Experimental test of kinetic theories for heterogeneous freezing in silicon

The crystallization rate of liquid silicon has been measured during epitaxial explosive crystallization of amorphous silicon. The measurements, together with numerical temperature calculations indicate that freezing in silicon saturates at 15.8 m/s for large undercooling (>130 K) below the equilibrium melting temperature. These data, as well as a variety of experimental results of other investigators, are used to test two models describing the kinetics of heterogeneous freezing. A transition-state theory in which the phase transformations are assumed to go through an intermediate state at a rate limited by the sound velocity is not consistent with the data. A theory in which the rate-limiting factor in freezing of liquid silicon is atomic diffusion in the liquid close to the interface describes the data well. The activation energy for self-diffusion of atoms in the liquid near the interface is found to be 0.7--1.1 eV.