P. C. Kelires

Insights into the fracture mechanisms and strength of amorphous and nanocomposite carbon.

Tight-binding molecular dynamics simulations shed light into the fracture mechanisms and the ideal strength of tetrahedral amorphous carbon and of nanocomposite carbon containing diamond crystallites, two of the hardest materials. It is found that fracture in the nanocomposites, under tensile or shear load, occurs intergrain and so their ideal strength is similar to the pure amorphous phase. The onset of fracture takes place at weakly bonded sites in the amorphous matrix. On the other hand, the nanodiamond inclusions significantly enhance the elastic moduli, which approach those of diamond.

Thermodynamics of C incorporation on Si(100) from ab initio calculations.

We study the thermodynamics of C incorporation on Si(100), a system where strain and chemical effects are both important. Our analysis is based on first-principles atomistic calculations to obtain the important lowest-energy structures, and a classical effective Hamiltonian which is employed to represent the long-range strain effects and incorporate the thermodynamic aspects. We determine the equilibrium phase diagram in temperature and C chemical potential, which allows us to predict the mesoscopic structure of the system that should be observed under experimentally relevant conditions.

Structure and Chemical Ordering in Amorphous Silicon Carbide Alloys

Simulations of amorphous silicon carbide alloys indicate that the amorphous network deviates from an ideal tetrahedral geometry due to the presence of threefold-coordinated carbon atoms. All samples exhibit a significant degree of chemical ordering, most of it arising from fourfold-coordinated carbon atoms. The results show that there is no phase separation and that hydrogenation might promote tetrahedral carbon coordination, as well as chemical ordering.

Optical and elastic properties of diamond-like carbon with metallic inclusions: A theoretical study

A tough material commonly used in coatings is diamond-like carbon (DLC), that is, amorphous carbon with content in four-fold coordinated C higher than ∼70%, and its composites with metal inclusions. This study aims to offer useful guidelines for the design and development of metal-containing DLC coatings for solar collectors, where the efficiency of the collector depends critically on the performance of the absorber coating. We use first-principles calculations based on density functional theory to study the structural, electronic, optical, and elastic properties of DLC and its composites with Ag and Cu inclusions at 1.5% and 3.0% atomic concentration, to evaluate their suitability for solar thermal energy harvesting. We find that with increasing metal concentration optical absorption is significantly enhanced while at the same time, the composite retains good mechanical strength: DLC with 70–80% content in four-fold coordinated C and small metal concentrations (<3 at. %) will show high absorption in the ...

Substitutional carbon impurities in thin silicon films: Equilibrium structure and properties

We discuss a set of atomistic calculations of the structure of Si geometries with substitutional carbon atoms, involving the (100) surface or bulk features related to thin films grown in the (100) direction. We use both quantum mechanical density functional theory and empirical potential calculations at finite temperature and constant pressure to study the local structure, bonding characteristics and overall distribution of the carbon atoms in the host silicon lattice. These calculations reveal a strong nearest neighbor repulsion between substitutional carbon atoms, to the point where these atoms prefer to have fewer bonds than normally in order to avoid each other. This effect still holds for high temperatures and high carbon concentrations. As a result, bulk ordering of the type observed in Si–Ge alloys is unlikely to occur.

Physical origin of trench formation in Ge∕Si(100) islands

Monte Carlo simulations of stress buildup and relief shed light onto the physical origin of trench formation in Ge∕Si(100) islands. By monitoring the stress evolution as the island grows layer by layer, we find that a trench is most likely being formed halfway during growth. The primary driving force for this phenomenon is the reduction of the concentrated stress below the edges of the island, but not the need to provide Si into it, as is widely believed. However, once the trench is formed, subsequent intermixing through it is enhanced, and nearly compensates for the stress in the island.

Intrinsic stress and stiffness variations in amorphous carbon

Abstract We have studied the problem of intrinsic stress in tetrahedral amorphous carbon. Our methodology was based on the concept of atomic level stresses. These are extracted from the local energetics within the empirical potential approach. The finite temperature statistics of the system are described by Monte Carlo simulations. The universal finding of our investigations was that equilibrated, annealed films that relax the external constraints and pressure possess zero total intrinsic stress, but still contain a high fraction of sp 3 sites. This is in contrast to the case of non-equilibrium as-grown structures that are left intrinsically stressed by the deposition process. We also studied the variation of stiffness in the amorphous carbon network as a function of the average coordination. It was found that this variation deviates from a mean-field-like behavior.

Theoretical investigation of the equilibrium surface structure of Si1−x−yGexCy alloys

Abstract We investigate the surface structure of SiGeC alloys at typical growth temperatures using Monte Carlo simulations within the empirical potential formalism. We find a strong tendency of carbon to segregate to the top layer. As a consequence, germanium segregation is drastically reduced. The carbon composition profile oscillates in the subsurface layers, with a marked enhancement in the third layer. Si–C dimers are the favored surface bonding geometry, and are mostly found in (2×2) and c (4×2) arrangements.

Softening of elastic moduli of amorphous semiconductors

Abstract We study the rigidity problem of amorphous semiconductors using Monte Carlo (MC) simulations and empirical potentials. We find that networks of tetrahedral a-C, a-Si, and a-Ge consistently have smaller elastic moduli than their crystalline counterparts. The reduction of rigidity seems to be associated with the reduced density and the random orientation of sp3 hybrids in the fully tetrahedral amorphous networks and, in addition, with the presence of sp2 sites in tetrahedral a-C.

Stress variations near surfaces in diamond-like amorphous carbon

Abstract Using Monte Carlo (MC) simulations within the empirical potential approach, we examine the effect produced by the surface environment on the atomic level stresses in tetrahedral amorphous carbon. Both the distribution of stresses and the distributions of sp 2 and sp 3 atoms as a function of depth from the surface are highly inhomogeneous. They show the same close relationship between local stress and bonding hybridization found previously in the bulk of the material. Compressive local stress favors the formation of sp 3 sites, while tensile stress favors the formation of sp 2 sites.