Wake-Up in a Hf_0.5Zr_0.5O₂ Film: A Cycle-by-Cycle Emergence of the Remnant Polarization via the Domain Depinning and the Vanishing of the Anomalous Polarization Switching

Abstract

The mechanism of the remnant polarization (P r) growth during the first stage of ferroelectric HfO2-based memory cell operation (the wake-up effect) is still unclear. In this work, we reveal the microscopic nature of the P r growth in functional ferroelectric capacitors based on a polycrystalline 10 nm thick (111) out-of-plane textured Hf0.5Zr0.5O2 film during electric cycling. We observe the cycle-by-cycle evolution of the domain structure with the piezoresponse force microscopy (PFM). During the early stage of the wake-up, three types of domains are found: (i) normal domains (polarization aligned along the applied electric field), (ii) nonswitchable domains with upward and downward polarization, and (iii) domains with anomalous polarization switching (polarization aligned against the applied electric field) that are commonly surrounded by nonswitchable domains. Initially, nonswitchable and “anomalous” domains are 200–300 nm in width, and they occupy ∼70% of the capacitor area. During electric field cycling, these domains reduce in area, which is accompanied by the P r growth. We attribute the domain pinning and the anomalous polarization reversal to the internal bias field of the oxygen vacancies. The local density of the oxygen vacancies decreases during electric cycling, thus producing the reduction of the internal bias field. The correlation of the PFM data with both the results of the structural analysis of fresh and cycled Hf0.5Zr0.5O2 film by transmission electron microscopy and the performance of the ferroelectric capacitor indicates that after the first cycle of the wake-up the P r growth is not associated with phase transformations, but only with the transformation of the domain structure. The obtained results elucidate the physical mechanism of the emergence of P r during the wake-up of the ferroelectric HfO2-based memory cell.

On the Origin of Wake‐Up and Antiferroelectric‐Like Behavior in Ferroelectric Hafnium Oxide

“…Furthermore, it can be described by two current peaks popping up at either direction of the current-voltage (I-V ) curve that might even merge to a single peak then resembling the standard distribution of coercive fields (E C ), as shown in Figure 1b. This behavior has been reported already by multiple groups [6,18,22,26,29,[31][32][33][34] and will be referenced to as the classical wake-up effect.…”

Nonlinear domain wall velocity in ferroelectric Si-doped HfO2 thin film capacitors

“…Figures 1(a) and 1(b) show the P-V hysteresis loops and the corresponding current-voltage (I-V) curves of the Si:HfO2 capacitors, respectively. All data were measured by triangular pulses with a frequency of 1 kHz and an amplitude of ±2.5 V. To check the effects of electric field cycling on the ferroelectric properties, i.e., the so-called wake-up effect, 9,10,26 1000 cycles of rectangular pulses at an amplitude of ±2.5 V and a frequency of 20 kHz were applied to the pristine specimen. Note that the P-V and I-V data did not show any further change after 1000 cycles (up to ~ 10 5 cycles, not shown).…”

Wake‐Up Effect in HfO2‐Based Ferroelectric Films

“…In addition to FORCs, the in situ technique is also an effective way to characterize the dynamic behavior of oxygen vacancies during cycling, and it is also helpful to understand the mechanism of a phase transformation. [109,110] Consequently, it has become a common way to investigate the root causes of the wake-up effect in HfO 2 -based ferroelectric films in recent years. Lin et al investigated the role of oxygen vacancies during the wake-up process using in situ synchrotron X-ray techniques.…”