Observation of electrically-inactive interstitials in Nb-doped SrTiO _3

摘要

Despite rapid recent progress, controlled dopant incorporation and attainment of high mobility in thin films of the prototypical complex oxide semiconductor SrTiO_3 remain problematic. Here, analytical scanning transmission electron microscopy is used to study the local atomic and electronic structure of Nb-doped SrTiO_3 both in ideally substitutionally doped bulk single crystals and epitaxial thin films. The films are deposited under conditions that would yield highly stoichiometric undoped SrTiO_3, 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 SrTiO_3 films compared to undoped SrTiO_3, but without the clear shift in the Fermi edge seen in bulk single crystal Nb-doped SrTiO _3. Analysis of atomic-resolution annular dark-field images allows us to conclude that the interstitial Nb is in the Nb~0 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 SrTiO _3 thin films.