Study On Tl-based HTS Thin Films For Microwave Applications

Large area Tl2Ba2CaCu2Og (Tl-2212) superconducting thin film is one of the best material candidates for the fabrication of passive microwave device due to its several advantages such as high critical temperature, high critical current desity, low microwave surface resistance and the stability in the working environment. Among the many substrate materials, sapphire and MgO sigle-crystal substrate are more suitable for microwave device application because of theire moderate dielectric constant and low dielectric loss.This dissertation is mainly focused on the fabrication of Thallium-based HTS thin films on sapphire and MgO substrates for microwave application. The influence of parameters during the fabrication processes on the crystal structure and the superconducting properties of the HTS films are studied. Based on these works, large area double-sided Tl-2212 HTS films with the diameter of 2 inch are fabricated. The main contents and results are as follows:1,Using the metal cerium target as the sputtering source, CeO2 buffer layer is deposited on r-cut sapphire substrates by RF magnetron reactive sputtering technique, and the influence of different sputtering parameters on the crystalline structure and surface morphology of CeO2 film is studied. The conclusion can be drawn that CeO2 buffer layer deposited under optimum conditions has excellent crystalline quality and surface morphology, and the 40 nm thick CeO2 buffer layer can be used as effective barrier to prevent the elements diffusion to Tl-2212 superconducting film.2,The influence of CeO2 buffer layer deposited on sapphire substrate under different conditions on the crystalline structure and superconducting properties of Tl-2212 films is studied. High quality Tl-2212 superconducting thin films are fabricated under optimum conditions. The test results indicate that the highly c-axis oriented Tl-2212 thin films are epitaxial grown on the CeO2 buffer layers, and the optimal Tc reach to 107 K,ΔT=0.3 K, Jc is around 6 MA/cm2 (77 K,0T).3,Double-sided Tl-2212 superconducting thin film is fabricated on the CeO2-buffered sapphire substrate with the area of 10 mm×10 mm. Test results show that the film is pure Tl-2212 phase with the c-axis perpendicular to the substrate surface. The film has good crystalline quality, and the films on both sides are uniform. Tc of the film reaches to 106.7 K and the Jc value at 77 K of each side is 4.25 MA/cm2 and 5 MA/cm2, respectively.4,Large area double-sided Tl-2212 superconducting thin film is fabricated on CeO2-buffered sapphire substrate. The films on both sides exhibit excellent uniform superconductivity distribution. The superconducting transition temperature Tc of the film is above 105 K,ΔT<0.4 K, and the Jc value at 77 K and 0 T of the film is above 1.5 MA/cm2, the Rs is less than 450μΩ(77 K,10 GHz).5,The effects of high-temperature anneal to MgO substrate on its crystalline structure and the CeO2 film are studied, and the growth parameters of epitaxial CeO2 film are studied; CeO2 buffer layer is fabricated on MgO substrate with the diameter of 2 inch under optimum conditions. The test result shows that high-temperature treatment to the MgO substrate could removed the deliquescence and damaged layer on the MgO surface effectively, and improved its crystalline structure. Large area CeO2 films grown under optimum conditions show excellent thickness and crystalline structure uniformity.6,The influence of CeO2 buffer layer deposited under different condition on the Tl-2212 film is studied, which provides important basis for the fabrication of large area Tl-2212 superconducting thin film.