Evolutionary design of interfacial phase change van der Waals heterostructures

We use an evolutionary algorithm to explore the design space of hexagonal Ge₂Sb₂Te₅; a van der Waals layered two dimensional crystal heterostructure. The Ge₂Sb₂Te₅ structure is more complicated than previously thought. Predominant features include layers of Ge₃Sb₂Te₆ and Ge₁Sb₂Te₄ two dimensional crystals that interact through Te-Te van der Waals bonds. Interestingly, (Ge/Sb)-Te-(Ge/Sb)-Te alternation is a common feature for the most stable structures of each generation's evolution. This emergent rule provides an important structural motif that must be included in the design of high performance Sb₂Te₃-GeTe van der Waals heterostructure superlattices with interfacial atomic switching capability. The structures predicted by the algorithm agree well with experimental measurements on highly oriented, and single crystal Ge₂Sb₂Te₅ samples. By analysing the evolutionary algorithm optimised structures, we show that diffusive atomic switching is probable by Ge atoms undergoing a transition at the van der Waals interface from layers of Ge₃Sb₂Te₆ to Ge₁Sb₂Te₄ thus producing two blocks of Ge₂Sb₂Te₅. Evolutionary methods present an efficient approach to explore the enormous multi-dimensional design parameter space of van der Waals bonded heterostructure superlattices.