Real‐time measurements of the melt and solidification dynamics during pulsed laser irradiation have permitted elucidation of the processes which lead to the formation of very thin (∼2 nm) buried impurity layers in amorphous silicon. These novel structures are found to result from interfacial segregation at two moving liquid silicon‐amorphous silicon interfaces—one propagating toward the surface and the other propagating from the surface inward. The velocities of these interfaces are found to be remarkably low, ranging from 1 to 8 m/s for melts of <20 ns duration.