Abstract
The electrical transport and resistance switching mechanism in amorphous carbon (a-C) is investigated at the nanoscale. The electrical conduction in a-C thin films is shown to be captured well by a Poole–Frenkel transport model that involves nonisolated traps. Moreover, at high electric fields a field-induced threshold switching phenomenon is observed. The following resistance change is attributed to Joule heating and subsequent localized thermal annealing. We demonstrate that the mechanism is mostly due to clustering of the existing sp2 sites within the sp3 matrix. The electrical conduction behaviour, field-induced switching and Joule-heating-induced rearrangement of atomic order resulting in a resistance change are all reminiscent of conventional phase-change memory materials. This suggests the potential of a-C as a similar nonvolatile memory candidate material.
Highlights
The electrical transport and resistance switching mechanism in amorphous carbon (a-C) is investigated at the nanoscale
We present a thorough investigation of the electrical transport and resistance switching in a-C films
Using this local probe technique, we can switch the resistance of a-C with a spatial resolution of tens of nanometres. Conductive-mode atomic force microscopy (c-AFM) and our stack configuration enable the study of the electrical transport mechanism in a-C films across the thickness of these films
Summary
The electrical transport mechanism is studied first, followed by an in-depth investigation of the permanent resistance-change mechanism. Even though the threshold switching behaviour in a-C has not been reported before, the field-dependent change in resistance observed in the ‘subthreshold regime’ has been studied by several groups assuming Poole–Frenkel (P–F)-type conduction [19, 20]. The average inter-trap distance turns out to be approximately 2.2 nm, the smallness of which validates the assumption of high trap density At this value of the mean intertrap distance, the barrier lowering is mostly independent of the permittivity value. It is unclear as to which mechanism is responsible for this switching behaviour, but it is clearly field driven, possibly due to tunnelling of carriers from deep traps to shallow traps at high electric fields From these studies, it can be seen that the subthreshold conduction behaviour and threshold switching are remarkably similar to that observed in GST. The higher current values prior to threshold switching in the doped-carbon stack are indicative of a reduced energy barrier for hopping due to nitrogen doping [6]
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