Observation of Electrically-Inactive Interstitials in Nb-Doped SrTiO₃

Abstract

Despite rapid recent progress, controlled dopant incorporation and attainment of high mobility in thin films of the prototypical complex oxide semiconductor SrTiO₃ remain problematic. Here, analytical scanning transmission electron microscopy is used to study the local atomic and electronic structure of Nb-doped SrTiO₃ both in ideally substitutionally doped bulk single crystals and epitaxial thin films. The films are deposited under conditions that would yield highly stoichiometric <i>undoped</i> SrTiO₃, but are nevertheless insulating. The Nb incorporation in such films was found to be highly inhomogeneous on nanoscopic length-scales, with large quantities of what we deduce to be interstitial Nb. Electron energy loss spectroscopy reveals changes in the electronic density of states in Nb-doped SrTiO₃ films compared to undoped SrTiO₃, but without the clear shift in the Fermi edge seen in bulk single crystal Nb-doped SrTiO₃. Analysis of atomic-resolution annular dark-field images allows us to conclude that the interstitial Nb is in the Nb⁰ state, confirming that it is electrically inactive. We argue that this approach should enable future work establishing the vitally needed relationships between synthesis/processing conditions and electronic properties of Nb-doped SrTiO₃ thin films