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.