Thesis defense

Title : Growth and ferroelectricity of GeTe on Si(111)

Language : french

Members of the jury :

Antoine Barbier (CEA) : Rapporteur
Alain Marty (CEA) : Rapporteur
Salia Cherifi-Hertel (IPCMS) : Examinatrice
Yannick Fagot-Revurat (IJL) : Examinateur
Vincent Garcia (CNRS-Thalès) : Examinateur
Olivier Thomas (IM2NP) : Examinateur
Fabien Cheynis (CINaM) : Co-directeur de thèse
Frédéric Leroy (CINaM) : Directeur de thèse

Abstract :

The recently measured giant Rashba effect in GeTe has shown the potential for spintronics of ferroelectric materials with large spin-orbit coupling. In order to electrically control the electrical polarization of the material and thus the spin structure of the valence and conduction bands, it is essential to characterize the structure of the ferroelectric domains and their spatial organization. In this thesis we have grown GeTe thin films on Si(111)-Sb by molecular beam epitaxy in a wide range of thickness. At 270°C, we observe a layer-by-layer growth and a main epitaxial relationship GeTe(111) ∥Si(111) and GeTe[110] ∥Si[110]. The growth of GeTe thin films shows that initially about 40% of the grains are rotated in-plane by 180°. This proportion decreases rapidly and reaches about 4% for a thickness of 200 nm. In addition, ferroelastic nanodomains appear which volume fraction and size can be controlled by fine tuning the thickness and the deposition temperature. These domains have an elongated needle shape in the <110> direction. Their unit cell is identical to that of the main domain (rhombohedral with ar = 4.29 ̊A and θ = 58.3°). The [111] axis of their unit cell and their electrical polarization are tilted by about 72° with respect to the [111] axis of the main domain. While the majority domain has a pure tellurium surface termination Te (1 × 1), we show by scanning tunneling microscopy that the surface of the ferroelastic nanodomains have three different surface structures, including a pure germanium termination associated with a (2 × 2) reconstruction, a missing row structure of Te, and a more complexe scale structure that is the most stable. In situ measurements by low energy  electron microscopy and X-ray diffraction during thermal cycling show the appearance and disappearance of the ferroelectric nanodomains at a temperature around 220°C. By a detailed analysis of the GeTe/Si interface by transmission electron microscopy, we demonstrate that misfit dislocations localized at the interface formed during growth play a key role in the stability of ferroelectric nanodomains. Finally, we have characterized the electronic band structure of the GeTe majority domains and measured a Rashba constant αr = 4.9 eV. ̊A that decreases with film thickness. First measurements to characterize the Berry curvature are performed by circular dichroism in angle-resolved photoemission spectroscopy.