50 High-quality As-grown MgB 2 Film Fabrication at Low temperature using In-plane-lattice Near-matched Epitaxial-buffer Layer To date a process for fabricating a superconducting MgB 2 [1] thin film has required a growth and/or annealing temperature higher than 600 º C [2]. When such a process is applied to the fabrication of an integrated device, interdiffusion occurs at interfaces between the integrated layers, which degrades the quality of the device. This is one problem faced in fabricating a high-quality integrated device. We succeeded in the fabrication of high-quality as-grown MgB 2 films [3] and their structural and superconducting characterization [4]. One of the keys to success was the utilization of low-temperature crystal growth combined with a near-matched epitaxial buffer layer. We clarified the function of the buffer layer from a structural point of view. A TiZr buffer layer was deposited on a sapphire Al 2 O 3 (0001) substrate at 815 º C by evaporation of a Ti-50 at % Zr alloy source. After the substrate was cooled to room temperature, MgB 2 films were deposited at 270 º C by coevaporation of Mg and B metals, which were evaporated using an effusion cell and an electron beam gun, respectively. The film and buffer layer were determined to be about 300 and 100 nm in thickness, respectively, by high-resolution SEM observation. For a control experiment, we also prepared a MgB 2 film (without the buffer layer) grown on a sapphire (0 0 0 1) substrate. Superconducting properties were examined resistively and magnetically. The critical temperature T c of MgB 2 /TiZr/Al 2 O 3 was about 1 K higher than that of MgB 2 /TiZr/Al 2 O 3 (not shown here). The applied magnetic field dependences of magnetizations were measured under a perpendicular magnetic field to the film surface. The critical current density J c was estimated from a magnetization hysteresis loop based on the Bean critical state model. The larger hysteresis loop for MgB 2 /TiZr/Al 2 O 3 in comparison with that for MgB 2 /Al 2 O 3 corresponds to a higher capacity for the flow of current in the former MgB 2 film. J c at 5 K under 1 T magnetic flux density is 6 × 10 5 A/cm 2 for MgB 2 /TiZr/Al 2 O 3 . J c obtained from MgB 2 /Al 2 O 3 is much smaller than this value at 9 × 10 4 A/cm 2 , suggesting that the crystallinity of the MgB 2 film in the MgB 2 /TiZr/Al 2 O 3 could be much improved. X-ray measurements for film-structural analysis were performed with a six-circle diffractometer at beamline BL13XU [5]. An X-ray wavelength of 0.102 nm was used. We located the sample at the 0 1 1 — 1 Bragg position and rotated it around the [0 0 0 1] direction L (almost the surface normal) to record X-ray intensities diffracted from the film (Fig. 1). The MgB 2 film without the buffer layer was a c -axis oriented crystal and had no epitaxial relation. Accordingly, we concluded that the crystallinity of the film with the Rotation Angle (deg) around L of MgB 2 Film 0 -40 -20 0 20 40 60 80 100 120 140 160 Without buffer With buffer layer X-ray Intensity (arb. unit) Fig. 1. L vs . X-ray intensity scattered from MgB 2 film on the buffer layer (solid circles) and without the buffer layer (crosses). The peaks came from the 0 1 1 — 1 reflection [4]. 51 Osami Sakata a, *, Shigeru Kimura a and Shugo Kubo b (a) SPring-8 / JASRI (b) Department of Materials Science, Shimane University *E-mail: o-sakata@spring8.or.jp References [1] J. Nagamatsu et al .: Nature 410 (2001) 558. [2] W.N. Kang et al .: Science 292 (2001) 1521. [3] S. Yata et al .: Physica C 388-389 (2003) 155. [4] O. Sakata, S. Kimura, M. Takata, S. Yata, T. Kato, K. Yamanaka, Y. Yamada, A. Matsushita and S. Kubo: J. Appl. Phys. 96 (2004) 3580. [5] O. Sakata et al .: Surface Rev. Lett. 10 (2003) 543. buffer layer was improved. By similar measurements, it was determined that the buffer layer was also crystalline. The epitaxial relationship in a plane perpendicular to L was obtained: MgB 2 (majority domain) [0 1 1 — 0] // TiZr [0 1 1 — 0] and TiZr [0 1 1 — 0] // sapphire [1 1 2 — 0]. q R , q T and q ⊥ scans were carried out in the reciprocal-lattice space to determine in-plane lattice spacings of MgB 2 and the TiZr buffer layer. The q R and q T scans were parallel to the radius an d tangential direction in the L-constant plane (almost in-plane) passing through a desired Bragg position, respectively. The q ⊥ scan was parallel to L around the position. The obtained in-plane lattice spacings, d // MgB 2 , d // TiZr , and d // sapphire are 0.268, 0.258, and 0.239 nm for the MgB 2 (0 1 1 — 0), TiZr (0 1 1 — 0), and sapphire (1 1 2 — 0) planes, respectively. It is noted that d // TiZr was between d // MgB 2 and d // sapphire . The crystallinity of the film with the buffer layer became higher accordingly. From FWHMs of ∆ q R , ∆ q T , and q ⊥ , we estimated crystal mosaic spreads. The spreads are 6.1 º and 0.38 º for the MgB 2 film and the TiZr layer, respectively. We plot the spreads with respect to the in-plane lattice spacings (Fig. 2). The spread of the MgB 2 film was around 6 º , while the film without the buffer layer was the c -axis oriented crystal. This suggests that the buffer layer improves the crystallinity of the MgB 2 film. In summary, we have found that the in-plane- lattice near-matched TiZr buffer layer aided the fabrication of the as-grown MgB 2 crystalline film. This is because the in-plane lattice spacing of the buffer layer was between those of MgB 2 and the substrate crystal. T c and J c of MgB 2 /TiZr/Al 2 O 3 were found to be higher than those of MgB 2 /Al 2 O 3 . The near-matched epitaxial crystallinity of the buffer-layered TiZr was of cardinal importance in the low-temperature growth of the high-quality as-grown MgB 2 film. In-plane d-spacing (nm) 0.235 0.240 0.245 0.250 0.255 0.260 0.265 0.270 0 – 4 – 3 – 2 – 1 1 2 3 4 sapphire TiZr MgB2 1120 0110 0110 Mosaic Spread (deg) Fig. 2. Measured in-plane lattice spacings and mosaic spreads [4].
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