Carbon nanotubes grow on the C face of SiC (0001̄) during sublimation decomposition: Quantum chemical molecular dynamics simulations

High-temperature quantum chemical molecular dynamics simulations (QM/MD) based on the density functional tight binding (DFTB) method were performed on SiC surfaces and compared with experimental observations. Following the nucleation of nanocaps on the C face of SiC(000l̄), substantial carbon nanotube (CNT) growth is observed during evaporation of Si between 2000 and 3000 K, while the Si face appears not capable of nanocap formation and perpendicular tube growth under otherwise identical conditions. Instead, graphene sheet growth parallel to the surface is observed in this case. Si evaporation is modeled by two approaches to Si atom removal, one where all Si atoms in one surface layer are removed simultaneously and another one where Si atoms are individually removed from random positions in selected surface layers. The tubes directly "grown" in our simulations display many sidewall defects, consistent with experimental findings. During random removal of Si on the C face, we also observe first indications for growth of a second inner tube, and we observe buildup of amorphous carbon around the tube/surface interface. The present simulations provide atomic-level, time-resolved insight into the interactions of graphitic material on SiC surfaces in the 100 ps domain. Analysis of our simulations under consideration of the geometry of the SiC lattice allows qualitative understanding of the origins for different carbon growth modes, namely, perpendicular tube growth on the C face and parallel slab growth on the Si face.