Direct observation of resolidification from the surface upon pulsed-laser melting of amorphous silicon

Amorphized Si has been irradiated using a 7.5-ns frequency-doubled neodymium:yttrium aluminum garnet laser. For low energy density pulses, time-resolved reflectivity measurements and Rutherford backscattering spectrometry of Cu implantation profiles show that the melted layer solidifies from the surface as well as from the liquid-solid interface. From interferences in the reflectivity, growth from the surface is found to occur at a velocity of 1.5 m/s. At higher energy densities sufficient to obtain epitaxial regrowth of the amorphous layer, solidification from the surface does not occur.