Structural insights into the formation and evolution of amorphous phase-change materials

Reduction of programming current is a major research goal in the development of phase-change random-access memory devices. The power-limiting step is the amorphization of the phase-change material (PCM), where a significant energy input is required to induce melting prior to amorphization. To address the challenge of reducing power consumption while retaining switching speed, a detailed understanding of the physics underpinning the amorphization process is required. As yet, little has been done to study the dynamics of the melt-quench process at the atomic level. In this article, we report a detailed study of the melting mechanism and kinetics, and the effect of quench rate on the amorphization process in the prototypical PCM Ge₂Sb₂Te₅, using ab initio molecular-dynamics simulations. We also study the evolution of the amorphous phase under low-temperature annealing, shedding light on the structural changes, which may occur after amorphization at device operating temperatures. Our results give microscopic insight into the amorphization of PCMs, and should inform future work to understand and resolve important issues in device engineering. Effect of quench rate on the structure of Ge₂Sb₂Te₅: While quenching at -5Kps^-1 leads to successful amorphization (left), quenching at a slower -1Kps^-1 leads to crystallization as the temperature is lowered (right).