Growth of biaxially-textured MgO buffer layers by Inclined Substrate Deposition

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1 Available online at Physics Procedia 36 (2012 ) Superconductivity Centennial Conference Growth of biaxially-textured MgO buffer layers by Inclined Substrate Deposition M. Dürrschnabel a,*, Z. Aabdin a, V. Große b, M. Bauer b, G. Sigl b, W. Prusseit b, O. Eibl a a Institut für Angewandte Physik, Universität Tübingen, Auf der Morgenstelle 10, D Tübingen, Germany b THEVA Dünnschichttechnik GmbH, Rote-Kreuz-Str. 8, D-85737, Ismaning, Germany Abstract MgO buffer layers were grown by Inclined Substrate Deposition (ISD) for superconducting coated conductors. The surface structure, texture, and extended defects were analyzed for films with different MgO layer thicknesses. A rooftile structure was only observed for films with layer thicknesses exceeding 800 nm. MgO facet widths and amplitudes were determined for different MgO buffer layer thicknesses. At 800 nm the facets height was 18 nm and the facet width was 71 nm. The corresponding values measured for a 5 μm thick MgO film were 64 nm and 234 nm, respectively. For layers with layer thicknesses smaller than 200 nm the films were polycrystalline with a grain size of 20 nm. For larger layer thicknesses the ISD MgO film grew in the form of columns with diameter of 50 to 100 nm. The texture of the layers was analyzed by electron diffraction in the TEM. From radial intensity profiles obtained from electron diffraction patterns the background-corrected (002)/(111) intensity ratio was extracted. It was found that the (002)/(111) intensity ratio allows to quantify the degree of texture Published by Elsevier B.V. Ltd. Selection and/or peer-review under responsibility of of Horst the Guest Rogalla Editors. and Peter Open access Kes. under CC BY-NC-ND license. Keywords: ISD; MgO; buffer layer; texture; TEM 1. Introduction With the second generation of coated conductors grown on flexible metal tapes practical applications in the electric power industry came into reach [1, 2]. Large efforts were needed for texturing the superconducting layer consisting of REBa 2 Cu 3 O 7-x (RE=rare earth) layers. A common problem in the * Corresponding author. address: michael.duerrschnabel@uni-tuebingen.de Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors. Open access under CC BY-NC-ND license. doi: /j.phpro

2 M. Dürrschnabel et al. / Physics Procedia 36 ( 2012 ) early days of coated conductors was limited currents due to weak links created by large angle grain boundaries [3]. Major improvements on current density were made by technologies such as RABiTS (Rolling Assisted biaxially Textured Substrates) [4] or IBAD (Ion Beam Assisted Deposition) [5] and, also, ISD [6-8]. After more than 30 years the ISD technique [9] was rediscovered and nowadays used by several groups and companies for coated conductor deposition [10-12]. An untextured substrate needs to be inclined under vacuum conditions vs. the incoming vapor. Anisotropy of diffusion will lead to a faceted surface structure and biaxially textured grains on a randomly orientated, polycrystalline substrate. Not all materials will yield a high-quality biaxial textured surface. Bauer [13] studied three materials grown by ISD, namely MgO, CeO 2 and yttria-stabilized zirconia (YSZ) on Hastelloy substrates. The results were compared to each other and it was found that YSZ and CeO 2 were not suitable for coated conductor growth due to bad texture transfer and crystallographic improper orientation. The different behavior of compounds during ISD deposition is mainly due to their anisotropy of surface energies. 2. Experimental All MgO films included in this work were grown by the ISD technique. As a substrate material a nonmagnetic HASTELLOY C276 (a Ni-Mo-Cr alloy) tape was used. In a first step the substrate was cleaned and flattened by electro-polishing to get better adhesion of the subsequently grown MgO film. For the subsequent MgO growth the substrate was tilted by an angle α towards the incoming MgO vapor. MgO was evaporated by e-beam evaporation and deposited at ambient temperature under vacuum conditions for semi-processed samples. For fully processed samples an additional MgO and a REBa 2 Cu 3 O 7-x layer were grown in an untilted position at elevated temperatures. Further growth details can be found in [14]. Samples were characterized using a Jeol 6500F Scanning Electron Microscopy (SEM), a JPK Nanowizard Atomic Force Microscopy (AFM), and a Zeiss EM912Ω Transmission Electron Microscopy (TEM) with an in-column omega energy filter operated at 120 kv. SEM and AFM were used to characterize the sample surfaces, whereas TEM was used to study volume properties of the samples. TEM samples were prepared in cross-section either by conventional grinding, polishing and ion beam milling techniques or by the Focused Ion Beam (FIB) method. TEM images and diffraction patterns were acquired using a 5 or 10 ev energy slit aperture to reduce inelastically scattered electrons contributing to the image background. For bright and dark field imaging a 3.5 mrad objective aperture was used. Diffraction patterns were acquired with a 700 nm selected area aperture. For analysis of radial and polar intensities within diffraction patterns the Gatan Digital Micrograph DiffTools script was used [15]. 3. Results and Discussion Along the white arrow inside Fig. 1 (a) an intensity profile was acquired. From this intensity profile a mean facet length was determined. A similar procedure was applied to the AFM images (not shown here) from which a height profile was obtained to determine the facet height. Both results are plotted inside Fig. 1 (b). Note that the profile direction in both cases was along the cross-sectional direction of the tape. The error bars in Fig. 1 (b) denote the standard deviations determined from several facet length measurements.

1548 M. Dürrschnabel et al. / Physics Procedia 36 ( 2012 ) 1546 1551 Fig. 1. (a) SEM plan-view image of an ISD MgO surface. A line scans was acquired along the white to determine the facet length.

3 1548 M. Dürrschnabel et al. / Physics Procedia 36 ( 2012 ) Fig. 1. (a) SEM plan-view image of an ISD MgO surface. A line scans was acquired along the white to determine the facet length. (b) Facet height (extracted from AFM images not shown here) and facet length for different MgO layer thicknesses. For MgO film thicknesses less than 800 nm no facet structure was visible. At an MgO layer thickness of 800 nm a facet length of 71 nm and a facet height of 18 nm were measured, whereas at an MgO layer thickness of 5 μm a facet length of 230 nm and a facet height of 64 nm were found. Consequently, the MgO film roughness, i.e. facet height, and facet length both increase with the MgO film thickness as can be seen in Fig. 1 (b). At MgO film thicknesses larger than 2 μm also a facet fine structure was observed. Koritala et al. [16] observed an increase of MgO grain size i.e. MgO column diameter from 20 nm in a 50 nm thick MgO film to nm for a MgO film of 3 μm thickness. We also found an MgO grain size of 20 nm in a 100 nm thick semi-processed film. This suggests that the growth for all ISD MgO films follows a common model. The MgO texture evolution is of crucial importance since it influences severely the subsequent superconductor growth. The choice of growth parameters controls totally the overall tape quality within the ISD buffer-layer growth. For texture analysis electron diffraction techniques were applied for both well-textured and poorly textured samples. Fig. 2 (a) shows a cross sectional TEM bright-field image of the MgO buffer layer of a well-textured sample. The dashed white circles indicate the position and size of the selected area aperture used for acquiring the diffraction patterns shown in Fig. 2 (b, e, h, k). All diffraction patterns were analyzed with respect to radial and (002) polar intensities. The results are shown in Fig. 2 (c) to (m). It was expected that with increasing distance from the Hastelloy/MgO interface the texture would increase. As a consequence the (002) reflection gets more and more pronounced in the diffraction patterns whereas the (111) reflection decreases in intensity. The radial intensity profiles in Fig. 2 nicely prove this. At the same time in the polar intensity profiles the intensity of all reflections except the (002) reflections decrease. This indicates that for a well-textured sample the only slightly textured structure in the nucleation layer changed into an almost single crystalline one. Fig. 2 (d, g, i, m) show this transition very clearly. The explanation of the increase of (002) reflections is that (002) type of facets will decrease the surface free energy [16]. The (002)/(111) peak ratio was calculated as function of the distance from the interface and quantifies the MgO texture. It is large for well-textured MgO films and small for poorly textured MgO films. Table 1 summarizes the (002)/(111) peak ratio results obtained from the well textured and the poorly textured sample. For the well-textured sample the (002)/(111) peak ratio increased with the distance from the interface from 2.5 to 9.1. In contrast for the poorly textured sample it increased only slightly from 1.1 to 3.3.

4 M. Dürrschnabel et al. / Physics Procedia 36 ( 2012 ) Fig. 2. Texture analysis of the MgO buffer layer by electron diffraction in the TEM. (a) Low magnification bright-field image. (b) Diffraction pattern displayed in false colors acquired at area (b). (c) Radial intensity profile of (b) and (d) (002) polar intensity profile of (b). Subfigures (e) to (g), (h) to (j) and (k) to (m) show corresponding images as subfigures (b) to (d) at different distances from the Hastelloy/MgO interface. The red arrow indicates the starting point and direction of the polar intensity profiles. Table 1. Background corrected (002)/(111) ratios obtained from electron diffraction patterns indicating the texture development with increasing MgO film thickness. MgO film thickness [nm] Well-textured Poorly textured Bauer et al. [12] proposed a model of ISD growth considering the influence of the substrate inclination angle, deposition rate, and film thickness on the MgO texture. Bauer et al. explained that the preferred orientation arises from mutual shadowing of the columns and thermally activated directional surface

5 1550 M. Dürrschnabel et al. / Physics Procedia 36 ( 2012 ) diffusion. To confirm this assumption they performed 3D Monte Carlo simulations. The simulations yielded a columnar growth with (002) terminated surface facets similar our experimental observations. Bauer et al. [12] plotted the Full Width Half Maximum (FWHM) of the in-plane texture extracted from X-ray pole figures vs. the MgO film thickness. They found an exponential decrease of the in-plane texture with thickness saturating at about 750 nm MgO film thickness i.e. at 750 nm the MgO is biaxially textured. The (002)/(111) peak ratio obtained from our TEM diffraction experiments saturates around that value and in SEM no faceting was observed below 800 nm film thickness. Thus, for a good biaxial texture the ISD MgO films should be thicker than 800 nm. 4. Conclusions The surface structure of ISD grown MgO films were studied by SEM, AFM, and TEM. Only for MgO films thicker than 800 nm surface faceting was observed by SEM and AFM. At an MgO layer thickness of 800 nm a facet length of 71 nm and a facet height of 18 nm were measured, whereas at an MgO layer thickness of 5 μm a facet length of 230 nm and a facet height of 64 nm were found. The (002)/(111) peak ratio obtained from TEM diffraction experiments revealed the amount of texture present in ISD MgO films. It was shown that all observations fit perfectly to the ISD growth model and the Monte Carlo simulations of Bauer et al. [12].

6 M. Dürrschnabel et al. / Physics Procedia 36 ( 2012 ) Acknowledgements The authors acknowledge financial support from the Bundesministerium für Wirtschaft und Technologie, the project HIGHWAY, FK C. They thank the NMI Reutlingen for preparing the FIB samples. References [1] P. Corsaro, M. Bechis, P. Caracino, W. Castiglioni, G. Cavalleri, G. Coletta, et al., Manufacturing and commissioning of 24 kv superconducting cable in Detroit, Physica C, 378 (2002) [2] V. Sokolovsky, V. Meerovich, V. Beilin, I. Vajda, Applications of an HTS thin film switching element in the inductive current limiter, Physica C, 386 (2003) [3] M.F. Chisholm, S.J. Pennycook, Structural Origin of Reduced Critical Currents at YBa 2Cu 3O 7- Grain-Boundaries, Nature, 351 (1991) [4] S. Sathyamurthy, M. Paranthaman, H.Y. Zhai, S. Kang, T. Aytug, C. Cantoni, et al., Chemical solution deposition of lanthanum zirconate barrier layers applied to low-cost coated-conductor fabrication, J Mater Res, 19 (2004) [5] Y. Iijima, N. Tanabe, O. Kohno, Y. Ikeno, Inplane Aligned YBa 2Cu 3O 7-x Thin-Films Deposited on Polycrystalline Metallic Substrates, Appl Phys Lett, 60 (1992) [6] W. Prusseit, C. Hoffmann, R. Nemetschek, G. Sigl, J. Handke, A. Lumkemann, et al., Reel to reel coated conductor fabrication by evaporation, Ieee T Appl Supercon, 16 (2006) [7] W. Prusseit, R. Nemetschek, C. Hoffmann, G. Sigl, A. Lumkemann, H. Kinder, ISD process development for coated conductors, Physica C, 426 (2005) [8] W. Prusseit, G. Sigl, R. Nemetschek, C. Hoffmann, J. Handke, A. Lumkemann, et al., Commercial coated conductor fabrication based on inclined substrate deposition, Ieee T Appl Supercon, 15 (2005) [9] D.O. Smith, M.S. Cohen, G.P. Weiss, Oblique-Incidence Anisotropy in Evaporated Permalloy Films, J Appl Phys, 31 (1960) [10] U. Balachandran, B. Ma, M. Li, B.L. Fisher, R.E. Koritala, D.J. Miller, et al., Development of coated conductors by inclined substrate deposition, Physica C, 392 (2003) [11] K. Hasegawa, N. Hobara, Y. Nakamura, T. Izumi, Y. Shiohara, Preparation of MgO films on metal substrate as a buffer layer for liquid phase epitaxy processed RE123 coated conductor, Physica C, 354 (2001) [12] M. Bauer, R. Metzger, R. Semerad, P. Berberich, H. Kinder, Inclined Substrate Deposition by Evaporation of Magnesium Oxide for Coated Conductors, MRS Proceedings, 585 (1999). [13] M. Bauer, Herstellung und Charakterisierung von YBCO-Schichten und biaxial texturierten Pufferschichten auf technischen Substraten, in, Technische Universität, Munich, Germany, [14] W. Prusseit, C. Hoffmann, R. Nemetschek, G. Sigl, J. Handke, A. Lumkemann, et al., Long length coated conductor fabrication by inclined substrate deposition and evaporation, Journal of Physics: Conference Series, 43 (2006) [15] D.R.G. Mitchell, DiffTools: Electron diffraction software tools for DigitalMicrograph, Microscopy Research and Technique, 71 (2008) [16] R.E. Koritala, M.P. Chudzik, Z.P. Luo, D.J. Miller, C.R. Kannewurf, U. Balachandran, Transmission electron microscopy investigation of texture development in magnesium oxide buffer layers, Ieee T Appl Supercon, 11 (2001)

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Available online at www.sciencedirect.com Physics Procedia 36 (2012 ) 1655 1660 Superconductivity Centennial Conference Microstructure of BaZrO 3 -doped DyBa 2 Cu 3 O 7-x coated conductors deposited by