The origin of conductivity in ion-irradiated diamond-like carbon – Phase transformation and atomic ordering

Focused ion irradiation of insulating diamond-like carbon (DLC) films is a well-recognized approach to write defined nanostructures with drastically changed properties compared to the unexposed matrix. In particular, the electrical conductivity of the film areas that are irradiated increases by several orders of magnitude. Furthermore it is known that the conductivity increase is directly related to the irradiation-induced sp³–sp² rehybridization. This experimental work shows in detail that ion-induced microstructural ordering of the carbon atom arrangement far beyond the sp³–sp² conversion saturation is an important microscopic mechanism for the alteration of physical properties, including an increase in the conductivity. The atomic ordering correlates with the local energy density deposited during the ion impact. Thus the ion-induced phase transformation of DLC is proposed to comprise a rehybridization stage, caused by nuclear collisions, and a rearrangement stage (graphite-like ordering) that is thermally driven by the ion impact. These conclusions are based on in-depth investigations of amorphous DLC films that were locally irradiated by a number of ion species employing several temperature regimes. The ion irradiation experiments cover a wide range of fluences. X-ray photoelectron spectroscopy, μ-Raman spectroscopy, transmission electron microscopy, and low temperature transport measurements have been applied to characterize the films.