SEM, AFM and TEM studies for repeated irradiation effect of femtosecond laser on 4H-SiC surface morphology at near threshold fluence

In order to investigate the interaction of femtosecond (fs) laser and hard-to-process semiconductor material 4H-SiC at near-threshold fluence, fs laser was repeatedly irradiated to SiC surface at different scanning velocities and scan times. The evolutions of surface morphologies were observed and discussed according to Scanning Electron Microscope (SEM), Atomic Force Microscope (AFM) and Transmission Electron Microscope (TEM). Discontinuous zones were ablated in SiC surface after laser irradiation at near-threshold fluence 1.1 J/cm². High spatial frequency rippled structures substantially shorter than the wavelength of incident fs laser were fabricated. The width of the ablated zones increased with lower scanning velocities and more scan times. The mechanism was discussed. Incubation effect occurred in the subsurface of SiC triggered inhomogeneous energy deposition accumulation, which was responsible for the discontinuous ablated zones. Moreover, an amorphous layer with a thickness of about 30 nm was observed in 4H-SiC surface where no ablation was induced after repeated irradiation. This was discussed and explained from the aspects of molecular dynamics simulations of fs laser irradiation to semiconductor materials.