Materials technologies for making GaAs-based solar cells on low-cost ceramics

Large-grain (>1-mm) germanium-on-ceramic substrates are developed for GaAs solar cells. Thin (2- to 5- micron) films of germanium are deposited on thermal-expansion matched polycrystalline alumina (Al/sub 2/O/sub 3/) substrates coated with a tungsten film that promotes wetting and adhesion. The Ge films are capped with various metal and oxide films, and then recrystallized in a rapid thermal processing (RTP) system. Average grain sizes greater than 1 mm are achieved. Epitaxial layers of GaAs epitaxy are grown on these structures using close-spaced vapor transport (CSVT) and liquid-phase epitaxy (LPE).