Models for phase-change of Ge₂Sb₂Te₅ in optical and electrical memory devices

We investigated three different modeling approaches to simulate the crystallization behavior of Ge₂Sb₂Te₅ in optical and, very recently, electrical phase-change memories. The first of these models is based on the Johnson–Mehl–Avrami–Kolmogorov (JMAK) formalism to calculate the fraction of crystallized material during isothermal anneals. In the literature, this model is widely used, but parameters of the model reported by different investigators vary considerably. We have shown that these discrepancies can be attributed to the inappropriate use of the JMAK approach. In order to overcome the restrictions imposed by JMAK theory, generalizations based on classical nucleation theory have been suggested. Material parameters required by the theory, such as viscosity, diffusivity, and fusion enthalpy of Ge₂Sb₂Te₅, have been deduced from published experiments. Uncertainty in the material parameters in combination with approximate expressions used by the classical nucleation theory, however, lead us to suggest a comprehensive model based on rate equations. Although it is more complicated, this modeling approach has yielded more favorable and reliable results. We have discussed different simulation–experiment comparisons to illustrate the capabilities of the model. © 2004 American Institute of Physics.