Chemical and electronic properties of interfaces between RuO2 and Hf0.5Zr0.5O2 studied by x-ray photoelectron spectroscopy

Recently, the extensive study of the platinum group metal oxide electrodes for TiN replacement in Hf0.5Zr0.5O2 (HZO)-based ferroelectric memory stacks began. In this work, we use high sensitivity of x-ray photoelectron spectroscopy to the interface chemical and electronic states to investigate the chemistry and band alignment between HZO and RuO2, which is one of the most suitable noble metal oxide electrodes. We report that RuO2 is easily reduced to metallic Ru during the atomic layer deposition (ALD) of HZO because of the chemical reaction between the oxygen stored in RuO2 and metallic precursors used for HZO deposition. This process makes it impossible to use RuO2 as a bottom electrode, when HZO is grown by the conventional thermal ALD process (with H2O as a reactant). At the same time, RuO2 may be effectively conserved if one uses more active oxygen reactants for the HZO growth because of the re-oxidation of the newly formed Ru. RuO2 is also effectively conserved at the top interface, at least within the thermal budget of the growth of all functional layers. We also report the detailed investigation of the band alignment between RuO2 and HZO in both type of stacks (with reduced and conserved RuO2) and both interfaces with HZO (top and bottom), which may be useful for the understanding of the ferroelectric properties of the related stacks in the future. In addition, the potential of RuO2 utilization for the reliability improvement of HZO-based stacks is discussed.