Wake-up Free Ferroelectric Rhombohedral Phase in Epitaxially Strained ZrO₂ Thin Films

Abstract

Zirconia- and hafnia-based thin films have attracted tremendous attention in the past decade because of their unexpected ferroelectric behavior at the nanoscale, which enables the downscaling of ferroelectric devices. The present work reports an unprecedented ferroelectric rhombohedral phase of ZrO2 that can be achieved in thin films grown directly on (111)-Nb:SrTiO3 substrates by ion-beam sputtering. Structural and ferroelectric characterizations reveal (111)-oriented ZrO2 films under epitaxial compressive strain exhibiting switchable ferroelectric polarization of about 20.2 μC/cm2 with a coercive field of 1.5 MV/cm. Moreover, the time-dependent polarization reversal characteristics of Nb:SrTiO3/ZrO2/Au film capacitors exhibit typical bell-shaped curve features associated with the ferroelectric domain reversal and agree well with the nucleation limited switching (NLS) model. The polarization-electric field hysteresis loops point to an activation field comparable to the coercive field. Interestingly, the studied films show ferroelectric behavior per se, without the need to apply the wake-up cycle found in the orthorhombic phase of ZrO2. Overall, the rhombohedral ferroelectric ZrO2 films present technological advantages over the previously studied zirconia- and hafnia-based thin films and may be attractive for nanoscale ferroelectric devices.

Influence of Si-Doping on 45 nm Thick Ferroelectric ZrO2 Films

“…Since ferroelectric Si-doped HfO 2 and Hf 1– x Zr x O 2 (HZO) thin films have been discovered, they have attracted abundant interest due to their remarkable ferroelectricity and excellent compatibility with the complementary metal–oxide–semiconductor process, , leading to new developments in future and emerging non-volatile memory technologies. − Even though there are a few reports of rhombohedral polar phases in epitaxial films, − the origin of ferroelectricity in HfO 2 -based polycrystalline thin films is mostly reported to be due to a non-centrosymmetric Pbc 2 1 orthorhombic (o) phase. ,, A paraelectric P 2 1 / c monoclinic (m) phase is the major phase in bulk fluorite HfO 2 and ZrO 2 ceramics at room temperature . In pure HfO 2 thin films, the m-phase is the primary phase, which results in paraelectric film properties. , As a component in the HZO system, undoped ZrO 2 thin films have been investigated less intensely due to the loss of ferroelectric properties at thicknesses of 10 nm .…”

“…Overall, the formation of the o- and the t-phase in thin ZrO 2 films seems to depend on different factors. For instance, electrode materials , and interface layers can play a role, but stress and strain can favor the o-phase too. In contrast, crystallization in the m-phase is often observed for atomic layer deposited (ALD)-doped HfO 2 or mixed Hf x Zr 1– x O 2 films above 20 nm. ,, …”

Impact of non-ferroelectric phases on switching dynamics in epitaxial ferroelectric Hf0.5Zr0.5O2 films

“…[28][29][30] Fast switching (around 10 ns) has also been observed in ferroelectric ZrO 2 films. 31,32 It is commonly found that switching dynamics follows Nucleation Limited Switching (NLS) model, 33 as expected for polycrystalline films, contrary to the Kolmogorov-Avrami-Ishibashi (KAI) model 34,35 expected for films of high crystalline quality. 36 It has been reported that the increase of remanent polarization ascribed to the increase of non-pinned ferroelectric orthorhombic domains by doping or electric cycling is correlated with an increase of the characteristic switching time.…”

Novel Fluorite‐Structured Materials for Solid‐State Refrigeration

“…[121] HfO 2 -and ZrO 2 -based films exhibit rapid switching (in the nanosecond range). [122] Moreover, HZO films show good FE properties even at an ultralow thickness (≈1 nm), which is not the case for perovskite FE thin films. [123] Flexible HZO films can acquire FE properties up to 420 K and are suitable for flexible cooling devices.…”

“…[ 121 ] HfO 2 ‐ and ZrO 2 ‐based films exhibit rapid switching (in the nanosecond range). [ 122 ] Moreover, HZO films show good FE properties even at an ultralow thickness (≈1 nm), which is not the case for perovskite FE thin films. [ 123 ]…”