Oxygen Vacancy Energetics and Kinetics at Hf _0.5 Zr _0.5 O ₂ –WO ₃ Interfaces: Impact on Ferroelectric Behavior and Device Functionality

Tungsten is a common electrode material for Hf0.5Zr0.5O2 (hafnium zirconium oxide) ferroelectric devices, and WO3 can form at the interfaces. Here, the role of WO3 interfacial layers in controlling the oxygen vacancy behavior in Hf0.5Zr0.5O2–WO3 (HZO/WO3) heterostructures is investigated using density functional theory (DFT). Charge transition level (CTL) analysis shows that the (+2/0) level lies above the conduction band minimum of WO3, suggesting that oxygen vacancies are thermodynamically stable in the doubly charged state (VO2+) throughout the entire accessible Fermi-level range of the heterostructure. Corresponding formation energy calculations reveal that, at the Fermi level determined by the charge neutrality level calculations, these charged vacancies exhibit the lowest formation energies in the WO3 region (−0.44 eV) and progressively higher energies toward HZO (up to ∼1.26 eV), demonstrating a strong thermodynamic driving force for vacancy localization in WO3. Migration barrier calculations further reveal that VO2+ defects can diffuse readily from HZO to WO3, with interfacial barriers as low as 0.45–0.68 eV. These results demonstrate that WO3 serves as both a thermodynamic sink and a kinetic trap for charged oxygen vacancies, offering a promising pathway to suppress defect-induced degradation and improve endurance in HZO-based ferroelectric devices: instead of striving for an abrupt HZO/W interface, an engineered interlayer of WO3 would be advantageous.