2D Two-Temperature Model Numerical Study Of Formation Mechanisms Of Periodic Surface Structures Induced On Silicon Under Femtosecond Laser Irradiation T. JY. Derrien1, R. Torres1, T. Sarnet1, M. Sentis1, T. E. Itina2 1 Lasers, Plasmas and Photonic Processes Laboratory (LP3 UMR 6182 CNRS), 163 av. de Luminy, Case 917, 13288 Marseille, France 2 Hubert Curien laboratory, UMR CNRS 5516/Université Jean Monnet, 18 rue du Professeur Benoit Lauras, 42000, Saint Etienne, France Contacts PhD std: thibault.derrien@lp3.univ-mrs.fr CNRS researcher: tatiana.itina@univ-st-etienne.fr Surface wave investigations Context Lifetime of a fs interference in Si Does melting occur under fs pulse on Si ? CINES computational center and French Ministry of Research PhD support are gratefully acknowledged. Additional thanks to D. Grojo, I. Bogatyrev, and A. Nikitin. « Spikes » Black silicon. « Agregation » Amplification Increasing number of 100 fs, 1 kHz shots « Ripples » LIPSS « Beads » Saturation Problem: Formation mechanisms are not well-known. -- Objective: Identify the mechanisms of : 1. Periodic modification of the electron-hole plasma created by femtosecond laser irradiation 2. Transmission of the pattern from electrons to lattice 3. Change of order and amplification of the nanostructures Interplay between Electron-lattice coupling and Diffusion of Electrons Conclusion Fusion Resolidification Recrystallisation References. - ~45 nm melted and resolidifcated depth (0.42 J/cm2, 130 fs) [J. Bonse, Applied Surface Science 221, 215 (2004)] - Melting occurs from 0.2-0.5 J/cm2 [NM Bulgakova, Physical Review B 69, 054102 (2004)], [DP Korfiatis, Journal of Physics D 40, 6803 (2007)] Bulk silicon Melting starts at ~0.4 J/cm². ~115 nm melted at 0.5 J/cm². Laser fluence (J/cm²) Electron temperature Free – carrier density Lattice temp. Melting Critical density Model includes: •Electron and Lattice temperatures (heating, coupling, diffusion, cooling). •Free-electron and hole density (1-2 photon ionization, electronic avalanche, convection, recombination) •Fresnel formulas of reflectivity and inverse Bremmstrahlung absorption •Drude model (EM behaviour) •Electron-phonon and electron-electron collisions are considered. EM plasmonic interference is modeled and the possibility of printing the pattern from electrons to lattice is checked. Result is : for high fluence, convection-diffusion is dominant on electron-lattice coupling, for low-fluence, it is possible to observe a priodic modification in lattice temperature profile. On these figures, F=500 mJ/cm². Calculated free-electron density, electron and lattice temperature are presented for a 500 mJ/cm² laser fluence, 800 nm NIR wavelength, and 100 fs pulse duration (FWHM). It has been reported that c-Si (100) melts from 0.2 J/cm² under ~100 fs laser irradiation. Our calculations based on a melting temperature criterium showed that, if melting occurs, resolidifcation front velocities are higher than maximum speed of recrystallisation, thus should be observed. However, calculations based on fusion enthalpy showed that no melting occurs. Our experimental measurements show that no melting/recrystallisation front is observable on Transverse Electron Microscopy pictures of the Si-protection layer interface. - Calculations showed that thermal processes (melting, evaporation, …) are not the mechanisms responsible for pattern transmission from electron to the lattice. Ablation may occur, but an ultrafast mechanism of ablation (~200 fs at 500 mJ/cm²) is required to support the interference hypothesis. - Calculations confirm the possibility of the surface electromagnetic wave (plasmon) to be responsible for ripple formation under fs laser irradiation, through the modification of absorption provided by a seed (defects, roughness, nanoparticle) under our conditions. - Calculations and experiments show that no melting occurs during ripple formation. - Experiments show that mechanism of ripple-to-beads transition occurs in a crysta