Decoupling effects of the resistive-switching behavior on the polarization reversal in ultrathin ferroelectric Hf0.5Zr0.5O2 films

HfO2-based ferroelectric films have attracted considerable attention as their nanoscale ferroelectricity and compatibility with cmos technology, fulfilling demands of emerging memory technologies. However, as films scale down, resistive-switching behavior becomes increasingly pronounced, intricately intertwining with the polarization-switching process and affecting ferroelectric switching factors often overlooked yet crucial for device performance optimization. By characterizing resistive-switching behavior and oxygen vacancy motion using tailored electric pulse schemes, we decouple the resistive-switching behavior from the overall switching process in ultrathin ferroelectric HZO films, which would otherwise erroneously inflate polarization values and increase coercive fields. Building on this, we elucidate endurance degradation mechanisms from dual perspectives of resistive switching and defect migration. Furthermore, we demonstrate the mitigated resistive switching activity by designing HfO2-based devices with symmetric oxide electrodes, achieving reduced coercive fields and improved cycling performances. This work provides crucial insights into the origins of inflated polarizations and reliability challenges in HfO2-based devices while offering a viable strategy to enhance ferroelectric properties for advanced memory applications.

💡 Research Summary

This paper investigates the intertwined phenomena of resistive switching (RS) and polarization reversal (PR) in ultrathin (≈6 nm) Hf₀.₅Zr₀.₅O₂ (HZO) ferroelectric films, a problem that the authors term the CPR (combined polarization‑resistive) effect. Using Pt/HZO/La₀.₆₇Sr₀.₃₃MnO₃ (LSMO) metal‑ferroelectric‑metal capacitors, the authors first demonstrate that, as the film thickness approaches the nanometer scale, oxygen‑vacancy (V_O) migration becomes dominant, giving rise to a pronounced memristive RS that co‑exists with ferroelectric switching. Conductive atomic‑force microscopy (C‑AFM) reveals spatially localized conductive filaments under a negative bias (‑2 V) that disappear under a positive bias (+2 V), directly visualizing the formation and rupture of V_O‑rich conductive paths.

A parallel‑circuit model comprising a ferroelectric capacitor and a memristor captures the device’s electrical response. The voltage drop across the capacitor (U_c) depends on the memristance (M); a low‑resistance state (LRS) reduces U_c below the intrinsic coercive voltage (E_c), suppressing true polarization reversal and producing “lossy” P‑E loops with inflated apparent polarization. Conversely, when the device switches to a high‑resistance state (HRS), M increases, U_c rises above E_c, and clean ferroelectric hysteresis is recovered. This explains the anomalous early‑cycle behavior where measured polarization appears reduced, yet the apparent polarization values are artificially enlarged due to the superposition of RS currents.

The authors further show that the CPR effect lengthens switching times and raises the effective coercive field because the RS‑induced voltage division forces the ferroelectric domain to experience a weaker driving field. Leakage currents also acquire hysteresis, further distorting the intrinsic ferroelectric current loops.

To mitigate these detrimental interactions, two strategies are introduced. First, symmetric oxide electrodes (LSMO on both sides) are employed, which suppress the asymmetric V_O drift that fuels RS. Second, a “dielectric training” protocol—repeated low‑amplitude voltage pulses—is applied to pre‑condition the V_O distribution, driving the device into a stable HRS before normal operation. After training, the devices exhibit markedly lower leakage, reduced coercive fields (by ~0.8 MV cm⁻¹), and stable ferroelectric loops over >10⁶ switching cycles.

Overall, the work provides a quantitative framework for separating resistive and ferroelectric contributions in ultrathin Hf‑based ferroelectrics, identifies oxygen‑vacancy migration as the root cause of inflated polarization and endurance degradation, and demonstrates practical device‑level solutions that improve reliability for emerging FeRAM, FTJ, and in‑memory computing applications. The findings suggest that future scaling of HfO₂‑based ferroelectrics must consider defect‑engineered electrode symmetry and pre‑conditioning schemes to achieve both high polarization and long‑term endurance.