Ruthenium oxide and strontium ruthenate electrodes for ferroelectric thin-films capacitors

Metallic ruthenium and ruthenium oxides, such as SrRuO3 and RuO2, are potential electrode materials for ferroelectric capacitors. The electrical properties (e.g. leakage currents) of such thin film devices are dependent on the electronic properties of the electrode/ferroelectric junctions and therefore also on the electrode work functions. During growth and processing of film-electrode layer structures the formation of sub-oxides within the electrode is possible, with their work functions being unknown. In order to obtain information for predicting device properties, we have systematically analysed the valence bands and work functions of RuOx and SrRuOy thin films with different oxidation states by using photoelectron spectroscopy techniques. The results suggest that Ru-0 and Ru4+ ions are present in co-existence at the surfaces of oxygen-deficient polycrystalline films (inhomogeneous oxidation). For both oxygen-deficient materials the work function coincides with that of metallic ruthenium (4.6 +/- 0.1 eV). Only for fully oxidised ruthenium oxide and strontium ruthenate films (no Ru-0 present at the surface) is the work function increased to 5.0 or 4.9 eV, respectively. As an example of importance for new dynamic random access memory applications, the junctions of Ba1-xSrxTiO3 with SrRuOy and RuOx are discussed.