In-Situ Processing and Theoretical Model for Deposition of Laser Ablated High-T_c YBa₂Cu₃O₇ Superconducting Thin Films

Abstract

We have theoretically analyzed and developed a model for the interaction of pulsed laser beams with bulk YBa₂Cu₃O₇ targets resulting in evaporation, plasma formation and deposition of thin films. In this model, the laser generated plasma is treated as an ideal gas initially at high temperature and pressure, and is then allowed to expand in vacuum. The three-dimensional isoentropic expansion of this plasma gives rise to spatial thickness and composition variations observed in YBa₂Cu₃O₇ thin films. The solution of the hydrodynamic gas equations show that the deposition characteristics are dependent on the dimensions and temperature of the plasma, and the mass of the plasma species. The effect of pulse energy density and other parameters on the deposition characteristics is also analyzed. Some of the theoretical results are compared with the experimental values obtained by pulsed excimer laser evaporation of bulk YBa₂Cu₃0₇ targets. We also discuss some of the superconducting and the microstructural characteristics of in-situ processed epitaxial films fabricated at substrate temperatures between 500–650^°C by XeC1 laser irradiation on bulk YBa₂Cu₃O₇ targets in an oxygen ambient of 200 millitorr. An application of a positively biased ring between the substrate and target was found to reduce the deposition temperature to 500^°C, although the quality of epitaxial growth decreased significantly below 500^°C. Excellent quality epitaxial single crystal superconducting thin films were fabricated on (100) SrTiO₃ and (100) LaA1O₃ substrates at temperatures between 550–650^°C. The films were nearly defect free with minimum ion channeling yields of 3% and 3.5% for YBa₂Cu₃O₇ films on (100) LaA10₃ and (100) SrTiO₃, respectively. Superconducting YBa₂Cu₃0₇ films on (100) YSZ substrates, although epitaxial, possessed much larger defect concentrations as a result of lattice mismatch. Very high critical current densities (at 77K and zero magnetic field) exceeding 6.0 × 10⁶ Amps/cm² were obtained on YBa₂Cu₃O₇ films on (100) LaA1O₃ substrates which represents the best values quoted in the literature. The effect of silver doping on critical current densities in these films is also discussed.