Anisotropic in-plane lattice strain relaxation in brownmillerite SrFeO _2.5 epitaxial thin films

Anisotropic in-plane lattice relaxation behavior of brownmillerite SrFeO_2.5 epitaxial thin films grown on (110) DyScO₃ substrates was investigated. The in-plane lattices in the films less than 50 nm thick are fixed by the substrate lattice, whereas partial in-plane lattice relaxation along the [010] direction occurs in a 50 nm thick film. When the thickness reaches 98 nm, the film eventually exhibits lattice relaxation in both the [010] and the [10-1] in-plane directions. In the bottom region of the partially relaxed film, a dislocation, at which additional Fe atoms are seen, leads to formations of the stacking faults. In the surface region of the film, the complicated lattice defects propagated from the bottom result in the partial in-plane lattice relaxation associated with the disordered arrangements of the FeO₄ tetrahedra and the FeO₆ octahedra in the surface region. The preferential generation of the dislocations in the (10-1) plane can be explained by taking into account the anisotropic thermal expansion of SrFeO_2.5, which results in the increase in the lattice mismatch between the film and the substrate only along the [010] direction in the cooling process after the film deposition.