Structural role of vacancies in the phase transition of Ge2Sb2Te5 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.