Atomic bonding in amorphous carbon alloys: A thermodynamic approach

Abstract

The free energy model previously developed for the prediction of the bonding in amorphous Si-based alloys is extended here to amorphous carbon alloys, a-CxH1−x, containing carbon atoms with sp3 and sp2 hybridization. Predictions have been made for the bonds present in the alloys, with the case of ‘‘chemical’’ ordering at T=0 K corresponding to phase separation into separate C (sp3) and C(sp2) regions. For T≳0 K phase separation is eliminated and there is no evidence for the clustering of graphitic carbon, indicating the importance of the configurational entropy in influencing the bonding in the alloys. Hydrogen atoms are predicted to bond preferentially to C (sp3) atoms for all T. The sp3/sp2 ratio is predicted to increase with increasing H content, as observed experimentally, and also with increasing T due to entropy effects. Predictions have been made for the distribution of bonds in tetrahedral C(sp3)- and planar C(sp2)=C(sp2)-centered units. It is found that essentially no aromatic or graphitic structures are present in typical alloys. The a-CxH1−x alloys have been proposed to consist of five amorphous components: diamondlike, graphitic, polymeric, olefinic, and mixed diamond–graphitic (d–g) components. It is predicted that the polymeric and mixed d–g components dominate in typical plasma-deposited alloy films while the mixed d–g component dominates in hydrogen-free a-C films.