Ferroelectricity in SrTiO₃ epitaxial thin films via Sr-vacancy-induced tetragonality

SrTiO₃ is one of the most widely studied perovskite oxides that exhibit quantum paraelectric behavior. Being at the border of the paraelectric-ferroelectric transition, SrTiO₃ is known to become ferroelectric at a finite temperature by exploiting various approaches including epitaxial strain, oxygen isotope exchange, and dimensionality. In this study, owing to the state-of-the-art pulsed laser epitaxy, a selective engineering of elemental vacancies is proposed for the emergence and systematic control of the ferroelectricity in SrTiO₃ thin films. Therein, Sr vacancy plays an essential role in inducing the cubic-to-tetragonal transition, resulting in the inversion symmetry breaking necessary for the switchable electric polarization. Furthermore, the tetragonality is enhanced with the increase in the Sr vacancies, eventually strengthening the ferroelectricity. Our research summarizes the tetragonality-induced ferroelectricity in SrTiO₃ and provides facile growth control of the behavior.