Anomalous thermoelectric power in conducting amorphous carbon thin films incorporating a high concentration of nitrogen

摘要

We study the large enhancement of thermoelectric power (S) at low temperatures of a batch of conducting amorphous carbon (a-CNx) thin films containing high concentrations of nitrogen (nitrogen atomic fraction x = 15-23) as well as the anomalous variation of the sign of S with x. A transport model is proposed to explain trends in both S and conductivity (sigma), particularly their weak temperature dependence, which showed deviations from Mott's variable range hopping towards an extended-state conduction. The proposed expression for S(T) of this inhomogeneous carbon system describes a specific energy dependence of density of states, including contributions from delocalized states more clearly than that obtained from the analysis of sigma(T). Although localized states at the Fermi level largely contribute to (hopping) transport at this high level of nitrogen incorporation, as suggested by the proposed electronic structure, subsequent annealing of these films at high temperature substantially modifies the films' electronic structure. Upon reducing the effect of defects both high value of S and its linear-temperature dependence together with an activated electrical conductivity are shown for the first time in a disordered carbon system.