Takanobu Watanabe

Novel Interatomic Potential Energy Function for Si, O Mixed Systems

A novel interatomic potential energy function is proposed for condensed systems composed of silicon and oxygen atoms, from SiO2 to Si crystal. The potential function is an extension of the Stillinger-Weber potential, which was originally designed for pure Si systems. All parameters in the potential function were determined based on ab initio molecular orbital calculations of small clusters. Without any adjustment to empirical data, the order of stability of five silica polymorphs is correctly reproduced. This potential realizes a large-scale modeling of SiO2/Si interface structures on average workstation computers.

Strain distribution around SiO2/Si interface in si nanowires : A molecular dynamics study

We have performed three-dimensional molecular dynamics simulations to investigate strain and stress distributions in silicon nanostructures covered with thermal oxide films, by using our original molecular force field for Si, O mixed systems. We have modeled a wire-shaped nanostructure by carving a Si(001) substrate, and then an oxide film with a uniform thickness was formed by inserting oxygen atom into Si–Si bonds from the surface. The simulation results show that a compressive stress is concentrated on the oxide region in the vicinity of the side SiO2/Si interface of the nanowire. At the top interface, there is also a compressive stress in the [110] direction, whereas the [001] component of the normal stress tensor is almost relaxed. These results suggest that the oxidation is strongly suppressed at the side faces of the silicon nanowire.

Adsorption mechanism of ribosomal protein L2 onto a silica surface: A molecular dynamics simulation study

A large-scale molecular dynamics simulation was carried out in order to investigate the adsorption mechanism of ribosomal protein L2 (RPL2) onto a silica surface at various pH values. RPL2 is a constituent protein of the 50S large ribosomal subunit, and a recent experimental report showed that it adsorbs strongly to silica surfaces and that it can be used to immobilize proteins on silica surfaces. The simulation results show that RPL2, especially domains 1 (residues 1-60) and 3 (residues 203-273), adsorbed more tightly to the silica surface above pH 7. We found that a major driving force for the adsorption of RPL2 onto the silica surface is the electrostatic interaction and that the structural flexibility of domains 1 and 3 may further contribute to the high affinity.

Strain-induced transconductance enhancement by pattern dependent oxidation in silicon nanowire field-effect transistors

Transconductance (gm) enhancement in n-type and p-type nanowire field-effect-transistors (nwFETs) is demonstrated by introducing controlled tensile strain into channel regions by pattern dependent oxidation (PADOX). Values of gm are enhanced relative to control devices by a factor of 1.5 in p-nwFETs and 3.0 in n-nwFETs. Strain distributions calculated by a three-dimensional molecular dynamics simulation reveal predominantly horizontal tensile stress in the nwFET channels. The Raman lines in the strain controlled devices display an increase in the full width at half maximum and a shift to lower wavenumber, confirming that gm enhancement is due to tensile stress introduced by the PADOX approach.

Modeling of SiO2/Si(100) interface structure by using extended -Stillinger-Weber potential

Abstract Large scale modeling of ultrathin SiO 2 films on Si(100) surfaces has been performed using our original potential, which was developed to simulate both Si and SiO 2 crystal systems. A SiO 2 film was formed by layer-by-layer insertion of oxygen atoms into Si-Si bonds in a Si wafer from one of the surfaces. The thickness of the obtained SiO 2 layer was about 17.2 A, and it showed the presence of the structural transition layer; the average Si O Si bond angle becomes smaller in the region closer to the SiO 2 /Si interface. The peak of Si O Si bond angle distribution is shifted toward a narrower angle from the equilibrium angle of 144 °, in agreement with experimental results reported so far.

Diffusion of Molecular and Atomic Oxygen in Silicon Oxide

Density functional calculations using model clusters were performed to clarify the atomic-scale diffusion mechanism of an O2 molecule or an O atom in SiO2. The activation energy required for the atomic O diffusion was estimated to be 1.3 eV, whereas that for the molecular O2 diffusion was revealed to be fairly low, 0.3 eV. This strongly suggests that the diffusing oxygen in SiO2 is primarily in a molecular form. The computational results were confirmed to be consistent between two SiO2 configurations of the cristobalite and quartz structures. Diffusion pathways and the related activation energies are shown to be well compatible with many recent works.

Impact of Structural Strained Layer near SiO2/Si Interface on Activation Energy of Time-Dependent Dielectric Breakdown

A structural transition region near the SiO2/Si interface has been considered to play an important role with respect to gate oxide reliability. We clarify the effects of the structural transition region on the time-dependent dielectric breakdown (TDDB) characteristics, particularly the activation energy of the oxide breakdown for ultrathin gate oxides formed by different oxidation processes, i.e., pyrogenic oxidation, rapid thermal O2 oxidation and N2O oxynitridation. Furthermore, we investigate the properties of the structural transition region, such as the density of SiO2 as measured by the grazing incidence X-ray-scattering reflectivity (GIXR) method, the Si–O–Si bond angle by Fourier-transform infrared attenuated total reflection (FTIR-ATR), the etching rate by chemical etching and X-ray photoelectron spectroscopy (XPS). Through these investigations, it is clarified that the oxide breakdown tends to occur at the Si–O–Si network with a lower bond angle (<115°) and that the strain in the structural transition region reduces the barrier to the oxide breakdown. A 1-nm-thick strained layer is found to have a strong effect on the oxide reliability and to limit oxide scaling in future ultra-large-scale integrated circuits (ULSIs).

Large-scale atomistic modeling of thermally grown SiO2 on Si(111) substrate

Large-scale SiO2/Si(111) models were constructed by introducing oxygen atoms in c-Si models in an atom-by-atom manner. Molecular dynamics calculation at a constant temperature was repeatedly carried out for the growing oxide model. By comparing the oxidation simulation of Si(111) substrate with that of Si(001) substrate performed previously, the influence of substrate orientation on the oxide structure was discussed. Owing to the significant feature of bonding arrangement within a Si bilayer in the Si(111) substrate, the inherent stress induced at the SiO2/Si interface by oxygen insertions is originally higher for the Si(111) oxidation than for the Si(001) oxidation, resulting in frequent changes in the bonding network. The resulting structure of bulk SiO2 on Si(111) has less strain and a lower density than that on Si(001), but involves a larger number of dangling bonds. The X-ray diffraction pattern calculated for the SiO2/Si(111) model exhibits a diffraction peak with a Laue-function-like profile on each of the crystal-truncation-rods from the 111 and 111 points, agreeing well with experimental results. These diffraction peaks indicate that the thermally grown SiO2 retains the residual order emanating from the {111} atomic planes in the original c-Si. Because of differences in the angles between the surface and the {111} atomic planes, the residual order within the SiO2 differs depending on the substrate orientation.

Effects of Thermal History on Residual Order of Thermally Grown Silicon Dioxide

By simulation of silicon oxidation and measurement of X-ray crystal-truncation-rod (CTR) scattering, the structures of silicon dioxide films grown at different temperatures and the structural changes due to thermal annealing have been investigated. Large-scale SiO2/Si(001) models were formed by introducing oxygen atoms, atom-by-atom, in crystalline Si from the surfaces. Molecular dynamics (MD) calculation at a constant temperature was repeatedly carried out for the growing oxide model. The intensity and position of the extra diffraction peak observed for the oxide, correlating with the residual order emanating from the parent Si crystal, depend on the growth temperature and change after thermal annealing. The peak intensity becomes smaller with increasing growth temperature. Thermal annealing monotonically decreases the peak intensity and shifts the position along the CTR, toward the lower angle side. There is a good agreement between the results of simulation and experiment. It is shown that (1) the oxide grown at a higher temperature has a lower degree of residual order, (2) thermal annealing decreases the residual order, ultimately leads to complete amorphization and never restores the ordering, and (3) the peak shift along the CTR corresponds to the volumetric expansion of the SiO2 in the surface-normal direction.

A molecular simulation study of an organosilane self-assembled monolayer/ SiO2 substrate interface

The bonding network of an alkylsilane self-assembled monolayer (SAM)SiO(2) substrate interface is investigated by means of canonical Monte Carlo (MC) simulations. SAMSiO(2) systems with different interfacial bonding topologies are sampled by the Metropolis MC method, and the AMBER potential with a newly developed organosilicon parameters are used to obtain an optimized structure with a given bonding topology. The underlying substrates are modeled as hydroxy-terminated (100) or (111) cristobalites. The SAMSiO(2) interface is characterized by a polysiloxane bonding network which comprises anchoring bonds and cross-linking bonds, namely, molecule-substrate and molecule-molecule Si-O-Si bonds, respectively. We show that at thermal equilibrium, the ratio of the number of anchoring bonds to cross-linking bonds decreases as a total Si-O-Si bond density increases, and that nevertheless, number of anchoring bonds always dominate over that of cross-linking bonds. Moreover we show that the total Si-O-Si bond density strongly affects the lateral ordering of the alkylsilane molecules, and that increase in the Si-O-Si bond density disorders the molecular packing. Our results imply that a lab-to-lab variation in the experimentally prepared SAMs can be attributed to different Si-O-Si bond densities at the SAMSiO(2) interface.