14 Externe wissenschaftliche Einrichtungen
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/15
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Item Open Access Molecular dynamics simulations of precursor-derived Si-C-N ceramics(2005) Resta, Nicoletta; Trebin, Hans-Rainer (Prof. Dr.)The microscopic mechanisms behind the transformation from the amorphous Si-C-N ceramics to the polycrystalline material are studied by isolating its fundamental steps. Si-C-N amorphous ceramics, amorphous and crystalline silicon carbide, and carbon, are numerically modeled by means of classical molecular dynamics. The interatomic interactions are modeled by the Tersoff many-body empirical potential. In the first part amorphous Si-C-N materials are studied. We show how the atomic structures of these ceramics depend on their chemical composition. Amorphous systems are obtained by rapid cooling from high temperatures. We find that the atomic structures depend on the relative concentration of silicon and nitrogen, regardless of the carbon amount. In particular, for a stoichiometric nitrogen/silicon greater or equal than 4/3, the atoms separate into two amorphous phases, a sp2 hybridized C-rich, and a Si/N-rich one. In this phase, silicon atoms form mainly SiN4 and mixed Si(C,N)4 tetrahedra. Far above the 4/3 ratio, we find Si/N-rich domains, where Si atoms are more than four-fold coordinated, and are surrounded by graphitic monolayers. In another set of simulations, the thermal evolution of a Si-C-N system is followed by annealing it at high temperatures. Volume shrinkage of about 12\% is observed and diffusion activation energies of 3.43 eV for Si, 3.63 eV for C, and 2.94 eV for N are calculated. We find that carbon atoms are the slowest atomic species in the amorphous network. In the second part of this work we study the crystal growth of silicon carbide from the amorphous phase. In a preliminary set of simulations, the study of diamond crystal growth demonstrates that empirical potentials can predict crystallization processes in complex covalent compounds. The crystal growth of a seed of cubic SiC into the amorphous material is then investigated. The dependence of the growth process on the crystallographic orientation of the crystalline/amorphous interface is studied by considering three different crystal planes, namely the {100}, {110}, and {111} planes. We observe the crystal growth only for the {110} and the {111} orientations, but not for the {100} ones. All interfaces after the annealing have a common atomic structure: Silicon {111} layers, triple bonded to the bulk. Moreover, we find that the preferential growth directions are the <110> ones, perpendicular to the {110} surfaces. Crystal growth proceeds by faceting on the {111} planes.