Systematic study on Al-doped $PrBa_2Cu_3O_7$ thin film for SIS Josephson junction applications

Abstract

I have vacuum-deposited c-axis and (110) oriented $PrBa_2(Cu_{0.8}A_{l0.2})_3O_7$ thin films on (001) $LaAlO_3$ and (110) $SrTiO_3$ substrates by rf-sputtering and Pulsed Laser Deposition (PLD). The initial growth stage in PLD process was studied by Atomic Force Microscopy (AFM) method. The oxygen partial pressure and thickness effect on thin film surface and property also has been studied. The surface smoothness and crystallinity decreased as thin film thickness increases. Thickness and stoichiometry distribution of the thin films were studied by Rutherford Backscattering Spectrometry (RBS) measurement. From the temperature dependence of electrical resistivities for both the target and the films, we found that the electrical conduction was mainly through the variable range phonon-assisted hopping mechanism. This suggests that Al-doping in this material has given rise to extensive impurity states in the band gap, which, in turn, localized the charge carriers, especially at lower temperatures. The resistivity of the doped $PrBa_2Cu_3O_7$ (PBCO) is much higher than the pure PBCO, ranged from ~1 Ω -cm at room temperature to about five orders of magnitudes higher at ~30 K. The electronic structure of the samples was studied through measuring the optical absorption coefficients vs. photon energy from which the band gap and defect state energies were inferred. Soft x-ray emission spectra obtained for pure and Al-doped PBCO thin films show shift in Cu Lα peak position and change in Cu Lα / $L_β$ intensity ratio upon doping. This indicates the change of Cu-O bonding due to the partial Al substitution. In addition, Raman spectra from the Al-doped PBCO thin films exhibit an additional peak near the apical oxygen peak, a result suggestive of the broken symmetry of the Cu-O chains caused by the partial replacement of Cu-atoms with the Al on the Cu-O chains. C-axis oriented YBCO films are very common but not well suited for the fabrication of sandwich-type superconductor-no...