Tuning Thermal Induced Porous-Ge Reconstruction for Layer Transfer and Substrate Re-use

Owing to their high efficiency, and heat and radiation resistance, III-V semiconductor multi-junction solar cells are dominating the space PV market. However, a lot of work has still to be done in terms of mass and cost reduction. Accordingly, reliable reduction of the substrate thickness can be obtained by solar cell detachment and substrate reuse allowing reducing both solar cells' weight. and cost. The use of porous germanium as weak layer for solar cell detachment is one of the most promising approaches ensuring scalability and cost-effectiveness. In seek of Ge substrate design providing both epitaxial seed layer and voided weak layer underneath suitable for III-V materials growth and subsequent detachment, we provide systematic investigation of thermal induced reorganization of porous germanium with various porosity levels and thicknesses. Indeed, high porosity structure shows fast reconstruction rate with increasing the thermal budget testifying its aptitude to form controllable voided separation layer. Meanwhile, low porosity structure’ reconstruction is found to be mediated by pores transformation to faceted small voids, giving rise to monocrystalline material with stable thickness potentially useful as a template for epitaxial growth. Epitaxial template on weak layer design with tunable morphological and mechanical properties has been fabricated by considering a structure with gradual low porosity on top to high porosity in depth. Almost non-porous, suspended thin Ge layer connected to the bulk substrate trough pillars of few tenths of nm in diameter with micrometer scale spacing, has been successfully demonstrated. Our results show that Ge layer with gradual porosity constitute a viable approach for solar cell detachment offering tunable properties depending on the porous layers thicknesses and porosity.