Croissance épitaxiale de Germanium et de matériaux III-V sur des substrats de Germanium mésoporeux, pour des applications photovoltaïques

In recent years, our society has been facing a continuous growth in energy demand across various sectors such as transportation, telecommunications and more. To meet this demand and gradually replace the use of fossil fuels (oil, gas, coal, etc.), the renewable energy sector is increasingly in demand. Among the available renewable energies, photovoltaic energy has undergone numerous advancements, especially in the civil and space domain. Currently, the most efficient solar cells are the III-V multijunctions type solar cells, based on Germanium (Ge) and/or Gallium Arsenide (GaAs) substrates. With this type of cell, efficiencies exceeding 45% can now be achieved. However, their use is limited to niche markets such as space industry due to their high production costs. Ge is considered as a rare material, significantly impacting cell prices. Additionally, the substrates used typically have thicknesses ranging from 150 µm to 180 µm, while only a few micrometers would be necessary to maintain cell performances. This substantial thickness, representing about 95% of the total weight, poses a significant limitation in popularizing cells in sectors where weight reduction is a challenge, such as in space or automotive applications. Furthermore, these substrates cannot be reused, resulting in a substantial loss of materials. Therefore, it is essential to find a reliable, cost effective and industrially scalable approach to reduce the amount of material used and enable the reuse of the Ge substrate. In the context of this thesis, we optimized the homoepitaxy approach of Ge on a porous Germanium monolayer at low and high temperature, now known as PEELER approach. This innovative approach demonstrated the creation of an epitaxial growth template in the form of a detachable Ge membrane with monocrystalline quality and low surface roughness (< 1 nm), enabling the growth of high quality III-V materials. It will also be shown that the substrate can be reused multiple times to repeat the process. The parameters used for this demonstration arise from the study of the impact of various factors, such as the Ge growth temperature and the thickness of the mesoporous Ge layer, on the porous structure reorganization and the membrane adhesion strength. It was observed that increasing the growth temperature leads to more significant porous structure reorganization, resulting in an increased membrane adhesion strength. By varying the porous Ge layer thickness, it was demonstrated that a wide range of Ge membrane adhesion strength can be obtained, ranging from a detachable membrane with a simple adhesive strip, with a low adhesion strength, to a nondetachable membrane. This study has shown the possibility of controlling the adhesion strength of the Ge membrane, opening up the potential use of the PEELER approach in other domains beyond that of solar cells.

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