Plasma characterization during laser ablation of graphite in nitrogen for the growth of fullerene-like CNx films

Abstract

Chemistry, energy, and spatial distributions of species in carbon–nitrogen plasma plumes were investigated to define plasma conditions for growth of carbon nitride ${\mathrm{CN}}_x$ films with a fullerene-like structure. Plumes were generated by ablation of graphite using a 248 nm excimer laser in the presence of low-pressure nitrogen. The plumes were investigated using element specific imaging, time-of-flight experiments, fluorescence spectroscopy, and molecular vibration sequence analyses. Studies showed the importance of plume/substrate interaction in causing secondary excitation phenomena. For ${\mathrm{N}}_2$ pressures within the 5–50 mTorr range, plasmas at the substrate vicinity were found to consist mostly of atomic carbon, CN and ${\mathrm{C}}_2$ molecules. Kinetic energies were calculated within 10–20 eV for mono atomic carbon, 30–55 eV for CN, and 20–40 eV for ${\mathrm{C}}_2.$ Excited CN and ${\mathrm{C}}_2$ molecules were generated by laser ablation and by collisions of the plume with the substrate surface. Their vibrational energies were strongly influenced by nitrogen pressure and time after a laser pulse. For pressures below 30 mTorr, vibrational energy was as high as 4.0 eV at 2–4 μs for CN and 2.5 eV at 8–10 μs for ${\mathrm{C}}_2.$ This low pressure was suggested for the growth of fullerene-like ${\mathrm{CN}}_x$ films based on correlations between plasma parameters and film composition and bonding. Synthesis of the fullerene-like structure required high molecular temperatures at the condensation surface. High concentrations of CN radicals in the plasma promoted nitrogen incorporation into the films. Correlations among ${\mathrm{CN}}_x$ film composition/bonding, excitation maximums, and kinetic/vibrational energies of atomic carbon, CN and ${\mathrm{C}}_2$ species located near the condensation surface are discussed.