Search

Different SrTiO3 thin films are investigated to unravel the nature of ultra-low conductivities recently found in SrTiO3 films prepared by pulsed laser deposition. Impedance spectroscopy reveals electronically pseudo-intrinsic conductivities for a broad range of different dopants (Fe, Al, Ni) and partly high dopant concentrations up to several percent. Using inductively-coupled plasma optical emission spectroscopy and reciprocal space mapping, a severe Sr deficiency is found and positron annihilation lifetime spectroscopy revealed Sr vacancies as predominant point defects. From synchrotron-based X-ray standing wave and X-ray absorption spectroscopy measurements, a change in site occupation is deduced for Fe-doped SrTiO3 films, accompanied by a change in the dopant type. Based on these experiments, a model is deduced, which explains the almost ubiquitous pseudo-intrinsic conductivity of these films. Sr deficiency is suggested as key driver by introducing Sr vacancies and causing site changes (FeSr and TiSr) to accommodate nonstoichiometry. Sr vacancies act as mid-gap acceptor states, pinning the Fermi level, provided that additional donor states (most probably Ti_Sr) are present. Defect chemical modeling revealed that such a Fermi level pinning also causes a self-limitation of the Ti site change and leads to a very robust pseudo-intrinsic situation, irrespective of Sr/Ti ratios and doping.

The defect chemistry and electronic trapping energies in undoped single crystalline SrTiO3 were examined by electrochemical impedance spectroscopy (EIS) at low (25-160°C) and intermediate (500-700°C) temperatures. At intermediate temperatures, the electronic and ionic conductivity as well as the chemical capacitance of SrTiO3 were determined as a function of T and p(O2) by employing a modified transmission line equivalent circuit to accurately describe the measured system. Defect modelling based on chemical capacitance measurements is established as a new method to determine the concentrations and the thermodynamic properties of ionic and electronic defects in SrTiO3. This method has potential for a wide application for mixed ionic and electronic conducting materials. Impedance spectroscopy at low temperatures was used to further quantify the electronic trapping energies of the main ionic defects of SrTiO3. Utilization of the chemical capacitance allows the establishment of a defect model based solely on electrochemical measurements, which correctly predicts the conductivity and the chemical capacitance, unveiling the concentrations of internal defects. This analysis yields a concentration of 6 ppm for acceptor-type titanium vacancies in the investigated SrTiO3 single crystals, which was experimentally confirmed by complementary Positron Annihilation Lifetime Spectroscopy measurements. The employed method is sensitive for electronically relevant defects in concentrations even below 1 ppm.

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 RSM experiments 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.