Epitaxial (La, Sr)TiO₃ on textured Ni–W as a conductive buffer architecture for high temperature superconducting coated conductor

Abstract

The formation of epitaxial (La, Sr)TiO₃ on biaxially-textured Ni–W metal alloy tape was examined as a conductive buffer layer for YBa₂Cu₃O₇ film growth. A TiN epitaxial nucleation layer served as an effective template layer for subsequent (La, Sr)TiO₃ film growth on the Ni–W tape. The TiN/(La, Sr)TiO₃ multilayer was epitaxial on the Ni–W substrate as confirmed via four-circle X-ray diffraction. A high temperature superconducting YBa₂Cu₃O₇ film was deposited epitaxially on the (La, Sr)TiO₃/TiN/Ni–W tape. The YBa₂Cu₃O₇ film exhibited a superconducting transition temperature of 86 K and critical current density of 0.42 × 10⁶ A/cm² at 77 K.

Introduction

Coated high temperature superconducting conductors, based on the epitaxial growth of YBa₂Cu₃O₇ on biaxially-textured metal tape, is presently being pursued as a viable means of achieving superconducting wires for high field, high temperature applications [1], [2], [3], [4]. This technique, known as Rolling Assisted Biaxially-Textured Substrates (RABiTS) is dependent on translating a sharp biaxially-texture in a metal tape [5] into a superconducting oxide film through the growth of an epitaxial buffer layer architecture [6], [7]. In this way, detrimental effects of high angle grain boundaries on critical current in cuprate superconductors can be eliminated [8], [9]. For some conductor configurations, there is interest in maintaining normal metal conductivity between the superconducting film and the metal substrate. This requires the development of conducting buffer layer architectures [10], [11]. (La, Sr)TiO_3+x is an interesting candidate material for a conducting buffer layer configuration. LaTiO_3+x exhibits metallic behavior over a wide temperature range with an oxygen stoichiometric of 0.1 < x < 0.25 [12]. With a pseudo-cubic lattice parameter that is well matched with YBa₂Cu₃O₇, LaTiO_3+x is an attractive buffer layer material for coated conductor application. Previous activities with this material mapped out the relationship between the resistivity in LaTiO_3+x films and the oxygen pressure used during growth. The results illustrated how oxygen content plays a crucial role in conducting properties [13]. Cation doping in the compound can largely alleviate the oxygen sensitivity of LaTiO_3+x. Doping with a divalent element increases the Ti³⁺/Ti⁴⁺ ratio and can make the compound less sensitive to ambient oxygen pressure during deposition. Electrical conductivity becomes a function of the La/Sr ratio in the La_1-xSr_xTiO₃ compound [14]. Previous work showed that the room temperature resistivity of La_0.5Sr_0.5TiO₃ remained below 1.0 × 10⁻³ Ω cm for deposition in an oxygen pressure of 10⁻⁴–10⁻² Torr [15]. In functioning as a conducting buffer layer in a coated conductor structure, the conductive oxide layer architecture must satisfy specific criteria. First, the buffer layer must be reasonably well lattice matched to both the metal substrate and the superconducting film, thus enabling epitaxy. Second, the material must be conductive not only as-deposited, but after subsequent HTS film growth and oxygen annealing. Third, the interaction between the buffer layer and the metal substrate must be such as to minimize formation of any native interfacial oxide that would serve as an insulating barrier to shunted current flow. (La, Sr)TiO₃ appears to satisfy these criteria. At room temperature, LaTiO₃ possesses the orthorhombic GdFeO₃ perovskite structure with a = 5.604 Å, b = 5.595 Å, and c = 7.906 Å. The pseudo-cubic lattice parameter of 3.96 Å represents a relatively large (12%) lattice mismatch to Ni (a = 3.524 Å), although similar mismatched parameters have proven useful in other RABiTS architectures. La, Sr, and Ti have a high affinity for oxygen relative to Ni, suggesting that native NiO formation at the interface should be minimal. As such, metallic (La, Sr)TiO₃ film may be attractive for various electronic applications [16], [17], [18], including coated conductors based on epitaxial high temperature superconducting films deposited on metal tapes. In this paper, the growth and properties of (La, Sr)TiO₃ based buffer layer architectures employing TiN on Ni–W alloy substrates, introduced earlier [19], is described in detail.