Effect of Thickness Reduction in UV Absorption Spectrum for a-C:H Thin Films

Some peculiarities in ultraviolet (UV) absorption spectra for amorphous hydrogenated carbon thin films are presented. The films were deposited by the rf magnetron sputtering method, with a gas pressure between 0.1 and 0.3 Torr. The most significant result found was that the films deposited at high pressures (greater than 0.2 Torr) and small thickness (bellow 400 nm) presents some peculiarities in the UV absorption spectrum. The Tauc plot for these films, compared with the films with thickness greater than 1000 nm, presents two different regions corresponding to two different optical transitions. Films deposited under similar conditions, with a thickness greater than 1 μm, present an optical band gap around 2.5 eV and a large sp3 ratio (greater than 85%). The results suggest a dimensional effect indicating optical absorption behaviour closer to diamond.