Daisuke Yamasaki

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.

Magnetometer Utilizing Cooled Normal Pickup Coil and High-Tc Superconducting Quantum Interference Device Picovoltmeter for Nondestructive Evaluation

A magnetometer utilizing a cooled normal pickup coil and a high-Tc superconducting quantum interference device (SQUID) picovoltmeter was applied to nondestructive evaluation in an unshielded environment. The pickup coil of copper wire was cooled to T=77 K in order to decrease thermal noise from resistance. The magnetic field noise of the magnetometer was 130 pT/Hz1/2 and 10 pT/Hz1/2 at 100 Hz and 1 kHz, respectively, for the pickup coil with a 4.4 mm diameter. It was shown that the magnetometer could be moved in an unshielded environment without degradation of its performance. By moving the cooled pickup coil, we successfully detected a small crack on the backsurface of the Cu plate in an unshielded environment.

Eddy Current Testing Utilizing Cooled Normal Pickup Coil and Superconducting Quantum Interference Device Picovoltmeter: Comparison between Experiment and Analysis

Eddy current testing utilizing a cooled normal pickup coil and a high-Tc superconducting quantum interference device (SQUID) picovoltmeter was performed both experimentally and analytically. In the experiment, we successfully detected a small crack on the back surface of a Cu plate by moving the coil in an unshielded environment. First, we developed a method of avoiding the drift of the detected signal that was caused by the variance of lift-off. Next, we clarified the dependences of the detected signal on the excitation frequency and the thickness of the Cu plate. It was shown that an optimum frequency that maximizes the detected signal exists. Because this frequency changed with the thickness of the Cu plate, the frequency dependence could be used to estimate the depth of the crack from the surface of the Cu plate. The experimental results were analyzed taking into account the phase and amplitude of the signal field caused by the crack. Good agreement was obtained between experiment and analysis.

Possible mechanisms for the pharmacokinetic interaction between phenytoin and folinate in rats

The plasma concentration of phenytoin (PHT) is decreased by coadministration of folinate (leucovorin; LV), a folate (FA) analogue. The aim of this study was to examine the effect of LV on the pharmacokinetics of PHT in rats in vivo and to investigate the mechanism of the interaction. LV (50 mg/kg) was administered orally to rats concomitantly given intravenous PHT (50 mg/kg) to evaluate the effect of LV on the pharmacokinetics of PHT. The effect of LV on the plasma protein binding of PHT was investigated by using plasma from rats that had received oral LV. We also examined the effects of LV on the uptake of PHT into isolated rat hepatocytes and on the metabolism of PHT in isolated rat hepatocytes and rat hepatic microsomes. LV significantly increased the systemic clearance (2-fold) and liver-to-blood partition coefficient (1.24-fold) of PHT. However, it did not affect the plasma protein binding or hepatic uptake of PHT. LV increased the metabolism of PHT in isolated rat hepatocytes, with a significant 1.41-fold increase in the maximum rate of metabolism and a decrease in the Michaelis-Menten constant. On the other hand, 5-methyltetrahydrofolate (5-MTHF), a primary metabolite of LV and FA, significantly increased p-hydroxylation of PHT in rat hepatic microsomes, whereas LV and FA themselves had no effect. In conclusion, these results suggest that, in rats, LV, an FA analogue, decreases the plasma concentration of PHT by increasing the hepatic metabolism of PHT, and the increase in the PHT metabolism is, at least in part, attributable to 5-MTHF.

Eddy Current Testing Utilizing Cooled Normal Pickup Coil and Superconducting Quantum Interference Device Picovoltmeter

An eddy current probe utilizing a cooled normal pickup coil and a high-Tc superconducting quantum interference device (SQUID) picovoltmeter was developed. The pickup coil could be moved in an unshielded environment, during which the signal voltage across the pickup coil was transferred to the SQUID picovoltmeter, which was fixed in a cylindrical magnetic shield. By moving the pickup coil, we successfully detected a small crack on the back surface of a Cu plate in the unshielded environment. The dependences of the detected signal on the excitation frequency and thickness of the Cu plate were clarified. The frequency dependence could be used to estimate the depth of the crack from the surface of the Cu plate.