Defect accommodation in off-stoichiometric (SrTiO 3 )n SrO Ruddlesden–Popper superlattices studied with positron annihilation spectroscopy

he low dielectric loss underlying the record performance of strained (SrTiO3)nSrO Ruddlesden–Popper films as tunable microwave dielectrics was postulated to arise from (SrO)2 faults accommodating local non-stoichiometric defects. Here, we explore the effect of non-stoichiometry on (SrTiO3)nSrO using positron annihilation lifetime spectroscopy on a composition series of 300 nm thick n = 6 (Sr1+δTiO3)nSrO thin films. These films show titanium-site vacancies across the stoichiometry series, with evidence that TiOx vacancy complexes dominate. Little change in defect populations is observed across the series, indicating the ability of Ruddlesden–Popper phases to accommodate ± 5% off-stoichiometry. This ability for defect accommodation is corroborated by scanning transmission electron microscopy with electron energy loss spectroscopy.