Colossal Room-Temperature Ferroelectric Polarizations in SrTiO₃/SrRuO₃ Superlattices Induced by Oxygen Vacancies

Artificial superlattices have demonstrated many unique phenomena not found in bulk materials. For this investigation, SrTiO₃/SrRuO₃ paraelectric/metallic superlattices with various stacking periods were synthesized via pulsed laser deposition. A robust room-temperature ferroelectric polarization (∼46 μC/cm²) was found in the superlattices with 2 unit cell (u.c.) thick SrRuO₃ layers, despite the fact that neither SrTiO₃ nor SrRuO₃ is inherently ferroelectric. Results obtained from atomically resolved elemental mapping and X-ray photoelectron spectroscopy verified that oxygen vacancies accumulated at the SrTiO₃/SrRuO₃ interfaces, causing lattice distortions and increased tetragonality (c/a). The observed ferroelectric responses can be mainly attributed to the broken spatial inversion symmetry induced by the ordered distribution of oxygen vacancies at the SrTiO₃/SrRuO₃ interfaces, coupled with the triggering of external electric field. The resulting polarization mechanism induced by oxygen vacancies suggests viable ways for improving the electrical properties of ferroelectric materials, with the goal of expanding the functionality of a range of electronic devices.