Second generation (2G), high-temperature superconducting (HTS) wires or coated conductors are based on epitaxial growth of YBa₂Cu₃O_7−δ (YBCO) films on biaxially-textured metallic substrates with multilayer buffers.^[Reference Goyal^1] The purpose of the buffer layers is to provide a continuous, smooth, and chemically inert surface for epitaxial growth of YBCO films, while transferring the biaxial texture from the substrate to YBCO. Buffer layers typically consist of several functional layers that include: (i) a seed layer (e.g., Y₂O₃), a good epitaxial oxide layer on biaxially-textured NiW substrate without the formation of undesirable NiO and NiWO₄ phases; (ii) a barrier layer (e.g., yttrium-stabilized ZrO₂, YSZ) to provide an excellent chemical barrier to prevent diffusion of Ni from the NiW substrate to the YBCO layer; and (iii) a thin cap layer (e.g., CeO₂) with lattice-matched with YBCO to provide chemical and structural compatibility with YBCO and to promote significantly cube-on-cube epitaxy of the YBCO layer.^[Reference Goyal^1] Since each layer performs a single function, multiple buffer layers are typically required to fulfill all the required functions. However, it is desirable to combine two or more functionalities into one layer and thereby, reduce the number of the layers in terms of minimizing processing steps and cost-effective manufacturing for 2G-HTS wires.^[Reference Wee, Shin, Cantoni, Zuev, Cook and Goyal^2, Reference Zhao, Pu, Li, Du, Zhou, Zhang, Yang, Wang, Sun and Cheng^3]

Ba₂RENbO₆ (BRENO, RE = rare earth elements) are a group of ordered double perovskites with cubic or distorted cubic cells depending on RE³⁺ ionic radius.^[Reference Henmi and Hiantsu^4, Reference Koshy, Kurian, Thomas, Yadava and Damdaran^5] Due to their excellent chemical inertness,^[Reference Koshy, Kurian, Thomas, Yadava and Damdaran^5, Reference Paulose, Koshy and Damodaran^6] BRENO are reported as the promising substrate materials and buffer layers for YBCO film growth.^[Reference Koshy, Kurian, Thomas, Yadava and Damdaran^5–Reference Sathiraju, Barnes, Varanasi and Wheeler^9] In addition, it has been recently reported that Ba₂YNb₂O₆ (BYNO) addition in YBCO films produces robust columnar defects comprised of self-aligned BYNO nanodots and results in strong flux-pinning and consequently, massive improvement in critical current density (J _c) over the entire field and angular ranges.^[Reference Wee, Goyal, Zuev, Cantoni, Selvamanickam and Specht^10, Reference Feldmann, Holesinger, Maiorov, Foltyn, Coulter and Apodaca^11]

In this study, we report on epitaxial growth of the BYNO film directly on biaxially-textured NiW substrates, and its robust role as a single buffer layer that performs multi-functionalities provided typically by all three seed, barrier, and cap layers, for epitaxial growth of high-quality YBCO films. Epitaxial BYNO films were directly deposited on biaxially-textured NiW substrates using pulsed laser ablation (PLD). Biaxially-textured NiW substrates were prepared by annealing as-rolled Ni–3at.%W tape in a reel-to-reel high-vacuum chamber under 3 × 10⁻⁷ Torr of H₂S at 1250 °C for 20 min to obtain the desired cube texture.^[Reference Goyal, Feenstra, List, Paranthaman, Lee, Kroeger, Beach, Morrell, Chirayil, Verebelyi, Cui, Specht, Christen, Martin and Schoop^12] Stoichiometric BYNO powder was synthesized by using commercially available powders of BaCO₃, Y₂O₃, and Nb₂O₅ with high purities over 99.9% via solid-state synthesis. The powder was pressed and sintered at 1450 °C in air for 12 h to prepare BYNO targets with 1″ diameter. BYNO films were prepared on NiW substrates via PLD with a KrF laser (λ = 248 nm) at a repetition rate of 10 Hz. Laser energy density and substrate-to-target distance were 1.6 J/cm² and 5 cm, respectively. The films were grown at the substrate temperature, T _s of 650 °C and in oxygen partial pressure, P(O₂) at or below 3 × 10⁻⁵ Torr to preclude surface oxidation of the substrates during the growth. On BYNO buffered NiW, YBCO films with 0.2 µm thickness were deposited by PLD at T _s of 760 °C and in P(O₂) of ~230 mTorr. After deposition, the films were in situ annealed at T _s = 500 °C and P(O₂) = 500 Torr, and ex situ annealed at 500 °C for 1 h in flowing O₂ gas after depositing sputtered Ag electrodes onto the films. The standard four-point probe method with a voltage criterion of 1 µV/cm was used for the transport measurements. Phase and textures of the films were characterized by the X-ray diffraction (XRD) method. Microstructures were characterized by atomic force microscopy (AFM) and transmission electron microscopy (TEM) carried out on a Hitachi HF-3300 TEM/STEM operated at 300 keV.

Figure 1 summarizes XRD results for an epitaxial BYNO film directly grown on NiW substrate, indicating that the film has excellent cube-on-cube epitaxy with improved out-of-plane textured compared with the substrate. The θ−2θ scan [Fig. 1(a)] shows that the film is strongly out-of-plane textured with only c-axis-oriented BYNO peaks and no additional peaks related to other oriented grains. Figures 1(b) and 1(c) illustrate (002) ω- and (110) φ-scans of the film and the substrate. Compared with the NiW substrate, BYNO film has significantly improved out-of-plane texture with much smaller Δω values, while the film has comparable in-plane texture with similar Δφ values around ~9°. The Δω values of the film were measured to have 3.3° (φ = 0°) and 4.5° (φ = 90°) which are more than 2° (φ = 0°) and 3° (φ = 90°) smaller than the values of NiW substrate. The improvement in out-of-plane texture of films directly deposited on the NiW substrates has been previously reported.^[Reference Cantoni, Goyal, Schoop, Li, Rupich, Thieme, Gapud, Kodenkandath, Aytug, Paranthaman, Kim, Budai and Christen^13, Reference Cantoni, Specht, Goyal, Li and Rupich^14] Cantoni et al. demonstrated the texture sharpening of epitaxial TiN layer grown on NiW substrates.^[Reference Cantoni, Goyal, Schoop, Li, Rupich, Thieme, Gapud, Kodenkandath, Aytug, Paranthaman, Kim, Budai and Christen^13] The c-axis tilting of the TiN layer toward the direction of the sample's surface normal, caused by a large lattice mismatch (~20%) between the  TiN layer and the NiW substrate, has been thought to give rise to the out-of-plane texture sharpening in the film. In general, the origin of the c-axis tilting by a large lattice mismatch, possibly leading to the texture sharpening, is ascribed to two types of microstructural mechanisms: step-mismatch mechanism and biased dislocation mechanism.^[Reference Cantoni, Specht, Goyal, Li and Rupich^14–Reference Olsen and Smith^16] When the lattice parameter of the film (a _f) is significantly larger than that of the substrate (a _s), a _f > a _s, the c-axis tilting toward the surface normal of the sample is likely to be occurred via the biased dislocation mechanism. In this case, misfit dislocations form readily during the nucleation and subsequent growth of the film, and the strong biased distribution of the dislocations with Burgers vector normal to the interface is responsible for the large tilts.^[Reference Cantoni, Specht, Goyal, Li and Rupich^14, Reference Olsen and Smith^16]

Figure 1.

(a) θ−2θ scan for an epitaxial BYNO film grown on a bare NiW substrate at 650 °C by PLD. (b) BYNO (004) and NiW (002) ω-scans for BYNO layer and NiW substrate of the sample. Left and right figures are ω-scans at rolling (φ = 0°) and transverse (φ = 90°) directions, respectively. (c) (220) φ-scans for BYNO layer and NiW substrate for the sample.

Similar to the TiN–NiW case,^[Reference Cantoni, Goyal, Schoop, Li, Rupich, Thieme, Gapud, Kodenkandath, Aytug, Paranthaman, Kim, Budai and Christen^13] a large lattice mismatch (~20%) between BYNO (a ₀/2 = 4.218 Å) and NiW (a ₀ ~ 3.524 Å) with the condition of a _f > a _s is expected to lead to the tilted epitaxy by the biased dislocation mechanism and consequently, sharpening the out-of-plane texture of the film. As shown in Fig. 2, the cross-sectional TEM images for BYNO film on NiW confirm that the texture sharpening indeed results from such tilted epitaxy by the biased dislocation mechanism. A TEM image shown in Fig. 2(a) shows diffraction contrast at the BYNO/NiW interface caused by the presence of misfit dislocations that promotes the tilted epitaxy of BYNO film. A high-resolution TEM image shown in Fig. 2(b) shows that BYNO film is ~5° tilted with respect to the surface normal of the NiW substrate toward the surface normal of the film, and the tilt initiates from incoherent BYNO–NiW interface.

Figure 2.

Cross-sectional TEM images for BYNO film grown on NiW substrate. (a) Low-magnification image shows the diffraction contrast at the BYNO/NiW interface caused by formation of misfit dislocations. In the inset of the figure, a selected area diffraction pattern of BYNO phase shows additional spots which correspond to (001) superstructure reflection indicating that BYNO structure is ordered. (b) High-resolution image indicates that BYNO film is ~5° tilted with respect to the surface normal of NiW substrate toward the surface normal of the film, and the tilt initiates from incoherent BYNO–NiW interface.

For subsequent growth of high-quality epitaxial films on top of BYNO buffer, smooth surface morphology of the BYNO film is required. As shown in Fig. 3, AFM images for ~0.2 µm thick BYNO film deposited on the NiW substrate show that the film has no surface particles and smooth surface morphology with root mean square (rms) roughness (R _q) of ~2.2 nm for 5 × 5 µm² scan and ~1.4 nm for 3 × 3 µm² scan.

Figure 3.

AFM images for BYNO film grown on NiW substrate. Scan sizes are 5 × 5 µm² (a) and 3 × 3 µm² (b).

To evaluate BYNO layer as a single multifunctional buffer layer for epitaxial growth of YBCO films on NiW substrates, ~0.2 µm thick YBCO films were subsequently deposited on BYNO (~0.2 µm) single-buffered NiW substrates via PLD. Figures 4 and 5 summarize XRD results for the YBCO film on the BYNO buffered NiW substrate. The θ−2θ XRD scan (Fig. 4) shows strong (00l) peak intensities from BYNO and YBCO films indicating that both layers are highly out-of-plane textured. In addition to c-axis-oriented BYNO and YBCO peaks, observation of small additional peaks in the scan is related to NiWO₄ and NiO phases formed at the BYNO/NiW interface by the self-limited oxidation of NiW substrate during YBCO film deposition in high P(O₂) condition.^[Reference Wee, Goyal, Martin and Heatherly^17, Reference Leonard, Goyal, Kang, Yaborough and Kroeger^18] The ω- and φ-scans [Figs. 5(a) and 5(b)] show that the YBCO film has an excellent cube-on-cube, epitaxial relationship with BYNO layer and NiW substrate, and the in-plane and out-of-plane textures of YBCO film are further improved as compared with BYNO layer. YBCO film has Δω of ~1.9° (φ = 0°) and ~2.2° (φ = 90°) and Δφ of ~6.1° which are 1.5–2.5° smaller than those of BYNO film.

Figure 4.

θ−2θ scan for YBCO film on NiW substrate with BYNO single layer.

Figure 5.

(a) ω- and (b) φ-scans of YBCO layer, BYNO layer, and NiW substrate for YBCO (~0.2 µm)/BYNO (~0.2 µm)/NiW (~50 µm). In Fig. 5(a), the left and right figures are ω-scans at rolling (φ = 0°) and transverse (φ = 90°) directions, respectively.

Superconducting properties for the YBCO film grown on the BYNO buffered NiW substrate were measured and compared with those for a reference sample, YBCO film with the same thickness on standard AMSC template comprised of CeO₂ (75 nm)/YSZ (75 nm)/Y₂O₃ (75 nm)/NiW. The reference sample was grown at the same time by mounting the samples side by side on the heater via PLD. Figure 6 shows the field and angular dependent J _c, J _c (H, θ), at 77 K for the samples. YBCO films on both BYNO single buffer and standard AMSC template show very similar J _c (H, θ) behavior with the self-field J _c values of 1.8 MA/cm² (for the film on BYNO single buffer) and 1.5 MA/cm² (for the reference sample). Both samples were also measured to have similar superconducting transition temperature, T _c of ~90 K. These results demonstrate the robustness of BYNO buffer layer providing comparable performance to the standard buffer structure with three different oxide buffer layers.

Figure 6.

Field and angular dependences of J _c at 77 K for the ~0.2 µm thick YBCO film grown on BYNO-buffered NiW substrate. For comparison, J _c data for YBCO film with the same thickness on standard AMSC template comprised of CeO₂ (75 nm)/YSZ (75 nm)/Y₂O₃ (75 nm)/NiW are also plotted.

In summary, our study demonstrates epitaxial growth of highly textured, BYNO films directly on bare, textured NiW substrates. The films show very dense and smooth surface structure, and more importantly, massive out-of-plane texture sharpening with 2–3° smaller Δω values as compared with the underneath NiW substrate texture. TEM results confirm that BYNO lattice is tilted toward the surface normal with formation of misfit dislocations at the NiW/BYNO interface, suggesting that the out-of-plane texture sharpening is attributed to the tilted epitaxy via the biased dislocation mechanism. Highly textured, epitaxial YBCO films are demonstrated on BYNO single-buffered NiW substrates. YBCO film has high T _c of ~90 K and J _c of 1.8 MA/cm² at 77 K, self-field which are comparable with or slightly better than those of the reference sample with the standard three-layer buffer architecture. The results suggest that the epitaxial BYNO layer should be potentially used as a multifunctional, single buffer layer for epitaxial growth of complex oxide, functional materials including superconducting YBCO films on the single crystal like, flexible, metallic substrates.