Electronic characteristics of the SrBi2Ta2O9–Pt junction

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Abstract
The voltage and film composition dependence of leakage currents of ferroelectric SrBi2Ta2O9 thin films, sandwiched between Pt has been studied. Schottky emission dominated the leakage current at voltages above the ohmic conduction regime, while space charge limited currents (SCLC), for which the observed temperature dependence is correctly predicted in Rose’s theory, appeared to dominate the leakage current in high conductivity SrBi2Ta2O9 thin films including bismuth-excess samples. A consequence of the latter was the observation of negative differential resistivity in high conductivity SrBi2Ta2O9 thin films. X-ray photoemission spectroscopy depth profiling indicated that Bi has diffused into the ferroelectric–metal interface and also influenced the electronic conduction mechanism of the ferroelectric capacitors. Confirmation of this was found through the current–voltage dependence of Pt/SrBi2Ta2O9/Bi, which is compatible with space charge limited currents. The theory of Rose was successfully applied to the temperature and voltage dependencies of leakage currents in Bi-excess SBT; this is the first application of such theories to ferroelectric films and results in a generalization of the Child’s Law approximation for space-charge-limited currents. A single fitting parameter T*=315±20 K satisfied data at the voltages 3<V<4 and temperatures 300 K<T<400 K most useful for engineering device applications.