Abstract
A study on charge transport properties of thin film Fe-doped SrTiO3 epitaxially grown on Nb-doped SrTiO3 is reported. Electric measurements between 350 °C and 750 °C show a transition from predominant ionic to electronic conduction and lower conductivity of the thin films compared to the bulk of polycrystalline samples. Defect chemical changes at elevated temperature were investigated by applying a bias voltage. A model is described which successfully predicts additional features such as inductive loops or extra semicircles measureable by impedance spectroscopy as well as the complicated time dependence of electric DC-measurements. With this model it is also possible to calculate the negligibly small ionic conductivity next to the dominating electronic conductivity in the high temperature regime. The ionic conductivity is referenced by oxygen isotope depth profiling. Changes of resistive states in Fe-doped SrTiO3 thin films at high temperature and moderate fields are compared to room temperature resistive switching phenomena at high electric fields. A conductive filament based switching process is observed at room temperature, and the capability for forming such filaments and their electric properties is further analysed using microelectrodes.
Highlights
Strontium titanate is a model electroceramic and among the best investigated oxide materials [1, 2]
Cation non-stoichiometry in STO can result from preparation at high temperature where a certain equilibrium non-stoichiometry is frozen-in at temperature where cations become immobile
For techniques working at low temperatures such as pulsed laser deposition (PLD) in this work, A:B cation ratios in perovskites which are not perfectly 1 can occur and lead to a certain low amount of cation vacancies which act as acceptor dopand and increase the lattice parameter
Summary
Strontium titanate is a model electroceramic and among the best investigated oxide materials [1, 2]. Undoped SrTiO3 behaves like a slightly p-doped material, but by varying temperature and oxygen partial pressure, its electric conductivity can be changed between predominantly electronic via either electrons or electron holes or predominantly ionic via oxygen vacancies [3,4,5,6,7]. The defect concentrations can be further modified by doping SrTiO3 with either acceptors or donors (e.g. Fe3+ or Nb5+ on the Ti4+ site). This is regularly done to achieve certain well defined electrical properties [6]. Mixed ionic electronic conductivity makes especially p-doped SrTiO3 an interesting material for solid oxide fuel cell electrodes and for sensors [8,9,10,11,12]
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