Defects, doping, and conduction mechanisms in nitrogen-doped tetrahedral amorphous carbon

First principles methods are used to study N doping of diamondlike amorphous carbon. A structural model containing 216 atoms is introduced, whose properties are in agreement with the available experimental data. The topological and electronic properties for different N doping concentrations are investigated. We find that N occurring in tetrahedral sites or chains of an even number of π bonded sites results in an increase of the Fermi energy, while N incorporation in strained network sites induces structural changes that lead to an increase in the sp2 fraction of the material. The prevalent conduction mechanisms are identified and discussed. While the Fermi energy increases upon N doping, the localization of the conduction-band-tail states limits extended state conduction. These results are compared to the recent experimental reports on N doping of ta-C and we find that the nondoping threefold N incorporation (N30) is energetically most likely, which explains the low doping efficiency seen in experiments.