Molecular-dynamics study of nitrogen impurities in tetrahedral amorphous carbon

We discuss the properties of nitrogen impurities inside tetrahedral amorphous carbon, ta-C. In contrast to previous studies on this subject, we have incorporated the N atoms in the disordered structure during evolution of the amorphous system, which is studied by simulated annealing of a high-temperature carbon-nitrogen phase at various N concentrations between 3 and 11 at. %. We find two- and threefold coordinated nondoping N sites that become stabilized by saturation of lone pair orbitals. These configurations appear to be isoenergetic with other local geometries, which potentially could act as dopants. The latter are either ${\mathrm{sp}}^2$-bonded nitrogen atoms with two σ-single and one π-double bond to next-nearest neighbors or ${\mathrm{sp}}^3$-hybridized atoms, which develop four σ-single bonds. Doping through these configurations is too difficult to control and should have a low efficiency, since the donated extra electrons can easily migrate to remote π-bonded clusters in the carbon host matrix. Increasing dopant concentration promotes the formation and growth of such clusters which counteracts the desired doping behavior.