Anisotropic-strain-relaxation-induced crosshatch morphology in epitaxial SrTiO₃/NdGaO₃ thin films

We investigate the strain relaxation and surface morphology of epitaxial SrTiO₃ (STO) films grown on (001)_O and (110)_O planes of orthorhombic NdGaO₃ (NGO), and (001) plane of cubic (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 (LSAT) substrates. Although the average lattice mismatches are similar, strikingly regular crosshatched surface patterns can be found on STO/NGO(001)_O[(110)_O] films, contrary to the uniform surface of STO/LSAT(001). Based on the orientation and thickness dependent patterns and high-resolution x-ray diffractions, we ascribe the crosshatch morphology to the anisotropic strain relaxation with possibly the 60° misfit dislocation formation and lateral surface step flow in STO/NGO films, while an isotropic strain relaxation in STO/LSAT. Further, we show that the crosshatched STO/NGO(110)_O surface could be utilized as a template to modify the magnetotransport properties of epitaxial La_0.6Ca_0.4MnO₃ films. This study highlights the crucial role of symmetry mismatch in determining the surface morphology of the perovskite oxide films, in addition to their epitaxial strain states, and offers a different route for designing and fabricating functional perovskite-oxide devices.