抄録
Aiming at the fabrication of SIS tunnel. Josephson junctions, complexity of c-/a-axis oriented YBa_2Cu_3O_x (YBCO) thin film growth with T_c=90 K is discussed from epitaxy points of view. The preferred orientation of YBCO thin films is strongly depending on growth parameters, namely growth temperature and oxygen partial pressure, independent of substrate materials. a-axis oriented films on perovskite-related substrates shows usually T_c as low as 30 K, which may be caused by 90°-domains where each c-axis intersects at right angles to each other. "Pure" a-axis oriented (c-axis in-plane aligned, a-axis oriented) films with a T_c of 90 K, which is though to be favorable to realize SIS struclure, are successfully grown on tetragonal K_2NiF_4-typed (100) substrates such as SrLaGaO_4 and Nd_2CuO_4. This epitaxy can be explained by a novel mechanism called Atomic Graphoepitaxy, which provides a coherent ( structural continuity and elctronical disconitinuity) interfaces nescessary to SIS tunnel junction fabrications. Thus grown "purc" a-axis oriented films on (l00) SrLaGaO_4 substrates consist of epiaxial grains surrounded by anti-phase and stacking-fault defects boundaries. The formation of such defect boundaries id closely connected with cobbled substrate surface during film depositio, and the atomic graphoepitaxy model gives a consistent full explanation of their formations. Efforts on the search of new K_2NiF_4-type substrate materials like SrLaGaO_4 are still needed. They are also usuiful to select insulating I-layer materials in SIS tunnel junctions, because K_2NiF_4-type materials allow coherent interfaces with structural continuity and electronical discontinuity.
