Defectand Strain Engineering Coenhanced NanoscaleFerroelectricity in SrTiO₃ Thin Films

Tensile biaxial strain has been demonstrated to induce in-plane ferroelectricity in SrTiO₃ thin films at room temperature. However, out-of-plane ferroelectricity is more favorable for electronic device applications. Here, we report the achievement of room-temperature out-of-plane ferroelectric SrTiO₃ thin films with giant tetragonality (<i>c</i>/<i>a</i> ∼ 1.061) and an ultrahigh ferroelectric stablity temperature (>1000 K) through epitaxial strain and defect engineering. Optical second-harmonic generation (SHG) proves that the enhancement of tetragonality enables improved ferroelectricity. Moreover, a combination of scanning transmission electron microscopy (STEM) and X-ray absorption near-edge spectroscopy (XANES) reveals the origin of enhanced tetragonality and strong ferroelectricity in defect- and strain-codriven supertetragonal SrTiO₃ thin films. Our findings present an approach to material design that can be extended to other material systems for the enhancement of ferroelectricity and the observation of emergent phenomena.