Agathe VIRFEU: Modeling and characterization of the properties of new solar absorbers II-IV-N2 (II = Mg or Zn, IV = Ge or Sn)

Abstract:
To replace the massive use of polluting fossil fuels and to satisfy the growing energy needs of the world population, it is essential to develop and democratize the use of solar energy. In order to achieve this goal, new solar cells must be developed that combine high efficiency and low production costs with the use of environmentally friendly chemical elements. Nitride compounds of the II-IV-N₂ family are potential candidates since they are made of non-toxic and abundant elements (zinc, tin, magnesium, etc...). The growth of these materials was carried out in an ultra-high vacuum equipment, unique in Europe, which guarantee an excellent chemical quality of the materials. The research are focused on three ternary compounds which are ZnSnN₂, ZnGeN₂ and MgSnN₂ deposited by reactive magnetron sputtering. First of all, the effect of substrate bias has been studied during the growth of ZnSnN₂ films in order to decrease the oxygen contamination and improve the properties. Then, the growth of ZnGeN₂ at different substrate temperatures revealed differences in local compositions and structures due to zinc evaporation at high temperatures. Finally, the growth of a new compound: MgSnN₂ has been performed at different temperatures. In this study, the structure and the chemical environment of tin atoms were characterized with new techniques. Its properties are intermediate between the compounds derived from zinc, such as the band gap energy. The second specificity of this work is the understanding of the macroscopic phenomena by ab initio calculations of the electronic and optical properties. The theoretical predictions were then compared with the experimental results and showed a good agreement with the measured properties. This work provides new knowledge, which completes the previous studies and nuances those reported in the literature.

Composition of the jury:
> Reporters :
- Marie-Paule BESLAND, Directeur de Recherche, Institut des Matériaux de Nantes Jean Rouxel, Université de Nantes,
- Valérie POTIN, Maître de Conférences, Laboratoire Interdisciplinaire Carnot de Bourgogne, Université de Bourgogne Franche-Comté,
> Examiners :
- Angélique BOUSQUET, Maître de Conférences, Institut de Chimie de Clermont-Ferrand, Université Clermont Auvergne,
- Stéphanos KONSTANTINIDIS, Professeur, Laboratoire de Chimie des Interactions Plasma-Surface, Université de Mons,
- Sébastien LEBEGUE, Directeur de Recherche, Laboratoire de Physique et Chimie Théorique, Université de Lorraine
> Direction of thesis :
- Jean-François PIERSON, Directeur de thèse, Professeur, Institut Jean Lamour, Université de Lorraine
- Jean-Pierre VILCOT, Co-encadrant, Directeur de recherche, Institut d'Electronique de Micro-électronique et de nanotechnologie, Université de Lille 1