Optical properties of diamond-like carbon thin films deposited by PECVD

Diamond-like carbon (DLC) thin films were deposited at different substrate temperatures using methane gas in DC-PECVD at 1x10-1 Torr. From the light transmission using UV-VIS spectroscopy it was found that the optical transition had changed from allowed indirect transition to forbidden direct transition as the substrate temperature increased. The photoluminescence study has shown that the recombination centre occurred at 1.738 eV. The observation in the present work shows that the Urbach energy decreases with increasing substrate temperature provides evidence that such films have a narrowed band tail. This decrease in Urbach energy is due to a more ordered structure with lower hydrogen concentration and larger cluster by the increase in the degree of cross-linking of the C-C network. Distorted cluster which contribute to the broadening of the band tail may also contribute to the PL peak width. As the substrate temperature increases, the cluster size in the tetrahedral hydrogenated amorphous carbon (ta-C:H) increases, resulting in smaller band gap and the structure is more ordered as the decrease in Urbach energy, give the decrease in PL peak energy.