7 - Strain engineering during epitaxial growth of oxides

Abstract

Advances in atomically controlled synthesis of complex oxide heterostructures made possible to utilize the so-called strain engineering and this way stabilize novel structures of ABO3-type perovskite oxide films as well as unique atomic distortions at the oxide-oxide interfaces. Typically, the strain accommodation is driven by the lattice mismatch between substrate and the film. In perovskite oxides such strain accommodation is achieved not only by varying the length of interatomic bonding but also by changing B-O-B bond angle and this way modify physical properties in a distinct way. Moreover, the atomic arrangement, or symmetry, of the ABO3 lattices for the substrate and the film might be crystallographically distinct which creates unique atomic distortions at the substrate-film interfaces. Here, using advanced high-resolution x-ray diffraction (HR-XRD) we identified unique structural distortions in the lattice mismatched epitaxial perovskite oxide films and at the heteroepitaxial oxide interfaces. The distortions in the film’s bulk and at the interface are generated by two different phenomena: the lattice mismatch and the interface crystallographic symmetry mismatch. We show that the unit cell distortions at the heteroepitaxial oxide interfaces significantly differ from structural distortions away from the interface region observed in thicker coherent epitaxial oxide films. HR-XRD results reveal a formation of the novel structures at the interfaces between crystallographically dissimilar perovskite oxides. These structures are stabilized due to a unique coupling between octahedral rotations/deformations induced by the symmetry mismatch and by the octahedral rotations due to lattice mismatch. The combination of crystallographic symmetry mismatch at the interfaces with the lattice mismatch offers new routes for strain engineering of functional complex oxide heterostructures that enables emergent physical phenomena and offers potential for future electronic devices.