Influence of ion energy and substrate temperature on the optical and electronic properties of tetrahedral amorphous carbon (ta-C) films

The properties of amorphous carbon (a-C) deposited using a filtered cathodic vacuum arc as a function of the ion energy and substrate temperature are reported. The sp³ fraction was found to strongly depend on the ion energy, giving a highly sp³ bonded a-C denoted as tetrahedral amorphous carbon (ta-C) at ion energies around 100 eV. The optical band gap was found to follow similar trends to other diamondlike carbon films, varying almost linearly with sp² fraction. The dependence of the electronic properties are discussed in terms of models of the electronic structure of a-C. The structure of ta-C was also strongly dependent on the deposition temperature, changing sharply to sp² above a transition temperature, T₁, of ≈200°C. Furthermore, T₁ was found to decrease with increasing ion energy. Most film properties, such as compressive stress and plasmon energy, were correlated to the sp³ fraction. However, the optical and electrical properties were found to undergo a more gradual transition with the deposition temperature which we attribute to the medium range order of sp² sites. We attribute the variation in film properties with the deposition temperature to diffusion of interstitials to the surface above T₁ due to thermal activation, leading to the relaxation of density in context of a growth model.