Polar Domains in Strained SrTiO₃ Films

We report on the microscopic mechanisms of how the incipient ferroelectric SrTiO₃ transforms into a ferroelectric by epitaxial strain. These films have recently been shown to have enhanced superconducting transition temperatures when doped, pointing to a relationship between superconductivity and ferroelectricity order parameters. Here, using high-angle annual dark-field scanning transmission electron microscopy, we detect local polar regions at room-temperature in compressively strained and unstrained SrTiO₃ films, respectively. Our image analysis uses the average deviation of the Ti column from the cubic position as the polar order parameter. We find that unstrained SrTiO₃ stays paraelectric while compressively strained films show short-range cooperative displacements within nano polar regions at room temperature. The observation of polar regions may explain optical second harmonic generation (SHG) studies, showing a residual SHG signal at room temperature in strained SrTiO₃. Oxygen annealing suppresses the formation of polar regions, highlighting the importance of free carriers for the stability of the polar regions.