Intermodulation Distortion in Epitaxial Y-Ba-Cu-O Thick Films and Multilayers

We report intermodulation distortion (IMD) in high-quality epitaxial YBa₂Cu₃O_7-delta (YBCO) thick films and multilayers prepared by pulsed-laser deposition (PLD) using multiple targets. Two sets of films were prepared: single-layer and multilayer YBCO films in which a 20-nm-thick CeO₂ inter layer is inserted between every 100-nm-thick YBCO layer. With increasing YBCO thickness from 200 nm to 1200 nm, single-layer films exhibited improvement of IMD, whereas multilayer films showed degradation of IMD. Similarly, YBCO crystalline quality in single-layer films was improved with increasing the thickness, while that in multi-layers films degraded. These results suggest that there is a strong correlation between crystalline quality and IMD for YBCO films. The comparison of experimental data with the theory of IMD suggests that improving crystalline quality is essential for YBCO films with thickness greater than 1 mum to achieve an intrinsic nonlinear behavior. PLD using multiple targets is a very useful method to grow thick YBCO films with high crystalline quality and low IMD.