Optical-absorption edge and disorder effects in hydrogenated amorphous diamondlike carbon films

Optical absorption (∼1–5 eV) in rf-deposited hydrogenated (20–40 at. %) amorphous ‘‘diamondlike’’ carbon films has been studied as a function of heat treatment. For E≳2.5 eV a Tauc behavior is exhibited. Optical band gaps $E_g$ are ≲2.4 eV. At lower energies, band tailing with an Urbach focus is found. $E_0$, the width of the tail, is quantitatively explained in terms of the annealing temperature $T_H$. Increasing $T_H$ (<700 K) reduces the hydrogen content, lowers $E_g$, and appears to harden the films. $E_g$ is determined to be a linear function of $E_0$. Additional analysis was performed in terms of a divergence temperature, $T_0$; the results of this analysis are comparable to those observed in a-Si:H.