The mechanism of the amorphous-to-crystalline transition in Ge²Sb²Te⁵ ternary alloys

The kinetics of the amorphous-to-crystalline phase transformation in alloys with compositions around Ge₂Sb₂Te₅, used as erasable optical memories, was investigated by electric impedance, Raman spectroscopy, and optical and transmission electron microscopy. To induce the transformation, isothermal and isokinetic experiments were carried out at various temperatures and various heating rates. The results obtained agree with the Johnson-Mehl-Avrami model and show that the overall activation energy for the transition is about 8 eV, which exceeds previous estimations. The nucleation process changes during the transition and depends on both temperature and heating rate. The available data indicate that nuclei are formed both in the bulk and at the surface of crystallites, and that the structural details of the material (the crystallite size, in particular) depend greatly on the transformation conditions. Intercrystalline inclusions, which appear after the transition, contain Te precipitates within a structurally disordered phase. These inclusions have electrical and optical properties essentially different as compared to the rest of the sample; they could be residual phases formed due to the nonstoichiometric nature of the samples.