The chemical reaction of an O2 molecule on the Si(001) surfaces is studied by the ab initio molecular dynamics with the spin-polarized generalized gradient approximation. The dissociative adsorption of an O2 molecule has been found to occur over two adjacent Si(001) dimers with a spin flip-flop transition. The spin-state transition could cause a substantial retardation for dissociative adsorption of an O2 molecule. The C-type defect after chemisorption of a dissociated O2 molecule, however, shows a spin-triplet state, indicating a barrier-less dissociative chemisorption. These results explain well the overall aspects including preferential oxidation of C-type defects found by recent experimental reports.

Initial processes of Si dimer row growth on Si(001) surface is studied by the first principles molecular dynamics method. We optimize several different ad-Si clusters composed of one to four atoms on the surface and estimate activation energies for some important growth processes. At lower temperatures, a metastable ad-Si dimer in the trough between substrate dimer rows attracts monomers and tends to grow into a short diluted-dimer row in the perpendicular direction to the substrate dimer rows. In high temperatures as ad-Si dimers can diffuse, a direct dimer condensation process is possible to elongate the dense-dimer rows also in the perpendicular direction.

The motion of a Si adatom over the reconstructed Si(100) surface with single-height rebonded (SB) step is studied using the pseudopotential total energy method. The step is shown to act as a good sink for adatoms descending onto the lower ledge. This is due to the presence of deep traps at the rebonded dimer row on the lower terrace and to the negative Ehrlich-Schwoebel barrier (the activation barrier for descent from the edge is 0.23 eV lower than for the motion on a flat surface). The diffusion characteristics of the adatom on both terraces are virtually unaffected by the presence of the step. The dimer buckling sequence on a lower terrace depends strongly on the position of the adatom along the diffusion path.

We have performed molecular-dynamics (MD) simulations of hydrogenated amorphous silicon (a-Si:H) thin-film growth using realistic many-body semiclassical potentials developed to describe Si-H interactions. In our MD model, it was assumed that SiH3, SiH2 and the H radicals are main precursors for the thin-film growth. In MD simulations of a-Si:H thin-film growth by many significant precursor SiH3 radicals, we have evaluated average radical migration distances, defect ratios, hydrogen contents, and film growth rates as a function of different incident radical energies to know the effect of the radical energization on the properties. As a result of the comparison between the numerical and experimental results, it was observed that the agreement is fairly good, and that an increase of radical migration distance due to the radical energization is effective on a- Si:H thin-film growth with a low defect.

The energetics of both dissociative adsorption of C12 molecules on reconstructed GaAs(001) surfaces and desorption of chlorides from the chlorinated GaAs(001) surfaces is theoretically investigated, employing the first-principles pseudopotential density-functional approach within the generalized gradient approximation. On the Ga-rich GaAs(001)-(4x2) surface a C12 molecule is found to dissociate with no potential barrier over a Ga dimer and the Ga dimer is also simultaneously broken upon chlorination, resulting in formation of GaC1 with two backbonds to the As layer below. The desorption energy of a GaCl molecule is smaller than that of a Cl atom, although the former involves breaking two Ga-As backbonds and the latter involves the breakup of one Ga-Cl bond. On the As-rich (2x4) surface, the dissociation of a C12 molecule over an As dimer is an activated process and the As dimer is not broken by the Cl adsorption. The desorption energy of a AsCI molecule is larger than that of a Cl atom. The obtained results are consistent with recent experiments such as temperature-programmed desorption measurements.

Ab initio local-density- functional- theory calculations of formation energies, surface stress, and multilayer relaxations are reported for the (111), (100), and (110) surfaces of Rh. The study is performed using ultrasoft pseudopotentials and plane waves in a parallel implementation.

Solid solution precipitates, including Ni3Al in Ni, play an important role in modifying and improving properties of structural and high temperature alloys. The nature of the interface between the host alloy and the precipitate has a large influence on the nature of the precipitate properties, and upon the energetics of their formation. We present here a brief summary of initial ab-initio electronic structure calculations of the Ni/Ni3Al interface, and present results for the interfacial energy. Our results indicate that that the spin-moment transition from high moment Ni to low moment Ni3Al accounts for much of the (zero temperature) interface energy. Corresponding paramagnetic calculations give a significantly lower interfacial energy, and one that is more consistent with high temperature (above the Curie temperature) experimental results.

The structures and thermodynamic properties of the (100), (010), and (110) lateral surfaces of extended-chain polyethylene crystals between 0 K and 300 K were determined by free energy minimization using consistent quasi-harmonic lattice dynamics. Slight rotations of the outermost chains from their corresponding orientations in the bulk were observed. These deviations from bulk structure were confined to the first three molecular layers (approximately 10 Å) at the surface. Surface free energy calculations found the (110) surface to be the most stable over the entire temperature range modeled, with free energies ranging from 95.4 erg/cm2 to 103.5 erg/cm2, at temperatures of 0 K and 300 K, respectively. Surface free energies of the (100) and (010) surfaces were found to be at least 15% higher than the (110) surface, with the (100) surface slightly more stable than the (010) surface, over all temperatures considered. Surface free energy increases as the density of chains at the surface decreases. The surface free energy at low temperatures was determined predominantly by intermolecular potential energy; at higher temperatures, excess entropy accounted for nearly half the surface free energy. Surface entropy was almost entirely due to lattice mode motions.

Molecular crystals are studied using the Car-Parrinello ab initio molecular dynamics simulation technique. In particular, the motion of protons in a variety of systems is considered. Results are presented on the rotation of the methyl group in solid nitromethane and proton transfer through a hydrogen bond in hydrogen chloride dihydrate crystal.

Structural and mechanical behavior of SiSe2 nanowires is investigated with the moleculardynamics (MD) method. Nanowires contain finite number of non-intersecting chains of edgesharing Si(Sel/2)4 tetrahedra. The simulations are based on an effective interatomic potential containing both 2- and 3-body interactions. It is found that the nanowires remain highly crystalline and stay in the elastic deformation regime up to a critical strain. Under large uniaxial strain, fracture of the nanowires is initiated by broken bonds in one of the chains at the outermost layer. This induces cross-linking among the neighboring chains, which leads to the presence of cornersharing tetrahedra and local amorphization. Local amorphization propagates across nanowires while multiple cracks start at the boundaries of the amorphous region. The dynamics of amorphization and fracture are discussed.

Core-level shifts and core-hole screening effects in alloy formation are studied ab initio by constrainedl-density-functional total-energy calculations. For our case study, the ordered intermetallic alloy MgAu, final-state effects are essential to account for the experimental Mg 1s shift, while they are negligible for Au 4f. We explain the differences in the screening by analyzing the calculated charge density response to the core hole perturbation.

Native point defect densities (including vacancies, antisites and interstitials) in ZnSe are calculated using a quasichemical formalism, including both vibrational and electronic contributions to the defect free energy. The electronic contribution to the defect formation free energy is calculated using the self-consistent first-principles full-potential linearized muffin-tin orbital (FP-LMTO) method and the local-density approximation (LDA). Gradient corrections are included so that absolute reference to zinc atoms in the vapor phase can be made. We find that the Frenkel defect formation energy is ∼0.3 eV lower at a stacking fault than in the bulk lattice. Nonradiative-recombination-induced Frenkel defect generation at stacking faults is proposed as a mechanism responsible for the limited device lifetimes.

The atomic and electronic structure of the radiation-induced interstitial atoms in MgO and KCl crystals representing two broad classes of ionic solids are calculated and compared. The first-principles full potential LMTO method is applied to a 16-atom supercell. For both crystals the energetically most favourable configuration is a dumbbell centered at a regular anion site. Its (110) and (111) orientations are very close in energy which permits the dumbbell to rotate easily on a lattice site. The mechanism and the relevant activation energy for thermally activated diffusion hops from the dumbbell equilibrium position to the cube face and cube center are discussed in the light of the available experimental data for MgO. In order to interpret recent experimental data on Raman spectroscopy, the local vibrational frequences are calculated for the dumbbell in KCl (the so-called H center). A strong coupling is found between its stretching molecular mode and the breathing mode of the nearest cations whose frequency is predicted.

We present a molecular dynamics investigation on hydrogen diffusivity in crystalline quartz by computing the diffusion coefficient over a wide range of temperatures (700K < T < 1500K) and by characterizing the diffusion path and mechanism. Our main findings are: (i) hydrogen diffusion is anisotropically confined along the c-axis in α- and β-quartz; (ii) hydrogen diffuses through a jump-like mechanism; (iii) the temperature-dependent diffusivity follows an Arrhenius law with activation energy of 0.56 eV and 0.27 eV for α-and β-quartz, respectively.

Crystalline and molecular silicon-oxygen compounds are investigated using a simplified LCAO-LDA scheme for the construction of a nonorthogonal tight-binding (TB) Hamiltonian within a two-center approximation. The applicability of this method to the important class of silicon oxides and related molecules is demonstrated. In particular, the properties of the equilibrium structure of α-quartz and several Siloxane molecules are calculated and found to agree well with experiments and self-consistent calculations. To obtain such a good agreement it is necessary to include also the 3d states of Si into the LCAO basis of the wavefunctions.

A Method of Moments (MoM) electromagnetic model of percolating conducting films was applied to calculate the effective parameters of the composite formed by conducting inclusions placed within a dispersive magnetic but nondispersive dielectric matrix. The MoM calculations demonstrate a coupling between the magnetic properties of the matrix and the effective composite permittivity and frequency dispersion of the composite. The coupling of permittivity and permeability is observed near the percolation threshold of the composite and for high conductivity inclusions. The prediction agrees with physical expectations since near percolation the conduction correlation length dominates the effective permittivity of the composite and this correlation length is determined by both the permittivity and permeability of the composite.

We have performed an ab initio geometry optimization of hydrogenated silicon clusters Si6Hx,(O ≤ x ≤ 12), within the generalized gradient approximation (GGA-PW91) in the density-functional theory. We have found that hydrogenation of Si6 clusters provides contrasting effects on their stable structures depending on x. From calculation of zeropoint corrected formation energies, we also found bistability of silicon clusters in the existence of hydrogen.

We present a first-principles calculation of the valence-band offset at the (001) interface between ZnSe and III-V lattice-matched alloys based on Al, Ca, In, P elements, namely Al0.5In0. 5P and Ga0.1n0.5P. Among the different possible interface geometries we have focused on the P-terminated alloy in contact with the Zn-terminated ZnSe crystal. The results of this study show the existence of a very low band offset at the abrupt interface, both for the ideal geometry and for the relaxed atomic positions. The investigation of the electronic interface band structure reveals the presence of a low-lying half-filled interface band related to the Zn-P bond.

A recently reported metastable phase of NbN with a superconducting Tc=16.4 K is characterized using full potential electronic structure methods. This new phase, which has Pm3m (cubic) symmetry, can be described as the B1 (rocksalt) structure with 25% ordered vacancies on each sublattice. We compare the equation of state and electronic spectrum of this Pm3m phase with its rocksalt counterpart [1] and with Nb4N3 in the 14/mmm (tetragonal) phase, which allows the characterization of N vacancies without accompanying Nb vacancies. For Pm3m NbN, the calculated lattice constant is 5% smaller than reported and the energy is 1.00 eV/molecule higher than B1 NbN, suggesting that the newly reported phase is something other than a stoichiometric Pm3m phase of NbN. We report on the energy surface for tetragonal distortions of this phase, from which we evaluate its structural stability and obtain Poisson's ratio.

Atomistic simulations using both tight-binding and density-functional approaches have been performed to investigate boron-related defects in silicon. In agreement with experiment, the boron interstitial is shown to be a negative- U center in the sense that its neutral charge state, with an associated Jahn-Teller distortion off the ideal tetrahedral site, is never the ground state for any value of the chemical potential in the gap. The possible consequences for an electron-assisted migration of the interstitial are discussed. We also find the boron substitutional defect to be a next-nearest neighbor of a silicon vacancy in agreement with EPR spectra.

A semi-empirical tight-binding model of the boron-silicon system is validated by direct comparison with the accurate density-functional results and is then used to perform molecular dynamics simulations of boron diffusion at high temperatures. The mobility of the interstitial is found to be strongly charge-state dependent. Termination of the boron interstitial migration path by recombination with a silicon vacancy is shown to be a very likely process with a number of configurations having no barrier to capture when the boron is a near-neighbor of the vacancy.