Evolution of sp² networks with substrate temperate in amorphous carbon films: theory and experiment

Amorphous carbon (a-C) film deposition was studied theoretically by molecular-dynamics simulations in the temperature range of 100-873 K. For each temperature the simulations yield a mixture of sp² and sp³ hybrids, which allows for the sp² and sp³ fractions, and size and type of the sp² clusters to be calculated. In parallel, the bonding structure of a-C thin films prepared by filtered cathodic vacuum arc (FCVA) at different substrate temperatures was analysed by x-ray absorption near edge spectroscopy (XANES) and Raman techniques. In addition, the optical properties of these films were determined from spectroscopic ellipsometry (SE).

Both, theory and experiment, show a transition from diamond-like to graphite-like films as the substrate temperature exceeds a certain threshold, in agreement with previous observations. The evolution of the sp² clusters, as defined by the number of consecutive sp² atoms, and of sp² sites rings with substrate temperature was computed from the simulated films. The results indicate an increase of sp² clustering with temperature and a pronounced promotion of six to ten membered rings for substrate temperatures above the transition threshold. The further formation of these rings above the transition temperature does not correlate with the sp² content and suggests an in-situ sp² rearrangement process during deposition. The evolution of the sp² networks predicted by the simulations will be contrasted with the data from the XANES and Raman investigations. Finally, the correlation of sp² arrangement and of observed optical properties will be discussed.