NiO layers on Ni RABiTS for epitaxial buffer deposition by LS MOCVD

Abstract
The direct LS MOCVD deposition of oxide buffer layers, e.g. YSZ, CeO₂ and Y₂O₃, on Ni RABiTS tapes leads to epitaxially grown layers with columnar structure. This columnar structure is unsuitable for epitaxial growth of multilayers. Further, the layers are not dense enough so that Ni diffuses along the grain boundaries to the surface. By this means Ni interferes the superconducting properties of the YBCO layers deposited thereon. In order to suppress this detrimental effect of Ni on YBCO a NiO layer was used as epitaxial basis for the subsequent deposition of oxide buffer layers. It was shown that biaxially oriented NiO layers can be produced by oxidation of pure Ni RABiTS and of Ni tapes micro-alloyed with 0.1 at.-% Mo or W, respectively, as well as alloyed with 5 at.-% W. The cube textured NiO layers achieved FWHM values down to 4.7°. The sharpness of cube texture of the NiO layers was strongly dependent of the substrate texture and the grain size of the substrate material as well as the processing conditions during oxidation. The resulting NiO layers had a characteristic smooth surface with a small fraction of grains having a columnar structure. The transfer of the NiO texture and structure to the epitaxially deposited YSZ and Y₂O₃/YSZ layers was achieved at deposition temperatures of 650° C and 600° C, respectively. The differences in the FWHM values between Ni RABiTS tapes and oxide layers grown on them were minimum, not more than 1°. YBCO layers deposited by LS MOCVD on YSZ/NiO/0.1at.-%W Ni RABiTS tapes proved critical current densities up to 0.5 MA/cm² at 77 K with critical temperatures of T_c = 90 K.