DC electrical properties of amorphous carbon with different bonding hybridization

Temperature dependence of de conductivity in hydrogenated amorphous carbon (a-C:H) samples, deposited in the range of -200 to -700 V self-bias voltages, was studied above room temperature from 300 K to 650 K. Because a non-Arrhenius dependence on 1/T was found, actual activation energy E(act) was determined by derivation of the log sigma versus T(-1). This E(act) increased gradually with temperature by a rate depending on self bias. Strong fluctuations of E(act), observed above 500 K, indicates a transition to another conduction mechanism. Below 500 K the plot of log sigma versus T(-1/4) could be fitted to linear functions, with an unrealistic density of states at Fermi energy, supporting, that underlying mechanism cannot be variable range hopping near E(F) rather it is related to conduction by hopping of carriers excited to band tails. (C) 1998 Elsevier Science B.V. All rights reserved.