Fabrication of ultrathin Ge template for growth of multijunction solar cells based on wafer-scale porous Ge

Multijunction solar cells (MJSC) currently hold the highest efficiency on the market. However, their widespread in terrestrial applications is getting held back by the high devices cost. A considerable part of the cost mainly comes from the substrate materials such as Ge and GaAs making them nonviable for terrestrial application compared to much cheaper silicon-based solar cells. Consequently, the Ge based MJSC deployment is restrained to niche domains such as spatial applications. Accordingly, the development of nanostructured substrates allowing MJSC detachment and wafer reuse stands out as a promising approach to overcoming these limitations. In this work, we demonstrate the formation of homogenous edge-to-edge porous Ge (PGe) layers on an industry-standard 100 mm wafer-scale produced by bipolar electrochemical etching. The produced nanostructured substrates' properties are easily assessable by production line compatible, fast, and nondestructive techniques such as ellipsometry. The PGe layers have been found to exhibit excellent uniformity over the wafer' surface with a relative variation of 1% in porosity and 2% in thickness. Furthermore, we show that the PGe structural properties can be finely tuned to create on-demand characteristics including the suitability for epitaxial growth. Accordingly, low-temperature growth of ultrathin crystalline Ge layer on top of PGe structure is demonstrated. The fabricated structure has been shown to be compatible with III-V heterostructures growth drawing the way for wafer-scale detachable MJSC and substrate reuse.