Ferroelectricity Induced by Oxygen Vacancies in Rhombohedral ZrO₂ Thin Films

Abstract

Rhombohedral phase HfxZr1‐xO2 (HZO, x from 0 to 1) films are promising for achieving robust ferroelectric polarization without the need for an initial wake‐up pre‐cycling, as is normally the case for the more commonly studied orthorhombic phase. However, a large spontaneous polarization observed in rhombohedral films is not fully understood, and there are also large discrepancies between experimental and theoretical predictions. In this work, in rhombohedral ZrO2 thin films, we show that oxygen vacancies are not only a key factor for stabilizing the phase, but they are also a source of ferroelectric polarization in the films. This is shown experimentally through the investigation of the structural properties, chemical composition and the ferroelectric properties of the films before and after an annealing at moderate temperature (400 °C) in an oxygen environment to reduce the VO concentration compared. The experimental work is supported by density functional theory (DFT) calculations which show that the rhombohedral phase is the most stable one in highly oxygen defective ZrO2 films. The DFT calculations also show that VO contribute to the ferroelectric polarization. Our findings reveal the importance of VO for stabilizing rhombohedral ZrO2 thin films with superior ferroelectric properties.

Ferroelectric ZrO2 phases from infrared spectroscopy

“…This distorted t to r phase transition might offer a new path to investigate the high polarization reported experimentally in the r phase, which is in discrepancy with DFT predictions of low polarization. 34,92 However, other factors, such as oxygen vacancies and the interaction between the ZrO 2 film and the substrate, may also contribute to stabilizing the rhombohedral phase.…”

Ferroelectricity in Epitaxial Tetragonal ZrO2Thin Films

“…investigated the influence of oxygen vacancies on the ferroelectric polarization of both the r and t ‐phases of ZrO 2 , revealing that oxygen vacancies can indeed promote a polarization in the t ‐phase of ZrO 2 . [ 24 ] However, at two distinct oxygen vacancy concentrations of 4.16% and 0.51%, the resulting polarization was found to remain lower than 1 µC cm −2 . [ 24 ] Thus, oxygen vacancies alone cannot account for the polarization levels measured in our ZrO 2 films (Figure 4).…”

“…[ 24 ] However, at two distinct oxygen vacancy concentrations of 4.16% and 0.51%, the resulting polarization was found to remain lower than 1 µC cm −2 . [ 24 ] Thus, oxygen vacancies alone cannot account for the polarization levels measured in our ZrO 2 films (Figure 4). Another factor is electric‐field induced oxygen migration.…”

“…[21][22][23] In recent years, the ferroelectric properties of pure ZrO 2 have garnered considerable attention, primarily due to its resemblance to HfO 2 , as well as its greater abundance in nature compared to HfO 2 . [24] Notably, investigations of ZrO 2 have been conducted in various forms, such as pure ZrO 2 in polycrystalline thin films, leading to the identification of the o-phase. [25][26][27][28] The same ophase has also been observed in epitaxial thin films, [29] for which both the r-phase [23,30] and the P4 2 /nmc t-phase (t-phase) [31] have been reported as well.…”

Ferroelectric Orthorhombic ZrO2 Thin Films Achieved Through Nanosecond Laser Annealing

“…[20] Although HZO shows the largest polarisation values, ZrO 2 is much more abundant in nature and therefore is a more appealing choice for large-scale use. [21] The emergence of the ferroelectricity in ZrO 2 arises from the o-phase (space group: Pca2 1 ) [22] and the polar rhombohedral (r-)phase (space group: R3m) [23] that can be only achieved in a constrained environment in the thin film form. It was recently revealed that ferroelectricity and hysteretic polarization switching can be achieved in ultrathin ZrO 2 with the fluorite-structure unit-cell size [24] for a film thickness of 5 Å.…”