Self-assembled surface patterning and structural modification upon femtosecond laser processing of crystalline silicon

Upon femtosecond laser ablation (Ti:Sapphire; 800 nm, 100 fs, under ultra-high vacuum) from crystalline silicon (001), the surface morphology and structural changes were examined ex-situ by optical, scanning electron microscopy, and Raman spectroscopy. After repetitive illumination with several thousand laser pulses at an intensity below the single shot damage threshold (1012 W/cm(2)), self-assembled periodic nanostructures with periods of 200 nm resp. 600-700 nm develop at the crater bottom. Raman spectroscopy reveals a phase transformation inside the crater from Si-I to the polymorphs Si-III, Si-XII, hexagonal Si-wurtzite (Si-IV), and amorphous silicon. The ablation dynamics was monitored by time-of-flight mass spectroscopy, showing the emission of superthermal positive ions with a kinetic energy of about 7 eV. The results suggest that the ablation, leaves behind a severely perturbed crystal surface. The resulting instability relaxes by a self-organization, independent of the initial, and surrounding, crystal structure.