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

The possibility to engineer (GeTe)_m(Sb₂Te₃)_n phase-change materials to co-host ferroelectricity is extremely attractive. The combination of these functionalities holds great technological impact, potentially enabling the design of novel multifunctional devices. In a recent paper published in Advanced Science to which researchers of PDI contributed, it has been demonstrated that molecular beam epitaxy allows fabricating lamellar GeTe-rich (GeTe)_m(Sb₂Te₃)_n alloys which show ferroelectric properties. 

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 neuromorphiccomputing. Moreover, these materials feature crystalline lamellar phases which are similar to other two-dimensional materials. At the same time, 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.

The material has been developed at PDI within the European project BeforeHand (GA 824957, Horizon2020), which involved six other 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 stabilized by epitaxy (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.

Overall, 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.

Publication: Thick does the trick: genesis of ferroelectricity in two-dimensional GeTe-rich
Authors: 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, R. Calarco 
Source: Adv. Sci., 2304785 (2023)
DOI: 10.1002/advs.202304785