Structural uniformity and grain formation in inch-scale few-layer YBa2Cu3O7 <b>−</b> <i>x</i> epitaxial films on MgO

Here, we report a systematic study of structural uniformity and grain formation in the initial layers of epitaxial YBa2Cu3O7−x (YBCO) films grown on MgO substrates by pulsed laser deposition. On a 2-in. YBCO film, we performed synchrotron x-ray crystal truncation rod measurements and three-dimensional reciprocal space mapping to assess the spatial homogeneity and grain texture across the entire 2-in. film. The results reveal an obvious film quality variation from center to edge, with significantly better crystallinity and atomic coverage near the center. More importantly, we find that the ultra-thin YBCO film is predominantly composed of c-axis-oriented R0° grains. The extracted lattice parameters are close to those of bulk YBCO, indicating that substrate-induced strain is largely relaxed within the initial few unit cells. These findings strongly suggest that the formation of R0° grains at the early growth stage is primarily governed by interfacial effects at the YBCO/MgO interface, whereas the emergence of R45° grains in thicker films is driven mainly by the film growth specifics. Our insights provide critical guidelines for further optimizing the fabrication of the large-area YBCO film on MgO for applications in ultra-low-loss superconducting microwave devices.