Inclined‐Substrate Pulsed Laser Deposition of Yttriastabilized Zirconia Template Film for YBCO Coated Conductors

Abstract

Biaxially textured films of yttria-stabilized zirconia (YSZ) were deposited on Hastelloy C276 (HC) substrates by inclined-substrate pulsed laser deposition (ISPLD). This method is promising for fabrication of YSZ templates on polycrystalline metallic tapes for coated conductor applications. Scanning electron microscopy showed columnar grains in ISPLD-YSZ films. X-ray pole figure analysis revealed good biaxial alignment in these films. The in-plane texture was determined to be ≈16° from the full-width at half maximum (FWHM) in the YSZ (111) f-scan; and the out-of-plane texture was ≈8° from the FWHM in the YSZ (002) w-scan. Before the deposition of YBCO films by pulsed laser deposition, a thin layer of CeO2 was deposited on the ISPLD-YSZ. The YBCO deposited on ISPLD-YSZ-buffered HC substrates were biaxially textured. Tc = 90 K and Jc = 180 kA/cm at 77 K in self-field were measured. INTRODUCTION YBa2Cu3O7-d (YBCO) coated conductors are promising for high-current carrying wires and other electric power devices operating at temperatures that approach liquid nitrogen [1-3]. Textured template films or buffer layers are needed for deposition of biaxially aligned YBCO films to overcome weak links at the grain boundaries and, therefore, to achieve high critical current density (Jc) in the YBCO films on metallic substrates [4]. Several techniques, including ion-beam-assisted deposition (IBAD), rolling-assisted biaxially textured substrates (RABiTS), and inclined-substrate deposition (ISD), have been developed in recent years [5-9]. We grew biaxially textured YSZ thin films on mechanically polished Hastelloy C276 (HC) substrates by inclined-substrate pulsed laser deposition (ISPLD). Ceria cap layers and YBCO films were subsequently deposited on ISPLD-YSZ-buffered metallic substrates by pulsed laser deposition (PLD). X-ray pole figures, f-scan and w-scan were used to analyze texture. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were utilized to study morphology and surface roughness. In this paper, we discuss the growth conditions, microstructure, and crystalline texture of ISPLD YSZ buffer layers and YBCO films deposited on polished HC substrates. EXPERIMENTAL PROCEDURE HC coupons (≈5 mm wide and 10 mm long) were mechanically polished to a mirror finish with 0.25-μm diamond paste for use as substrates. Surface roughness of ≈3 nm was measured by AFM. A schematic illustration of the experimental setup is shown in Fig. 1. The polished substrate was mounted on a tiltable heater stage using silver paste. The substrate inclination angle (a), substrate normal with respect to the laser plume axle, was set to a desired value (45-55°). A Lambda Physik LPX 210i excimer laser, with a Kr-F2 gas premixture as the lasing medium, was used for ablation of targets (Superconductive Components, 99.99% pure). YSZ, CeO2 and YBCO targets are 45 mm in diameter and 6 mm thick. The size of laser spot focused at the rotating target was ≈12 mm, which produced an energy density of ≈2.0 J/cm. The distance between the target and the substrates was 4-7 cm. The desired oxygen partial pressure was obtained by flowing ultra-high-purity oxygen through the chamber. Details of deposition conditions are listed in Table 1. Layers of CeO2 and YBCO were subsequently deposited by PLD on the ISPLDYSZ buffered HC substrate at zero degree inclination angle. FIGURE 1. Schematic illustration of experimental setup for ISPLD system. Table 1. PLD Conditions for Growth of Biaxially Textured YSZ Films Laser System Lambda Physik LPX 210i Laser Wavelength 248 nm (KrF) Pulse Duration 25 ns Energy Density 1-3 J/cm Repetition Rate 20-90 Hz Substrate Temperature 400-650°C Inclination Angle 45-55° Oxygen Partial Pressure 1-200 mTorr Target-to-Substrate Distance 4-7 cm Crystalline texture was measured by X-ray diffraction pole-figure analysis using Cu-Ka radiation. In-plane texture was characterized by the FWHM of f-scans for the YSZ (111) reflection, and out-of-plane texture was characterized by the FWHM of w-scans for YSZ (002). Surface morphology was investigated by SEM using a Hitachi S-4700-II. Surface roughness was measured by AFM using a Digital Instruments D3100 scanning probe microscope (operated in tapping mode). The superconducting critical transition temperature (Tc) and Jc for the YBCO films were determined by the inductive method reported earlier [9]. RESULTS AND DISCUSSION The X-ray diffraction 2q scan patterns for the ISPLD YSZ films deposited at different temperature are shown in Fig. 2. Data indicate that out-of-plane orientation FIGURE 2. X-ray diffraction 2q scan patterns for YSZ films deposited on HC substrate at different temperatures using ISPLD. is dependent on deposition temperature. C-axis oriented films were obtained with a deposition temperature of ≈600°C. The (111) oriented YSZ films, whose texture was not in favor for the coated conductor applications, were obtained at higher (above 650°C) or lower (below 400°C) deposition temperatures. We will limit our discussion to c-axis oriented films in this paper. FIGURE 3. YSZ (111) pole figure for ISPLD YSZ film deposited at 600°C. Figure 3 shows a typical X-ray diffraction pole figure of YSZ (111) for an ISPLD-YSZ film deposited at 600°C on an HC substrate. The four distinct poles indicate that the ISPLD-YSZ film was biaxially textured. In-plane texture measured FIGURE 4. (a) Plan-view and (b) fracture cross-sectional view SEM images of ISPLD YSZ film deposited at 600°C. from the FWHM of the YSZ (111) f-scan was ≈16.0°. The c-axis of ISPLD-YSZ film was slightly tilted, with a tilt angle of ≈6°, similar to that reported by Hasegawa et al. [10]. Out-of-plane texture measured from the FWHM of the YSZ (002) wscan was ≈7.8°. Gaps between grains were observed from the plan-view SEM image (Fig. 4a) of an ISPLD YSZ film. Columnar grains were observed from the cross-sectional fracture surface (Fig. 4b). This funding suggests that the texturing mechanism for ISPLD-YSZ films is most likely due to grain self-shadowing effect, similar to that for the IDS MgO films [11,12]. However, unlike the ISD MgO, textured YSZ films can hardly be grown by simple inclination of substrates and using electron beam evaporation, where the momentum energy of the atoms in YSZ was not sufficent. In order to have atoms in YSZ to have a better arrangement at their sublattices, additional momentum other than thermal energy must be supplied to these atoms. In the case of ISPLD, the energy needed for preferred lattice arrangement in YSZ was provided by the pulsed laser plasma/plume. Root-meansquare (RMS) surface roughness of 16 nm was measured by tapping mode AFM on the as-deposited YSZ films. FIGURE 5. YBCO (103) pole figure of YBCO deposited on ISPLD YBZ buffered HC