Undercooling of Liquid Germanium

AbstractSmall liquid germanium (ℓ-Ge) droplets (0.3–0.5mm diameter) have been undercooled in a B2O3 flux a maximum of 415 ± 20°C below Tm before recalescence. Although the droplets undercooled to temperatures well below the temperature expected for metastable equilibrium with amorphous Ge, Taℓ, the solidified structures were always polycrystalline. There was no evidence of a continuous liquid-to-amorphous transition at or below Taℓ. The nucleation frequency was calculated to be ˜8 × 106 /cm 3-sec at the maximum undercooling. This should be an upper limit to the homogeneous nucleation frequency of the crystal phase since we did not establish that the nucleation was homogeneous in our experiments. However, this limiting I for Ge indicates that homogeneous nucleation of the crystal or amorphous phase would not become appreciable in nsec laser pulsing experiments until ℓ-Ge is undercooled to well below Taℓ. The structure of solidified Ge and Ge/Sn alloy droplets indicates that dendritic growth does not occur in Ge until the liquid is undercooled at least 300°C, and in Ge/Sn until the liquid is undercooled at least 250°C. Greater undercoolings lead to increased dendritic breakup, which results in grain refinement in the final structure.