Structure and bonding in amorphous iron carbide thin films

TL;DR

This study characterizes amorphous Fe-C thin films, revealing their nanocomposite structure, chemical bonding, and electrical properties, and shows how resistivity can be tuned by controlling amorphous domain structures.

Contribution

It provides detailed insights into the amorphous structure, bonding, and electrical behavior of Fe-C thin films, highlighting the influence of domain structures on resistivity.

Findings

Films are amorphous nanocomposites with Fe-rich carbides and carbon-rich matrix.

Resistivity increases exponentially with carbon content, depending on total carbon.

Charge-transfer effects at interfaces influence electronic structure.

Abstract

We investigate the amorphous structure, chemical bonding, and electrical properties of magnetron sputtered Fe1-xCx (0.21<x<0.72) thin films. X-ray, electron diffraction and transmission electron microscopy show that the Fe1-xCx films are amorphous nanocomposites, consisting of a two-phase domain structure with Fe-rich carbidic FeCy, and a carbon-rich matrix. Pair distribution function analysis indicates a close-range order similar to those of crystalline Fe3C carbides in all films with additional graphene-like structures at high carbon content (71.8 at% C). From X-ray photoelectron spectroscopy measurements, we find that the amorphous carbidic phase has a composition of 15-25 at% carbon that slightly increases with total carbon content. X-ray absorption spectra exhibit increasing number of unoccupied 3d states and decreasing number of C 2p states as a function of carbon content. These changes signify a systematic redistribution in orbital occupation due to charge-transfer effects at the domain-size dependent carbide/matrix interfaces. Four-point probe resistivity of the Fe1-xCx films increases exponentially with carbon content from 200 mu-Ohm-cm (x=0.21) to 1200 mu-Ohm-cm (x=0.72), and is found to depend on the total carbon content rather than the composition of the carbide. Our findings open new possibilities for modifying the resistivity of amorphous thin film coatings based on transition metal carbides by control of amorphous domain structures.