Electrical conductivity of amorphous hydrogenated carbon

Abstract

Abstract Amorphous hydrogenated carbon films (a-C: H) were deposited in a capacitively coupled r.f. discharge reactor. The precursor gas and the amount of doping and inert gas added during deposition were changed in a systematic manner. The d.c. conductivity was measured in the temperature range 80 to 500 K. The conductivity behaviour of the investigated samples, as well as to our knowledge that of all samples investigated by other authors, can be described in a uniform way by a characteristic temperature Θ (≊ 1500 K). At this temperature the extrapolated conductivity of all samples should be the same. This behaviour will be discussed in the context of different conductivity models as, for example the variable-range hopping, transport in band-tail states and hopping between graphite-like clusters, a statistical shift of the Fermi level, and the multiphonon tunnelling model. It is shown that only the latter model can, under certain conditions, satisfactorily explain the common behaviour of the samples investigated. The uniform description includes doped samples too. Because of the unspecific behaviour of the doped a-C: H samples it is probable that structural changes in the medium-range order are responsible for the dramatic increase of the conductivity with increasing doping concentration.