Characterization of epitaxial SrTiO 3 /YBa 2 Cu 3 O 7 layers deposited on (001) MgO by laser ablation
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L. Correra and S. Nicoletti CNR-Istituto LAMEL, via Castagnoli 1, 1-40127 Bologna, Italy (Received 29 March 1993; accepted 14 July 1993)
We discuss the results obtained for SrTiO 3 /YBa 2 Cu 3 O7 layers deposited on (001) MgO substrates by UV pulsed laser ablation. Different samples were prepared to study both the growth of a thin (55 nm) layer of SrTiO3 on MgO and the successive epitaxy of a 220 nm YBa 2 Cu 3 0 7 (YBCO) film on the SrTiO3 layer. An x-ray diffraction (XRD) texture analysis is reported for the bilayers together with resistivity versus temperature and critical current density (/ c ) measurements of the superconducting films. The results show that YBCO grains grow with c-axis normal to the surface; the main in-plane orientations are [100] MgO//[100] SrTiO 3 //[100] YBCO ([010] YBCO). The XRD line broadening analysis suggests that YBCO columnar grains grow along the whole thickness of the film, also evidencing dislocations and/or faulting separated by a mean distance of 80 nm. The values obtained for the critical current of the superconductor demonstrate the effectiveness of the SrTiO3 intermediate layer in improving the structural quality of the YBCO film.
I. INTRODUCTION High temperature superconducting materials have demonstrated superior performances in microwave applications. Because of YBCO low surface resistance, thin films can be patterned in planar, compact devices such as resonators, delay lines, or filters with high-Q and low losses.1"3 With this aim, the availability of high-quality films on substrates with satisfactory dielectric properties represents the crucial point in the microwave applications of high Tc superconductors. While perovskite materials are the best candidates for film deposition, they often exhibit poor dielectric properties at high frequencies (50-100 GHz). LaA10 3 has recently been demonstrated to satisfy rf requirements such as low dielectric constant and loss tangent, but it has a twinned structure that results from a phase transition at 470 °C.4 On the other hand, MgO and A12O3 crystals show good dielectric properties, but the dependence on substrate surface preparation and the interdiffusion make them inappropriate for high Tc thin film deposition. In fact, the high reactivity between film and substrate heavily affects the superconducting properties of YBCO grown on bare sapphire.5'6 Meanwhile, the large mismatch between YBCO and MgO makes the epitaxial growth strongly dependent on the surface preparation.7'8 Even if highquality superconducting films with a very low surface resistance have already been obtained on this substrate,1 a
'Address correspondence to this author.
2780 http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 11, Nov 1993
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the YBCO layer generally shows a high density of large angle grain boundaries resulting in poorer transport properties than those of films grown on perovskites. To prevent these problems oxide intermediate layers, such as CeO 2 , SrTiO3, or YSZ, are generally deposited before the YBCO depositio
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