Operando Spectroscopic Investigation of the Valence Change Mechanism in La2NiO4+δ -Based Memristive Devices

Valence change memory devices, based on redox reactions and oxygen dynamics, are considered to be one of the most promising candidates for the next generation of non-volatile memory devices and neuromorphic architectures. Devices based on La2NiO4+delta have demonstrated analog resistive switching behavior, but the underlying mechanism is not fully understood. To get a profound understanding of the device's behavior, the employment of element-selective techniques to provide direct information on oxygen migration is of paramount importance. In this work, TiN/La2NiO4+delta/Pt devices are studied using an original operando X-ray absorption near edge spectroscopy (XANES) methodology based on monitoring absorbance intensity changes at a fixed energy position which, in combination with in situ electron energy-loss spectroscopy (EELS) measurements, has provided valuable insights into the resistive switching mechanism. This approach allows to study the formation of the TiNxOy interlayer at the TiN/La2NiO4+delta interface and directly monitor oxygen migration between TiNxOy and La2NiO4+delta. An energy shift of the Ni K-edge spectra is consistently measured during the device operation as it underwent cycling in both voltage polarities, thus confirming the pivotal role of the valence change mechanism in the resistive switching behavior of these devices. Furthermore, a switching model based on the coexistence of filamentary and interfacial switching is proposed.