A comparison of Schottky-limited and space-charge-limited currents in SrBi2Ta2O9 thin films

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Abstract
Stoichiometric SrBi2Ta2O9 (SBT)-Pt contact exhibits a Schottky contact with a barrier height of 0.9 eV, but small amounts of excess Bi produce ohmic contacts at low voltages, followed by perfectly quadratic space-charge-limited current-voltage dependences (SCLC). Attempts to fit Schottky models to the temperature dependence of leakage currents in Bi-rich SBT films yield unphysical results, including a negative barrier height. We have generalized the SCLC theory of Rose to incorporate the temperature dependence of the dielectric constant. This theory has a single parameter T* which is a measure of trap energy distribution. A single T* value fits our data at all temperatures and all voltages. These macroscopic leakage current data are correlated with X-ray photoelectron spectroscopy (XPS) depth profiles that show how excess Bi has diffused into the electrode-ferroelectric interface region.