Buffer layers on silicon for growth of YBCO films

Pulsed laser deposition is a powerful technique for the epitaxial growth of superconducting oxide films on numerous oxide substrates. Until recently, epitaxy on silicon substrates was made impossible by the chemical reactivity and thermal expansion mismatch of silicon and superconducting oxides. By interleaving an epitaxial buffer layer of yttria-stabilized zirconia in a fully in-situ laser deposition process, we have addressed both of these problems to grow c-axis oriented, epitaxial Y 1 Ba 2 Cu 3 O 7-δ (YBCO) films on silicon. X-ray diffraction indicates an in-plane, epitaxial alignment within 2°. The films not only have high superconducting transition temperatures of 88 K but also have critical current densities of 2.2×10 6 A/cm 2 at 77 K. Transition widths are 1 K and normal state resistivities are 0.28 mOhm-cm at 300 K. Additional studies of YBCO on silicon-on-sapphire show that the limit on thickness of about 50 nm, set by thermal strain, for oxide films on bulk silicon can be overcome, and films with thicknesses far beyond this critical thickness have been grown. The surface resistance of these films at 4.2K and 11.8GHz is 72 μω cm and the critical current at 77 K is 4.6×10 6 A/cm 2 . The values of the above parameters are now high enough to enable the fabrication of micro-electronic devices with superior performance.