Large-grain (>1-mm), recrystallized germanium films on alumina, fused silica, oxide-coated silicon substrates for III–V solar cell applications

Abstract

Recrystallized germanium films on ceramics and other materials can serve as a low-cost replacement for the single-crystal germanium wafers currently used for high-efficiency III–V semiconductor solar cells. Related work implies that Ge films with large-grain sizes (>1 mm) formed on thermal-expansion matched substrates, such as sintered alumina ceramic, would function as suitable alternative substrates for epitaxy of high-performance GaAs-based solar cells. For this purpose, we describe a process wherein Ge layers, 0.5–5 μm thick, are deposited by close-spaced vapor transport or sputtering on sintered alumina ceramics, fused silica, or thermally oxidized polysilicon substrates, capped with metal layers deposited by electron-beam evaporation, and recrystallized in an annealing step. Post-growth, the metal cap is stripped with a selective liquid etchant. Recrystallized Ge films have large (1–5 mm) grains and highly oriented textures. These structures provide suitable substrates for epitaxy of GaAs solar cells.