LASER-INDUCED NUCLEATION OF CRYSTALS IN AMORPHOUS-GE FILMS

Amorphous Ge films were crystallized by laser pulses and the evolution of crystals within the laser spot was visualized by time-resolved transmission electron microscopy. At the lowest energies employed the film crystallizes by slow solid-state reaction during a microsecond. At medium energies crystallization starts delayed by 10-50 ns and is completed within 100-150 ns. This transition is interpreted as melting of the a-Ge film between the melting points of the amorphous and crystalline material and subsequent freezing of the supercooled liquid by homogeneous crystal nucleation and diffusion-limited growth. The mode of transformation again changes, when a melt with a temperature above T(mc) is generated. Crystallization starts delayed by microseconds as the melt supercools, and proceeds from its periphery towards the center, accumulating liquid material ahead. The initially plane phase boundary breaks down and crystallization is completed by inward growth of single-crystal plates with velocities approximately 0.5 m/s.