Silylene (SiH2) radicals created by electron impact dissociation of silane in reactive gas discharges can play an important role in plasma deposition of amorphous and nanocrystalline silicon thin films. In this article, we present a systematic computational analysis of the interactions of SiH2 radicals with a variety of crystalline and amorphous silicon surfaces based on atomistic simulations. The hydrogen coverage of the surface and, hence, the availability of surface dangling bonds is shown to exert the strongest influence on the radical-surface reaction mechanisms and the corresponding reaction probabilities. The SiH2 radical reacts with unit probability on the pristine Si(001)-(2×1) surface which has one dangling bond per Si atom; upon reaction, the Si atom of the radical forms strong Si–Si bonds with either one or two surface Si atoms. On the H-terminated Si(001)-(2×1) surface, the radical is found to react with a probability of approximately 50%. The SiH2 radical attaches itself to the surface either by forming two bonds with Si atoms of adjacent dimers in the same dimer row or through Si–Si bonds with one or both atoms of a surface dimer. In addition, the SiH2 radical can attach itself in the trough between dimer rows, forming two Si–Si bonds with second-layer Si atoms. The energetics and dynamics of these surface reactions are analyzed in detail. A reaction probability of approximately 70% is calculated for SiH2 radicals impinging on surfaces of hydrogenated amorphous silicon (a-Si:H) films with varying concentrations of hydrogen. Recent experimental measurements have reported a 60% loss probability for the SiH2 radical on the reactor walls through laser induced fluorescence. The experimentally obtained reaction probability falls within the range for the sticking coefficients on the H-terminated and amorphous film surfaces as determined by our atomistic calculations. Molecular-dynamics (MD) simulations of a-Si:H film growth by repeated impingement of SiH2 radicals have revealed adsorption reactions at early stages to occur with similar energetics as the corresponding reactions of isolated radicals on crystalline surfaces. The reaction probability of SiH2 on a-Si:H films deposited through MD simulations is approximately 30%. Finally, it is found that the SiH2 radical is much more mobile on surfaces of a-Si:H films than on crystalline surfaces, especially when the hydrogen concentration in the amorphous film and, thus, on the surface is high.

Abstract:Since the discovery of carbon nanotubes in 1991 by Iijima, they have been of great interest both from the fundamental point of view and for future applications. The most eye catching features of this structure are their electronic, mechanical, optical and chemical characteristics, which opens a way to future applications. These properties can even be measured on single nanotubes. For commercial applications, large quantities of purified nanotubes are needed. In this paper, recent research on preparation of carbon nanotubes with special reference to low temperature synthesis of high purity is reviewed. The reported achievements in this area will open up more knowledge on carbon nanostructured materials in many areas of emerging nanoscale science and nanotechnology.

The diamond chemical vapor deposition (CVD) process has been investigated theoretically and the morphological instabilities associated with the growth of diamond films have been examined with a model based on the continuum species conservation equation coupled to surface reaction kinetics. A linear stability analysis and numerical calculations have been carried out to determine critical parameters affecting the diamond deposition layer morphology. A two-dimensional model describes the evolution of the gas-solid interface. The dynamic behavior of the interface depends on the reactants' diffusivity and surface kinetics. These factors depend upon the reactant material properties and film growth conditions such as the reactor temperature and pressure. From the analyses, it has been found that the ratio (2/k) of gas phase diffusivity (9) to the surface reaction rate constant (k) plays the critical role in promoting diamond morphological instabilities because the film morphology stabilizing processes of surface diffusion and re-evaporation are absent or negligible during diamond CVD. It is found that the film nonuniformity increases as the ratio (2/k) decreases. Increasing growth rates also result in increasing morphological instability. leading to rough surfaces. It is shown that increasing reactor pressure and decreasing gas-phase temperature and/or substrate temperature promote deposition layer nonuniformity.An approach to avoiding these instabilities is proposed. 0 1997 Elsevier Science S.A.

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A theoretical study of the nucleation, size, and structure of diamond phase carbon clusters on Si͑111͒ substrates is presented. Molecular mechanics analysis has been utilized to predict energetically and entropically feasible pathways for nucleation of the carbon clusters. Several mechanistic pathways for nucleation of carbon clusters are examined with CH 3 and/or C 2 H 2 as the nucleation precursors. A possible model for the nucleation mechanism of diamond-phase carbon clusters on the ␤-SiC͑111͒ surface, which forms epitaxially on Si͑111͒ substrates, is presented. The critical size of the carbon clusters is computed based on the atomistic theory of nucleation and the proposed nucleation mechanisms.