The effects of annealing and wake-up cycling on the ferroelectricity of zirconium hafnium oxide ultrathin films prepared by remote plasma atomic layer deposition

Abstract

The crystalline phases and ferroelectric properties of Zr x Hf 1-x O 2 (ZHO) ultrathin films (5.6-5.8 nm in thickness) prepared by remote plasma atomic layer deposition (RP-ALD) before and after post-annealing and wake-up cycling have been investigated in this study. For the films with high ZrO 2 percentages, the plasma bombardment from RP-ALD was able to induce partial ferroelectric orthorhombic (o) crystallization during the film deposition stage. The as-deposited pure ZrO 2 ultrathin film exhibited a fully developed ferroelectric polarization hysteresis after wake-up cycling. For the films containing the ferroelectric o-phase in the as-deposited state, post-annealing the films followed by a wake-up cycling procedure could greatly promote ferroelectric switching while maintaining low leakage. The ferroelectric properties of the ZHO ultrathin films were highly dependent on the ZrO 2 -to-HfO 2 ratio and could be tailored by various combinations of post-annealing and wake-up cycling. An increase in the amount of HfO 2 has been found to be detrimental to the ferroelectricity, mainly due to the high crystallization temperature of HfO 2 . For the post-annealed ZHO ultrathin films, the ferroelectricity was governed by the relative amounts of the ferroelectric o-phase and the non-ferroelectric monoclinic (m) phase. The Zr 0.5 H 0.5 O 2 ultrathin film was the only composition which exhibited a large, stable polarization hysteresis after post-annealing but before wake-up cycling. The annealed Zr 0.5 H 0.5 O 2 ultrathin film is believed to contain a suitable mix of o and m phases to produce loose grain boundaries, allowing the ferroelectric o-phase crystallites to switch freely under electrical loading.

Stabilization of the Ferroelectric Phase in Epitaxial Hf1–xZrxO2 Enabling Coexistence of Ferroelectric and Enhanced Piezoelectric Properties

“…The second is electric field-induced t−o transition, which is usually considered the cause of the high polarization that some polycrystalline ZrO 2 and Zr-rich Hf 1−x Zr x O 2 films can present. 2,6,11,32 It has to be noted that field-induced t−o transition in polycrystalline films requires a large number of cycles, ranging from 160 to thousands. 6,32 Field-induced t−o transition has been also reported very recently in epitaxial Ydoped HZO, presenting the films' wake-up effect too, up to about 100 cycles.…”

“…2,6,11,32 It has to be noted that field-induced t−o transition in polycrystalline films requires a large number of cycles, ranging from 160 to thousands. 6,32 Field-induced t−o transition has been also reported very recently in epitaxial Ydoped HZO, presenting the films' wake-up effect too, up to about 100 cycles. 33 In contrast, there is no wake-up effect in our epitaxial ZrO 2 films (Supporting Information, Figure S6).…”

An Overview of Ferroelectric Hafnia and Epitaxial Growth

“…[ 87 ] Therefore, the wake‐up effect can also be considered as a thermally activated process whose degree is proportional to the process time and temperature. [ 104 ] Other factors such as change in radiation of γ‐ray, [ 113 ] change in annealing temperature, [ 114,115 ] change in electrode materials, [ 116 ] and NH 3 plasma treatment [ 117 ] can also help eliminate the wake‐up effect.…”

Defects in ferroelectric HfO2

“…Physical mechanisms behind fatigue, breakdown, wake-up and retention loss have been studied in a number of papers. [169][170][171][172][173][174][175] In the next section, we will briefly overview the results.…”