A molecular dynamics study of N-incorporation into carbon systems: doping, diamond growth and nitride formation

An ab-initio-based tight-binding molecular-dynamics method is used to describe the behaviour of a variety of carbon-nitrogen systems, ranging from doping in CVD diamond and the effect of N on CVD diamond growth to that of the possible fabrication of super-hard C3N4 crystals. We describe why N does not dope in CVD diamond though it is incorporated predominantly on-site. This also has consequences for growth processes, in particular the (100) Harris growth mechanism. Towards higher N densities, we observe clear trends counteracting the formation of a low-compressibility crystalline phase: (1) N-incorporation strongly catalyses C-under-coordination, which, in turn; (2) causes the nitrogens to develop in a paracyanogen-like manner (CN-double and -triple) bonding; and (3) the most favourable densities for a-CN appear to be much lower than the desired hard crystalline ones. Finally, we investigate the possibility that an admixture of Si catalyses sp(3) carbon formation, thus favouring increased network connectivity and crystallinity. (C) 1998 Elsevier Science S.A.