Structural role of vacancies in the phase transition of Ge ₂Sb₂Te₅ memory materials

Crystallization in amorphous materials requires significant atomic diffusion for structural ordering to occur. Vacancies can play a critical role during the crystallization process, although little is known for phase-change materials. Here, using ab initio molecular-dynamics simulations, we have observed how vacancies evolve and influence the crystallization process in Ge₂Sb₂Te₅ (GST) materials. It was found that vacant sites have mostly Te atoms as neighbors. The diffusion of Ge/Sb atoms in the amorphous phase to vacancies at the crystal-glass interface helps in the formation of stable cubic clusters that potentially grow as nuclei for crystallization. Such selective vacancy diffusion with its particular redistribution facilitates the crystal-nucleation process, thereby significantly contributing to the fast speed of crystallization in this material.