CHARACTERIZATION OF DIAMOND-LIKE FILMS PREPARED BY LASER ABLATION OF GRAPHITE

Diamond-like films were produced by graphite ablation using 200 mJ per pulse, 308 nm excimer radiation in a background gas mixture (typically 5-100 Torr) consisting of pure helium (to obtain amorphous carbon films (a-C)), variable ratios of helium to hydrogen, or pure hydrogen (to obtain hydrogenated amorphous carbon films (a-C:H)). Typical deposition rates of up to 10 nm s-1 were easily attainable. Film characterization techniques included vacuum UV-visible measurements, Fourier transform IR (FTIR) spectroscopy and scanning electron microscopy; in addition, measurements of reactivity with atomic oxygen using the quartz crystal microbalance technique are presented. The measured optical band gaps range from 0.5 eV for a-C (5 Torr helium), 1.5 eV for a-C:H (20 Torr) and up to 3 eV for a-C:H at 100 Torr. FTIR spectra with and without hydrogen present showed dramatic differences in feature linewidth, related to the mechanism of film growth. While a-C films displayed C-C frequencies typical of ring structures with some aromatic character, a-C:H films have additional C-H frequencies with mostly olefinic sp2 or sp3 character and weak sp1 features. Vacuum-deposited films show broad absorption features in the vicinity of the sharper features displayed in a-C:H films which clearly arise from film growth by gas phase radicals translationally cooled to room temperature.