Microstructure and Current Transport Properties of YBa 2 Cu 3 O 7-x /(Ba 0.05 , Sr 0.95 )TiO 3 Multiple-Layer Thin Films
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U11.46.1
Microstructure and Current Transport Properties of YBa2Cu3O7-x/(Ba0.05, Sr0.95)TiO3 Multiple-Layer Thin Films Y. Luo, R. A. Hughes, J. S. Preston and G. A. Botton Brockhouse Institute for Materials Research, McMaster University, Hamilton, Ont., Canada L8S 4L7
Abstract YBa2Cu3O7-x (YBCO) films grown by pulsed laser deposition (PLD) on (100) LaAlO3 (LAO) substrates show a strong thickness dependence on the electrical properties. For example, for films in excess of 0.3 µm, the critical current density decreases with increasing thickness. In contrast, nano-composite films consisting of a series of multiple layers of YBa2Cu3O7-x and (Ba0.05, Sr0.95)TiO3 (BSTO) thin films having a total thickness of 5 µm show improved electrical properties. In order to understand this phenomenon, a detailed microstructural characterization has been undertaken. Transmission electron microscopy (TEM) observations show that cracks, stacking faults, c-║ crystals and secondary phase precipitates are present on the single-layer films, while a high-quality microstructure is observed for the nanocomposite multiple-layer films although defects at YBCO/BSTO interface are still present. In addition, nanocomposite films have a reduced surface roughness. In this complex microstructure, the YBCO/BSTO interfaces and the lattice mismatch strain play a crucial role in controlling the nature of the defects and stability of phases. In order to understand the role of the BSTO layer has on the microstructure, the interfacial mismatch strain and defects are analyzed by high-resolution transmission electron microscopy (HRTEM) in combination with the Moiré fringe technique. Introduction There are many applications for the YBCO superconductor thin films, including microwave devices, electronic signal filters, and superconducting transistors [1, 2]. An obstacle to the commercialization of some of these applications has been the limitation of the critical current ( Ic ) in the films. The logical approach to increasing Ic is to simply increase the film thickness. This has been previously shown to be an effective solution up to a critical thickness [3]. Beyond this thickness, the critical current density ( Jc ) drops rapidly, resulting in limited returns to Ic. This trend seems to be independent of the YBCO deposition method [4, 5]. The degradation to the transport properties with increased film thickness has been investigated by several groups [6-9]. As the film thickness increase, the growth of YBCO with the c-axis perpendicular ( ┴ ) to the substrate surface transforms to the c-axis parallel (║ ) to the substrate surface above a critical thickness [8]. Since the Jc that can be carried within the a-b planes of YBCO is known to be orders of magnitude greater than that perpendicular to the planes [6], the appearance of c-║ growth is an important factor in the degradation of the electrical properties of thick YBCO films. Cracks also appear in films thicker than the critical thickness, resulting in lowering of the Jc [3]. In order to circumvent the deteriora
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