Beispiele zur Verknüpfung atomistischer Simulationen mit größeren Skalen

The goal of this dissertation is the development and evaluation of scale hopping computational models to supplement or replace experimental research. In this work this is realized for two separate topics.The first model deals with thin silicon carbon layers on silicon wafers and their thermal stability. Modern transistors require a strained channel of the NMOS technology, because of increasingly small transistor size. This tensile strain can be achieved by decreasing the average lattice constant of silicon by alloying with carbon. Experiments indicated a loss in tensile strain during heat treatment, and especially layers doted with phosphorus are thermodynamically unstable. A model is proposed as the reason for relaxation such that substitutional carbon atoms form silicon-carbon dumbbells, creating a vacancy in the process. The acceleration of the process by phosphorus atoms is explained by the formation of phosphorus vacancy pairs, which shift the thermodynamic equilibrium in favor of the silicon carbon dumbbells.The underlying reactions of this model are simulated on three different scales. By molecular dynamics these mechanisms are identified as a possible reason for relaxation. By ab initio methods the formation energies of the foreign atoms are determined, as well as the energies of select atom combinations which were previously found plausible. By a nudged elastic band approach the migration energies of the proposed defect reactions are calculated. On the biggest length scale a statistical model is developed which describes the kinetics of the relaxation process in dependence of temperature and initial defect concentration. The previously calculated formation and migration energies are input parameters of the statistic model. In a comparison to experimental data the statistical model describes the relaxation behavior well and therefore validates the assumptions of the responsible mechanisms.The second topic deals with simulation of grain boundary mobility by molecular dynamics. These mobilities are an important property for mesoscopic recrystallization simulations and can only be determined laboriously by experiments. To get a statistical overview of the mobilities in dependence of the complex grain boundary geometry, a preferably automated method is necessary. In a common procedure to achieve this in molecular dynamics two fcc crystals are created, which form the desired grain boundary on their interface. In this bicrystal system a potential energy between the crystals is applied, using a defined orientation parameter, which acts as a driving pressure to move the grain boundary.During this dissertation potentially severe problems with this method were recognized. The problems of the until now commonly used orientation parameter were eliminated. Other potential error sources introduced by the approximative nature of molecular dynamics are investigated, because in contrast to the atomic silicon defects ab initio calculations are not possible for grain boundaries. The found strong dependency of the calculated mobilities on the molecular dynamics potential suggest the determined mobility values are unreliable and that molecular dynamics should only be used for qualitative research on grain boundary motion at the current state of the art of MD potentials