Improvement in crystal orientation and in-plane alignment of La-doped YBCO thin film on BaZrO 3 -buffered MgO substrate
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Y0.9Ba1.9La0.2Cu3Oy (La-YBCO) thin films were prepared by an off-axis magnetron sputtering method on MgO substrates with and without a BaZrO3 buffer layer. Insertion of BaZrO3 buffer layer was effective for obtaining La-YBCO films with pure c-axis orientation and in-plane alignment at low temperatures below 600 °C. We prepared La-YBCO thin films on BaZrO3-buffered MgO substrates using a temperature-gradient method, in which a template La-YBCO layer was first deposited at 600 °C, and then the temperature was continuously raised to 700 °C. By this method, La-YBCO thin films with improved crystallinity were successfully prepared. It was also proved that the insertion of the BaZrO3 buffer layer enables us to prepare high-quality La-YBCO films with high reproducibility.
I. INTRODUCTION
For application of high-Tc superconductors to electronic digital devices such as single flux quantum (SFQ) circuits, multilayer structures must be fabricated on substrates. Multilayer structures for high-Tc superconducting electronic digital devices consist of superconducting layers and insulating layers. In most instances, the first superconducting layer on a substrate is used as a ground plane.1–3 A ground plane must consist of high-quality films with desired single orientation, in-plane alignment, and a flat surface, etc., because the plane works not only as a superconducting ground but also as a foundation for multilayer structures of epitaxial films. Preparation of YBa2Cu3Oy (YBCO) thin films on MgO substrates has been extensively studied to fabricate microwave filters and small-scale SFQ circuits such as samplers. MgO is one of the most commonly used substrates for high-Tc devices treating high-frequency signals because it has lower permittivity and loss tangent than SrTiO3 (STO), (LaAlO3)0.3–(SrAl0.5Ta0.5O3)0.7, (LSAT) etc. It has been, however, pointed out that inplane misoriented grains often exist in YBCO thin films on MgO substrates. Insertion of buffer layers such as BaSnO3, SrSnO3, and BaZrO3 between YBCO film and MgO was examined.4–6 Of these, BaZrO3 is particularly interesting because its lattice parameter (0.419 nm) is very close to that of MgO (0.421 nm). BaZrO3 was also used as a buffer layer for SmBa2Cu3Oy (SmBCO) films
DOI: 10.1557/JMR.2005.0207 J. Mater. Res., Vol. 20, No. 6, Jun 2005
http://journals.cambridge.org
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on MgO substrates.7,8 These previous reports described effective utilization of the BaZrO3 buffer layer to obtain c-axis-oriented YBCO and SmBCO films with in-plane alignment. We studied the effects of BaZrO3 buffer layer between MgO substrate and Y0.9Ba1.9La0.2Cu3Oy (La-YBCO) films. The La substitution for both the RE and Ba sites was previously found effective for obtaining smooth films without precipitates.9 The preparation conditions yielding La-YBCO films with c-axis orientation, in-plane alignment, and a flat surface, which are extremely important characteristics for a ground plane of SFQ devices, have been investigated in detail. We confirmed that the use of BaZrO3 buffer
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