Anderson Localization of electrons in Chalcogenide Phase Change Materials

Phase-change materials (PCMs) are capable of undergoing fast and reversible transitions between the amorphous and crystalline phase upon heating. This property has been exploited in rewritable optical discs and non-volatile electronic memories of new concept, based on the strong optical and electrical contrast between the two phases respectively[1,2]. Recently, compelling evidence for a metal-insulator transition (MIT) solely due to disorder has been observed in the crystalline GeSbTe phase change compounds through electrical transport measurements [3] and density functional theory (DFT) simulations [4,5]. All these works rely on key assumptions about structural properties of GST. Here we report a direct atomic scale chemical identification experiment on the rocksalt structure obtained upon fast crystallization of amorphous GST. Our results firmly establish the two-sublattice structure resolving the distribution of chemical species, and unequivocally demonstrate the existence of atomic disorder on the Ge/Sb/vacancy sublattice. Moreover, we identify a gradual vacancy ordering process upon further annealing that was predicted to trigger the transition to extended electronic states. These findings not only provide a structural underpinning of the observed Anderson localization, but also have implications for the development of novel multi-level data storage within the crystalline phases.[1] M. Wuttig, N. Yamada, Nat. Mater. 6, 824-832 (2007).[2] W. Zhang, V. L. Deringer, R. Dronskowski, R. Mazzarello, E. Ma, M. Wuttig, MRS Bulletin40, 856-865 (2015).[3] T. Siegrist, P. Jost, H. Volker, M. Woda, P. Merkelbach, C. Schlockermann, M. Wuttig, Nat. Mater. 10, 202-208 (2011).[4] W. Zhang, A. Thiess, P. Zalden, R. Zeller, P. H. Dederichs, J. Y. Raty, M. Wuttig, S. Blügel, R. Mazzarello, Nat. Mater. 11, 952-956 (2012).[5] W. Zhang, M. Wuttig, R. Mazzarello, Sci. Rep. 5, 13496 (2015).