Journal of Materials Chemistry C 2018-04-11

Evolution of short- and medium-range order in melt-quenching amorphization of Ge2Sb2Te5

Chong Qiao, Y. R. Guo, Fei Dong, Jinjin Wang, Hong Shen, Songyou Wang, Ming Xu, Xiangshui Miao, Y. X. Zheng, Rongjun Zhang, Liangyao Chen, Cai Zhuang Wang, Kai Ming Ho

文献索引：10.1039/C8TC00549D

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摘要

Phase-change memory takes advantage of fast phase transition between amorphous and crystalline phases of the phase-change materials (e.g., Ge2Sb2Te5 or GST). To date, while the “SET” process (crystallization of GST glass) has been intensively studied, studies of “RESET” process (melt-quenching amorphization of GST) are still limited. In this work, we explored the structural changes of GST upon rapid cooling by ab initio molecular dynamics simulations and an atomistic cluster alignment (ACA) analysis. Different from other methods which only focus on the nearest bonding atoms, the ACA method can study both the short- and medium-range order clusters containing atoms beyond the first-neighbor shell and enables us to explore the changes of cluster structures in a larger scale. The results reveal that the low-coordinated octahedral clusters tend to become high-coordinated ones, and Ge-centered octahedral structures change to tetrahedrons whereas Sb-centered tetrahedrons transform to octahedral structures during the amorphization process. Interestingly, the tetrahedrons show an aggregation in liquid and supercooled liquid in contrast to the 6-fold octahedrons which present a notable aggregation in amorphous GST. Moreover, our study showed that wrong bonds (Ge-Ge, Sb-Sb, Ge-Sb and Te-Te bonds) can promote the formation of large rings, and irreducible rings tend to separate into smaller and larger rings as the temperature is decreased. Our findings provide useful insights into the formation process and the structure of amorphous GST, which is valuable for facilitating the application of phase change materials.