Showing papers by "Fabrizio Cleri published in 2006"

Point-defect recombination efficiency at grain boundaries in irradiated SiC

Abstract: We studied the atomic-scale mechanisms of radiation damage recovery, by molecular dynamics simulations of irradiation cascades in a $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}$ model system, containing one general (001) twist grain boundary in the direction approximately perpendicular to the cascade. The (001) grain boundary has a disordered atomic structure, representative of high-angle, high-energy boundaries in cubic silicon carbide. Compared to the perfect crystal model system, we find a relevant effect of grain boundaries on the annealing of cascade defects, both in terms of localization of defects, which are preferentially concentrated around the grain boundary, and of relative defect recovery efficiency. In general, C interstitials are the prevalent type of defect over the whole range of energies explored. A slight grain boundary expansion is observed, accompanied by a broadening of the central atomic planes.

Screening and surface states in molecular monolayers adsorbed on silicon.

Abstract: We performed density functional theory calculations of the atomic and electronic structure of a dense monolayer of phenyl-terminated alkyl chains chemisorbed onto the (100) Si surface. Different adsorption sites were characterized for both the pristine and (2 × 1) reconstructed surface. A strong effect on the ordering and alignment of the molecular energy levels with respect to the Fermi level of silicon is observed, consequent to intermolecular screening in the monolayer and of the appearance of surface localized states, as a function of the different bonding arrangements. Some possible consequences of these findings are discussed in the framework of the experimental synthesis of such monolayers as molecular current rectifiers in silicon-integrated nanoscale electronics.

Diffusion of boron in silicon: Compatibility of empirical molecular dynamics with continuum simulations

Abstract: The compatibility of atomistic simulations with continuum methods is tested by applying empirical molecular dynamics to the diffusion of a boron dopant atom in silicon. Extended timescale simulations of the diffusion path are performed. The analysis of the position of boron during the migration events reveals a preference for a kick-out mechanism. The deduced migration length is in excellent agreement with the classical value, a promising conclusion encouraging the transition to all-atomistic process simulations. The diffusion coefficient of boron is analyzed in light of an accelerated diffusion in the presence of a silicon self-interstitial oversaturation.

What's so special about nanocrystalline semiconductors?

Abstract: Nanocrystalline semiconductors display unique features compared to coarse-grained microstructures and even to their monocrystalline counterparts. We contend that such peculiarities are due to: (1) the extremely large fraction of atoms located at Grain Boundaries (GBs) and (2) the 'character distribution' of GBs, which are mostly high-energy, random interfaces. Initially, we study the structure of random GBs in nanocrystalline semiconductors by means of large-scale Molecular Dynamics (MD) simulations. Subsequently, the atomic structure and electronic properties of some typical high-energy GBs in Si- and C-based nanostructures are characterised by means of a semi-empirical tight-binding Hamiltonian. We show that relevant properties of nanocrystalline semiconductors containing a large fraction of high-energy GBs are quite distinct with respect to those of coarse-grained and bulk semiconductors.

Atomic-Scale Investigation on Fracture Toughness in Nanocomposite Silicon Carbide

Abstract: Ceramic materials are attractive for structural applications because of their low density, chemical inertness, high strenght, high hardness and high-temperature stability. However they have inherently low fracture toughness, so that plastic deformation in ceramics is found to be extremely limited. Ceramic matrix composites (CMC) have been therefore developed to overcome the intrinsic brittleness and lack of reliability of monolithic samples. CMC’s consist of a ceramic matrix reinforced with inclusions, such as particles, whiskers or chopped fibers (fiber thoughening).