Energetics and kinetics of defects, impurities, nanostructures and interfaces

Computer simulations using classical interatomic potentials are an efficient and promising tool to investigate and understand atomic-level properties and processes in advanced materials. They allow the consideration of length and time scales which are often hardly accessible by experiments. However, the accuracy of the interatomic potentials employed in such type of simulations determines decisively the quality of the obtained results. Therefore, these potentials must be continuously improved and evaluated.
In the present contribution three applications of atomistic computer simulations are illustrated. The focus is on energetics and kinetics of defects, impurities, nanostructures and interfaces in materials for micro- and nanoelectronics and in structural materials for fission reactors. Implications of the simulation results for the explanation of experimental findings are discussed.

The first example deals with molecular dynamics simulations on basic migration mechanisms of mono- and di-(self-)interstitials in Si. Both the atomic mobility due to the presence of the defect and the defect mobility itself are determined. The mechanism of di-interstitial migration depends on temperature, in contrast to that of the mono-interstitial.

In the second example amorphous Si and Ge as well as their solid-phase epitaxial recrystallization (SPER) are considered. Results obtained by different interatomic potentials are compared. The molecular dynamics simulations yield amorphous material with realistic structural and thermodynamic properties, but the SPER rate is strongly overestimated. It is shown that a more realistic SPER rate can be obtained using a modified interatomic potential which yields a higher melting temperature of the amorphous phase. This is explained by the fact that both melting and SPER are essentially determined by the flexibility of atomic bonds.

The subject of the third example is the formation of coherent Cu-rich precipitates in bcc-Fe, i.e. nanostructures containing Cu and vacancies. For pure vacancy and pure Cu clusters as well as for mixed clusters up to a maximum size of 200 the free binding energy and nucleation free energy are determined, using a combination of on-lattice Metropolis Monte Carlo simulations and off-lattice molecular dynamics calculations. The data which are obtained for the binding energy of a single Cu atom and a single vacancy to a cluster are important input parameters of the rate theory. This type of simulation is an efficient multi-scale modeling tool to simulate the cluster evolution on realistic length and time scales.