Ferroelectricity in Hf_0.5Zr_0.5O₂ Thin Films: A Microscopic Study of the Polarization Switching Phenomenon and Field-Induced Phase Transformations

Abstract

Because of their full compatibility with the modern Si-based technology, the HfO-based ferroelectric films have recently emerged as viable candidates for application in nonvolatile memory devices. However, despite significant efforts, the mechanism of the polarization switching in this material is still under debate. In this work, we elucidate the microscopic nature of the polarization switching process in functional HfZrO-based ferroelectric capacitors during its operation. In particular, the static domain structure and its switching dynamics following the application of the external electric field have been monitored with the advanced piezoresponse force microscopy (PFM) technique providing a nm resolution. Separate domains with strong built-in electric field have been found. Piezoresponse mapping of pristine HfZrO films revealed the mixture of polar phase grains and regions with low piezoresponse as well as the continuum of polarization orientations in the grains of polar orthorhombic phase. PFM data combined with the structural analysis of pristine versus trained film by plan-view transmission electron microscopy both speak in support of a monoclinic-to-orthorhombic phase transition in ferroelectric HfZrO layer during the wake-up process under an electrical stress.

Defect Engineering to Achieve Wake‐up Free HfO2‐Based Ferroelectrics

“…A strong electric field induces structural change in hafnia‐based thin films. Indeed, the transition from m‐ and t‐ to o‐phase was observed by different research groups, [ 22,23 ] which increases P r in HfO 2 ‐based ferroelectrics. Moreover, the oxygen vacancy diffusion into different regions changes the relative stability of polymorphs and consequently causes phase transition.…”

“…Another mechanism which is responsible for wake‐up effect is the structural transition from nonferroelectric (m or t) to ferroelectric (o) phase driven by a strong electric field. [ 22,23 ] Therefore, the wake‐up effect can be substantially reduced by the reduction of m‐ and t‐phase during thin‐film processing. [ 24 ] Goh et al [ 25 ] evaluated the effect of metal oxide (as an electrode) on the wake‐up behavior of the Hf 0.5 Zr 0.5 O 2 (HZO) film and they observed an improve in wake‐up behavior which could be due to the suppression of oxygen vacancy at the interfacial region.…”

Genuinely Ferroelectric Sub-1-Volt-Switchable Nanodomains in HfxZr(1–x)O2 Ultrathin Capacitors

“…For conventional ferroelectrics, there are techniques for verifying the ferroelectric origin of hysteretic response of the capacitor relying on the comparison of field-on (also called DC-on) and field-off hysteresis loops 23,25 . Recently, PFM study of domain dynamics in 10nm HZO in the capacitor geometry had been carried out 26 , however, no in-depth analysis of the field-on/off loops is available so far.…”

Ferroelectricity in Si-Doped Hafnia: Probing Challenges in Absence of Screening Charges

“…In this scenario, the probe being a non-blocking contact, a strong interaction of oxygen vacancies with the environment should be expected for the presence of water and oxygen radicals. Therefore, with the objective of tracking dynamic domain evolution, or operando analysis over hundreds of cycles, a passive BE-PFM analysis in the presence of a thin top electrode is likely to be more recommended, as recently reported by Chouprik and co-workers [35].…”

“…In agreement with previous reports on the limitations of PFM for the analysis of thin films, this indicates that our attempts to use PFM in ultrathin doped-HfO 2 are affected by severe electrostatic parasitic, with the response likely dominated by cantilever motion arising from long range electrostatics [32][33][34]. It must be noted that the applied polarization is a static dc stress applied by the probe, thus representing a different stress condition compared to what is commonly associated with the increase in volume fraction of orthorhombic phase by field-induced conversion or domain de-pinning [35].…”