THE EPITAXIAL-GROWTH OF GE SINGLE-CRYSTAL FILMS ON A CAF2 SAPPHIRE SUBSTRATE

Ge(100) single-crystal films, up to 30-mu-m thick, have been obtained by molecular-beam epitaxy (MBE) on a CaF2 (100) or BaF2 (100) single-crystal film substrate, 2000 angstrom thick, grown in turn by MBE on a sapphire (1102BAR) single-crystal wafer. By dissolving the CaF2 or BaF2 in water, this double epitaxy procedure allowed the recovery and the subsequent reuse of the expensive sapphire substrate. This is of interest for large area devices, such as solar cells. The Ge single-crystal films served as substrates for the growth of GaAs single-crystals by metalorganic chemical vapor deposition (MOCVD). Furthermore, by doping during growth, tandem double energy gap photovoltaic junctions, which may be suitable for high-efficiency solar cells, may be obtained. The growth conditions, orientation relationship, crystalline quality and microstructure of these films was examined by in situ reflection high-energy electron diffraction (RHEED) and by transmission electron diffraction and microscopy and by scanning electron microscopy. The Ge films grew in a parallel orientation to that of the CaF2 or BaF2 substrate and were composed of high perfection. The final film of 3-30-mu-m thickness had a mirror like appearance and was free of cracks due to thermal stresses. The intrinsic p-type carrier concentration was 2.2 x 10(15) cm-3 and the mobility 1300 cm2 V-1 s-1, at room temperature.