Byung-Hyun Kim

Stress evolution during the oxidation of silicon nanowires in the sub-10 nm diameter regime

Using a reactive molecular dynamics simulation, the oxidation of Si nanowires (Si-NWs) with diameters of 5, 10, and 20 nm was investigated. The compressive stress at the interface between the oxide and the Si core decreased with increasing curvature in the sub-10 nm regime of the diameter, in contrast to the theory of self-limiting oxidation where rigid mechanical constraint of the Si core was assumed. The Si core of the thinner Si-NW was deformed more with surface oxidation, resulting in a lower compressive stress at the interface. These results explain the experimental observation of full oxidation of very thin Si-NWs.

Reactive molecular dynamics simulation of early stage of dry oxidation of Si (100) surface

Initial stage of oxidation of Si (100) surface by O2 molecules was investigated in atomic scale by molecular dynamics (MD) simulation at 300 K and 1200 K without external constraint on the oxygen molecules. A reactive force field was used for the simulation to handle charge variation as well as breaking and forming of the chemical bonds associated with the oxidation reaction. Results of the present simulation are in good agreement with previous first principle calculations and experimental observations: the oxygen molecules spontaneously dissociated on the Si (100) surface and reacted with Si first layer without energy barrier. The simulation also exhibited that the reacted oxygen preferentially located in the back bonds of the surface dimer. Consecutive oxidation simulation with 300 O2 molecules showed that the diffusion of oxygen atom into the subsurface of clean Si surface can occur during very short time of the present oxidation simulation. The present MD simulation also revealed that the oxidation at...

Effects of interfacial bonding in the Si-carbon nanotube nanocomposite: A molecular dynamics approach

We investigated the effects of interfacial bonding on the mechanical properties in the Si-carbon nanotube (CNT) nanocomposite by a molecular dynamics approach. To describe the system appropriately, we used a hybrid potential that includes Tersoff, AIREBO (adaptive intermolecular reactive empirical bond order), and Lennard–Jones potentials. With increasing bonding strength at the interface of Si matrix and CNT, toughness as well as Young’s modulus and maximum strength increased steadily. CNT pull-out and load transfer on the strong CNT were identified as the main mechanisms for the enhanced properties. At optimum bonding, crack tip was deflected around CNT and the fracture proceeded in plastic mode through Si matrix owing to the strong reinforcement of CNT, and resulted in a further enhancement of toughness. At maximum bonding, however, only load transfer is operative and the fracture returned to brittle mode. We concluded that a strong interface as long as the CNT maintains its structural integrity is des...

Reactive molecular dynamic simulations of early stage of wet oxidation of Si (001) surface

We have investigated the initial stage of oxidation of Si (001) surface by water (H2O) molecules using reactive molecular dynamics (MD) simulation at 300 K and 1200 K without any external constraint on the water molecules. Previously, reported water reaction behaviors on silicon surface by ab initio calculations or experimental observations were reproduced by the present MD simulation. The present simulation further revealed that the hydrogen atom in H2O is more attractive than oxygen atom in O2 to bond with Si, such that it accelerates the dissociation process of H2O. It was also observed that the oxidation reaction was enhanced with increased number of the supplied water molecules. It was suggested that the repulsion between water molecules and their fragments facilitates the dissociation of both water molecules and hydroxyl decomposition on the Si surface. Therefore, the wet oxidation behavior appeared to have more temperature dependence even in the early stage of oxidation.

Interfacial reaction-dominated full oxidation of 5 nm diameter silicon nanowires

While almost all Si nanostructures, including Si nanowires (SiNWs), Si nanocrystals, and Si nanotrench-like structures on a supra- or sub-10 nm scale exhibit self-limiting oxidative behavior, herein we report full oxidation of SiNWs 5 nm in diameter. We investigated the oxidative behavior of SiNWs with diameters of 5 nm and compared our findings with those for SiNWs with diameters of 30 nm. Single-crystalline SiNWs 5 and 30 nm in diameter were grown by a chemical vapor deposition (CVD) process using Ti as a catalyst. The SiNWs were then oxidized at 600–1000 °C for 30 min to 240 min in O2. The thicknesses of the resulting oxide layers were determined by transmission electron microscopy (TEM). As expected, the SiNWs 30 nm in underwent self-limiting oxidation that was parabolic in nature. However, under the same conditions, the SiNWs 5 nm in diameter underwent full oxidation that was linear in nature. Atomic-scale molecular dynamic simulations revealed that the compressive stress in the oxide layer, which is...

Characteristic analysis of electrodynamic suspension device with permanent magnet Halbach array

This article deals with the characteristic analysis of electrodynamic suspension (EDS) device with the permanent magnet (PM) Halbach array system. On the basis of transfer relation theorem with magnetic vector potential, the magnetic field quantities were obtained and then confirmed with the corresponding two-dimensional and three-dimensional finite element analysis result. In order to validate the characteristic analysis scheme for PM-EDS device, the dynamic performance was tested by using a high-speed rotary-type dynamic test facility with linear peripheral speed up to 250 km/h. A comparison is made between the analysis and the experimental results to demonstrate the design considerations of PM-EDS device for high-speed maglev.

Effects of suboxide layers on the electronic properties of Si(100)/SiO2 interfaces: Atomistic multi-scale approach

A multi-scale approach connecting the atomistic process simulations to the device-level simulations has been applied to the Si(100)/SiO2 interface system. The oxidation of Si(100) surface was simulated by the atomic level molecular dynamics, the electronic structure of the resultant Si/suboxide/SiO2 interface was then obtained by the first-principles calculations, and finally, the leakage currents through the SiO2 gate dielectric were evaluated, with the obtained interface model, by the non-equilibrium Greens function method. We have found that the suboxide layers play a significant role for the electronic properties of the interface system and hence the leakage currents through the gate dielectric.