A-Axis oriented superconductive YBCO thin films : Growth mechanism on MgO substrate

Abstract

The growth mechanism of a -axis oriented YBCO thin films has been studied by TEM. At 650°C, a disordered cubic perovskite is first formed with a p‖ a MgO, then a strained tetragonal a -axis oriented perovskite is observed, with c=3 a p, slightl y misoriented with respect to MgO and showing a marquetry-like contrast. At 750°C, a [1 1 0] axis oriented perovskite is formed whose lattice exhibits a rotation with respect to MgO lattice, but also a tilting of the [CuO 2] ∞ layers with respect to the substrate plane. Introduction of oxygen at this temperature leads to an a -axis oriented orthorhombic perovskite and involves the formation of a poorly crystallized interface layer. The almost perfect a -axis orientation is achieved after slow cooling (30') down to room temperature, the lattice of the “123” phase being disoriented with respect to the MgO lattice.