High critical currents in Y-Ba-Cu-O films on silicon using YSZ buffer layers

Exceptionally high-quality films of Y₁Ba₂Cu ₃O_7-x (YBCO) were successfully grown epitaxially on Si(100) wafers with a buffer layer of yttria-stabilized zirconia (YSZ) using a fully in-situ pulsed laser deposition (PLD) process. Critical current densities of a 30-nm-thick film are 2×10⁷ at 4.2 K and 2.2×10⁶ at 77 K. Zero-resistance critical temperatures are about 87 K, the transition width is 1 K, and normal-state resistivity is 0.28 mΩ-cm at 300 K. X-ray diffraction phi scans indicate in-plane epitaxial alignment within 1.0° and 2.0° for YSZ and YBCO, respectively. Lattice constant and thermal expansion mismatches occur at both subsurface interfaces. causing strain cracks to occur in YBCO films thicker than about 50 nm. The Si surface preparation utilizes an oxide etch and hydrogen termination that is an essential step in the process. as is careful control of the deposition environment temperature and oxygen pressure. The crystal quality of YSZ buffer layers on Si is not degraded by decomposition reactions, as is YBCO directly on Si or SiO₂, and cube-on-cube orientation of the YSZ cubic fluorite on Si(100) surfaces can be made to occur very effectively