Novel, single-crystalline-like templates on low-cost, flexible substrates for high efficiency photovoltaics

A novel template of single-crystalline-like germanium has been developed on low-cost, flexible metal substrates for growth of high efficiency photovoltaics. The template technology is enabled by the process of ion beam assisted deposition (IBAD) of MgO followed by epitaxial deposition of a series of oxide films. The IBAD process as well as all epitaxial deposition processes up to the Ge layer was conducted by reel-to-reel continuous processing. A high degree of biaxial texture is achieved in the Ge film when it is grown epitaxially on intermediate epitaxial fluorite films such as CeO₂ which have a good basal-plane structural match with Ge. The in-plane texture is improved to 1.5° full-width-at-half-maximum (FWHM) i.e. grain-to-grain misorientation of about 1° when the Ge film thickness is increased to 6 μm. Etch-pit measurements show a defect density in the Ge film in order of 10⁸ cm^-2. Cross-sectional Transmission Electron Microscopy analysis show that the defects are primarily concentrated near the Ge-CeO₂ interface and that the defects could be propagating from layers underneath. The defect density is seen to be considerably reduced in the top part of thicker Ge films. Epitaxial GaAs has been successfully grown on the Ge film on metal substrate by molecular beam epitaxy.