Phase-field modeling of the non-congruent crystallization of a ternary Ge-Sb-Te alloy for phase-change memory applications

The ternary alloy of germanium, antimony, and tellurium (GST) is widely used as material for phase-change memories. In particular, the stoichiometric compound Ge₂Sb₂Te₅ exhibits a rapid congruent crystallization. To increase the temperature at which spontaneous crystallization erases the stored information, alloys that are enriched in germanium have been investigated. Their crystallization is accompanied by segregation and eventually the nucleation of a new, germanium-rich phase. In order to model the redistribution of alloy components and the time evolution of the microstructure during device operations, we develop a multi-phase-field model for the crystallization of GST that includes segregation, and couple it with orientation fields that describe the grain structure. We demonstrate that this model is capable to capture both the emergence of a two-phase polycrystalline structure starting from an initially amorphous material, and the melting and recrystallization during the SET and RESET operations in a memory cell of the "wall" type.