Atomic Level Modelling of Structure and Growth of Materials

The research of our group is concerned with the study of structural properties of and growth phenomena in solid materials. All studies rely on microscopic modeling, in which the interatomic interactions are described through pairwise and many-body model potentials. In generic studies we have used Lennard-Jones potentials, while in more specific cases the Tersoff, Stillinger-Weber and Valence Force Field potentials as regards to the study of semiconductors and Effective Medium and the Embedded Atom model potentials as regards to studies of metals have been used. Because these semiempirical potential models have their drawbacks and limitations when attempting to describe bonding in certain materials (especially carbon in tubular form) the more accurate tight-binding approach has been used as a complementary method to describe inter-atomic interactions.

The main topics of investigation in the case of semiconductor materials have been structure and mechanical properties of surfaces and various nanostructures – e.g. thin films, quantum dots and carbon nanotubes. In addition to crystalline materials recrystallization of amorphous silicon has been studied by computational methods. As regards to metals, the focus has been on detailed microscopic structure and dynamics of dislocations.

Large scale Molecular Dynamics (MD) and Monte Carlo (MC) simulations have been the standard tools employed in all these studies and their execution have been done using mostly our in-house parallel cluster computers. This has entailed program development especially for parallel computing puposes. In connection with the MD simulations, development of the graphical user interface for the simulation programs has been continued. In addition interactive simulation programs have been developed and used to study dislocation dynamics and strain relaxation in two and three-dimensional heteroepitaxial systems and mechanical and structural properties of carbon nanotubes. A more general development of scientific visualization has also been done using the open-source program package OpenDX.

Strain relief in mismatched heterostructures

Researchers: Antti Kuronen, Marco Patriarca, and Kimmo Kaski

We study the conditions for nucleation of dislocations in lattice-mismatched heterostructures, which have recently risen a great interest due to their technological importance. In particular, we study the effect of lattice misfit, overlayer thickness and surface structure on the dislocation nucleation. To this aim we use simulation programs with a graphical user interface coupled to a molecular dynamics and minimum energy path determination code. In Fig. 23 the effect of surface structure on the dislocation nucleation activation energy in a 2D system with misfit = -3.5%. The stress concentration caused by the defects on the surface (notch or step) lowers the activation energy considerably. We study misfit induced

Figure 23

Figure 23: Left: Three different systems used to study the effect of surface structure on the dislocation nucleation barrier. Right: Minimum energy paths for dislocation nucleation for the three systems depicted on the left.

dislocations by changing continuously the misfit, rather than temperature or the applied stress. This technique allows one to observe clearly the transition from a perfect crystal to a state with one or more dislocations and vice versa. This is shown in Fig. 24 (right), where the internal energy at T=0 is plotted as a function of the misfit . We observe a hysteresis effect, due to the activation barrier for dislocation nucleation.

Figure 24

Figure 24: Left: Sample structure made up of a quantum dot and a substrate. Colors are according to potential energy. Right: Internal energy versus , where is the misfit and the lattice constants.

Clusters of amorphous Si in crystalline Si: stability and collapse

Researchers: Sebastian von Alfthan, Adrian Sutton, Antti Kuronen and Kimmo Kaski

We have studied the thermal stability of small spheres of amorphous silicon embedded in crystalline silicon using atomistic computer simulations. We use a molecular dynamics simulations to follow the time dependent behavior of these clusters. In order to study this problem we have developed a novel method for automatically identifying the position of the amorphous-crystalline interface at different time-steps.

We calculate the surface and bulk energies of amorphous clusters and also study the dynamical behavior of the interface between the amorphous and crystalline phases at different temperatures. This enables us to calculate the mobility and activation energy of the interface movement. Using these results we can predict the long time behavior of larger clusters at room temperature. Also the atomistic structure of the interface and the process by which the growth happens is of interest. We identify some typical features by which the growth happens such as a ledge growth mechanism and the formation of prefered atomic planes at the interface.

Figure 25

Figure 25: A slice of a system with an amorphous cluster is depicted at three different time steps when simulated at a temperature of 1250 K.

Dislocations in FCC Metals: Peierls Stress and Interaction with Defects

Researchers: Péter Szelestey, Marco Patriarca, and Kimmo Kaski

type of dislocations, the most common in face-centered-cubic materials, generally dissociate into two partials connected by a stacking-fault region. These dislocations have been the object of much research in recent years. We use atomic level, Molecular Dynamics simulations, with the aid of the previously developed Embedded Atom potentials and visualization and tracking methods, in order to proceed along two distinct research directions.

Mobility: An essential factor affecting the mobility of dislocations is the Peierls potential, the periodic potential experienced by a gliding dislocation, due to the discrete nature of the crystal. Under external stress, dislocation motion in the Peierls potential represents a complex problem, especially for a dissociated dislocation, as its structure changes during motion. We have studied this phenomena using for the case of a screw dislocation. We considered various configurations with different separation distances and analyzed its implications for low temperature plastic behaviour.

Interaction with defects: Another research direction is aimed to study the interaction of a moving dislocation with sessile defects. We are particularly interested in vacancy-formed defects, such as Frank’s dislocation loops and stacking-fault tetrahedra. One one hand, these defects represents obstacles for the moving dislocation, act as pinning sites, and on the other hand, the defects can undergo transitions due to intersection with a moving dislocation.

Figure 26

Figure 26: Snapshots of a stacking-fault tetrahedron (pyramid shaped object) intersected by a dissociated screw dislocation. The two partials can be identified by the blue, originally straight lines. The screw dislocation moves from the left to the right side. Only atoms in the core region are visualized.

Modeling of thin semiconductor films

Researchers: Laura Juvonen, Francesca Tavazza*, David P. Landau*, Antti Kuronen, and Kimmo Kaski
*Center for Simulational Physics, The University of Georgia, Athens, GA, USA

Understanding the growth of thin semiconductor films is crucial for developing new types of nanoelectronic devices. We are studying the structure and properties of heteroepitaxial Ge/Si(001) systems which are estimated to be one of the most promising materials for novel electronic and optoelectronic components. These systems are strongly influenced by the strain-related effects and long-range elastic interactions which requires using large-scale simulation methods. Our approach is to use empirical potentials in connection with advanced Monte Carlo techniques which allows us to reach sufficiently large system sixes (up to 100000 atoms). Moreover, temperature dependence is inherently included in the simulations.

Semiconductor surfaces are often difficult to study because these systems are characterized by complicated energy landscapes, and consequently conventional algorithms can easily get trapped into metastable states. In order to overcome these problems, we have developed advanced Monte Carlo algorithms which can be applied to problems where substantial configurational rearrangement is required. In the dimer-jump algorithm, trial MC moves consist of long displacements of atom pairs followed by a local relaxation using Molecular Dynamics. Figure 27 shows a snapshot of a relaxing Si island surrounded by addimers. All red particles are mobile and can migrate on the surface.

Figure 27

Figure 27: Relaxation of a Si island on Si(001) using the dimer-jump algorithm. All red atoms are mobile (atom pairs are moved as a unit). The color coding of the atoms in the underlying layers reflects their energy (green is higher and blue lower in energy).

Mechanical Properties of Carbon Nanotubes

Researchers: Maria Huhtala, Jussi Aittoniemi, Antti Kuronen, and Kimmo Kaski

Carbon nanotubes are cylindrical all-carbon molecules composed of concentric graphitic shells with extremely strong covalent bonding of atoms within the shells but very weak van der Waals type interaction between them. Due to the unique atomic structure nanotubes have exceptional electronic and mechanical properties which imply a broad range of possible applications as constituents of nanometer-scale devices and novel composite materials.

The properties of a carbon nanotube depend on the local atomic configuration, on local strain and defects. For composite and device development it is essential to understand how these structural changes affect the properties and our work strives after shedding some more light on the occurring phenomena. Current projects include studies of carbon nanotube local bending and buckling and studies of irradiation induced defects in carbon nanotube strength, load transfer and inter-shell friction. The tools employed are both classical molecular dynamics and dynamical tight binding methods. Figs. 28 and 29 show examples of the defect structures which have been studied.

Figure 28

Figure 28: Vacancy reconstructions in a single-walled (17,0)-tube. From the left: single vacancy (one atom missing), double vacancy (two atoms missing) and triple vacancy (three atoms missing).

Figure 29

Figure 29: Two views of a covalent inter-shell bond in a multi-walled (5,5)@(10,10)-nanotube. For example, two adjacent vacancies can form such a bond.