Unravelling the origin of ultra-low conductivity in SrTiO${}_3$ Thin films. Sr vacancies and Ti on A-sites cause Fermi level pinning

Different SrTiO${}_3$ thin films are investigated to unravel the nature of ultra-low conductivities recently found in SrTiO${}_3$ films prepared by pulsed laser deposition. Impedance spectroscopy reveals electronically pseudo-intrinsic conductivities for a broad range of different dopants (Fe, Al, Ni) and partly high dopant concentrations up to several percent. Using inductively-coupled plasma optical emission spectroscopy and reciprocal space mapping, a severe Sr deficiency is found and positron annihilation lifetime spectroscopy revealed Sr vacancies as predominant point defects. From synchrotron-based X-ray standing wave and X-ray absorption spectroscopy measurements, a change in site occupation is deduced for Fe-doped SrTiO${}_3$ films, accompanied by a change in the dopant type. Based on these experiments, a model is deduced, which explains the almost ubiquitous pseudo-intrinsic conductivity of these films. Sr deficiency is suggested as key driver by introducing Sr vacancies and causing site changes (Fe${}_{Sr}$ and Ti${}_{Sr}$) to accommodate nonstoichiometry. Sr vacancies act as mid-gap acceptor states, pinning the Fermi level, provided that additional donor states (most probably Ti${}_{Sr}^{\bullet \bullet }$) are present. Defect chemical modeling revealed that such a Fermi level pinning also causes a self-limitation of the Ti site change and leads to a very robust pseudo-intrinsic situation, irrespective of Sr/Ti ratios and doping. (© 2022 The Authors. Advanced Functional Materials published by Wiley‐VCH GmbH)