Formation of Periodic Surface Structures on Semiconductors under Mid-Infrared Free-Electron Laser Irradiation

In understanding the formation mechanism of laser-induced periodic surface structures (LIPSS), the dependence of the LIPSS formation threshold fluence F// on the material properties is crucial. In this work, the LIPSS generated upon the irradiation of various semiconductor materials with a train of 11.4 µm femtosecond laser pulses were examined. The laser source was a Mid-infrared free-electron laser (MIR-FEL). Eight semiconductor substrates (i.e., Si, Ge, ZnO, SiC-4H, GaP, GaN, CdTe, and SiO2) were used as the targets. On Si, Ge, SiC-4H, and GaN, the LIPSS were oriented parallel to the MIR-FEL polarization with interspaces of 1/8 to 1/4 of the MIR-FEL wavelength λFEL. These interspaces tended to match the wavelength of the second harmonic of the MIR-FEL light propagating in the substrates. The obtained F// were relatively correlated with the bandgap energies of materials, as were those for near-infrared femtosecond lasers. Another type of LIPSS oriented perpendicular to the MIR-FEL polarization with interspaces of λFEL to λFEL/9 was also observed in the same spot on SiC-4H or GaN at higher fluences than only //-LIPSS was formed.