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 undoped 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 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 SrTiO3 thin films.

Investigation of secondary hardening in Co–35Ni–20Cr–10Mo alloy using analytical scanning transmission electron microscopy

“…HAADF of cold worked and aged MP35N revealed high contrast at stacking faults and deformation twins. The contrast in a HAADF image may be a result of a higher average atomic number, sample thickness, sample preparation induced surface amorphous layer, or local crystal strain [38][39][40]. show the contrast differences observed in aged MP35N in two prior cold work conditions.…”

Defects, stoichiometry, and electronic transport in SrTiO3-δ epilayers: A high pressure oxygen sputter deposition study

“…26 These films have been incorporated in heterostructures that enable such advances as 2D normal state quantum oscillations in a superconducting system. 38 Open questions remain, however, including the origin of the frequently encountered low dopant activity, the role for Nb interstitials, 39 and the ultimate l possible in films. It should also be mentioned that a number of poorly understood technical factors appear to play a role in the STO growth.…”

Homogeneity and variation of donor doping in Verneuil-grown SrTiO3:Nb single crystals

“…Epitaxial thin films grown by pulsed laser deposition at relatively low temperatures often show cluster-like distortions of the perovskite lattice. In some cases, a correlation with inhomogeneities of the Nb concentration16 or the Nb valence17 has been reported, while in other cases no such correlation was observed18.…”