Impact of Thermal Annealing on Porous-Ge Reconstruction for Layer Transfer and Substrate Re-use

Owing to their high efficiency, III-V multi-junction solar cells on Ge substrate are widely used for space PV applications. However, weight and cost reduction are still encountered. Porous Ge (PGe) lift-off is a reliable approach for solar cell detachment allowing Ge mass reduction and offering substrate reuses. In seek of Ge substrate structure providing both epitaxial seed layer and voided weak layer underneath suitable for III-V materials growth and subsequent detachment, we provide detailed investigation of thermal induced reorganization of porous germanium with different conditions. Annealing step is crucial to have controllable reorganization of PGe. We varied the annealing thermal budget (temperature and time), the annealing atmosphere composition oh H2 and the annealing technique (furnace). We demonstrate fast reorganization of only 4 s with continuous laser annealing allowing the creation of epitaxial template for III-V on weak layer by considering a structure with low porosity on top to high porosity in depth. Suspended thin Ge layer connected to the bulk substrate trough pillars of few tenths of nm in diameter, has been successfully demonstrated.