Heteroepitaxial Ge and GaAs terminated, flexible, large-area, single-crystal-like substrates for device applications including III-V solar cells |
Amit Goyal1,2, Gokul Radhakrishnan2, Kyunghoon Kim2, Ravi Droopad3 1State University of New York (SUNY) at Buffalo, Buffalo, NY, United States /2TapeSolar Inc., Knoxville, TN, United States /3Texas State University, San Marcos, TX, United States |
High-quality, III-V epitaxial thin-films deposited on single-crystal substrates have yielded extremely high performance, but are extremely expensive and rigid. Here we demonstrate heteroepitaxial deposition of Ge and GaAs thin-films on large-grained, single-crystal-like, biaxially-aligned, flexible, metallic substrates. First, we demonstrate heteroepitaxially grown Ge films on flexible, very large-area, single-crystal-like metallic substrates. Multiple, heteroepitaxial, buffer layers of nanoscale dimensions are deposited on the triaxially textured, single-crystal-like, thermomechanically processed Ni-W alloy substrates. Ge films were deposited on a CeO2-terminated, heteroepitaxial buffer stack on the metallic substrate using electron beam evaporation. X-ray diffraction θ-2θ scans showed a very strong Ge (400) peak and the full-width-half-maximum (FWHM) of the Ge (400) rocking curve was 0.93°. The Ge (111) ϕ-scan showed a FWHM value ~ 4°. Reflection-high-energy-diffraction (RHEED) patterns from the Ge layer was streaky indicative of a smooth and essentially single-crystal-like Ge film. Cross-section TEM examination revealed a sharp interface between the Ge film and the topmost buffer layer, CeO2, with a low defect density. The CeO2 layer serves as a highly compliant layer that modulates its lattice parameter to attain excellent lattice-matching to the heteroepitaxial Ge layer. Second, we demonstrate heteroepitaxial deposition of GaAs thin-films on large-grained, single-crystal-like, biaxially-aligned, flexible, metallic substrates. We use molecular beam epitaxy (MBE) for the controlled growth of high quality GaAs layers on lattice matched Ge capped, flexible metal substrates. The structural, optical, interfacial and electrical characteristics and properties of the heteroepitaxial GaAs layers are analyzed and discussed. The results show that heteroepitaxial GaAs layers with good crystalline and optoelectronic properties can be realized for flexible, III-V based semiconductor devices. III-V materials integrated on large-grained, single-crystal-like, flexible, metallic substrates offer a potential route towards fabrication of large-area, high-performance electronic devices including III-V solar cells. |