Structural changes produced in silicon by intense 1-µm ps pulses

The numerous bulk and surface structural changes observed in c-Si following melting with 1-μm pulses that range from 4 to 260 ps in duration and fluences from about 0.6 to 2.8 J cm^-2 are examined by Nomarski and transmission electron microscope techniques. For melting pulse widths 30 ps or longer, recrystallization from the melt was observed. By contrast, for the shorter pulses (∼7 ps), the steep temperature gradients that accompany the onset of two-photon absorption associated with pulses of this width produce an undercooled melt. Under these conditions, the resolidification velocities are evidently too high to allow epitaxial regrowth from the crystalline substrate and, for the first time, regions of amorphous and large- and fine-grain polycrystalline silicon are observed to form directly on a crystalline underlayer. In addition, alternate stripes of amorphous and crystalline material are produced by these short pulses. These are associated with localized melting, demonstrating that uniform surface melting is not always required before a spatial modulation of the absorbed surface energy can occur.