GROWTH MECHANISMS OF DIAMOND-LIKE FILMS FORMED BY LASER ABLATION OF GRAPHITE

Films have been produced by graphite ablation using 200 mJ/pulse, 308 nm excimer radiation. Diamond-like films were obtained by controlling the background gas mixture (typically 5 to 100 Torr), consisting of pure helium (amorphous C films, a-C), variable ratios of helium-to-hydrogen, or pure hydrogen (hydrogenated amorphous C films, a-C:H). Typical deposition rates up to 10 nm/s were easily attainable. Film characterization techniques included VUV-VIS, FTIR, and SEM. The measured optical band gaps range from 0.5 eV for a-C (5 Torr He), 1.5 eV for a-C:H (20 Torr) and up to 3 eV for a-C:H at 100 Torr. FTIR transmission spectra with and without hydrogen present showed dramatic differences in feature location and linewidth. 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. These differences are attributed to the dual role of the buffer gas in modifying the growth medium above the substrate by both translational cooling and chemistry within the ablation plume.