Microstructure and Electrophysical Properties of YBa₂Cu₃O_7–δ Films

Abstract

The quest for very rapid practical utilization of high-temperature superconducting (HTSC) films has driven many research groups thoroughly to study their properties. An important part of this activity is the investigation of the growth mechanism of HTSC layers and the effect of the microstructure on their electro-physical properties. The perfection of the crystalline structure of the grains and the presence of intermediate phases seem to determine the superconducting transition temperature and the width of the critical transition [1]. Furthermore, the size of the critical current depends on the state of the intergrain boundaries, i.e., the presence at them of amorphous interlayers, gaps, etc. and on the twinning structure and the orientation of the film relative to the direction of the transport current flow [2–5, 7]. In many studies, the films are epitaxial and the substrates have an orienting effect. On (001)SrTiO₃ substrates, the c axis is usually perpendicular to the substrate although a horizontal placement has also been noted [3, 4, 6, 7]. Film orientations for which the a, b, and c axes are parallel to {100} SrTiO₃, i.e., the c axis of YBa₂Cu₃O_{7_δ} (YBCO) is parallel to the substrate plane and at a 45° angle to it, have been observed [8]. Several efforts involved an investigation of the capabilities of controlling the oriented growth of HTSC films [5, 9, 10]. It has been reported [6, 11] that the growth rate of YBCO films in the c direction is much less than that in the direction of the small constants. Interdiffusion at the film-substrate junction has been studied. The diffusion becomes perceptible for SrTiO₃ at T _s ∼ 900°C [12]; for Si and A1₂O₃, atT _s ≤ 600°C [13].