Crystallography of YBa2Cu3O6+A; thin film-substrate interfaces Lisa A. Tietz and C. Barry Carter Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853 Daniel K. Lathrop, Stephen E. Russek, and Robert A. Buhrman School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853 Joseph R. Michael Homer Research Laboratory, Bethlehem Steel Corporation, Bethlehem, Pennsylvania 18016 (Received 6 February 1989; accepted 17 April 1989) The epitactic nature of the growth of YBa2Cu306+J (YBCO) superconducting thin films on ceramic substrates has been studied using high-resolution electron microscopy (HREM) and selected-area diffraction (SAD) of cross-sectional specimens. The films were grown in situ on (OOl)-oriented MgO and (OOl)-oriented Y2O3-stabilized cubic ZrO2 (YSZ) single-crystal substrates by electron beam evaporation. Both of these materials have large lattice misfits with respect to YBCO. Different orientation relationships were observed for films grown on the two types of substrates. These orientation relationships are shown to provide the best matching of the oxygen sublattices across the substrate-film interfaces. A crystalline intermediate layer, 6 nm thick, between the YBCO film and YSZ substrate was observed by HREM and shown by EDS to be a Ba-enriched phase, possibly barium zirconate formed by a reaction. In contrast, the YBCO-MgO interface was found to be sharp and free of any intermediate layers. I. INTRODUCTION Superconducting thin films of YBa2Cu306+, (YBCO) have been successfully grown, now, using almost all of the conventional thin-film techniques including electron-beam evaporation,1"4 thermal evaporation, 5 sputtering,6"8 and laser ablation.9"12 Typically, the films are deposited under vacuum onto heated substrates (T < 500 °C). Such as- deposited films are, in general, not superconducting and may even be amorphous. However, the superconducting phase can be obtained by an ex situ, high-temperature (800-900 °C) oxygen anneal. Recently, research has been focused on lowering the processing temperatures in order to minimize problems with cracking due to thermal stresses and reaction of the film with the substrate material that have been associated with degradation of the supercon- ducting properties of the film. Using the electron-beam evaporation technique, Lathrop et al. showed that high qual- ity superconducting thin films of YBCO can be grown in situ by depositing onto substrates heated to 600-700 °C and providing a source of oxygen to the film during deposition and cooling.3 Variations of this method have subsequently been developed and successfully applied to other thin film techniques.4"7' 12' 13 As-deposited superconducting YBCO thin films have been grown using these low-temperature methods with Tc up to 91 K and Jc > 106 A/cm2 at 77 K with no magnetic field applied.12 The success of thin-film technologies over preparation by bulk sintering in achieving high critical-current densi- ties has been attributed to the microstructural control and compositional purity, particularly at the grain boundaries, attainable with these techniques. In general, YBCO thin films have been found to be polycrystalline but they can exhibit large areas of highly oriented material. Such micro- structures take advantage of the anisotropy of the super- conducting properties of YBCO to provide a path for high current densities. Highly oriented films have been grown on a wide variety of single-crystal substrates. The most ex- tensive results of the effects of the substrate on thin-film microstructure have been reported for YBCO films grown on SrTiO3 which has a "perovskite"-type structure and only a 0.5-2.1% lattice misfit with respect to YBCO. Films grown on (lOO)-oriented SrTiO3 are usually predominantly oriented with their c-axis normal to the substrate, while (HO)-oriented SrTiO3 gives predominantly (HO)-oriented YBCO (i.e., the c-axis is parallel to the substrate surface), suggesting that the growth is epitactic.14"18 While these re- sults are encouraging, from a practical point of view, other substrate materials which often have high lattice misfits with respect to YBCO and quite different crystal structures (e.g., MgO, ZrO2, A12O3, Si) must also be considered. The question then arises, how does the substrate affect the microstructure of YBCO thin films and how can it be used to control it? In fact, microstructural control has been shown to be a complex proposition depending on deposition con- ditions, film composition, and choice of substrate. 1' 7' 19 This study addresses only the role of the substrate. In addition to microstructural effects, the nature of the substrate can affect the composition of the film. Numerous studies of YBCO thin films have shown that interdiffusion or reaction can occur with almost all substrate materials including Si, A12O3, SrTiO3, ZrO2, and MgO (see, for ex- ample, Refs. 1, 16, and 19-22). In some cases, the diffu- 1072 J. Mater. Res., Vol. 4, No. 5, Sep/Oct 1989 © 1989 Materials Research Society