New conductive IBAD buffer for HTS applications
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New conductive IBAD buffer for HTS applications Karola Thiele1, Sibylle Sievers1, Juergen Dzick2, Lars-Oliver Kautschor1, Christian Jooss1, Joerg Hoffmann2 and Herbert C. Freyhardt1,2 1 Institut fuer Materialphysik, Windausweg 2, 37073 Goettingen, Germany, 2 Zentrum fuer Funktionswerkstoffe, Windausweg 2, 37073 Goettingen, Germany ABSTRACT Biaxially aligned Indium Tin Oxide (ITO) thin films to be used as electrically conductive buffer layers were prepared by an Ion-Beam Assisted Deposition (IBAD) process on various substrates. Two Kaufman ion sources with 2.5 cm diameter were employed for the assisting and the sputtering beam, respectively. All deposited films revealed (001) oriented film growth with a strong in-plane alignment. The degree of the in-plane orientation was studied depending on the ion-beam parameters and the incident angle. Investigations (TEM and X-ray) of the texture evolution of these IBAD films during film growth were carried out. An in-plane texture of 12.6° FWHM for a 1 µm thick film has been achieved so far. The quality of the buffer has been demonstrated by the subsequent deposition of high-current carrying YBCO-films deposited by thermal coevaporation using a 3-5 nm thick Y2O3 interlayer. A jC of 0.76 MA/cm2 (77K, 0T) has been obtained for a 1 cm x 1 cm sample with ITO of 20° FWHM. The in-plane texture of ITO improves with increasing film thickness similar to the well established texture development in YSZ (Yttria-Stabilized Zirconia). However, the dependence on the etching rate and the angle of incidence is different in both materials, which indicates different mechanisms that might be responsible for the texturing.
INTRODUCTION The use of biaxially textured IBAD-YSZ thin films as a buffer layer for HTS has been widely studied and coated conductors of up to 10 m in length and more have been sucessfully prepared [1,2,3]. Nevertheless, further reduction of the deposition time, i.e. a faster texture evolution, and buffer layers with improved performance are of high interest. There are at least two points which require further investigation: Firstly, the mechanisms of the texture development are only partly understood. Therefore, it is important to follow the texture evolution of IBAD-YSZ films during film growth in detail and to use other IBAD materials for comparison. Secondly, YSZ is an insulator and, therefore, in coated conductor architectures, a shunt, e.g. an additional gold layer, is necessary for protecting the HTS film in case of switching to the normal state at high currents. An electrically conductive buffer would avoid this additional shunt layer and act as a bypass for the transfer of the electrical current into the metallic substrate. Therefore, ITO was investigated as a new electrically conductive buffer material that can be textured by an IBAD process and which serves with 7% lattice misfit as a suitable template for YBCO. ITO is well known from semiconductor technology as a transparent conducting oxide used for transparent electrodes in form of polycrystalline films. The struc
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