Spatial organization of the sp-hybridized carbon atoms and electronic density of states of hydrogenated amorphous carbon films

Hydrogenated amorphous carbon (a-C:H) films prepared by plasma decomposition of hydrocarbons exhibit a wide variety of electronic and mechanical properties depending on their deposition conditions, which makes them very interesting for applications in several domains. This versatility is essentially due to the presence of both sp²- and sp³-hybridized carbon atoms in variable proportions, and to the tendency of the sp² C atoms to gather into π-bonded clusters with different bonding configurations. The relationships between the film microstructure and their electronic density of states, as deduced from a detailed analysis of their optical properties over a large spectral range, are described and discussed, taking as reference materials the purely sp² (graphite) and purely sp³ (diamond) carbon crystalline phases, as well as the prototype hydrogenated amorphous tetra-coordinated semiconductor, hydrogenated amorphous silicon. It is shown that the type of clustering of the sp² C atoms is certainly more determinant for the electronic density of states, and especially for the optical gap value, than the proportion of these atoms in the material.