THE POSITION OF THE FUNDAMENTAL ABSORPTION-EDGE AND ACTIVATION-ENERGIES FOR THERMALLY ACTIVATED ELECTRICAL-CONDUCTIVITY IN AMORPHOUS-CARBONLAYERS

Amorphous carbon layers (a-C, a-C:H) with a hydrogen content between 3 at.% and 25 at.% were deposited by plasma decomposition processes, sp uttering and evaporation. Their mass density values were obtained from a flotation method. The refractive index and absorption coefficient w ere calculated from spectrophotometric data. Special attention was pai d to the Urbach tail and Tauc's plot absorption regions. The electrica l conductivity was investigated in the temperature range T = 80-350 K. The conductivity values of all types of layers are discussed in terms of thermally activated conduction processes. In this sense all layers behave like semiconductors. For interpreting the conductivity values of the high gap layers, a Davis-Mott model with broad band tails was a pplied. However, this model was insufficient for fitting the conductiv ity data of samples with vanishing gaps. Reproduction of the conductiv ity values of these layers was possible in terms of a band model consi dering a structureless band at a position of 10-50 meV above the Fermi level. The conductivity of low gap samples (optical gaps around 0.3 e V) could only be fitted by a superposition of the conductivity laws fo llowing from both models. Fortuitously, the superposition of these fun ctions yields a temperature dependence very similar to Mott's T-1/4 la w between 50 K and 300 K, which may be an explanation of this widely o bserved behaviour.