Pre-exponential factor of the hopping conductivity in disordered carbon films

The conductivity of carbon films grown by polymethylphenylsiloxane vapor decomposition in stimulated dc discharge plasma was studied. It is found that the Mott hopping conductivity $ sigma left( T right) = sigma _0 left( T right)exp left{ { - frac{{T_0 }} {T}^{{1 mathord{left/ {vphantom {1 4}} right. kern-nulldelimiterspace} 4}} } right} $ sigma left( T right) = sigma _0 left( T right)exp left{ { - frac{{T_0 }} {T}^{{1 mathord{left/ {vphantom {1 4}} right. kern-nulldelimiterspace} 4}} } right} is characteristic of the samples under study in the temperature range of 80–400 K in the electric field E to 5 · 10⁴ V/cm. An analysis of the pre-exponential factor σ ₀(T) = σ ₀₀(T ₀)T ^α allowed the conclusion that the hopping transport is most adequately described in the model with the exponential energy dependence of the density of localized states for which α = −1/2 and the universal relation ln σ ₀₀ −T ₀^1/4 0 is valid, which is satisfied in the range where the parameter σ ₀₀ varies by eight orders of magnitude.