Description du sujet de thèse

Thermoelectric modules are solid-state energy converters that allow direct and reversible conversion of heat into electricity and are considered as a promising clean-energy technology. The past decade has witnessed a sustained boom in new thermoelectric materials, among which GeTe has shown one of the highest performances. So far, thermoelectrics have been optimized via (i) the increase of electric conductivity, creating degenerate or resonant energy states close to the Fermi level, and (ii) a decrease of thermal conductivity through defects or meso-scale structures to increase phonon scattering. We propose a new strategy to enhance and drive the thermoelectric properties of GeTe via the control of the ferroelectricity in single crystalline thin films. Indeed ferroelectric materials such as rhombohedral GeTe bring an additional degree of freedom in the structure design, namely the electric polarization. Therefore we propose a new paradigm where domain engineering at the nanoscale can drive the thermoelectric performances.
In the perspective of both fundamental research and future technological applications, the manipulation of ferroelectric domains in thin films must fulfil two requirements: (i) be based on materials with a high technological potential and (ii) be studied on a model system to control and elucidate the basic mechanisms responsible for the targeted thermoelectric performances. Therefore, we propose to base our project on GeTe, that is already well-established in microelectronics, to tackle the ferroelectric properties of GeTe thin films: (i) the domain structure, the polarization dependence on (ii) temperature and (iii) thickness at the nanometer scale. Before studying the ferroelectric properties, the project will be first devoted to the growth and characterization of two dimensional layers of GeTe obtained by Molecular Beam Epitaxy grown on miscut Si substrates and SOI(111) single crystal. The growth parameters will be adjusted to improve the layer quality measuring in situ the diffraction pattern by Reflection High Energy Electron Diffraction. In addition the layer structure and ferroelectric domains will be analyzed through ex situ HRTEM measurements and X-ray diffraction.

Contexte de travail

The project is part of an ANR national project FETh in close collaboration with Institut Néel (Grenoble) and IPCMS (Strasbourg). The doctoral contract is already funded.
The « Surface and structure team » at the CINaM lab has already a strong expertise on GeTe thin films [15-17]. The experimental setup combines a MBE growth chamber coupled under UHV to LEEM and STM apparatus. The CINaM lab is equipped with HRTEM for structural characterization.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.

Contraintes et risques

None

Informations complémentaires

None