Development of Object-Oriented Atomistic Simulator: OHMMS

We develop object-oriented codes using C++ to perform molecular dynamics simulations with atomistic potentials (classical/tight-binding/first-principle). For a given system, the potential and efficient algorithms will be chosen based on
  • accuracy required to describe relevant physical quantities,
  • system size to capture important interactions and
  • time scale of dynamical phenomenon of interest.
    • Different problems should be investigated with appropriate data structures and algorithms, while maximizing reuse of the code. This can be accomplished efficiently by developing code in object-oriented frameworks. The MD codes are built on POOMA (Parallel Object Oriented Methods and Applications) developed by Los Alamos National Laboratory which provide underlying objects responsible for parallelism and node-level simulations for efficiency, portability and re-usability. Our plan includes implementations of recent algorithms in solving large scale electronic structure calculations and in performing accelerated molecular dynamics simulations to handle a wide range of length and time scales of simulations. In parallel, we have been working on implementation of linear scaling method on a tight-binding MD code in collaborations with F. Kirchhoff. This code will be mainly used for a test tool for tight-binding Hamiltonians for transition metals and compound systems.
    On-going projects using Atomistic Simulator based on POOMA
    • MD simulations of compliant substrates[animation]
    • Elastic properties of self-assembled III-V quantum dots

    Interstitial defects in Si: cluster to extended defects

    Rod-like {311} defects are commonly observed in ion-implanted silicon and are believed to play important roles in boron transient enhanced diffusion (TED) by providing interstitials during annealing processes. However, even under the implantation conditions to suppress the formation of extended defects, TED has been observed, suggesting that microscopic interstitial defects exist. While the structural properties of rod-like {311} defects are well characterized by experiments, e.g.,via high-resolution transmission electron microscopy (HRTEM), the existence and properties of small clusters are not well understood. Only recently, the presence of interstitial clusters has been evidenced by deep level transition spectroscopy.
    We found trends in the formation energies of n-interstitial defects. Using first-principle calculations within the local density approximation (LDA), we consider relevant interstitial defects - small clusters (n=2-5), chains, and planar {311} defects -- and we relate them to growth of interstitial defects in interstitial supersaturated silicon as achieved in ion-implanted samples. In general, the stability of interstitial clusters increases as the size n increases. The formation energy per interstitial ranges from Ef(n=2) = 2.46 eV for di-interstitials to Ef(infinity) = 0.7 eV for rod-like {311} defects. Preferred elongation of rod-like defects in the [011] direction is predicted in accordance with experiments. We find that the stable configuration for a given size can be (a) compact for small clusters, (b) elongated for medium clusters and (c) planar for large clusters.

    For more details, check out
    - Thermally activated reodientation of di-interstitial defects in silicon [reprint,pdf]
    - Stability of Si-inetrstitial defects: from point ot extended defects [reprint,pdf]
    - Animation of a di-interstitial diffusion at 1200 K: View-A[gif-17M] View-B[gif-11M]

    Dynamics simulation of defected materials

    We study thermal properties of the {311} extended defects by molecular dynamics simulations. In particular, we investigate the role of the extended defects in the interstitial diffusions to clarify the transient enhanced diffusion in ion-implanted silicon. Recently developed hyper-MD method [A. Voter, Phys. Rev. Lett. 78, 3908 (1997); M. Steiner, P.-A. Genilloud, and J. W. Wilkins, Phys. Rev. B 57, 10236 (1998)] is being tested to perform time scale of 10 -- 100 nsec, which has been beyond the scope of the conventional MD scheme. For classical, we achieve boost factors of 103 by exploiting the locality of interactions. In collaborations with F. Kirchhoff and J. W. Wilkins, we study diffusion of point defects essential for understanding the mechanical properties of transition metal alloys at elevated temperatures. Titanium and its alloys are an important class of metallic materials which are used extensively in energy, aerospace and biomedical applications. Titanium alloys have very high specific strength (i.e., ratio of yield strength/density), good fracture toughness, and excellent corrosion resistance.
    Local energy during a di-interstitial reorientation



    Hyperdynamics simulations: adatom diffusion on Si(001)[animation]


    To cite this page:
    Research, http://www.physics.ohio-state.edu/~jnkim/research.html

    Your comments and suggestions are appreciated.