Initial growth mechanism of Yba 2 Cu 3 O y crystal on MgO substrate by liquid-phase epitaxy
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Initial growth features of YBa2Cu3Oy (YBCO) crystal on an MgO substrate by a liquid-phase epitaxy (LPE) process were investigated and compared with homoepitaxial growth on a YBCO substrate. The partial dissolution of the seed grains in the initial stage of the LPE growth was influenced by the crystallinity of in-plane alignment of the seed grains, which could be explained by the preferential dissolution and growth mechanism. Concurrently, the slope angle of the growth grain varied with growth time. The opposite tendency of the slope angle change between the hetero- and the homoepitaxial growth was observed and could be explained by considering the difference in the step-advancing rates on each interface. It could be understood that the formation of entrapped liquid inclusions was the combination phenomena of both the small step-advancing rate of YBCO crystal on the MgO surface and the roughening of the MgO surface due to the partial dissolution of MgO to the solution.
I. INTRODUCTION
Since the discovery of the oxide superconductors in 1987,1 many researchers have been carried out by focusing on the next generation electronic devices.2–4 As the electronic device usually has a multilayered structure and its characteristics are affected by the crystallinity of the layers, a substrate/wafer on which the multiple layers are grown needs to have high crystallinity such as a single-crystalline substrate. The substrate/wafer is also required to have a large area for the high productivity of the device. In the case of the single flux quantum (SFQ) device applications using the superconductors, high crystallinity with c-axis alignment, 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 relatively poor crystallinity, low Tc, and large roughness (up to about 150 nm). The crystallinity of the above layers degrades further with growth on the ground plane with poor crystallinity. The ground plane needs to have a higher Tc than those of the above layers to reject excess magnetic fluxes from the circuit. In addition, the large roughness including the surface particles causes a serious problem such as a short circuit between the layers. J. Mater. Res., Vol. 16, No. 10, Oct 2001
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Among the various film preparation methods, liquidphase 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 fabricate the REBa2Cu3Oy (REBCO, RE ⳱ Y, Nd, etc.) films.5–18 The LPE grown film has achieved a critical temperature of over 90 K,10,11 an extremely flat
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