Optical properties of sputtered hydrogenated amorphous carbon

Films of hydrogenated amorphous carbon, a-C:H, have been deposited by dc magnetron sputtering from a graphite target in an Ar/H2 atmosphere. The partial pressure or relative flow rate of H was varied over the range 2.5×10−5–0.9 to prepare a set of samples with different H content. Surprisingly, the concentration of H incorporated in the films, as determined by elastic recoil detection analysis, was found to vary only between 18% and 37% and to decrease with increasing relative H flow rate at larger flow rates. The films have been investigated by reflection/transmission and by photothermal deflection spectroscopies. The optical absorption edges are broad and have Urbach slopes essentially independent of the H content. E04 varies from 1.9 to 3.5 eV and is correlated with the amount of H in the films.