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 tr ansmission electron microscopy. To induce the transformation, isothermal an d isokinetic experiments were carried out at various temperatures and vario us heating rates. The results obtained agree with the Johnson-Mehl-Avrami m odel and show that the overall activation energy for the transition is abou t 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 c onditions. Intercrystalline inclusions, which appear after the transition, contain Tr precipitates within a structurally disordered phase. These inclu sions have electrical and optical properties essentially different as compa red to the rest of the sample; they could be residual phases formed due to the nonstoichiometric nature of the samples.