DISORDER AND ABSORPTION EDGES IN ION-IRRADIATED HYDROGENATED AMORPHOUS-CARBON FILMS

The optical properties of ion-irradiated hydrogenated amorphous carbon films in the low and medium absorption coefficient region (1-105 cm-1) were investigated as functions of the hydrogen atomic fraction (5-50 H at. %) by using a photothermal deflection spectroscopy technique. The data are interpreted using Urbach and Tauc theories for the different spectral regions. It was found that an increase in hydrogen content induces an overall increase of order in the film, leading to a sharpening of the band edge slope, in agreement with the known tendency of trigonal carbon to cluster in ordered graphitic domains. Correspondingly a decrease of the Tauc optical energy gap (from 1.6 to 0.2 eV) was observed due to the formation of weakly bonded π electron states in these materials. The data are compared with those reported by several authors on a-Si and a-Si:H samples. © 1995 The American Physical Society.