Structure and energetics of the Si-SiO_2 interface

Silicon has long been synonymous with semiconductor technology. This unique role is due largely to the remarkable properties of the Si-SiO_2 interface, especially the (001)-oriented interface used in most devices. Although Si is crystalline and the oxide is amorphous, the interface is essentially perfect, with an extremely low density of dangling bonds or other electrically active defects. With the continual decrease of device size, the nanoscale structure of the silicon/oxide interface becomes more and more important. Yet despite its essential role, the atomic structure of this interface is still unclear. Using a novel Monte Carlo approach, we identify low-energy structures for the interface. The optimal structure found consists of Si-O-Si "bridges" ordered in a stripe pattern, with very low energy. This structure explains several puzzling experimental observations.Comment: LaTex file with 4 figures in GIF forma

Synthesis of highly oriented and dense conical carbon nanofibers by a DC bias-enhanced microwave plasma CVD method

Conical carbon nanofibers (CCNFs) have been synthesized on Si substrates coated with a Fe thin film of a few nanometers in thickness by a DC bias-enhanced microwave plasma chemical vapor deposition (CVD) method using H2 and CH4 as reactant gases under a chamber pressure of about 230 Pa. Without DC bias, only catalytic nanoparticles could be observed by field emission scanning electron microscopy (FE-SEM) on substrate surface after growth. Highly oriented and dense CCNFs could be deposited when a negative DC bias in the range of 150-230 V was applied to the substrate holder during growth. The average density of CCNFs was measured as being of the order of 1010 cm-2, while the average length was of the order of 102 nm, and the conical angle was in the range of 10-15°. The relationships between the density, the root diameter and the length of CCNFs are discussed. © 2004 Elsevier B.V. All right reserved

Residual order within thermally grown amorphous SiO2 on crystalline silicon

The origin of x-ray diffraction peaks observed on the crystal truncation rods (CTR’s) in reciprocal space for thermally grown SiO_2 films has been investigated by large-scale atomistic simulation of silicon oxidation. Three models of SiO_2 on Si(001), Si(111), and Si(113) were formed by introducing oxygen atoms in crystalline Si from the surfaces in an atom-by-atom manner. The SiO_2 structures are classified as being amorphous in conventional characterizations, but retain the residual order originating from the {111} atomic planes in their parent crystals. The calculated diffraction patterns exhibit intensity peaks with Laue-function-like fringe profiles along the CTR’s, at positions depending on the substrate orientations, agreeing quite well with experimental results.K. Tatsumura, T. Watanabe, D. Yamasaki, T. Shimura, M. Umeno and I. Ohdomari. Residual order within thermally grown amorphous SiO_2 on crystalline silicon. Physical Review B, 2004, 69(8), 085212. https://doi.org/10.1103/PhysRevB.69.085212. Copyright 2004 by the American Physical Society

Structural investigation of organosilane self-assembled monolayers by atomic scale simulation

Molecular mechanics (MM) and molecular dynamics (MD) simulations have been performed to investigate the two-dimensional structure of organosilane self-assembled monolayers (SAMs). Unlike alkanethiol SAMs, the arrangement of molecules in organosilane SAMs is not crystalline, and their molecular structure yet remains undetermined. AMBER 8 is employed with our newly developed Si parameters for the MM/MD simulations. Simulations performed for structures with different bonding networks in the polysiloxane layer shows that the ratio of hydrogen bonds has a profound effect on conformations and strain energies of optimized structures. Our results suggest that alkylsilane SAMs formed on substrates are not perfectly uniform but may have some defects

Synthesis of millimeter long vertically aligned single-walled carbon nanotubes by point-Arc microwave plasma Cvd

A decade has passed since the discovery of single-walled carbon nanotubes (SWNTs), but the main methods for synthesizing SWNTs such as laser ablation, arc discharge and thermal (or catalytic) chemical vapor deposition (CVD) are still subject to the following disadvantages; high (800-1200°C) growth temperature requirement, low production yield (the mass ratio of SWNTs to catalyst) or high catalyst contamination, and out of control of the as grown SWNTs (usually randomly oriented, entangled bundles or ropes). All these make the purification and application of SWNTs very difficult. Plasma assistant CVD is good at controlled growth of multi-walled carbon nanotubes (MWNTs) at low temperatures. However it rarely succeeded in the growth of SWNTs. In this study, we demonstrate the low temperature (600°C) synthesis of very dense (1E16/m2) and vertically aligned SWNTs by point-arc microwave plasma CVD [1], which overcomes all the above mentioned disadvantages. Vertically aligned SWNTs were synthesized at a low temperature of 600°C on Si substrates coated with a sandwich-like structure Al2O3 /Fe /Al2O3 (/Si). Al2O3 between Si and Fe is a buffer layer to prevent them from reacting. On the other hand, Al2O3 above the Fe film works as a barrier of the surface diffusion of catalytic atoms so that the aggregation of Fe atoms can be suppressed during the pre-heating time. As a result, dense catalytic particles can be formed and extremely dense and vertically aligned SWNTs can be synthesized. To identify the SWNT samples, TEM and Raman spectroscopy were used. TEM observations show that almost all tubes are single-walled. Raman spectra of as-grown SWNTs have fingerprint features of SWNTs: the sharp tangential mode G peak, the shoulder of G peak and the radial breathing mode (RBM) peaks. From the RBM peaks, their diameters range from 0.5 to 3.0 nm. The thickness of SWNTs can increase as the growth time increases, and the lifetime of the catalyst is more than 10 hours at the growth rate of 2 μm/min, so millimeter long vertically aligned SWNTs can be synthesized. A production yield (mass ratio of SWNTs to catalyst) and a volume density are 2,500,000% and 66 kg/m3, respectively. The production yield is 50 times as high as that reported by Hata[2], Up to now, the growth temperature about 600°C is the lowest, while both the volume density and the production yield are the highest for the synthesis of SWNTs. We have also succeeded in position control of SWNTs, showing potential for applied researches such as field emitters and vertically aligned field effect transistors using our as-grown vertically aligned SWNTs without further purification

Reactions and diffusion of atomic and molecular oxygen in the SiO2 network

To address the reactions and diffusion of atomic and molecular oxygen in SiO_2, the modification of the SiO_2 network on exposure to an atomic or molecular oxygen atmosphere is investigated by measuring the x-ray-diffraction profile of the residual order peak emanating from the oxide. Analyses of the peak intensity and its fringe pattern provide experimental evidence for the recent theoretical predictions, indicating that atomic oxygen is incorporated into the SiO_2 network near the surface and diffuses toward the interface along with modifying it even at a low temperature of 400°C, whereas molecular oxygen diffuses without reacting with the bulk SiO_2 even at a temperature of 850°C that is sufficiently high for oxidation reaction at the interface.Tatsumura K., Shimura T., Mishima E., et al. Reactions and diffusion of atomic and molecular oxygen in the SiO_2 network. Physical Review B - Condensed Matter and Materials Physics, 2005, 72(4), 045205. https://doi.org/10.1103/PhysRevB.72.045205. Copyright 2005 by the American Physical Society

Nano-fabrication of CDW and its negative resistance phenomenon

Nano-fabrication of CDW (nano-CDW) with sub 0.1 micrometer thick has been successfully formed by applying of focused-ion-beam to the one-dimensional conductor, molybdenum blue bronze (K0.3MoO3) crystal. The nano-CDW characteristically shows negative resistance above the threshold fields, which monotonously increased with decreasing temperature. The nano-fabrication is completed via two steps procedure. 1 st : Hydrogen ion beam irradiation to the K0.3MoO3 crystal makes no-CDW layer by hydrogen atoms as excess-donar at 40-100 nm depth, where CDW at the surface of the crystal was isolated from the bulk side. 2 nd : Scanning focused silicon ion beam makes amorphous MoO3 in line and space patterns, where CDW at the surface are isolated as real one-dimensional region. The one-dimensionality of electrons has been confirmed by negative magnetic resistance, which magnitude should be reduced with decreasing dimensionality

Large-area synthesis of carbon nanofibers by low-power microwave plasma-assisted CVD

The use of Low-power microwave plasma-assisted chemical vapor deposition (CVD) for the large-areas synthesis of Carbon Nanotubes was investigated. The results show that at a small microwave power input of 60 W, large-area synthesis of carbon nanofibers can be achieved using remote deposition. The size limitation of the substrate heater caused a growth area increase factor of 25