Amorphous diamond films deposited by pulsed-laser ablation: The optimum carbon-ion kinetic energy and effects of laser wavelength

A systematic study has been made of changes in the bonding and optical properties of hydrogen-free tetrahedral amorphous carbon (ta-C) films, as a function of the kinetic energy of the incident carbon ions measured under film-deposition conditions. Ion probe measurements of the carbon ion kinetic energies produced by ArF and KrF laser ablation of graphite are compared under identical beam-focusing conditions. Much higher C⁺ kinetic energies are produced by ArF-laser ablation than by KrF for any given fluence and spot size. Electron energy loss spectroscopy and scanning ellipsometry measurements of the sp³ bonding fraction, plasmon energy, and optical properties reveal a well-defined optimum kinetic energy of 90 eV to deposit ta-C films having the largest sp³ fraction and the widest optical (Tauc) energy gap (equivalent to minimum near-gap optical absorption). Tapping-mode atomic force microscope measurements show that films deposited at near-optimum kinetic energy are extremely smooth, with rms roughness of only approximately 1 angstrom over distances of several hundred nm, and are relatively free of particulates.