High Rate in situ YBa 2 Cu 3 O 7 Film Growth Assisted by Liquid Phase
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W. Jo Department of Physics and Division of Nano Sciences, Ewha Womans University, Seoul 120-750, Korea (Received 12 August 2003; accepted 16 January 2004)
High-rate (10 nm/s) in situ YBa2Cu3O7 (YBCO) film growth was demonstrated by molecular beam epitaxy with electron beam co-evaporation at a system pressure of approximately 5 × 10−5 Torr. To explain the phase stability observed, it is suggested that activated oxygen is generated in the process. Growth of very good YBCO, with a Jc of more than 2 MA/cm2, is possible at this very high rate because the growth is in a liquid (Ba–Cu–O), which forms along with the YBCO epitaxy. This liquid seems essential for high Jc-YBCO film growth at very high in situ growth rates and may be essential for all high-rate processes, including postanneal ex situ processes.
Our goal is to develop a process to grow in situ superconducting YBa2Cu3O7 (YBCO) films, directed towards coated conductor tape synthesis for electric power applications. The requirements are challenging due to the economic constraints: deposition rate of greater than 10 nm/s and thickness of microns over large areas while preserving excellent superconducting properties. The electron beam co-evaporation method used is promising in terms of relatively inexpensive material cost, easy wide-area expandability, and high-rate thick-film deposition. Here we report reproducible YBCO film fabrication with high superconducting critical current density (Jc > 2 MA/cm2) at around 10 nm/s deposition rate on single crystal substrates as a preliminary demonstration for coated conductors. This paper focuses on the phase stability relationships of YBCO films grown at high rates at reduced oxygen pressure but with a higher inferred oxygen activity. Films were grown by means of molecular-beam epitaxy (MBE) with electron beam co-evaporation of Y, Ba, and Cu metal sources in oxygen atmosphere, or so-called reactive co-evaporation. Details are described elsewhere.1,2 Y and Ba metal evaporation rates were controlled by laser atomic absorption sensors,3 and the Cu rate by chopped ion gauge monitor.2 Molecular oxygen was introduced in two ways: into the growth chamber a)
Present address: Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8581, Japan. e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2004.0127 J. Mater. Res., Vol. 19, No. 4, Apr 2004
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background generally, and alternatively by means of a 1⬙ diameter nozzle directed to the substrate surface to enhance the flux. A 500 W halogen-lamp-based radiation heater could heat the sample holder to 1000 °C. Most films were grown in a system background pressure of 5 × 10−5 Torr regardless of the nozzle or general chamber use, at oxygen flow rate of around 35 sccm for the general case, and 52 sccm for the nozzle case. Temperatures ranging from 860 to 1000 °C were explored in this report; a much wider range was used in developing the process. By turning off the heate
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