Non-Arrhenius temperature dependence of direct-current conductivity in amorphous carbon (a-C:H) above room temperature

Abstract

The temperature dependence of the direct-current (DC) electrical conductivity ( σ ) of hydrogenated amorphous carbon (a-C:H) does not exhibit Arrhenius-type behavior at and above room temperature. By increasing the temperature the actual activation energy, E act , defined as the local gradient in a plot of ln σ versus reciprocal temperature, increases continuously by a rate depending on the material properties. This sort of temperature dependence of DC conductivity is expected when the distribution of tail states is very broad and the mobility edge is not sharp. In such a case the energy at which the dominant transport process takes place may change drastically with temperature. This argument gains strong support from our present results: thermal annealing and higher deposition self-bias decrease those potential barriers which localize the π electrons and result in a much weaker dependence of E act on temperature. After annealing the actual activation energy decreases and, for samples deposited at −700 V self-bias, E act becomes nearly constant. The structural changes caused by thermal treatment and by deposition voltage were monitored by Raman scattering measurements.