GeTe Ferroelectric Rashba Semiconductor—From Growth to Electronic Properties

Among ferroelectrics, a new class of materials for spintronics has recently been introduced: The Ferroelectric Rashba semiconductors [1,2]. The main results, obtained on GeTe thin films, have demonstrated that reversal of the ferroelectric polarization leads to a change in the spin chirality of the band structure [3,4]. A spin-to-charge conversion has also been demonstrated at room temperature in ferromagnetic-GeTe structures [5-7].<br/>In this presentation, I will discuss the organization of ferroelectric nanodomains present in GeTe thin films grown on Si(111) [8], the type of domain wall and the structure of the interface with the substrate. Quasi-monocrystalline GeTe thin films can be produced on Si(111) by molecular beam epitaxy, by first depositing an atomic monolayer of Sb [9,10,11]. This substrate enables ferroelectric domains to be studied and controlled, as they are not limited by grain boundaries. Ferroelectric nanodomain volume fraction and domain size were measured by X-ray diffraction and low energy electron microscopy (LEEM) over a wide range of film thicknesses (10-1800 nm). Second harmonic generation (SHG) microscopy combined with polarimetric analysis revealed the local symmetry of these domains. Using high-resolution transmission electron microscopy (HR-TEM), we show that the domain walls are at 71° and that the GeTe/Si interface is stabilized by dislocations that relax the large parametric mismatch between the two crystal lattices. The reversible appearance/disappearance of ferroelectric nanodomains by thermal cycling, visualized by LEEM in situ, is attributed to thermal stresses induced by the thermal expansion differential between the two materials [12]. At last we demonstrate a giant Rashba effect in GeTe thin films by Angle Resolved Photo Emisson Spectroscopy for thin films down to a 1 nm thick layer. We also put in evidence significant THz emission through spin-to-charge conversion in a Fe/GeTe/Si(111) stack.<br/><br/><br/>[1] D. Di Sante et al., Adv. Mater. 2013, 25, 509<br/>[2] M. Liebmann et al. , Adv. Mater., 2016, 28, 560<br/>[3] C. Rinaldi et al., Nano Lett., 2018, 18, 2751<br/>[4] J. Krempasky et al., Phys. Rev. X, 2018 8, 021067<br/>[5] C. Rinaldi et al., APL Mater., 2016, 4, 032501<br/>[6] J. Slawinska et al., Phys. Rev. B., 2019, 99, 075306<br/>[7] S. Varotto et al., Nature Electronics, 2021, 4, 740<br/>[8] B. Croes et al., Phys. Rev. Materials, 2021, 5, 124415<br/>[9] R. Wang et al., J. Phys. Chem. C, 2014 118, 29724<br/>[10] B. Croes et al., Phys. Rev. Materials, 2022, 6, 064407<br/>[11] B. Croes et al., Phys. Rev. Materials, 2023, 7, 014409<br/>[12] B. Croes et al., J. Appl. Phys., 2023, 134, 204103