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 inductively-coupled plasma optical emission spectroscopy and positron annihilation lifetime spectroscopy were employed to reveal the cation composition and the predominant point defects of the thin films, respectively. A severe cation non-stoichiometry with Sr vacancies was found in films deposited from stoichiometric targets. The across plane electrical conductivity of such epitaxial films was studied in the temperature range of 250–720 °C by impedance spectroscopy. This revealed a pseudo-intrinsic electronic conductivity despite the substantial Fe acceptor doping, i.e. conductivities being several orders of magnitude lower than expected. Variation of PLD deposition parameters causes some changes of the cation stoichiometry, but the films still have conductivities much lower than expected. Targets with significant Sr excess (in the range of several percent) were employed to improve the cation stoichiometry in the films. The use of 7% Sr-excess targets resulted in near-stoichiometric films with conductivities close to the stoichiometric bulk counterpart. The measurements show that a fine-tuning of the film stoichiometry is required in order to obtain acceptor doped SrTiO3 thin films with bulk-like properties. One can conclude that, although reciprocal space maps give a first hint whether or not cation non-stoichiometry is present, conductivity measurements are more appropriate for assessing SrTiO3 film quality in terms of cation stoichiometry.
Highlights
SrTiO3 (STO) single crystals and thin lms are currently a cornerstone for the study of multiple phenomena such as resistive switching in memristive devices,[1,2,3,4] photoconductivity,[5] increased oxygen incorporation under UV light,[6,7] photovoltages in bulk SrTiO3,8,9 and interfacial conductivity, especially with LaAlO3.10,11 SrTiO3 thin lms are not fully understood yet and show great variability in properties depending on the fabrication conditions
We study Fe-doped SrTiO3 lms deposited using different parameters and target stoichiometries to elucidate their effect on the resulting layers
Variable energy positron annihilation lifetime spectroscopy (VEPALS) measurements were conducted on Fe:SrTiO3 samples at the Mono-Energetic Positron Source (MePS) beamline at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Germany.[29]
Summary
Despite the numerous studies performed so far, there is still only a limited understanding of the structure–composition– property relations in SrTiO3 thin lms. Conductivities of Nb-donor doped SrTiO3 thin lms have been varied by orders of magnitude by modifying the laser uence in PLD deposited layers, which was correlated with different lattice expansion and, cation non-stoichiometries.[20] the properties of conducting SrTiO3/LaAlO3 heterointerfaces have been proved to be strongly affected by small variations of the LaAlO3 composition.[21,22] Owing to this sensitivity of properties to the exact composition and/or structure of the thin lms, it is mandatory to have sensitive tools for quantitatively evaluating the quality of PLD lms and to control the deposition parameters for a proper tuning of the layers. The sensitivity of the different tools for quantitative evaluation of the cation nonstoichiometry is compared
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