Enabling technologies for making GaAs-based thin-film solar cells on ceramic and polysilicon substrates

We present results for the first phase of an effort to develop large-grain (>1-mm), highly-oriented, thin (0.5 to 5 micron) films of germanium on substrates of: 1. fused silica (SiO₂), 2. sintered alumina (Al₂O₃) ceramic, or 3. low-cost polysilicon sheet (Silicon-Film™) material. We use a water-vapor mediated, close-spaced vapor transport (CSVT) process to deposit Ge, followed by a recrystallization step. An alternative chemical vapor transport process using iodine vapor is also being developed for low-cost deposition and epitaxy of Ge and GaAs. Ge films with a highly oriented texture and lateral dimensions of grains in excess of 1-mm have been achieved on fused silica, alumina ceramic, and polysilicon substrates. These structures are ultimately intended for use as Ge (coated) surrogate substrates for epitaxial growth of high-performance GaAs/InGaP solar cells.