No abstract

The unique electronic properties of graphene offer the possibility that it could replace silicon when microelectronics evolves to nanoelectronics. Graphene grown epitaxially on silicon carbide is particularly attractive in this regard because SiC is itself a useful semiconductor and, by suitable manipulation of the growth conditions, epitaxial films can be produced that exhibit all the transport properties of ideal, two-dimensional graphene desired for device applications. Nevertheless, there is little or no understanding of the actual kinetics of growth, which is likely to be required for future process control. As a step in this direction, we propose a local heat release mechanism to explain finger-like structures observed when graphene is grown by step flow decomposition of SiC(0001). Using a continuum equation of motion for the shape evolution of a moving step, a linear stability analysis predicts whether a shape perturbation of a straight moving step grows or decays as a function of growth temperature, the background pressure of Si maintained during growth, and the effectiveness of an inert buffer gas to retard the escape of Si atoms from the crystal surface. The theory gives semi-quantitative agreement with experiment for the characteristic separation between fingers observed when graphene is grown in a low-pressure induction furnace or under ultrahigh vacuum conditions.Comment: 4 pages, 5 figure

As a potential first-wall fusion reactor material, tungsten will be subjected to high radiation flux and extreme mechanical stress. We propose that under these conditions, coupled grain boundary (GB) motion, in some cases enhanced by interstitial loading, can lead to a radiation-damage healing mechanism, in which a large stress activates coupled GB motion, and the GB sweeps up the defects, such as voids and vacancies, as it passes through the material. The stress-induced mobility characteristics of a number of GBs in tungsten are examined to investigate the likelihood of this scenario.

We use kinetic Monte Carlo simulations to understand growth-induced and etching-induced step bunching of 6H-SiC͕0001͖ vicinal surfaces oriented toward ͗1100͘ and ͗1120͘. By taking account of the different rates of surface diffusion on three inequivalent terraces, we reproduce the experimentally observed tendency for single bilayer height steps to bunch into half unit-cell height steps. By taking account of the different mobilities of steps with different structures, we reproduce the experimentally observed tendency for adjacent pairs of half unit-cell height steps to bunch into full unit-cell height steps. A prediction of our simulations is that growthinduced and etching-induced step bunching lead to different surface terminations for the exposed terraces when full unit-cell height steps are present.

The development of compressive strain in metal thin films grown at low temperature has recently been revealed via x-ray diffraction and explained by the assumption that a large number of vacancies were incorporated into the growing films. The results of our molecular dynamics and parallel temperature-accelerated dynamics simulations suggest that the experimentally observed strain arises from an increased nanoscale surface roughness caused by the suppression of thermally activated events at low temperature combined with the effects of shadowing due to off-normal deposition.

No abstract

Reflection and implantation of low energy helium (He) ions by tungsten (W) substrate are studied using molecular dynamics (MD) simulations. Motivated by the ITER divertor design, our study considers a range of W substrate temperatures (300 K, 1000 K, 1500 K), a range of He atom incidence energies ($\le$100 eV) and a range of angles of incidence ($0^{\circ}$-$75^{\circ}$) with respect to substrate normal. The MD simulations quantify the reflection and implantation function, the integrated moments such as the particle/energy reflection coefficients and average implantation depths. Distributions of implantation depths, reflected energy, polar and azimuthal angles of reflection are obtained, as functions of simulation parameters, such as W substrate temperature, polar angle of incidence, the energy of incident He, and the type of W substrate surface. Comparison between the MD simulation results, the results obtained using SRIM simulation package, and the existing experimental and theoretical results is provided.Comment: 51 pages, 6 tables, 27 figures. Submitted to Journal of Nuclear Material

When grain sizes are reduced to the nanoscale, grain boundaries (GB) become the dominant sources of the dislocations that enable plastic deformation. We present the first molecular dynamics (MD) study of the effect of substitutional solutes on the dislocation nucleation process from GBs during uniaxial tensile deformation. A simple bi-crystal geometry is utilized in which the nucleation and propagation of dislocations away from a GB is the only active mechanism of plastic deformation. Solutes with atomic radii both larger and smaller than the solvent atomic radius were considered. Although the segregation sites are different for the two cases, both produce increases in the stress required to nucleate a dislocation. MD simulations at room temperature revealed that this increase in the nucleation stress is associated with changes of the GB structure at the emission site caused by dislocation emission, leading to increases in the heats of segregation of the solute atoms, which cannot diffuse to lower-energy sites on the timescale of the nucleation event. These results contribute directly to understanding the strength of nanocrystalline materials, and suggest suitable directions for nanocrystalline alloy design leading toward structural applications.