Oxygen Vacancy Dynamics in Different Switching Modes of Hf_0.5Zr_0.5O_2-δ

HfO₂, one of the most common materials in resistive switching devices, can stabilize in a ferroelectric orthorhombic phase, enabling two nonvolatile polarization states via oxygen displacement in the unit cell. Under certain conditions, ferroelectric and resistive switching can coexist, independently addressable, within one device. This study employs operando spectroscopic analysis to elucidate the role of oxygen in both switching processes. A conductive filament is identified through a local valence change at the oxide surface via X-ray Photoelectron Emission Microscopy, allowing vacancy density and filament diameter evaluation. This provides well-founded experimental evidence of a conductive filament in orthorhombic Hf_0.5Zr_0.5O_2-δ (HZO) in application-relevant device geometry. Depth-dependent changes in the electronic signature of HZO and La_0.8Sr_0.2MnO_3-δ (LSMO) with ferroelectric field cycling are identified by Hard X-ray Photoelectron Spectroscopy. Polarization-dependent shifts in the Hf core level align with the oxygen vacancy migration during ferroelectric switching. Fatigue-related vacancy generation causes an inhomogeneous reduction that does not propagate into the bottom electrode and extended domain pinning at the HZO/LSMO interface. This highlights the importance of interface engineering for the ferroelectric performance and of the oxygen affinity of the bottom electrode for both switching regimes.