Kinetics of crystal nucleation in undercooled droplets of Sb- and Te-based alloys used for phase change recording

Droplets of molten alloys of composition Ge12Sb88, Ag0.055In0.065Sb0.59Te0.29, Ge4Sb1Te5, and Ge2Sb2Te5, used for optical data storage, surrounded by a molten dehydrated B2O3 flux were undercooled to 40–80K below their liquidus temperature in a differential thermal analyzer. The crystal-melt interfacial energy was calculated from the nucleation temperature using the classical nucleation theory, which gave values of around 0.20 times the heat of fusion per atom in the interface for all alloys. This value should be a lower limit since we did not establish that nucleation was homogeneous in the experiments. The steady-state nucleation rate was calculated between the liquidus and glass transition temperatures and was higher for the GeSbTe alloys than for the Sb-rich alloys. Nevertheless, the nucleation rates appear too high to allow amorphization under operating conditions for the highest achievable cooling rates. Therefore, we conclude that it is the presence of an incubation time that makes amorphization and therefore phase change recording possible in both optical and electronic media.