Preparation and comparison of YBa 2 Cu 4 O 8 films on MgO(100) and SrTiO 3 (100) by MOCVD
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The thin films (Y/Ba/Cu = 1.0/2.7/4.7) which have a strong intensity corresponding to the c-axis peaks of the 124 phase (YBa 2 Cu 4 0 8 ) in x-ray diffraction (XRD) patterns were prepared. The film structure and surface morphology of these films were observed by scanning electron microscopy (SEM), energy dispersive x-ray (EDX), transmission electron microscopy (TEM), and atomic force microscopy (AFM). It was found that the film on MgO(lOO) clearly has grain boundaries with many a-axis oriented grains of the 123 phase (YBa^usOv-^). However, the film on SrTiO3(100) had a smooth surface, and a-axis oriented grains of the 123 phase couldn't be observed. This difference could not be explained by only the difference in the lattice mismatch.
I. INTRODUCTION Since Karpinski et al.1 discovered the 124 phase (YBa 2 Cu 4 0 8 ), this superconductor has been prepared by many different kinds of methods. In the case of bulk processing, the 124 phase has been synthesized mostly under high oxygen partial pressure, such as the O 2 -HIP process.2 In the case of a chemical solution process, Cava et al.3 and Murakami et al.4~6 succeeded in normal-pressure synthesis of the 124 phase, though its synthesis had been considered to be impossible without the application of high pressure. YBCO (YBa 2 Cu 3 0 7 -^ or YBa 2 Cu 4 0 8 ) thin films have been studied extensively by using both physical vapor deposition (PVD) and chemical vapor deposition (CVD). However, except for our results,7"10 no report has been published on the preparation of the 124 thin films without postannealing. The 124 phase is superior to the 123 phase (YBa 2 Cu 3 0 7 _ j: ) in terms of the thermodynamical stability of oxygen content under normal conditions.11 The oxygen content of the 124 phase is stable at high temperatures (up to about 800 °C). On the contrary, the 123 (YBa 2 Cu 3 0 7 -^)oxygen loss is large at temperatures above 400 °C.4 Although the Tc value (80 K) of the 124 phase is lower than that (90 K) of the 123 phase, Miyatake et al.12 have reported that it could be increased from 80 K to 90 K by the partial substitution of Y by Ca. The 124 phase is consequently expected to be used for practical applications. We have studied processing to obtain the 124 single phase films by metal-organic chemical vapor deposition (MOCVD). The 124 phase diagrams and the relationship between oxygen partial pressure and film composition have been reported on previously.7 It was possible to obtain the films that had the 124 phase at the oxygen partial pressure of 7.5 Torr and higher. This pressure is J. Mater. Res., Vol. 8, No. 9, Sep 1993
much higher than those used by several other researchers fabricating YBCO thin films. This result indicates that it was very important to have a high oxygen partial pressure to obtain the as-grown 124 films. As a result of x-ray diffraction (XRD) analysis of the films, the c-axis peaks of the 124 phase and the 123 phase have been identified in the case of both MgO(100) and SrTiO3(100). In the case of the films on MgO(100), the 124 phase was id
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