Exploring Intergrowth Formation in Superconductor YBaCuO7-δ Thin Films

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Cuprate superconductors, particularly YBaCuO7-δ (YBCO), have attracted interest as high-temperature superconductors. However, strain in cuprate thin films poses challenges to their superconductivity property and their practical applications [1,2,3] . In YBCO, the superconducting current is along the Cu-O planes, resulting in anisotropic superconductivity in the a-b plane. The anisotropy nature of the superconductivity suggests that any distortion of the YBCO lattice, caused by substrate-induced strain, can negatively impact superconductivity. Therefore, it is critical to select an appropriate substrate material(s) that supports the epitaxial growth of the cuprate film, thereby minimizing strain and defect formation. By investigation of the effects of different substrates on the formation of defects and lattice strain in YBCO thin films can offer valuable insights for reducing YBCO defects, enhancing overall uniformity and promoting superconductivity in YBCO. This study examines the epitaxial growth of YBCO thin films using an aberration-corrected transmission electron microscope (AC-STEM). The YBCO thin films were grown on CeO2-buffered Al2O3 (Sapphire) and non-buffered MgO substrates. AC-STEM was employed to acquire high-angle annular dark field (HAADF) and energy dispersive spectroscopy (EDS) datasets. These data sets were used to characterize YBCO phases and obtain structural information about the interfaces and substrates, which contribute to the formation of defects and phases in YBCO thin films. The nominal YBCO stoichiometry (Y-123) of Figure 1a was determined to be present in both samples [4]. However, as demonstrated in Figures 1(c,d,f,g), the CeO2-buffered YBCO samples displayed the several missing-CuO chain intergrowths in addition to the nominal YBCO-123 phase, whereas YBCO grown on MgO exhibited high uniformity of a single Y-123 phase. Intergrowths have been demonstrated to reduce the superconducting critical temperature Tc in YBCO [5]. To explore the formation of the intergrowths within the CeO2 buffered sample and the MgO substrate sample, the structure of the substrate or buffer layers were analyzed using Fast Fourier Transformations (FFT), shown in Figures 1(e,h). By inspecting the logarithm of the modulus of the FFT we can observe the uniformity of the underlying support materials of YBCO films. The inverse-FFT images taken from different regions (1 and 2) across the CeO2 buffer layer demonstrated a greater amount of inhomogeneities and disorientation compared to a high structural uniformity in the MgO substrate in regions (3 and 4). Therefore, Y-123 phase growth is better structurally supported by MgO substrate rather than on CeO2. From observation of inhomogeneities in the support material, a greater degree of disorientation of the support materials was found to correlate with the formation of complex intergrowths in YBCO near the interface. To explain the formation of observed YBCO intergrowths in the YBCO films, we focus on interfacial effects. As shown in Figure 2 (a-c), c/a lattice parameter ratio maps were generated for each sample at interfacial regions with Y-123 phase. In comparison, the sapphire sample demonstrated greater out-of-plane elongation of the Ba-Cu pseudocubic (PC) unit cell, while the MgO sample demonstrated a more relaxed overall YBCO structure. These differences in lattice strain can facilitate the formation of the intergrowths observed in the YBCO film grown on CeO2/sapphire sample. Additionally, as displayed in Figure 2 (d-g), EDS data were collected for each sample, revealing elemental redistribution in the buffered substrate sample. In the sapphire sample, interdiffusion of Ba was observed between the YBCO film and CeO2 buffer layer. This resulted in the formation of a BaCeO3 buffer layer and alluded to the disorder at the interface. Moreover, the Ba non-stoichiometry in the YBCO film can lead to growth irregularities, helping to explain the formation of the observed intergrowths in YBCO. The interdiffusion between YBCO and CeO2 is in part enabled by the similar size of the Ba and Ce atoms. This also supports the observation of similar intergrowths in the YBCO films grown La0.67Ca0.33MnO3 substrates [5]. In contrast, the YBCO/MgO interface displayed no noticeable interfacial diffusion and suggested the importance of selecting supporting materials with a greater atomic size difference to Ba to minimize interfacial interdiffusion and prevent formation of intergrowths. This study focused on how interfacial effects and support material selection impact the YBCO film uniformity promote the growth of the nominal Y-123 phase. We emphasize that structural disorder in the supporting material, along with interfacial interdiffusion, plays a significant role in creating intergrowths in YBCO thin films. Additionally, the underlying material exerts varying levels of compressive strain on YBCO films, which can influence the formation of intergrowths through strain relaxation. The degree of strain in the YBCO films may reflect the inhomogeneity of the underlying substrate where a more structurally ordered substrate would likely produce a uniform YBCO film. Furthermore, to mitigate interdiffusion and prevent the degradation of the YBCO structure, materials with a significant atomic size difference should be considered for support materials. Overall, this study offers insights into controlling YBCO phase stability and promotes further research into material support selection to enable stable superconductive YBCO phases and maintain a high-Tc. Intergrowths in YBCO Thin Films. (a) Nominal YBCO-123 phase structure highlighting the superconductive CuO planes. (b) Expanded crystal structure of YBCO demonstrating the missing CuO layer intergrowths. (c,f) HAADF-STEM of the YBCO interfaces of CeO2 buffer layer and MgO substrate. (d,g) HAADF-STEM images at the blue and red box regions in (c,f), respectively. The yellow, green and blue dots correspond to the Y, Ba and Cu atoms. The missing atomic CuO layers are annotated by yellow arrows. (e,h) FFT and Log of Modulus inverse-FFT of YBCO grown on MgO and CeO2/Al2O3 substrates, respectively. The FFT peaks in the white circles were used to generate the corresponding iFFT images. Interfacial Strain and EDS Analysis (a) Atomic structure model highlighting the Ba-Cu and Y-Ba-Cu pseudocubic unit cells. (b,c) HAADF-STEM images overlayed with c/a lattice parameter ratio maps of the Ba-Cu and Y-Ba-Cu PC unit cells in YBCO grown on CeO2/Al2O3 and MgO substrates, respectively. (d,e) Simultaneously captured ADF images of the YBCO interfaces. (f,g) Corresponding EDS maps at the YBCO interfaces. Scale bar = 2nm.

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