Formation of conic microstructures upon pulsed laser evaporation of solids

The appearance and development of large-scale self-organising microstructures on the surface of monocrystalline Si and several other materials (Ge, Ti) are studied upon their evaporation by 20-ns pulses from a copper vapour laser at 510.6 nm. The structures were formed upon repeated pulsed laser irradiation (~ 10⁴ pulses with an energy density of 1 — 2 J cm^-2) in the absence of optical breakdown of the medium above the target surface in the 1 — 10⁵ Pa pressure range in a wide range of angles of laser radiation incidence on the surface. The structures are cones with an apex angle of ~20 — 25°, which grow towards the laser beam. It is shown that the spatial period of the structures developing during laser evaporation is determined by the period of the waves arising on the melt surface and is equal to 10 — 20 μm. The x-ray diffraction analysis showed that the modified substrate region has a polycrystalline structure and consists of crystallites with dimensions ranging from 40 to 70 nm, depending on the pressure of the ambient atmosphere.