Optimization of a single crystalline-like germanium thin film growth on inexpensive flexible substrates and fabrication of germanium bottom junction

III-V semiconductor multijunction solar cells utilizing a germanium (Ge) bottom junction show high efficiency, however, are limited to the expensive Ge single-crystal wafers. Our approach focuses on thin film-based multijunction photovoltaics to replace Ge wafers by depositing single-crystalline-like Ge thin film on flexible and inexpensive metal tapes. The deposition temperature and film thickness were optimized to achieve optimal surface roughness, carrier mobility and carrier concentration. It has been found that 850°C is an optimum deposition temperature to achieve the sharpest texture, the highest carrier mobility and film smoothness. The surface roughness decreases while hall mobility increases with increasing Ge layer thickness up to 1.25 μm. Finally, a p-Ge/n-Ge bottom junction was fabricated based on the optimized Ge thin film.