Ultrafast crystallization mechanism of amorphous Ge15Sb85 unraveled by pressure-driven simulations

Crystallization rate determines the switching speed of phase-change memory for next-generation cache-type applications. Using ab initio molecular dynamics simulations, we studied the pressure-driven rapid crystallization behavior of amorphous Ge₁₅Sb₈₅ (a-GS), a technologically important Te-free phase-change material (PCM) that exhibits rapid growth-controlled phase transition. We concentrated on the fluctuations and competitions between the short-range order (SRO) and medium-range order (MRO) in a-GS under different pressures. The results reveal that the a-GS at zero pressure is composed of Ge-centered tetrahedrons and defective Sb-centered octahedrons, and these special SRO motifs are connected by 5-fold rings. The pressure-driven rapid phase transition of a-GS involve two stages: the incubation stage and the fast growth stage. In the incubation stage, Ge-centered tetrahedrons transform into octahedrons and meanwhile Sb-centered octahedrons increase as well. The disordered 5-fold rings evolve into a specific ordered spatial conformation, in which four atoms lie on one plane and one atom is pushed out with a dihedral angle of 90°. During the fast growth stage, the formation of 4-fold rings is linked with the existing defective octahedrons, requiring only minimum atomic movements with disappearance of 5-fold rings. Using pressure, we reveal this transient process which bridges the missing gap in the formation of new MRO clusters that are yet unknown in experiments. The unique crystallization mechanism of a-GS under pressure is of great significance for the design of high-speed phase-change memory.