Enhanced ferroelectricity in ultrathin films grown directly on silicon

Abstract

The critical size limit of electric polarization remains a fundamental question in nanoscale ferroelectric research 1 . As such, the viability of ultrathin ferroelectricity greatly impacts emerging low-power logic and nonvolatile memories 2 . Size effects in ferroelectrics have been thoroughly investigated for perovskite oxides -the archetypal ferroelectric system 3 . Perovskites, however, have so far proved unsuitable for thickness-scaling and integration with modern semiconductor processes 4 . Here, we report ultrathin ferroelectricity in doped-HfO2, a fluorite-structure oxide grown by atomic layer deposition on silicon. We demonstrate the persistence of inversion symmetry breaking and spontaneous, switchable polarization down to 1 nm. Our results indicate not only the absence of a ferroelectric critical thickness, but also enhanced polar distortions as film thickness is reduced, contradictory to perovskite ferroelectrics. This work shifts the focus on the fundamental limits of ferroelectricity to simpler transition metal oxide systems -from perovskite-derived complex oxides to fluoritestructure binary oxides -in which 'reverse' size effects counter-intuitively stabilize polar symmetry in the ultrathin regime.Ferroelectric materials exhibit stable states of collectively ordered electrical dipoles whose polarization can be reversed under an applied electric field 5 . Consequently, ultrathin ferroelectrics are of great technological interest for high-density electronics, particularly field-effect transistors and nonvolatile memories 2 . However, ferroelectricity is typically suppressed at the few nanometer scale in the ubiquitous perovskite oxides 6 . First-principles calculations predict six unit cells as the critical thickness in perovskite ferroelectrics 1 due to incomplete screening of depolarization fields 3 . Atomic-scale ferroelectricity in perovskites often fail to demonstrate polarization switching 7,8 , a crucial ingredient for application. Furthermore, attempts to synthesize ferroelectric perovskite films on silicon 9,10 are plagued by chemical incompatibility 4,11 and high temperatures required for epitaxial growth. Since the discovery of ferroelectricity in HfO2-based thin films in 2011 12 , fluorite-structure binary oxides (fluorites) have attracted considerable interest 13 as they enable lowtemperature synthesis and conformal growth in three-dimensional (3D) structures on silicon 14,15 , thereby overcoming many of the issues that restrict its perovskite counterparts in terms of complementary metal-oxide-semiconductor (CMOS) compatibility and thickness scaling 16 .

Guidelines for the stabilization of a polar rhombohedral phase in epitaxial Hf0.5Zr0.5O2 thin films

“…It appears that the stabilization of the polar o-phase (red box in Figure 9(c)) is a result of the inhomogeneous strain fields originating at the intersection of various kinds of nanoscopic monoclinic domains that form accordion-like structures, as shown in Figure 9(c). Following this work, Cheema et al [61] have reported well textured ultrathin layers of HZO directly grown on Si using atomic layer deposition. The authors report that the films below 2.5 nm are well-oriented, perhaps predominantly along (100) (and definitely not (111)-from pole figures).…”

Exploring Ferroelectric Switching in α‐In2Se3 for Neuromorphic Computing

“…© 2020 Wiley-VCH GmbH it requires a specific capping layer (TiN) and high-temperature (>400 °C) annealing process. [48,49] Thus, there are advantages to explore 2D ferroelectrics based on α-In 2 Se 3 both from device as well as material processing perspectives.…”

Tunable Microwave Filters Using HfO2-Based Ferroelectrics

“…In order to test the ferroelectric behaviour of the HfZrO thin film, we obtained the polarization versus voltage (P–V) loop. During the initial electric field cycling, the remanent polarization ( P r ) was found at ~0.8 μC/cm 2 and the coercive voltage at ~2.6 V. A well-known phenomenon in ferroelectrics is the wake-up effect, which does not appear in the case when HfZrO is grown directly on Si (as in our case) even if the HfZrO has a thickness of 1 nm [ 17 ].…”