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, rangin g 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 d escribe why N does not dope in CVD diamond though it is incorporated p redominantly on-site. This also has consequences for growth processes, in particular the (100) Harris growth mechanism. Towards higher N den sities, we observe clear trends counteracting the formation of a low-c ompressibility crystalline phase: (1) N-incorporation strongly catalys es C-under-coordination, which, in turn; (2) causes the nitrogens to d evelop 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 p ossibility that an admixture of Si catalyses sp(3) carbon formation, t hus favouring increased network connectivity and crystallinity. (C) 19 98 Elsevier Science S.A.