Impact of Thermal Annealing on the Mechanical Properties of Ge epilayer on Mesoporous Germanium for Layer Separation and Substrate Re-use

Summary form only. III-V multi-junction solar cells on Ge substrate are widely used for space PV applications. However, challenges related to weight and cost reduction are still encountered. Porous Ge (PGe) lift-off is one of the reliable approaches allowing solar cell detachment (mass reduction) while offering substrate multiple reuses (cost reduction). Accordingly, the adhesion of the epilayer to the Ge substrate through PGe reconstructed weak layer (nm scale pillars) needs to be investigated and optimized to offer scalable and controllable detachment. In this context, detachable Ge epilayer on porosified 4" Ge substrate has been grown. The weak layer mechanical properties dependence on the post-growth thermal annealing at various temperatures (from 600ºC to 750ºC) is experimentally and numerically investigated. Pull test experiments were performed to measure the adhesion of the weak layer for each annealing temperature. Pillar densities and diameters were determined precisely for each temperature using plan view SEM observations of the detached substrate. The pillars density is found to decrease in favor of an overall diameter increase as the annealing temperature increases. The values are used as input for a 3D pull test model to simulate the detachment process and to study the influence of the pillar characteristics on the epilayer adhesion. Our results show that the epitaxial Ge template is very stable even at high annealing temperature while keeping low adhesion force allowing mechanical detachment which is very suitable to grow detachable multi-junction solar cell.