Bond nature of active metal ions in SiO2-based electrochemical metallization memory cells

Electrochemical metallization cells are candidates for the next-generation non-volatile memory devices based on resistive switching. Despite the intensive studies in recent years a microscopic model of the processes in these nanoscale electrochemical systems is still missing and the physicochemical properties of the active metal ions have been rarely reported. We examined the bonding characteristics of Cuz+ and Ag+ ions in SiO2-based cells using soft X-ray absorption spectroscopy. Whereas the Ag/SiO2 interfaces showed no chemical interaction of Ag ions, the Cu/SiO2 showed clear signatures of partial oxidation into two ionic species of Cu2+ and Cu+. The analyses on the orbital hybridization strength evidently showed that the Cu2+-O2- bonds in SiO2 are much weaker than the Cu+-O2- bonds, analogous to the case of bulk CuO and Cu2O. This suggests that the Cu2+ ions should be more mobile and with a dominating role in the process of resistive switching.