Preparation and Properties of Laser Deposited High T_c Films

Abstract

High T_c superconductor films were prepared by laser deposition using a SIEMENS XP 2020 excimer laser. Epitaxial YBaCuO films of high quality are obtained for a uniform laser beam energy density of about 4 J/cm², a substrate temperature above 730 °C and an oxygen pressure of about 0.4 mbar. These films show high DC critical current densities as 5×10⁶ A/cm² at 77 K or 6×10⁷ A/cm² at 4.2 K and zero field and excellent microwave properties as surface resistance or attenuation constant. The performance of first passive microwave devices made from these films will be discussed. Using identical parameters, LaAlO₃ inter-layers grow epitaxially on the YBaCuO films and are therefore used as insulating layers in multilayer structures. Also epitaxial Bi₂Sr₂CaCu₂O_x films were prepared on SrTiO₃ substrates. The inductively measured T_c,onset reached 83 K. Critical current densities up to 10⁷ A/cm² at 4.2 K and 10⁶ A/cm² at 50 K were obtained. In magnetic fields, these films show a dramatic dependence on the field direction, with j_C being almost field-independent (up to 7 T) at low temperatures, when the field lies within the film plane.