High-resolution observations of an amorphous layer and subsurface damage formed by femtosecond laser irradiation of silicon

Using transmission electron microscopy (TEM), we observed the micro- and nanostructures of silicon after irradiation by ∼150fs duration pulses centered at 800nm wavelength. Specimens irradiated with a single pulse of 11J∕cm2 fluence and with five pulses, each with a fluence of 1.3J∕cm2, exhibited various structures which included amorphous phases. The amorphous phases were pure silicon, as was revealed by high-resolution TEM imaging, nanobeam diffraction patterns, high-angle annular dark-field images, conventional diffraction images, and energy-dispersive x-ray spectra. Irradiation with a single pulse of 1.5J∕cm2 produced neither amorphous material nor lattice defects. Single-pulse irradiation at a fluence of 33J∕cm2 and irradiation by four pulses at 11J∕cm2 led to substantial subsurface damage around the center of the laser spot. It is concluded that multiple-pulse irradiation produces crystallographic damage more readily than a single pulse.