Twinning characteristics in YBaCuO superconducting thin films

We have analyzed the nature, distribution and number density of twins as a function of orientation of superconducting YBa₂Cu₃O₇ thin films deposited on strontium titanate (100), magnesium oxide (100) and ytria-stabilized zirconia substrates. The twinning characteristics in these high T_c superconducting thin films have been found to be different from those in bulk materials as a results of texturing, interfacial stresses and constraints during crystal growth. The free surface has been found to play an important role in the relaxation of stresses introduced in the film during tetragonal-to-orthorhombic phase transformation. Detailed electron microscopy studies were performed on superconducting thin films deposited by evaporation of bulk YBa₂Cu₃O₇ targets using a pulsed excimer laser (λ = 308 nm; τ_p = 45 × 10^-9 s). Calculations of stresses in thin films indicated that higher shear stresses and strain energies were introduced in films with the c axis normal to subtrate than in those parallel to the substrate, thereby resulting in a higher number density of twins in the first case. The electron diffraction patterns of twinned crystals were simulated to understand the crystallography of the twinning process, and high resolution transmission electron microscopy studies were performed to determine the bending of planes across the twin boundary. Good agreement between experimental observations and theoretical models was obtained.