Mechanism of the isotermic amorphous-to-crystalline phase transition in Ge: Sb : Te ternary alloys

The kinetics of the amorphous-to-crystalline (fcc) phase transition under i sothermal treatments has been investigated in alloys with the composition c lose to Ge22Sb22Te56, which is the most frequently used for the erasable op tical memory devices. For monitoring the transformation, electrical, ellips ometric, optical transmission and electron microscopy measurements were emp loyed. A procedure to determine the transformed volume fraction based on th e electrical conductivity data is proposed. It is shown that the widely acc epted assumption about the linear dependence between the conductivity varia tion and the transformed volume fraction is not correct. The use of this as sumption could give an error in the determination of the crystalline volume fraction of up to one order of magnitude. The amorphous-to-crystalline tra nsformation kinetics observed agrees with the Johnson-Mehl-Avrami formalism . The transformation exhibits two distinct stages that are attributed to th e surface and bulk nucleation, and gives an average Avrami exponent around 2. The activation energy of the process is about 4 eV, which exceeds previo us estimations. (C) 2001 American Vacuum Society.