Physica C 412–414 (2004) 1286–1290 www.elsevier.com/locate/physc
Relationship between composition and surface morphology in YBCO films deposited by large-area PLD K. Develos-Bagarinao *, H. Yamasaki, Y. Nakagawa, K. Endo Superconductor Technology Group, Energy Electronics Institute, National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan Received 29 October 2003; accepted 13 January 2004 Available online 10 June 2004
Abstract YBCO films deposited from a stoichiometric (Y123) target via a large-area PLD system have consistently been found to be Y-rich in composition as well as characterized by pores. Use of an off-stoichiometric target with increased Ba and Cu content proved to be an effective method to compensate for the deficiency in barium and copper in the films. Films with composition closer to the ideal stoichiometric ratio have been obtained from the off-stoichiometric target, including significantly reduced pore density, improved microstructure and higher critical current density (Jc ) in excess of 00 2 MA/cm2 at 77.3 K over a 2 -diameter area. Furthermore, it was found out that the dense and smooth YBCO film deposited from the off-stoichiometric target needed ex situ oxygen annealing to reach fully-oxygenated state. Ó 2004 Elsevier B.V. All rights reserved. PACS: 68.55.)a; 81.15.Fg; 74.72.Bk Keywords: YBCO; Yttrium-rich; Pores; Pulsed laser deposition
1. Introduction Currently, the pulsed laser deposition (PLD) method is a highly versatile way to deposit highquality YBCO thin films for various applications such as fault current limiters [1] and microwave devices [2]. One of the merits of this method is
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[email protected] (K. Develos-Bagarinao).
stoichiometric ablation above a certain threshold fluence [3], i.e., the composition of the deposited film usually mimics that of the original target. However, this seems to be applicable only to films grown using standard PLD systems which utilize a relatively short target–substrate distance, usually in the range of 50–60 mm. Scale-up of the PLD system to deposit large-area films, however, the employment of a long target–substrate distance (>80 mm) enforces conditions unique to large-area PLD, and is therefore expected to affect the properties of the YBCO films. In fact, a characteristic of the YBCO films deposited using the large-area PLD system is the existence of
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K. Develos-Bagarinao et al. / Physica C 412–414 (2004) 1286–1290
pores, ranging from submicron to micron sizes [4]. These films have also been found to be Y-rich [5]. In this study, we investigate the correlation between surface morphology and composition of the films, and furthermore demonstrate the use of an off-stoichiometric target with enriched barium and copper content as an effective method to reduce or eliminate pore density and further improve the properties of the films.
2. Experimental Details of the film preparation are described elsewhere [4]. For a fixed laser energy of 600 mJ, the laser fluence at the target’s surface was estimated to be 7.4 J/cm2 . The laser repetition rate was fixed at 25 Hz, and the oxygen pressure at 175 mTorr. The deposition temperature for YBCO was set at the temperature controller to be 750 °C; pyrometer measurements of the surface yielded temperature values of 830–850 °C. For large-area 00 deposition, we utilized 2 -diameter LAO (1 0 0) substrates. Two target-to-substrate distances were employed, i.e., D ¼ 142 and 112 mm. These values correspond to the maximum and minimum distances at which the substrate may be positioned from the target. Such large substrate–target distances are required for the PLD system to deposit 00 up to 5 -diameter films. Two YBCO targets with the following nominal compositions were used: YBa2 Cu3 Ox (Y123) and YBa2:3 Cu3:45 Oy (off-stoichiometric YBCO). The YBCO film composition was measured by ICP-AES (inductive coupled plasma atomic emission spectrometer, SPS7700 Seiko Instruments). Film structure was investigated by X-ray diffraction measurements. The surface microstructure of the films was examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM, Digital Instru00 ments Nanoscope III). Jc distribution of the 2 films was measured inductively at 77.3 K using third harmonic voltages (THEVA Cryoscan system). Ex situ annealing of the films was performed in an infrared tube furnace heated to 450 °C in flowing 1 atm oxygen, with total annealing time fixed at 3 h.
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3. Results and discussion 3.1. Film composition and surface morphology Fig. 1 shows the elemental ratios of Ba/Y, Cu/ Y, and Cu/Ba based on the total film composition for YBCO films, using the Y123 target (filled symbols) and the off-stoichiometric target (open symbols), deposited at the two target–substrate distances of 142 and 112 mm. At both D ¼ 142 and 112 mm, it is evident that the YBCO films deposited from the Y123 target contained excess yttrium in the composition, i.e., Ba/Y
