Diffusion-assisted displacive transformation in Yttrium-doped Sb2Te3 phase change materials

Phase transformations between cubic and rhombohedral states of phase-change chalcogenide materials have been considered to result in small resistance drift, fast read/write speeds, and low power consumption in phase-change memory devices. However, it is difficult to observe such dynamic processes by experiments. Especially, the ultra-fast speed of, and ultra-small area affected during, the displacive transformation involved in the phase transformations, lead to a lack of direct evidence for studying the transformation mechanism. Here, we directly observed the reversible displacive transformations in Yttrium-doped Sb ₂Te ₃ (Y-Sb ₂Te ₃) by ab initio molecular-dynamics (AIMD) simulations. The forward displacive transformation, from the cubic to the rhombohedral phase, can be spontaneously realized through a two-step transformation, the shearing, and contraction between Te-Sb-Te-Sb-Te building blocks. On the other hand, the backward transformation, from the rhombohedral to the cubic phase, requires high-temperature heating (above 900 K) and atomic diffusion. The diffusion of cation-site Sb atoms into interstitial sites, located in the interlayers of building blocks, triggers the backward transformation; thus, it is diffusion-assisted. Moreover, the enhancement of thermostability of the cubic phase by Y dopants promotes the backward transformation. This study can improve the understanding of the displacive transformations in phase-change materials and may offer new insights into developing phase-change memory devices based on Sb ₂Te ₃.