Germanium Nano-Membrane Growth on Porous Germanium with Unpolished Bulk Substrates for Substrate Reuse

Multi junction solar cells (MJSC) account for different materials used as semiconductors in their multi-layer structure. Among these layers is the porous germanium (Ge) substrate that plays a mechanical role and is the template for the growth of the active layers. These Ge substrates account for a substantial portion of the production cost in MJSC for two reasons, (i) Ge is a critical raw material, (ii) the production cost of these Ge substrates is due to some complicated steps (e.g., polishing). To address this issue, we studied the feasibility of utilizing unpolished (surface-ground and as-cut) Ge wafers as cost-effective alternatives to standard epi-ready substrates for Ge nanomembrane (NM) growth. The porous lift-off approach was successfully applied to 4-inch surface-ground and as-cut Ge wafers, yielding homogenous porous Ge (PGe) layers that served as templates for monocrystalline Ge NM epitaxial growth, preserving the orientation and crystalline quality of the parent substrate. Our results show the comparable surface quality obtained on lower-cost unpolished Ge wafers with the same conditions usually applied to epi-ready wafers. Therefore, these results pave the way for further studies in the challenges of III-V growth on rough surfaces for MJSC that may lead to the use of unpolished wafers in the pursuit of further reducing the high cost in solar cells for space application.