Abstract
This paper gives an extended overview of the electrical properties of ion tracks in hydrogen-free tetrahedral amorphous carbon (ta-C) with a sp3 bond fraction of about 80%. The films were grown by mass selected ion beam deposition of 100 eV 12C+ ions. The ion tracks are generated by irradiation of ta-C films with uranium ions of 1 GeV kinetic energy. Along the ion path a conversion from diamond-like (sp3) carbon to graphite-like (sp2) carbon takes place. Topography and current measurements of individual ion tracks were performed by atomic force microscopy at ambient temperature. The temperature dependence of the electric conductivity was studied between 15 and 390 K by means of 0.28 mm2 large contact pads averaging over about 107 tracks. For each sample and at each temperature the conductivity as a function of the applied electrical field (non-ohmic behaviour) was measured separately and the data were extrapolated to field zero. In this way, the zero-field conductivity was determined independent from the field dependence. In spite of large differences in the absolute values, the temperature dependence of the zero-field conductivities is found to be very similar in shape for all samples. The conductivities follow a law up to temperatures slightly below room temperature. At higher temperatures a transport mechanism based on over-barrier hopping dominates with an activation energy of about 220 meV for tracks and 260 meV for the ta-C matrix. The field dependence measurements show that the deviation of the I–V characteristics from ohmic behaviour decreases with increasing zero-field conductivity. We also tested Cu-doped ta-C samples and found that they conduct significantly better than pure ta-C. However, the doping also increases the zero-field conductivity resulting in a weaker contrast between the track and matrix. The data are interpreted within the so-called ‘barrier model’ where the electrons are assumed to move fairly freely in well-conducting sp2 regions but encounter barriers in track sections consisting of more sp3-like bonds.
Highlights
Under swift heavy ion irradiation, amorphous carbon materials undergo structural changes resulting in a modification of the sp3/sp2 ratio along the ion trajectories
A tendency to less bending shows up for the better conducting tracks; the current–voltage characteristics for tetrahedral amorphous carbon (ta-C) samples irradiated with C60 and for some doped samples irradiated with uranium ions are linear at room temperature
There is no indication in the C K-edge spectra of the narrow peak at 285 eV typical for sp2 carbon. These values are in good agreement with results of several other groups obtained for ta-C films grown by ion beam deposition or filtered vacuum arc deposition with carbon ion energies around 100 eV (Hofsäss et al 1998)
Summary
For each sample and at each temperature the conductivity as a function of the applied electrical field (non-ohmic behaviour) was measured separately and the data were extrapolated to field zero. In this way, the zero-field conductivity was determined independent from the field dependence. In spite of large differences in the absolute values, the temperature dependence of the zero-field conductivities is found to be very similar in shape for all samples. The field dependence measurements show that the deviation of the I–V characteristics from ohmic behaviour decreases with increasing zero-field conductivity. The doping increases the zero-field conductivity resulting in a weaker contrast between the track and matrix. The data are interpreted within the socalled ‘barrier model’ where the electrons are assumed to move fairly freely in well-conducting sp regions but encounter barriers in track sections consisting of more sp3-like bonds
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