Thick does the trick: Ferroelectricity in two-dimensional (GeTe)m(Sb2Te3)n lamellae

The results of the research published in Advanced Science
schema di lamelle 2D ferroelettriche e non ferroelettriche

The ternary Ge-Sb-Te alloys (GST) are employed as active material in non-volatile phase-change memory devices, which exploit the large difference in both electrical and optical properties between the amorphous and crystalline phases and the possibility to thermally induce a fast and reversible transition between the two phases. Recently, this technology has attracted great attention for the possible exploitation in neuromorphic computing. Moreover, these materials feature crystalline lamellar phases which are similar to other two-dimensional materials.

On the other hand, the binary compound GeTe is a well known ferroelectric material, as well as a Rashba semiconductor with a giant spin splitting of its bulk bands.

A crucial issue for the study of the multifunctional properties of this family of chalcogenide materials and its exploitation in advanced devices is the high crystal quality which can be achieved by molecular beam epitaxy on silicon substrate.

In a recent paper entitled “Thick Does the Trick: Genesis of Ferroelectricity in 2D GeTe-Rich (GeTe)m(Sb2Te3)n Lamellae” (doi: 10.1002/advs.202304785) published in Advanced Science (Impact factor 15.1 - 2022 Journal Impact Factor, Journal Citation Reports (Clarivate Analytics, 2023)) to which researchers of the Department of Materials Science of the University of Milano-Bicocca contributed, it has been demonstrated that molecular beam epitaxy allows  fabricating lamellar GeTe-rich (GeTe)m(Sb2Te3)n alloys which show ferroelectric properties. The material has been developed at the Paul-Drude-Institut in Berlin within the European project BeforeHand (GA 824957, Horizon2020), involving the Department of Materials Science together with other six European partners. Thanks to the analysis of the structural properties of single GST lamellae by scanning transmission electron microscopy (University of Groningen) and electronic structure calculations based on density functional theory (University of Milano-Bicocca), the presence of a structural asymmetry in very thick lamellae (e.g. with 25 or more atomic layers) was identified as responsible for the ferroelectric behavior of the material. The inversion of the polarization of the material by applying an external electric field was demonstrated by piezoresponse force microscopy, in collaboration with the Politecnico di Milano.

Hence, this study demonstrates the possibility to combine the well-known phase-change functionality of GST with the ferroelectric properties of GeTe, paving the way for the development of innovative multifunctional devices. The study, lead by Dr. Stefano Carlo Cecchi, involved Dr. Daniele Dragoni, Dr. Omar Abou El Kheir and Prof. Marco Bernasconi of the Department of Materials Science.