Cation non-stoichiometry in Fe:SrTiO₃ thin films and its effect on the electrical conductivity

Abstract

The interplay of structure, composition and electrical conductivity was investigated for Fe-doped SrTiO3 thin films prepared by pulsed laser deposition. Structural information was obtained by reciprocal space mapping while solution-based...

Unravelling the Origin of Ultra‐Low Conductivity in SrTiO3 Thin Films: Sr Vacancies and Ti on A‐Sites Cause Fermi Level Pinning

“…In our specific case, some moderate changes of laser fluence and pulse frequency were performed, but turned out to be not sufficient to achieve fully stoichiometric films, while growth from targets with Sr excess yielded stoichiometric films. [71] The compositional results were confirmed by RBS. Here, a Fe doped SrTiO 3 thin film (with a nominal Fe content of 5% Fe at the B-site) on a SrTiO 3 substrate was investigated.…”

“…Please note that the Sr deficiency is also lower in Al‐doped thin films compared to the other thin films, thus decreasing the difference in lattice parameter, as also observed in ref. [60] for similar cases.…”

“…Due to this similarity we denote the measured conductivities as “pseudo‐intrinsic.” Such low values were also reported in refs. [ 37, 60 ] , but reasons are not understood so far. There must be a fundamental difference between SrTiO 3 bulk samples with acceptor‐type bulk conductivity and the SrTiO 3 thin films with pseudo‐intrinsic conductivity.…”

“…However, a first hint was reported in a separate paper: When using highly off-stoichiometric PLD targets with 5 to 7% Sr excess, SrTiO 3 films result with "normal" conductivities (i.e., like for doped bulk samples rather than pseudo-intrinsic). [60] This Sr excess in the target was shown to compensate substantial amounts of Sr vacancies otherwise present in the films. This leads to the hypothesis that additional point defects present in our films, but not in bulk SrTiO 3 , strongly reduce the electron and hole concentrations by a Fermi level pinning close to mid-gap, thus leading to the observed "pseudo-intrinsic" behavior.…”

“…[37,52] A correlation with Sr stoichiometry was found, indicating that this ultra low conductivity appears for substantial Sr vacancy concentrations. [60] In this study, numerous doped as well as undoped SrTiO 3 thin films were characterized via electrochemical impedance spectroscopy (EIS) with regard to their electrical conductivity. For various dopants (Fe, Ni, Al) and dopant concentrations (up to 10%), pseudo-intrinsic behavior with extremely low conductivities was found.…”