Ge(100) single-crystal films, up to 30 μm thick, have been obtained by molecular-beam epitaxy (MBE) on a CaF2(100) or BaF2(100) single-crystal film substrate, 2000 A thick, grown in turn by MBE on a sapphire (1102) single-crystal wafer. By dissolving the CaF2or BaF2in 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 CaF2or BaF2substrate and were composed of three-dimensional nuclei. The sharp Kikuchi lines in the RHEED patterns demonstrated their high perfection. The final film of 3–30 μm 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 1015cm “3and the mobility 1300 cm2V-1S-1at room temperature.
|Number of pages
|Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films
|Published - Sep 1991