Solar energy, which is renewable, readily available and accessible to all, seems to possess all the qualities to become tomorrow’s energy. However, the improvement of photovoltaic technologies is crucial to promote solar cells as a major source of electricity. The goal of the so-called “third generation” cells, is twofold: affordability (abundant material, production techniques), and maximum efficiency. A way to achieve this goal is the insertion of semiconductor nanocrystals in a dielectric material by ion implantation, followed by annealing. Such systems should allow the generation of multiple excitons per incident photon, and thus promote cell efficiencies. Amongst the materials considered, silicon nanocrystals (Si-nc) embedded in silica are favorites, and germanium nanocrystals (Ge-nc) are full of advantages in terms of solar absorption. In this fundamental work, we have investigated the synthesis parameters and mechanisms of formation of these nanocrystals. We have combined ion beam analysis with physicochemical characterizations by XPS and μ-Raman, electron microscopy, optical characterizations by ellipsometry and photoluminescence spectroscopy, and have approached the study of photovoltaic properties of these materials. The promise delivered by Si nanocrystals was initially evaluated. We have implanted Si into SiO2 at fluences of 6 − 20 × 1016 Si.cm-2, followed by different annealing times in N2 at 1100 ° C. The formation of Si-nc whose size depends on the local implanted concentration was observed by TEM and PL, and measuring their IV response provided evidence of a photovoltaic effect in these structures. Depth profiling of Si was made possible by the development of an ion beam analysis method, based on BS and NRA, in order to profile easily Si ions implanted in a matrix containing Si. The turn of Ge nanocrystals in silica came in a second time : we have joined ion beam analysis to SEM and μ-Raman spectroscopy to reveal the desorption of Ge during annealing ( up to 65%), as well as the formation of Ge-nc and large spherical nanocavities at temperatures above the melting point of Ge, of diameters between 4 and 35 nm. The mechanisms of formation of these structures were revealed. A lower desorption of Ge was observed in thin films of SiO2/Si characterized by RBS and XPS. Migration of Ge to preferential sites at the interface was revealed. Third, co-implantation of Si prior to Ge provided a solution of prestige to the desorption of Ge. We have demonstrated, by RBS characterizations, a trapping of Ge by Si atoms introduced in excess, with a linear increase in the amount of Ge retained with the fluence of implanted Si, far as to retain the amount of Ge measured before annealing. In parallel to these characterizations, we have explored a new way for the characterization of Si-nc and nc-Ge: ionoluminescence (IL), or the study of light emissions in the ion beam. This method has revealed a strong potential for the characterization of damage to nanostructures and transfer of carriers in these materials in the case of Si-nc, and has revealed numerous agreements with μ-Raman and XPS characterizations for Ge-nc, allowing also a study of the charge transfer mechanisms in PL of these structures.
Synthèse et caractérisation fondamentale de nanocristaux de silicium et germanium synthétisés par implantation ionique pour le développement de capteurs photovoltaïques à haut rendement - Développement de l’ionoluminescence
Demarche, J. (Author). 28 Aug 2012
Student thesis: Doc types › Doctor of Sciences