The modeling of atomistic processes in semiconductors based on the density functional theory is outlined. The role of intrinsic defects in the self and dopant diffusion, as well as in the dopant activation is investigated for the case of silicon carbide. A hierarchy of annealing mechanisms for vacancies and interstitials is proposed. The identification of the microscopic origin of experimental defect centers by calculated defect signatures establishes a link between theoretical modeling and experiments.

Dislocation-density based multiple-slip constitutive formulations and specialized computational schemes are introduced to account for grain-boundary (GB) effects in polycrystalline aggregates. New kinematically based interfacial grain-boundary regions and formulations are introduced to account for dislocation-density transmission, absorption, and pile-ups that may occur due to CSL grain-boundary misorientations.

A first principles-based multiscale model is developed to examine mechanisms underlying Si nanocrystal formation in Si-rich SiO2. Using the multiscale approach, we have found that the embedded nanocrystal formation is mainly driven by suboxide penalty arising from incomplete O coordination, with a minor contribution of strain, and it is primarily controlled by O diffusion rather than excess Si diffusion and agglomeration. The overall behavior of Si cluster growth from our Monte Carlo simulations based on these fundamental findings agrees well with experiments.

To investigate the cooling mechanism and the local structural changes of excimer laser-annealed silicon (Si), molecular dynamics (MD) simulations were performed. Heat flow of molten Si showed a strong dependency of the local region during a natural cooling. An amorphous-to-liquid transition near an interface in the temperature range of 1600 K ∼ 1800 K was expected with the results of the local diffusion coefficients calculated by integrating the velocity autocorrelation functions. It was confirmed that the structure of the interface region affected the cooling rate of the overall system. The structural properties at the various local regions after a cooling were assessed in terms of the configurational properties including the coordination and bond-angle distributions. A spontaneous nucleation of Si near a interface was observed during a natural cooling.

Coiled-coil alpha-helical dimers are the elementary building blocks of intermediate filaments (IFs), an important component of the cell's cytoskeleton. Therefore, IFs play a leading role in the mechanical integrity of the cells. Here we use atomistic simulation to carry out tensile tests on coiled-coils as well as on single alpha-helices of the 2B segment of the vimentin dimer that has been shown to control the large-deformation behavior of cells. We compare the characteristic force-strain curves of both structures and suggest explanations for the differences on this fundamental level of hierarchical assembly. We further systematically explore the strain rate dependence of the mechanical properties of the vimentin coiled-coil protein. We develop a simple continuum model capable of reproducing the atomistic modeling results. The model enables us to extrapolate to much lower deformation rates approaching those used in experiment.

The growth of nanocrystalline materials has received much attention recently due to its importance in nanocrystalline thin film properties and fabrications. Research studies of this subject have so far focused on the face-centered-cubic metals. Two mechanisms, grain-rotation induced grain coalescence and curvature-driven grain-boundary migration are considered as the dominant mechanisms in the nanograin growth. In this work, we use molecular dynamics method to simulate the growth of Ni and Ni3Al alloy. We find the above mechanisms can describe the growth behavior well. A detailed comparison of the nanograin growth between the two systems is discussed in terms of grain rotation and grain sliding. We also study the temperature effect and the size effect in the nanograin growth. The tendency of twinning in the nanograin growth is discussed.

We will present a general formalism for deriving boundary conditions for molecular dynamics simulations of crystalline solids in the context of atomistic/continuum coupling. These boundary conditions are modeled by generalized Langevin equations, derived from Mori-Zwanzig's formalism. Such boundary conditions are useful in suppressing phonon reflections, and maintaining the system temperature.

The effect of stress on diffusion during semiconductor processing becomes important as device dimensions shrink from microns to nanometers. Simulating these effects requires accurate parameterization of the formation and migration volume tensors of the defects that mediate diffusion on the atomistic scale. We investigate the effect of boundary conditions on the accuracy of atomistic calculations of defect formation energies and formation volume tensors. Linear elasticity provides a correction to the effect of the boundaries on the resulting relaxation volume tensor. By a formal proof we show that the correction term is zero for free boundaries and for periodic boundary conditions with zero mean boundary stress. This is demonstrated in the far field for periodic and free boundary conditions for an isotropic (vacancy) and an anisotropic (<110> intersitial) defect in Stillinger-Weber silicon. For periodic boundary conditions, formation volume tensor components converge to within 5% in a 216 atom simulation cell. For free boundary conditions, slow convergence of elastic constants results in slow convergence of formation volumes. Most significantly, this provides a new method to calculate the formation volume from constant volume calculations. This removes the need for relaxing boundaries, allowing for simpler and more efficient algorithms. We apply this method to both the vacancy and the <110> interstitial in Stillinger-Weber silicon.