In this paper, we modeled the activation energies of GeSbTe type of phase change materials. This model is based on the analysis of the atomic bonding types in the amorphous state and the crystalline state. Both the bond numbers and bond energies were used to calculate the crystallization activation energies. The calculated crystallization activation energies of the as-deposited amorphous states match the experimental data in the literature. We also estimated the crystallization activation energies of the melt-quenched amorphous states for different GeTe-Sb2Te3 phase change materials. The results show that the activation energies of the melt-quenched amorphous state are 0.21~0.48 eV lower than those of the as-deposited amorphous state. This might explain the difference of the crystallization behaviors between the as-deposited and the melt-quenched amorphous states.
The crystallization process of two types of phase change materials (Ge2Sb2Te5 and AgInSbTe) has been studied by in-situ annealing experiments in the column of a transmission electron microscope. Under the same heating rate (7.5K/s), the Ge2Sb2Te5 material starts to nucleate at 170°C, while AgInSbTe starts to nucleate at 250°C. The measured nucleation rate of Ge2Sb2Te5 is 8.2×1014 cm-3s-1 larger than nucleation is dominant for both Ge2Sb2Te5 and AgInSbTe with ZnS-SiO2 as the dielectric layer in the thinned samples.