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

Metallic ruthenium and ruthenium oxides, such as SrRuO3 and RuO2, are poten tial electrode materials for ferroelectric capacitors. The electrical prope rties (e.g. leakage currents) of such thin film devices are dependent on th e electronic properties of the electrode/ferroelectric junctions and theref ore also on the electrode work functions. During growth and processing of f ilm-electrode layer structures the formation of sub-oxides within the elect rode is possible, with their work functions being unknown. In order to obta in information for predicting device properties, we have systematically ana lysed the valence bands and work functions of RuOx and SrRuOy thin films wi th different oxidation states by using photoelectron spectroscopy technique s. The results suggest that Ru-0 and Ru4+ ions are present in co-existence at the surfaces of oxygen-deficient polycrystalline films (inhomogeneous ox idation). For both oxygen-deficient materials the work function coincides w ith that of metallic ruthenium (4.6 +/- 0.1 eV). Only for fully oxidised ru thenium 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 exam ple of importance for new dynamic random access memory applications, the ju nctions of Ba1-xSrxTiO3 with SrRuOy and RuOx are discussed.