Influence of ZrO2 Seed and Nb2O5 Capping Layers on the Ferroelectric Characteristics of 4 nm La-Doped Hafnium Zirconium Oxide Capacitors

Abstract

While sub-5 nm hafnium zirconium oxide (HZO) films are important for scaled ferroelectric memory devices, stabilizing their ferroelectric characteristics becomes increasingly challenging because the reduced thickness makes phase stabilization highly sensitive to microstructure, stress, and defect chemistry. Here, we evaluate 4 nm La-doped HZO (La:HZO) capacitors using an interfacial engineering strategy combining post-metallization annealing (PMA), ZrO2 seed layer control, and Nb2O5 capping layer. PMA provides a mechanically confined crystallization environment through the presence of the top electrode during annealing. Building upon this confined crystallization environment, the effects of the ZrO2 seed layer deposition temperature and the introduction of the Nb2O5 capping layer on the structural and ferroelectric characteristics of La:HZO were systematically investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), grazing-incidence X-ray diffraction (GIXRD), in-plane XRD, X-ray photoelectron spectroscopy (XPS), and positive-up-negative-down (PUND) measurements. Capacitors incorporating only the ZrO2 seed layer exhibit weak ferroelectric switching at 3 MV/cm and early electrical breakdown, indicating that oxygen vacancy-related effects strongly limit polarization switching. Introducing a 2 nm Nb2O5 capping layer decreases the relative intensity of the low-binding-energy shoulders in the Hf 4f and Zr 3d spectra, consistent with a reduced contribution of oxygen-deficient local environments. PUND measurements further reveal a progressive reduction in the leakage current component during the ‘U’ pulse, which is attributed to the oxygen-donating role of the Nb2O5 capping layer, whereas only a weak change is observed during the ‘D’ pulse. This polarity-dependent behavior is consistent with oxygen vacancy compensation near the Nb2O5 capping layer during cycling. Among all investigated device configurations, the capacitor incorporating the 250 °C ZrO2 seed layer and the Nb2O5 capping layer exhibits the highest 2Pr,rel. These results indicate that grain evolution, nonswitching current behavior, and the oxygen-related defect environment are correlated with the ferroelectric response of 4 nm La:HZO capacitors and should be considered together when designing the stack.