João F. Justo

INTERATOMIC POTENTIAL FOR SILICON DEFECTS AND DISORDERED PHASES

We develop an empirical potential for silicon which represents a considerable improvement over existing models in describing local bonding for bulk defects and disordered phases. The model consists of two- and three-body interactions with theoretically motivated functional forms that capture chemical and physical trends as explained in a companion paper. The numerical parameters in the functional form are obtained by fitting to a set of ab initio results from quantum-mechanical calculations based on density-functional theory in the local-density approximation, which include various bulk phases and defect structures. We test the potential by applying it to the relaxation of point defects, core properties of partial dislocations and the structure of disordered phases, none of which are included in the fitting procedure. For dislocations, our model makes predictions in excellent agreement with ab initio and tight-binding calculations. It is the only potential known to describe both the 30°- and 90°-partial dislocations in the glide set

Group IV graphene- and graphane-like nanosheets

111%. The structural and thermodynamic properties of the liquid and amorphous phases are also in good agreement with experimental and ab initio results. Our potential is capable of simulating a quench directly from the liquid to the amorphous phase, and the resulting amorphous structure is more realistic than with existing empirical preparation methods. These advances in transferability come with no extra computational cost, since force evaluation with our model is faster than with the popular potential of Stillinger-Weber, thus allowing reliable atomistic simulations of very large atomic systems. @S0163-1829~98!04026-0#

Hydrogen role on the properties of amorphous silicon nitride

We performed a first-principles investigation on the structural and electronic properties of group IV (C, SiC, Si, Ge, and Sn) graphene-like sheets in flat and buckled configurations and the respective hydrogenated or fluorinated graphane-like ones. The analysis on the energetics, associated with the formation of those structures, showed that fluorinated graphane-like sheets are very stable and should be easily synthesized in the laboratory. We also studied the changes of the properties of the graphene-like sheets as a result of hydrogenation or fluorination. The interatomic distances in those graphane-like sheets are consistent with the respective crystalline ones, a property that may facilitate integration of those sheets within three-dimensional nanodevices.

A Palm Tree Antipodal Vivaldi Antenna With Exponential Slot Edge for Improved Radiation Pattern

We have developed an interatomic potential to investigate structural properties of hydrogenated amorphous silicon nitride. The interatomic potential used the Tersoff functional form to describe the Si–Si, Si–N, Si–H, N–H, and H–H interactions. The fitting parameters for all these interactions were found with a set of ab initio and experimental results of the silicon nitride crystalline phase, and of molecules involving hydrogen. We investigated the structural properties of unhydrogenated and hydrogenated amorphous silicon nitride through Monte Carlo simulations. The results show that depending on the nitrogen content, hydrogen has a different chemical preference to bind to either nitrogen or silicon, which is corroborated by experimental findings. Besides, hydrogen incorporation reduced considerably the concentration of undercoordinated atoms in the material, and consequently the concentration of dangling bonds.

Stability and plasticity of silicon nanowires : The role of wire perimeter

This letter presents an Exponential Slot Edge Antipodal Vivaldi Antenna (ESE-AVA), with improved radiative features as compared to the conventional Antipodal Vivaldi Antenna (AVA) design. It extends the low-end bandwidth limitation, mitigates the side and back lobe levels, corrects squint effect, and increases its main lobe gain. In order to confirm those features, a comparative study among the ESE-AVA, the low directivity conventional AVA and two popular modifications, regular slot edge (RSE) and the tapered slot edge (TSE) AVA is performed. A comparison between AVA and the proposed ESE-AVA at 6 GHz shows an improved gain of 8.3 dB, -15.5 dB of Side Lobe Level (SLL), and 0 degrees of main lobe squint (MLS), in contrast with 5 dB of gain, - 5 dB of SLL, and 5 degrees of MLS in the conventional AVA. By comparing the ESE-AVA with RSE-AVA and TSE-AVA, it was observed that its notches in exponential shape, similar to open the main radiator, besides mitigating the SLL also directs the E-fields distributions towards the main lobe. It reflects into a main lobe gain improvement.

Dislocation core reconstruction and its effect on dislocation mobility in silicon

We investigated the properties of stability and plasticity of silicon nanowires using molecular dynamics simulations. We considered nanowires with , and growth directions with several diameters and surface facet configurations. We found that the wire perimeter, and not the wire diameter, is the meaningful dimensional parameter. As a result, the surface facets play a central role on the nanowire energy, that follows a universal scaling law. Additionally, we have computed the response of a silicon nanowire to external load. The results were compared to available experimental and ab initio data.

Functionalized adamantane: Building blocks for nanostructure self-assembly

Through atomistic calculations of kink nucleation and migration in the core of partial dislocations in silicon we demonstrate that symmetry-breaking structural reconstructions will strongly affect dislocation mobility. Core reconstructions give rise to multiple kink species, and, relative to kinks in an unreconstructed dislocation, an increase in kink formation and migration energies. These factors provide additional resistance to dislocation motion which scales with the energy reconstruction. Our results indicate that the observed variations of dislocation mobility in going from elemental to IV–IV, and further to III–V and II–VI zinc-blende semiconductors, can be attributed in part to the weakening of core reconstruction across the series.

Finite-temperature molecular-dynamics study of unstable stacking fault free energies in silicon

We report first principles calculations on the electronic and structural properties of chemically functionalized adamantane molecules, either in isolated or crystalline forms. Boron and nitrogen functionalized molecules, aza-, tetra-aza-, bora-, and tetra-bora-adamantane, were found to be very stable in terms of energetics, consistent with available experimental data. Additionally, a hypothetical molecular crystal in a zincblende structure, involving the pair tetra-bora-adamantane and tetra-aza-adamantane, was investigated. This molecular crystal presented a direct and large electronic bandgap and a bulk modulus of 20 GPa. The viability of using those functionalized molecules as fundamental building blocks for nanostructure self-assembly is discussed.

Dislocation core properties in semiconductors

We calculate the free energies of unstable stacking fault (USF) configurations on the glide and shuffle slip planes in silicon as a function of temperature, using the recently developed Environment Dependent Interatomic Potential (EDIP). We employ the molecular dynamics (MD) adiabatic switching method with appropriate periodic boundary conditions and restrictions to atomic motion that guarantee stability and include volume relaxation of the USF configurations perpendicular to the slip plane. Our MD results using the EDIP model agree fairly well with earlier first-principles estimates for the transition from shuffle to glide plane dominance as a function of temperature. We use these results to make contact to brittle-ductile transition models.

Electronic properties and hyperfine fields of nickel-related complexes in diamond

Abstract Using ab initio calculations, we computed the core reconstruction energies of {111} 30° partial dislocations in zinc-blende semiconductors. Our results show a direct correlation between core reconstruction energies and the experimental activation energies for the velocity of 60° dislocations. The electronic structure of unreconstructed dislocation cores comprises a half-filled band, which splits up in bonding and antibonding levels upon reconstruction. The levels in the electronic gap come from the core of β dislocations, while the levels related to α dislocations lie on the valence band.