process with generating an oxygen vacancy. The energy barrier of the SiO incorporation is estimated to be 0.79–0.81 eV. The elementary process of oxygen vacancy diffusion leading to the complete SiO incorporation is also simulated, and the energy barriers are found to be relatively small, 0.71–0.79 eV. The energy changes of Si emissions into the substrate and SiO2 are estimated to be 2.97–7.81 eV, which are larger than the energy barrier of the SiO emission. This result suggests that, at the ideally flat Si/SiO2 interface, the SiO emission into the SiO2 region occurs prior to the Si emission, which is consistent with previous theoretical and experimental studies. The above mentioned typical atomic processes are successfully extracted from some (or one) of MD simulations among many trials in which a statistical procedure is partly employed. Our results give a unified understanding of Si oxidation processes from an atomistic point of view.

Large-Scale Electronic Transport Calculations of Finite-Length Carbon Nanotubes Bridged between Graphene Electrodes with Lithium-Intercalated Contact

We perform density-functional-theory-based large-scale calculations of finite-length carbon nanotubes with caps bridged between graphene electrodes. The electronic transport properties vary with the length of the nanotubes and the contact structure. Despite the use of thin nanotubes expected to show n-type behavior, the Fermi level of the shorter nanotubes is uniformly pinned to the cap state, forming a large conduction gap. Although the longer nanotubes still have a medium conduction gap after forming a Schottky-like contact, the lithium intercalation in the contact area brings about a good ohmic property due to not only doping but also the orbital hybridization.

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 .

Elucidation of the atomic-scale mechanism of the anisotropic oxidation rate of 4H-SiC between the (0001) Si-face and ( 000 1 ¯) C-face by using a new Si-O-C interatomic potential

Silicon carbide (SiC) is an attractive semiconductor material for applications in power electronic devices. However, fabrication of a high-quality SiC/SiO2 interface has been a challenge. It is well-known that there is a great difference in the oxidation rate between the Si-face and the C-face and that the quality of oxide on the Si-face is greater than that on the C-face. However, the atomistic mechanism of the thermal oxidation of SiC remains to be solved. In this paper, a new Si-O-C interatomic potential was developed to reproduce the kinetics of the thermal oxidation of SiC. Using this newly developed potential, large-scale SiC oxidation simulations at various temperatures were performed. The results showed that the activation energy of the Si-face is much larger than that of the C-face. In the case of the Si-face, a flat and aligned interface structure including Si1+ was created. Based on the estimated activation energies of the intermediate oxide states, it is proposed that the stability of the flat...

Characterization of millisecond-anneal-induced defects in SiON and SiON/Si interface by gate current fluctuation measurement

In this paper, we have investigated bulk trap and interface trap density (Dit) caused by millisecond annealing (MSA) using gate current fluctuation (GCF) and charge pumping measurements. We show that the high energy flash lamp annealing (FLA) creates the GCF with a long duration time and it is critical issue to get a stable SRAM operation. FLA creates interface traps localized at the gate edge of MOSFET.

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

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Ab initio Study of Boron Pile-up at the Si(001)/SiO2 Interface

We studied the atomistic structure of boron atom at the Si(001)/SiO2 interface using ab initio calculation method to investigate the mechanism of boron pile-up at the interface. We found that, if there is no defects, such as oxygen vacancy, at the interface, no stable sites of B would appear at Si/SiO2 interface and SiO2 layer, thus indicating that boron in silicon will only diffuse to the interface, but not segregate across the interface, unless additional defects or impurities exist. By introducing oxygen vacancy and H bonds, we found some stable configurations at Si/SiO2 interface, which can support the mechanism of boron segregation at Si(001)/SiO2 interface. Therefore, we assume that vacancy of O and H bonds may play a crucial role in segregation by opening additional trapping sites. Furthermore, we also found the largest energy difference between B at Si/SiO2 interface and that in deep bulk Si is about 2.9eV, which is in agreement with experimental boron activation energy of emission from Si/SiO2 value of 2.64eV