We have performed experiments on steady state leakage currents through insulating thin films of high permittivity dielectrics such as strontium-titanate (STO) and barium-strontium-titanate (BST) in order to contribute to the understanding of the mechanism limiting the current. Furthermore, as BST is a candidate as a dielectric in the capacitor of future Gb-generation DRAM cells, the absolute value of the current in dependence of external parameters is interesting for the application.
Besides external parameters such as applied voltage and temperature we have investigated the dependence on dielectric film thickness between 20 nm and 150 nm with symmetrical Pt electrodes. For some selected films the influence of asymmetric electrodes has also been investigated: While the bottom electrode was always Pt, the top electrodes were Pt, Au and Al. An epitaxial STO film with oxide electrode bottom electrode has also been tested.
The main results are as follows:
The field dependence of the leakage current quite often shows a linear behavior in a “Schot-tky”-plot, i.e. log (current density) vs. sqrt (applied field). At constant applied field the thinner films show much lower leakage currents than the thicker ones. The leakage currents increase using Pt, Au and Al as cathode, respectively. The leakage in the epitaxial STO film is much smaller with an oxide electrode (SrRuO3) as cathode than with a platinum one.
All these results will be discussed within a modified model, in which the current is bulk-limited, and compared to numerical simulation results of this model.