Leakage Current Experiments in STO and BST Thin Films
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U6.2.1
Leakage Current Experiments in STO and BST Thin Films
Herbert Schroeder and Sam Schmitz Institut für Elektrokeramische Materialien im Institut für Festkörperforschung, Forschungszentrum Jülich GmbH D-52425 Jülich, Germany
ABSTRACT
We have performed experiments on steady state leakage currents through insulating thin films of high permittivity dielectrics such as strontium-titanate (STO) and barium-strontiumtitanate (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 “Schottky”-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 bulklimited, and compared to numerical simulation results of this model.
INTRODUCTION Barium-strontium-titanate (BST) is a candidate as a high dielectric constant material for application in the capacitor of a DRAM cell in future Gb DRAM generations. Although leakage current data in thin film metal-insulator-metal (MIM) capacitor structures with BST as dielectric are sufficiently small and will probably meet the goals for application [1] there is no generally accepted mechanism describing the leakage current density through these thin films. Most often the current is assumed to be electrode limited [2-4], either controlled by fieldenhanced thermionic emission (i.e. injection over a “Schottky” barrier) only [2] or with additional tunnel injection [3,4]. But also bulk-limited mechanisms have been suggested for interpretation, e.g. “Poole-Frenkel” [5] or space charge limited currents [6].
U6.2.2
In order to contribute to the understanding, we have performed systematic investigations on leakage current densities in dependence of various parameters (applied field, temperature, and thickness of dielectric, electrode material and treatment) in BST thin films and also in strontiumtitanate (STO), which serves as a kind of model material. After shortly describing the preparation and characterization of the sa
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