N-doped oxides and/or oxinitrides constitute a booming class of compounds with a broad spectrum of useable properties and in particular for novel technologies of carbon-free energy production. Indeed, the insertion of nitrogen into the crystal lattice of a semiconductor oxide allows, in principle, to modulate the value of its band gap or to introduce additional electronic states and thus to obtain new functionalities and optical properties. The production of oxynitride single crystalline thin films is highly challenging. In this essentially experimental thesis work, thin films of oxynitrides will be developed by atomic plasma-assisted molecular beam epitaxy. We will start from BaTiO3, which synthesis is well mastered in the laboratory, to realize co-dopings with nitrogen and compensating metals in order to preserve the neutrality of the elementary unit cell. The resulting structures will be studied for their chemical compositions, crystalline structures and ferroelectric characteristics. These observations will be correlated with their performance for the photo-electrolysis of water, which allows the virtuous production of hydrogen. Finally, the corrosion resistance of these new materials will also be studied.
The student will acquire skills in a wide range of ultra-high vacuum techniques, molecular beam epitaxy growth, clean room lithography, ferroelectric measurements and photo-electrolysis of water, as well as in state-of-the-art synchrotron radiation techniques.

N-doped oxides and/or oxinitrides constitute a booming class of compounds with a broad spectrum of useable properties and in particular for novel technologies of carbon-free energy production, surface coatings for improving the mechanical strength of steels or protection against corrosion and multifunctional sensors. In this research field the search for new materials is particularly desirable because of unsatisfactory properties of current materials. The insertion of nitrogen in the crystal lattice of an oxide semiconductor allows in principle to modulate its electronic structure and transport properties enabling new functionalities. A detailed understanding of these aspects requires materials that are as perfect as possible. The production of corresponding single crystalline thin films is however highly challenging. In this thesis work, single crystalline oxynitride heterostructures will be grown by atomic plasma-assisted molecular beam epitaxy. The heterostructure will combine two N doped layers: a N doped BaTiO3 will provide ferroelectricity and a heavily doped ferrimagnetic ferrite whose magnetic properties can be modulated using N doping to obtain new artificial multiferroic materials better suited to applications. The resulting structures will be investigated with respect to their ferroelectric and magnetic characteristics as well as their magnetoelectric coupling, as a function of the N doping. These observations will be correlated with a detailed understanding of crystalline and electronic structures. The later will be modelled thanks to electronic structure calculation to reach a comprehensive description of this new class of materials.

The student will acquire skills in ultra-high vacuum techniques, molecular beam epitaxy, ferroelectric and magnetic characterizations as well as in state-of-the-art synchrotron radiation techniques. X-ray magnetic dichroism is particularly suited to this study and the project will give rise to close collaboration and/or co-supervision with the DEIMOS beamline of SOLEIL synchrotron.