Electrical-transport properties and microwave device performance of sputtered TlCaBaCuO superconducting thin films

Abstract

Tl-Ca-Ba-Cu-O high-temperature superconducting thin films were deposited on lanthanum aluminate substrates, by rf magnetron sputtering and postannealing methods. A reproducible fabrication process with low-resistance metal contacts has been established for high-Tc and high-Jc TlCaBaCuO thin films after patterning using standard microelectronic photolithography and wet chemical etching techniques. Low-resistance gold contacts on TlCaBaCuO thin films were obtained by annealing in an oxygen flow of 1 l/min followed by a slow furnace cooling. Specific contact resistivity was approximately 10−10 Ω cm2 below 77 K. High transition temperatures as high as 100 K, and current density at zero magnetic field greater than 105 A/cm2 are routinely obtained in 0.3–0.5 μm TlCaBaCuO thin films. The morphology studies of the films using scanning electron microscopy show the correlation between Jc and the microstructure of the films. Films with featureless morphology have larger zero-field transport currents. The microwave properties of TlCaBaCuO thin films were investigated by designing, fabricating, and characterizing microstrip ring resonators with a fundamental resonance frequency of 12 GHz on 10-mil-thick lanthanum aluminate (LaAlO3) substrates. Ring resonators with a superconducting ground plane of 0.3 μm thickness and a gold ground plane of 1 μm thickness were fabricated and characterized in the temperature range of 60–95 K. Typical unloaded quality factors Q for the ring resonators at 12 GHz were above 1500 at 65 K, compared to an unloaded Q of 370 for a gold ring resonator. A surface resistance as low as 1.5 mΩ at 12 GHz and 77 K was obtained in 0.3 μm TlCaBaCuO thin films using the ring resonator Q measurements. Typical values of penetration depth at 0 K in the TlCaBaCuO thin films were determined to be between 7000 and 8000 Å using the temperature variation of resonance frequency measurements.