Van der Waals Epitaxy of Ge-Sb-Te Alloys—A Powerful Way to Design 2D Heterostructures for Neuromorphic Applications
The (GeTe)m(Sb2Te3)n alloy (Ge-Sb-Te - GST) is a key phase change material (PCM), widely studied for its cutting-edge technological applications. It is a prominent material for phase change memories, and, very recently, has attracted new interest as the most advanced emerging non-volatile memory technology for neuromorphic applications [1-2]. The ability of growing high-quality epitaxial GST represents a key-point to tailor the material properties, such as a large programming window , improved cycling endurance and faster crystallization . Depending on n and m, prototypical crystalline GST can have various compositions along the pseudobinary tie line and, in its trigonal structure, exhibits a stacked structure along the  direction that involve weak van der Waals (vdW) interactions between adjacent blocks. vdW epitaxy represents a powerful way for growing heterostructures made of stacked sequences of 2D crystals, potentially exhibiting new phenomena and peculiar properties. In this study we aim at identifying the key parameters to predict the interaction between GST layered materials and substrate surface. In particular, we present the case of GST alloys grown by Molecular Beam Epitaxy on InAs(111). In this system the substrate can be efficiently prepared into self- and un-passivated surfaces to clarify the role of the surface interaction. Furthermore low-lattice mismatch conditions are fulfilled. Those are necessary to avoid relaxation due to formation of misfit dislocations and allow to correctly identify vdW epitaxy. It is known that GST exhibits two different highly ordered 2D structures and a three-dimensional disordered structure, allowing to directly infer the nature of the epitaxy. We will give evidence of the key role played by the substrate surface in vdW epitaxy. We will show that substrate symmetry influences the symmetry of the growing film and, for extremely well passivated covalent or pure 2D surfaces, vdW epitaxy can be fully achieved. When this requisite is relaxed an interaction with the substrate surface arises, which can lead to the formation of coincidence lattices or induce a quasi-pseudomorphic growth. In general, our study paves the way for mastering and design of vdW epitaxial growth of 2D heterostructures as well as hybrid 2D and non-layered materials. References  Abu Sebastian, Manuel Le Gallo, Geoffrey W. Burr, Sangbum Kim, Matthew BrightSky, and Evangelos Eleftheriou, Tutorial: Brain-inspired computing using phase-change memory devices, J. Appl. Phys. 124, 111101 (2018)..  W. Zhang, R. Mazzarello, M. Wuttig, and E. Ma, Designing crystallization in phase-change materials for universal memory and neuro-inspired computing, Nat. Rev. Mater. 4, 150 (2019).  V. Bragaglia, F. Arciprete, W. Zhang, A. M. Mio, E. Zallo, K. Perumal, A. Giussani, S. Cecchi, J. E. Boschker, H. Riechert, S. Privitera, E. Rimini, R. Mazzarello, and R. Calarco, Metal-Insulator Transition Driven by Vacancy Ordering in GeSbTe Phase Change Materials, Sci. Rep. 6, 23843 (2016).  K. Ding, J. Wang, Y. Zhou, H. Tian, L. Lu, R. Mazzarello, C. Jia, W. Zhang, F. Rao, and E. Ma, Phase-change heterostructure enables ultralow noise and drift for memory operation, Science 366, 210 (2019).