Control of porosity and composition in large-area YBCO films to achieve micrometer thickness and high Jc on sapphire substrates

Relatively thick YBCO thin films (thickness d>0.5 μm) ideal for fault current limiter as well as microwave applications have been successfully prepared by large-area pulsed laser deposition (PLD) on CeO₂-buffered sapphire substrates. The attainment of an unusually high film thickness (up to 2.0 μm) without microcracking is attributed in part to the presence of pores correlated with yttrium-rich composition in the films. The effect of using YBCO targets with varied Y:Ba:Cu ratios on film porosity and surface morphology was investigated in detail. Using the optimum target composition, uniform critical current densities (J_c) ranging from ∼4.5 MA/cm² at d∼0.23 μm to ∼1.75 MA/cm² at d∼0.6 μm have been achieved. Characterization of a film with d∼0.5 μm showed low microwave surface resistance values [R_s(77.3 K)≈2.09 mΩ and R_s(20 K)≈227 μΩ at 22 GHz] comparable to the best YBCO films reported by other studies.