Single-crystal-like germanium thin films on large-area, compliant, light-weight, flexible, single-crystal-like...

Germanium (Ge) films were heteroepitaxially grown on flexible, large-area, single-crystal-like metallic substrates. Multiple, heteroepitaxial, buffer layers of nanoscale dimensions were deposited on the triaxially textured, single-crystal-like, thermo-mechanically 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 at half-maximum (FWHM) of the Ge (400) rocking curve was 0.93°. The Ge (111) ϕ-scan showed a FWHM value ∼4°. Based on the X-ray ω-scan, ϕ-scan and (111), (110), and (001) X-ray pole-figures, the Ge film deposited on the flexible, metallic substrate had a cube-on-cube heteroepitaxial relationship with the single-crystal-like metallic substrate. 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. Ge films grown on these flexible metal substrates exhibited electron mobilities in the range of 175-250 cm²V^-1s^-1. Such single-crystal-like semiconductor films on low-cost, flexible, large-area, scalable, single-crystal-like metallic substrates could potentially enable high-performance electronic devices for a range of applications.