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.<br/>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).<br/>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)<i>_m</i>(Sb₂Te₃)<i>_n</i> (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.<br/><br/>¹ A. Koma, “Van der Waals epitaxy–a new epitaxial growth method for a highly lattice-mismatched system,” Thin Solid Films <b>216</b>(1), 72–76 (1992).<br/>² S.W. Fong, C.M. Neumann, and H.P. Wong, “Phase-change memory – Towards a storage-class memory,” IEEE Transactions on Electron Devices <b>64</b>(11), 4374–4385 (2017).<br/>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 <b>124</b>(11), 111101 (2018).<br/>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 <b>4</b>(10), 740–747 (2021).<br/>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 <b>5</b>(2), 026107 (2017).<br/>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 <b>28</b>(14), 1705901 (2018).<br/>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 Sb₂₊<i>_x</i>Te₃ Alloys,” Advanced Functional Materials <b>29</b>(2), 1805184 (2019).<br/>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 <b>11</b>(1), 2304785 (2024).