Strain evolution in non-stoichiometric heteroepitaxial thin-film perovskites

Abstract

Epitaxial strain has been extensively used to control and induce new properties in complex oxide thin films. Understanding strain evolution and how to manipulate it are essential to the continued development of strain-induced effects in materials. The chemical complexity that underlies the diverse functionality of such oxide materials can have complex and unexpected effects on strain evolution and the breakdown of classic models of strain relaxation (e.g., misfit dislocation formation). We explore the connection between the growth process and the diverse and extensive range of point and volumetric defects that can be generated and accommodated in oxide systems and how this ultimately impacts strain evolution. Pulsed-laser deposition was used to produce thin films of the prototypical perovskite oxide SrTiO₃ with chemical compositions ranging from 4% Sr-deficiency to 4% Sr-excess. Small variations in film composition are found to give rise to three distinct modes of strain relaxation, critical thicknesses for relaxation that vary from 60 to 300 nm, and have a notable impact on interfacial intermixing. Atomic-scale scanning transmission electron microscopy and spectroscopic studies provide information on defect structures, how the defect formations are connected to the epitaxial strain relaxation, and reveal that the presence of defect structures generally leads to a local chemical broadening of the interface.