Physics of bandtail hopping in disordered carbons

Electrical transport data in disordered forms of carbon are reviewed and compared with the bandtail hopping characteristics obtained from a modeling of phonon-assisted tunneling within an exponential density of states distribution. In the weak electric field limit, the dependence of the effective carrier mobility and the ohmic conductivity on the localization parameter N(E-F)gamma(-3) and the disorder parameter E-0 are computed using the filling rate method and the Kubo formula. Most of the data for amorphous carbon-based alloys and ion-implanted diamond match the bandtail hopping predictions and the changes in the localization parameter can thus be investigated as a function of the synthesis conditions. For nanotube mats or cluster beam deposited films, the situation is more complex and transport data probably reflect the heterogeneity of the microstructure. (C) 2003 Elsevier Science B.V. All rights reserved.