Kinetics of N‐ to M‐Polar Switching in Ferroelectric Al_1−xSc_xN Capacitors

Abstract

Ferroelectric wurtzite‐type aluminum scandium nitride (Al1−xScxN) presents unique properties that can enhance the performance of non‐volatile memory technologies. The realization of the full potential of Al1−xScxN requires a comprehensive understanding of the mechanism of polarization reversal and domain structure dynamics involved in the ferroelectric switching process. In this work, transient current integration measurements performed by a pulse switching method are combined with domain imaging by piezoresponse force microscopy (PFM) to investigate the kinetics of domain nucleation and wall motion during polarization reversal in Al0.85Sc0.15N capacitors. In the studied electric field range (from 4.4 to 5.6 MV cm−1), ferroelectric switching proceeds via domain nucleation and wall movement. The currently available phenomenological models are shown to not fully capture all the details of the complex dynamics of polarization reversal in Al0.85Sc0.15N. PFM reveals a non‐linear increase of both domain nucleation rate and lateral wall velocity during the switching process, as well as the dependency of the domain pattern on the polarization reversal direction. A continuously faster N‐ to M‐polar switching upon cycling is reported and ascribed to an increasing number of M‐polar nucleation sites and density of domain walls.

Ferroelectric Al0.85Sc0.15N and Hf0.5Zr0.5O2 Domain Switching Dynamics

“…The complex extrinsic nature of the ferroelectric switching process involving domain nucleation and domain wall motion is widely accepted. ,− The switching kinetics in ferroelectrics are determined by the relative contributions of domain nucleation and domain wall motion. When unrestricted sideways expansion of ferroelectric domains can be achieved after nucleation, the time-dependent normalized switched polarization follows the Kolmogorov–Avrami–Ishibashi (KAI) model Δ P norm = Δ P false( t false) 2 P s = 1 − e − true( t / t 0 true) n where P s is the spontaneous polarization, t 0 is the characteristic switching time, and n is the Avrami exponent, which is related to the effective dimension of domain growth.…”

“…We note that even if not discussed explicitly in the present analysis, grain boundaries represent another possible source of domain wall pinning and, hence, domain wall creep motion. PFM studies showed that ferroelectric domains in fluorite- and wurtzite-structured thin films encounter several grain boundaries during their sideways expansion and that larger lateral domain wall velocities can be achieved by increasing the grain size in fluorite-structured ferroelectrics. − The character of the grain boundary, and hence the crystalline regularity between neighboring grains, affects the way in which domain walls move . In contrast to Al 0.85 Sc 0.15 N in which the grains have a similar crystallite orientation along the direction of the applied field, the different crystallite orientations between neighboring Hf 0.5 Zr 0.5 O 2 grains may significantly slow down or even impede the domain wall motion for subcoercive applied electric field magnitudes.…”

“…The field-driven configurations of ferroelectric domains and dynamics control the number of achievable partial polarization states and can affect data storage reliability as well as the speed of the state update. ,− Hence, a thorough understanding of the domain switching dynamics involved in the polarization reversal mechanism is required to achieve full control of the multibit capability and understand reliability performances, such as the retention of the intermediate polarization states. In ferroelectrics, polarization reversal relies on the complex interplay between domain nucleation and domain wall motion. ,− Several phenomenological theories have been developed to predict the time-dependent evolution of the switched polarization. − Nonetheless, phenomenological theories do not account explicitly for the correlation between domain dynamics and the crystal structure of the film. Understanding the influence of the crystal structure on the switching kinetics in thin films is crucial for application-oriented material optimization toward a large number of reliable partial polarization states programmable at high speed with limited device-to-device variability. ,,, Given the significant crystallographic differences between wurtzite- and fluorite-structured ferroelectrics, the comparison of these two material systems can provide important insights in understanding the influence of the material crystal structure on the domain dynamics.…”