Wake-up and fatigue mechanisms in ferroelectric Hf0.5Zr0.5O2 films with symmetric RuO2 electrodes

Abstract

The mechanisms leading to wake-up and fatigue in ferroelectric hafnium zirconium oxide thin film devices with symmetric RuO2 electrodes are investigated via polarization, relative permittivity, dielectric nonlinearity, pyroelectric coefficient, and microfocus x-ray diffraction (XRD) measurements. The devices are observed to wake-up for up to 103 bipolar pulsed field cycles, after which fatigue occurs with polarization approaching zero following 108 cycles. Wake-up is accompanied by a decrease in both high-field permittivity and hysteresis loop pinching and an increase in the pyroelectric coefficient, indicating that the wake-up process involves a combination of transformations from the tetragonal to the orthorhombic phase and domain depinning from defect redistribution. Fatigue is observed to coincide with an increase in irreversible domain wall motion and a decrease in pyroelectric coefficient. Finite pyroelectric coefficients are measured on fully fatigued devices, indicating that domain pinning is a strong contributor to fatigue and that fatigued devices contain domain structures that are unable to switch under the fields applied for measurement. Microfocus XRD patterns measured on each device reveal that the phase constitution is qualitatively unaffected by field cycling and resultant polarization fatigue. These data indicate that the wake-up process has contributions from both phase transformations and domain depinning, whereas the fatigue process is driven primarily by domain pinning, and the near-zero measured switchable polarization is actually a poled device with immobile domains. These observations provide insight into the physical changes occurring during field cycling of HfO2-based ferroelectrics while examining a possible oxide electrode material for silicon CMOS device implementation.

Hafnium Oxide (HfO2) – A Multifunctional Oxide: A Review on the Prospect and Challenges of Hafnium Oxide in Resistive Switching and Ferroelectric Memories

“…Generally, several mechanisms have been presumed to be the cause of fatigue in different types of ferroelectrics during electric field cycling. [356][357][358]365,412,413] From the fabrication perspective, the formation of interfacial layers with nonpolar characteristics and low ε r is believed to be the main cause of fatigue. [356][357][358] Lou et al [357,358] suggested that the fatigue is initiated by charge injection from electrodes and induced by the subsequent local phase decomposition.…”

“…[357,358] Another mechanism presented is based on the accumulation of oxygen vacancies. [356,412,413] The oxygen vacancies were claimed to migrate and accumulate at the film/electrode interfaces and film grain boundaries during field cycling, which causes domain-wall pinning. [356,412,413] Metal electrodes were thought to block the migration of oxygen vacancies, resulting in a significant accumulation of these defects at the metal/dielectric interfaces and leading to pronounced fatigue.…”

“…[356] The accumulation of oxygen vacancies at the grain boundaries, which pin domain walls and decrease the polarization switching under applied electric field can gradually decrease P r value (Figure 24d). [412,413] Therefore, the formation of large grains during the fabrication process may increase the number of cycles to fatigue. [414][415][416] Huang et al [412] showed that the domain-wall pinning caused by carrier injection at [315] Copyright 2021, ACS.…”

“…Reproduced with permission. [413] Copyright 2021, AIP. b) A snapshot of the non-equilibrium transient state at the very early switching stage.…”

“…Fatigue in an HfO 2 -based ferroelectric; a) Switchable polarization ( | | | | P P r r = + + −) quantified as a function of the number electric field cycle, 2.0 MV cm -1 , and 10 kHz bipolar pulses. Reproduced with permission [413]. Copyright 2021, AIP.…”