Preferential growth mechanism of REBa 2 Cu 3 O y (RE = Y, Nd) crystal on MgO substrate by liquid phase epitaxy
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Growth of the REBa2Cu3Oy (REBCO, RE ⳱ Y, Nd) crystals on the MgO substrates by the liquid phase epitaxy (LPE) process was investigated to clarify the growth mechanism. The crystal orientation of in-plane alignment was improved during the LPE process due to the preferential dissolution and growth even from a polycrystalline seed film. The orientation of preferential growth depended on the kind of RE for the REBCO system. The phenomena could be explained by the coarsening model by introducing the difference in the interfacial energies, which were considered not only general lattice matching but the Coulomb force at the interface between the REBCO and the MgO crystals. The preferential growth model was developed, and the calculation results showed a good agreement with the experimental results.
I. INTRODUCTION
Since the discovery of the oxide superconductors in 1987,1 a lot of efforts have been focused on the next generation of electronic devices.2–4 For application devices, a substrate with high crystallinity is necessary, such as a single-crystalline substrate/wafer, since the electronic device usually has a multilayered structure and its characteristic is determined by the crystallinity of the layers. Among the various film preparation methods, liquid phase epitaxy (LPE), which has been successfully developed for fabrication of GaAs films, is expected to also work well for preparation of large, single-crystalline films with high crystallinity. Recently, the LPE method has been applied to fabrication of the REBa2Cu3Oy (REBCO, RE ⳱ Y, Nd, etc.) films.5–13 The LPE-growth film has achieved the critical temperature of over 90 K11,13 and an extremely flat surface with about 1-nm roughness after polishing.12 In the case of single flux quantum (SFQ) device applications, high crystallinity, high critical temperature (Tc), and flatness are required for a ground plane, which is a base layer for the multilayered structure. Both the crystallinity and the flatness of the ground plane strongly affect the crystallographic qualities of the above layers that influence the properties of the device. The ground plane prepared by vapor deposition methods on a singlecrystalline substrate has some problems, which are a)
Present address: Fujikura Ltd., 1-5-1, Kiba, Koto-ku, Tokyo 1358512, Japan J. Mater. Res., Vol. 16, No. 4, Apr 2001
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relatively poor crystallinity, low Tc, and large roughness (up to 100 nm). In particular, the large roughness including the surface particles causes a serious problem such as a short circuit between the layers. Because the LPE wafer has achieved the attractive properties mentioned above, it is expected to be the substrate/wafer for SFQ devices with a function of the ground plane. Several kinds of single-crystalline substrates, such as MgO, SrTiO3, and NdGaO3, are used for LPE growth of the REBCO film.5–10 In general, researchers considered that the lattice matching between the growth crystal and the substrate is one of the most important factors for
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