Microstructure of epitaxial YBa2Cu3O7−x thin films

Abstract

We have investigated the microstructure of epitaxial YBa2Cu3O7−x thin films on SrTiO3(100), LaGaO3(100), and LaAlO3 (100) with particular emphasis on how the final microstructure is developed. Cross-sectional transmission electron microscopy as well as plan-view transmission electron microscopy combined with sputter depth profiling were used to study the change in microstructure with the increase in film thickness. For a thin film or near the substrate/film interface of a thick film, the YBa2Cu3O7−x film is composed primarily of grains oriented with c axis normal to the substrate surface and a small volume fraction of grains with c axis parallel to the substrate surface. As the film grows thicker, the c-axis parallel grains increase in size and grow over the top of the c-axis normal grains. The volume fraction of c-axis parallel grains increases rapidly as the film thickness increases and eventually the entire film surface is covered by c-axis parallel grains. However, the number density of the c-axis parallel grains remains constant throughout the whole film thickness. A growth model is proposed to explain the observed microstructure. Based on this model, a computer simulation is carried out. The simulated microstructure agrees well with the experimental result.

Computer Simulation of Growth Mechanisms in Y-Ba-Cu-O Thin Films

“…At lower deposition rates, the "trend is again seen to reverse. At this second transition, substrate effects now lead to the nucleation of 'c' type domains, but the growth limited kinetics quickly lead to the formation of 'a' type growth thus producing the type of 'c' to 'a' type transformation through thickness in thin fllms which is commonly reported [11,12,15,16,18,19] and also observed in the simulation. As the deposition rate is decreased further, the growth proceeds via a two-dimensional layer mechanism under which the equilibrium free energy of formation of the 'c' type phase is great enough to overcome the kinetic path afforded to 'a' type domains by the growth rate anisotropy and only 'c' type grains nucleate and grow.…”

Influence of growth rate on the structural orientation of YBCO superconducting films

“…the growth rate in [100] direction is more than 10 times higher than that in [001] direction. The density of YBCO nucleation centres for the layer-by-layer growth mechanism was estimated at N 001 ≈ 10 9 cm −2 for the (001)-phase and N 100 ≈ 2 × 10 8 cm −2 for the (100)-phase [26].…”

“…The growth rate of an YBCO single crystal is highly anisotropic: V (ab) g 10V (c) g [26]; i.e. the growth rate in [100] direction is more than 10 times higher than that in [001] direction.…”

“…Since the ratio of nucleation centre densities for YBCO film phases with different orientations was estimated at N 001 /N 100 ≈ 5 [26], formation of c ⊥ -oriented films is more probable. To make lower the internal energy, the film has to build its monolayers at the maximal rate, so that film formation along the a−b plane prevails over YBCO film growth in all other possible directions.…”