Ab Initio Molecular Dynamics Approach Research Articles

Diffusion Mechanism of Si Adatoms on a Double-Layer Stepped Si(001) Surface

Using the ab initio molecular dynamics approach we investigate the diffusion mechanism of Si adatoms on a rebonded and a nonrebonded double-layer stepped Si(001) surface. The rebonded ${D}_{B}$ step shows two Schwoebel barriers, whereas the nonrebonded step reveals a single barrier. This is due to the severe tensile strain on the surface layers parallel to the dimer row near the rebonded step edge. Adatom-step interaction is more favorable on a nonrebonded step than on a rebonded one near the ascending step. Several pathways for diffusion of an ad-dimer to the rebonded step edge from an upper terrace are examined. We find that the exchange and crossing-over processes are favored over the rolling-over process.

Structural and vibrational properties of amorphousSi1−xGexalloys: Anab initiomolecular-dynamics study

The simulated quenching procedure in ab initio molecular-dynamics approach has been used to generate amorphous ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ alloys for various Ge compositions. The generated structures show a random covalent network rather than a chemically ordered network. The bond-length distributions and the vibrational properties of the generated networks do not follow a simple Vegard's rule, suggesting the presence of inhomogeneity in the networks. It has been found that the local defect structures are governed by the dangling bonds at the Ge sites and the Si-centered floating bonds for a wide range of Ge compositions of $x>~0.15$. The backbonding states of the Ge dangling-bond sites increase with the Ge composition. The Si-related high-frequency modes are softened at a low Ge composition of $x=0.15$. Results and discussion on the calculated structural and vibrational properties of the ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ alloys are presented.

Ab initiomolecular-dynamics study of liquidGeSe2

The structural, vibrational, and electronic properties of liquid GeSe${}_{2}$ are investigated using ab initio molecular dynamics. The static structure factor $S(Q)$ and the pair-correlation functions of our model are in good agreement with experiment. We find many similarities between the topology of the liquid and the glass state. In addition we introduce a way of characterizing the intermediate-range order of liquid and glassy GeSe${}_{2}$ through fourfold and sixfold rings. The overall vibrational density of states is found to be consistent with Raman experiments. The intensity of the low-frequency modes, splitting of the ${A}_{1}$ and ${A}_{1c}$ peaks, and the decrease in the intensity of the high-frequency modes are all reproduced. The electronic density of states is determined and compared to our results for glassy GeSe${}_{2}$. We find that an increase in Se bond length and bond-angle disorder significantly broadens the conduction band. The time-dependent behavior of the electronic eigenvalues is examined and transient events are observed in which an electronic state crosses the optical gap. The structural configurations which produce states in the optical gap are determined using an ab initio molecular-dynamics approach and are found to be in agreement with experimental photoluminescence and electron-spin-resonance data. We also find that a linear relationship exists between the root mean square of the thermal fluctuations of an electronic eigenvalue in time and its localization.

Microscopic role of a surfactant in epitaxial crystal growth

ABSTRACTWe have investigated the kinetic role of a surfactant in the epitaxial crystal growth of Si/Ge using an ab initio molecular dynamics approach. We find that adsorption and diffusion of an adatom on monohydride Si(001) surface is drastically altered from that on a clean surface in several ways. We particularly emphasize the role of H as a surfactant, and this is further compared to typical group V materials.

Sulfur adsorption on an iron (001) surface by ab initio molecular dynamics

We study the equilibrium structure for sulfur adsorption on an iron (001) surface. The equilibrium structure is calculated by an ab initio molecular dynamics approach with a mixed basis. The magnetic moment of the iron atoms changes from the bulk value and the spin-polarization of non-magnetic adsorbate sulfur atoms is also observed.

Surfactant-Mediated Si/Ge Epitaxial Crystal Growth

AbstractWe investigate the kinetic role of a surfactant in the epitaxial Si/Ge crystal growth using ab initio molecular dynamics approach. We examine the previously suggested dimer-exchange mechanisms and find that kinetics plays a crucial role in determining the exchange process. We further find that the diffusion of adatoms on an island in the presence of a surfactant is quite different from the dimer-exchange process on a flat surface.

Ab initio molecular dynamics simulation of the solvation and transport of hydronium and hydroxyl ions in water

Charge defects in water created by excess or missing protons appear in the form of solvated hydronium H3O+ and hydroxyl OH− ions. Using the method of ab initio molecular dynamics, we have investigated the structure and proton transfer dynamics of the solvation complexes, which embed the ions in the network of hydrogen bonds in the liquid. In our ab initio molecular dynamics approach, the interatomic forces are calculated each time step from the instantaneous electronic structure using density functional methods. All hydrogen atoms, including the excess proton, are treated as classical particles with the mass of a deuterium atom. For the H3O+ ion we find a dynamic solvation complex, which continuously fluctuates between a (H5O2)+ and a (H9O4)+ structure as a result of proton transfer. The OH− has a predominantly planar fourfold coordination forming a (H9O5)− complex. Occasionally this complex is transformed in a more open tetrahedral (H7O4)− structure. Proton transfer is observed only for the more waterlike (H7O4)− complex. Transport of the charge defects is a concerted dynamical process coupling proton transfer along hydrogen bonds and reorganization of the local environment. The simulation results strongly support the structural diffusion mechanism for charge transport. In this model, the entire structure—and not the constituent particles—of the charged complex migrates through the hydrogen bond network. For H3O+, we propose that transport of the excess proton is driven by coordination fluctuations in the first solvation shell (i.e., second solvation shell dynamics). The rate-limiting step for OH− diffusion is the formation of the (H7O4)− structure, which is the solvation state showing proton transfer activity.

Phase separation of Si1-xGex alloys

AbstractWe generate liquid and amorphous Si1_xGex alloys for various Ge compositions using ab initio molecular dynamics approach. The electronic bonding characters and structural properties are discussed in terms of radial distribution function, bond angle distribution, and order parameters. Although the order parameters suggest approximately random alloy for all compositions, the snapshots reveal clearly phase separation. We will discuss how the phase can be separated in SiGe alloy system.

Dynamics of dissociative chemisorption: Cl2/Si(111)-(2 x 1).

We report the first simulation of a surface chemical reaction performed within the ab initio molecular dynamics approach. A set of trajectories with different initial conditions has been generated for single ${\mathrm{Cl}}_{2}$ molecules impinging on the Si(111)-2\ifmmode\times\else\texttimes\fi{}1 surface with incident translational energy of 1 eV. We observe a high probability of dissociation, triggered by active sites on the \ensuremath{\pi}-bonded chains, and accompanied by a large surface response and local rehybridization effects.

Interplanar relaxation at the (0001) surface of Be

Using ab initio many body interatomic potential and molecular dynamics approach we have calculated the interplanar relaxation at the (0001) surface of Be. A slab of 392 atoms in seven layers with periodic boundary conditions was used to model the surface. All but two layers were allowed to relax. The results indicate an expansion of 5.9% of the top two layers. The relaxation between the second and third layers is considerably smaller, namely 0.9%, while the remaining layers do not relax at all. Our results are in excellent agreement with the experiment and provide additional insight into the nature of bonding between atoms with low coordination.

Interplanar relaxation at the (0001) surface of Be

Using ab initio many body interatomic potential and molecular dynamics approach we have calculated the interplanar relaxation at the (0001) surface of Be. A slab of 392 atoms in seven layers with periodic boundary conditions was used to model the surface. All but two layers were allowed to relax. The results indicate an expansion of 5.9% of the top two layers. The relaxation between the second and third layers is considerably smaller, namely 0.9%, while the remaining layers do not relax at all. Our results are in excellent agreement with the experiment and provide additional insight into the nature of bonding between atoms with low coordination.

Ab-Initio Molecular Dynamics Approach to the Study of Grain Boundaries in Semiconductors

AbstractUsing an ab-initio molecular dynamics approach based on the Car-Parrinello method, the detailed atomic and electronic structure of a high-angle grain boundary in germanium is determined by investigating its energy-translation surface. Information concerning the coordination of the lowest energy configuration, its translation state, volume change, structure factor and local density of states is obtained.