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 sp²-bonded nitrogen atoms with two {σ}-single and one {π}-double bond to next-nearest neighbors or sp³-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.