Van der Waals Epitaxy and Characterization of Quasi Two-Dimensional Ge-Sb-Te Materials and Heterostructures

The advent of two-dimensional materials redefined the horizons of materials science in the last decade, promising disrupting advances in many technological fields. Among the available synthesis techniques, van der Waals (vdW) epitaxy ¹ ensures high quality, purity and scalability, all crucial for the integration with microelectronic technology.
Chalcogenide phase change materials have been identified as promising candidates for the development of storage class memories, ² as well as brain-inspired computing. ³ Beyond the well-known phase change functionality used in non-volatile memories, the Ge-Sb-Te family possesses a unique variety of functional properties. As an example, the binary compound GeTe is the father of a new class of materials, namely ferroelectric Rashba semiconductors, in which ferroelectricity is used to control the spin texture at room temperature. ⁴ A key element for the exploitation of this rich playground is the high crystal quality and interface control achieved for the material deposited by molecular beam epitaxy (MBE).
In this presentation, I will first give an overview on the fabrication by MBE of Ge-Sb-Te layered materials and heterostructures on Sb-passivated Si(111) substrates. ^5–7 Next, I will discuss results on the vdW epitaxy and characterization of GeTe-rich (GeTe)_m(Sb₂Te₃)_n (GST) films, which recently provided breakthrough evidence of their composition-dependent ferroelectric behavior. ⁸ Finally, I will present the investigation of the electronic and vibrational properties of epitaxial GST films. The analysis, based on X-ray photoemission spectroscopy and THz spectroscopy, respectively, is supported by density functional theory calculations.

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² S.W. Fong, C.M. Neumann, and H.P. Wong, “Phase-change memory – Towards a storage-class memory,” IEEE Transactions on Electron Devices 64(11), 4374–4385 (2017).
³ A. Sebastian, M. Le Gallo, G.W. Burr, S. Kim, M. BrightSky, and E. Eleftheriou, “Tutorial: Brain-inspired computing using phase-change memory devices,” Journal of Applied Physics 124(11), 111101 (2018).
⁴ S. Varotto, L. Nessi, S. Cecchi, J. Slawinska, P. Noël, S. Petrò, F. Fagiani, A. Novati, M. Cantoni, D. Petti, E. Albisetti, M. Costa, R. Calarco, M. Buongiorno Nardelli, M. Bibes, S. Picozzi, J.-P. Attané, L. Vila, R. Bertacco, and C. Rinaldi, “Room-temperature ferroelectric switching of spin-to-charge conversion in germanium telluride,” Nature Electronics 4(10), 740–747 (2021).
⁵ S. Cecchi, E. Zallo, J. Momand, R. Wang, B.J. Kooi, M.A. Verheijen, and R. Calarco, “Improved structural and electrical properties in native Sb₂Te₃/Ge_xSb₂Te_3+x van der Waals superlattices due to intermixing mitigation,” APL Materials 5(2), 026107 (2017).
⁶ R. Wang, F.R.L. Lange, S. Cecchi, M. Hanke, M. Wuttig, and R. Calarco, “2D or not 2D: Strain tuning in weakly coupled heterostructures,” Advanced Functional Materials 28(14), 1705901 (2018).
⁷ S. Cecchi, D. Dragoni, D. Kriegner, E. Tisbi, E. Zallo, F. Arciprete, V. Holý, M. Bernasconi, and R. Calarco, “Interplay between Structural and Thermoelectric Properties in Epitaxial Sb_2+xTe₃ Alloys,” Advanced Functional Materials 29(2), 1805184 (2019).
⁸ S. Cecchi, J. Momand, D. Dragoni, O. Abou El Kheir, F. Fagiani, D. Kriegner, C. Rinaldi, F. Arciprete, V. Holý, B.J. Kooi, M. Bernasconi, and R. Calarco, “Thick Does the Trick: Genesis of Ferroelectricity in 2D GeTe-Rich (GeTe)m(Sb2Te3)n Lamellae,” Advanced Science 11(1), 2304785 (2024).