The crystallization of amorphous semiconductors is a strongly exothermic process. Once initiated the release of latent heat can be sufficient to drive a self-sustaining crystallization front through the material in a manner that has been described as explosive. Here, we perform a quantitative in situ study of explosive crystallization in amorphous germanium using dynamic transmission electron microscopy. Direct observations of the speed of the explosive crystallization front as it evolves along a laser-imprinted temperature gradient are used to experimentally determine the complete interface response function (i.e., the temperature-dependent front propagation speed) for this process, which reaches a peak of 16 m/s. Fitting to the Frenkel-Wilson kinetic law demonstrates that the diffusivity of the material locally/immediately in advance of the explosive crystallization front is inconsistent with those of a liquid phase. This result suggests a modification to the liquid-mediated mechanism commonly used to describe this process that replaces the phase change at the leading amorphous-liquid interface with a change in bonding character (from covalent to metallic) occurring in the hot amorphous material.
The temperature plotted in Fig. 7 is the crystallization front temperature that we extract from the lumped interface modeling one gridpoint in advance of the delta-function source. The horizontal/temperature error bars on the points in Fig. 7 represent our best estimate of the way that the uncertainty in the time-dependent position of the delta-function source (the uncertainties associated with the data in Fig. 3) propagates through the modeling into c-l front temperature. This was determined by computing the time-dependent temperature field (with the model described above) with the range of source trajectories that is consistent with the data in Fig. 3 in a monte-carlo like manner.