Microstructure studies of epitaxial YBa2Cu3O7–δ films

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Abstract
Thin films of YBaCuO were prepared as a superlattice of three constituents on SrTiO3 (100) substrates from three electron guns using a dedicated, computer‐controlled evaporator. The results indicate that the as‐deposited thin films grow as a superlattice of the three constituents, with a mixture of amorphous and polycrystalline regions. After annealing, however, the multilayer films are converted to the homogeneous superconducting phase; the morphology, studied with transmission electron microscopy, consisted of an array of orthogonal interconnecting rectangular bars. The better films had Tc(R=0)=90 K and Jc=2.9×106 A/cm2 at 4.2 K. The selected area electron diffraction pattern, and the high‐resolution electron microscopy images show that the interconnecting bars are single crystals of the 1‐2‐3 phase. During annealing, the growth rate along the a and b directions is faster than along the c direction. For the films with the a axis oriented perpendicular to the substrate, the grains nucleated directly on the (100) surface of the SrTiO3 and grow through to the film surface. The grain boundary of two interconnecting rectangular crystals (bars) is abrupt at the atomic level over most of the boundary. The superconducting films and the substrates have a well‐defined epitaxial relationship; the b and c axes are parallel to the in‐plane 〈001〉 substrate axes for the a‐axis oriented films. Depending on the film preparation conditions, the c‐axis can also be perpendicular to the substrate plane. The morphology of samples with poorer superconducting performance consists of somewhat randomly oriented crystal bars of the 1‐2‐3 phase.