Optical properties of high-density amorphous carbon films grown by nanosecond and femtosecond pulsed laser ablation

High-density tetrahedral amorphous carbon (ta-C) films have been prepared by nanosecond (17 ns) and femtosecond (150 fs) pulsed laser deposition (PLD) using fluences and repetition rates compatible with fast and homogeneous growth over large areas. Their optical properties were measured by spectroscopic ellipsometry from 1.0 to 4.7 eV and analyzed using a multi-layer Tauc-Lorentz model. In spite of very different ablation mechanisms, both PLD techniques produce high density bulk layers as revealed by a refractive index (n at 2 eV) of 2.7±0.1 for both fs-PLD and ns-PLD. Films are covered by a few nm-thick sp2-rich top layer which is denser and thicker in femtosecond PLD as compared to nanosecond PLD. The respective roles of low and high energies in the kinetic energy distribution of the incident carbon species are discussed in terms of densification and sp3↦sp2 configurational relaxation predicted by the subplantation growth model. The significantly higher optical gap found in the ns-PLD films is attributed to the larger contribution of energetic species with kinetic energies Ec≥200 eV, as revealed by time-of-flight optical studies.